Industrial Automation and Mechatronics

Motors and Drives Basics

2009-02-12T04:46:00.000-08:00

Selecting the right motor and drive combination can save energy and improve performance.

The venerable electric motor that was the muscle of the industrial revolution is becoming the smart muscle of the computer-controlled plant and commercial facility of the future. The advent of powerful, reliable electronic drives is keeping motors in the forefront of this technological evolution.

Electric motors have a tremendous impact on overall energy use. Between 30 to 40 percent of all fossil fuels burned are used to generate electricity, and two-thirds of that electricity is converted by motors into mechanical energy.

The Fundamentals of Energy Management series this month will focus on topics that will allow facility managers and engineers at commercial and industrial facilities to understand the basics of motors and drives. This information will help them to select and implement strategies with the goal of reducing motor and drive costs as well as decreasing downtime.

AC Induction Motors

AC induction motors are ideal for most industrial and commercial applications because of their simple construction and low number of parts, which reduce maintenance cost. Induction motors are frequently used for both constant-speed and adjustable speed drive (ASD) applications.

The two basic parts of an induction motor are the stationary stator located in the motor frame and the rotor that is free to rotate with the motor shaft. Today's motor design and construction are highly refined. For example, stator and rotor laminations have been designed to achieve maximum magnetic density with minimum core losses and heating. The basic simplicity of this design ensures high efficiency and makes them easily adaptable to a variety of shapes and enclosures.

A three-phase induction motor can best be understood by examining the three-phase voltage source that powers the motor. Three-phase currents flowing in the motor leads establish a rotating magnetic field in the stator coils. This magnetic field continuously pulsates across the air gap and into the rotor. As magnetic flux cuts across the rotor bars, a voltage is induced in them, much as a voltage is induced in the secondary winding of a transformer. Because the rotor bars are part of a closed circuit (including the end rings), a current begins to circulate in them. The rotor current in turn produces a magnetic field that interacts with the magnetic field of the stator. Since this field is rotating and magnetically interlocked with the rotor, the rotor is dragged around with the stator field.

When there is no mechanical load on the motor shaft (no-load condition), the rotor almost manages to keep up with the synchronous speed of the rotating magnetic field in the stator coils. Drag from bearing friction and air resistance prevents perfect synchronicity. As the load increases on the motor shaft, the actual speed of the rotor tends to fall further behind the speed of the rotating magnetic field in the stator. This difference in speed causes more magnetic lines to be cut, resulting in more torque being developed in the rotor and delivered to the shaft mechanical load. The rotor always turns at the exact speed necessary to produce the torque required to meet the load placed on the motor shaft at that moment in time. This is usually a dynamic situation, with the motor shaft speed constantly changing slightly to accommodate minor variations in load.

The rotor consists of copper or aluminum bars connected together at the ends with heavy rings. The construction is similar to that of a squirrel cage, a term often used to describe this type of ac induction motor.

The rotating magnetic field in the stator coils, in addition to inducing voltages in the rotor bars, also induces voltages in the stator and rotor cores. The voltages in these cores cause small currents, called eddy currents, to flow. The eddy currents serve no useful purpose and result in wasted power. To keep these currents to a minimum, the stator and rotor cores are made of thin steel discs called laminations. These laminations are coated with insulating varnish and then edge welded together to form a core. This type of core construction substantially reduces eddy current losses, but does not entirely eliminate them.

By varying the design of the basic squirrel-cage motor, almost any characteristic of speed, torque, and voltage can be controlled by the designer. To standardize motor features the National Electrical Manufacturers Association (NEMA) has established standards for a number of motor features.

The speed of an ac induction motor depends on the frequency of the supply voltage and the number of poles for which the motor is wound. The term poles refers to the manner in which the stator coils are connected to the three incoming power leads to create the desired rotating magnetic field. Motors are always wound with an even number of poles. The higher the input frequency, the faster the motor runs. The more poles a motor has, the slower it runs at a given input frequency. The synchronous speed of an ac induction motor is the speed at which the stator magnetic flux rotates around the stator core at the air gap. At 60 Hz the following synchronous speeds are obtained:

Number of poles RPM
2 3,600
4 1,800
6 1,200
8 900
10 720
12 600

Providing the motor is properly constructed, the output speed can be doubled for a given number of poles by running an ASD supplying the motor at an output frequency of 120 Hz.

The actual speed of an induction motor rotor and shaft is always somewhat less than its synchronous speed. The difference between the synchronous and actual speed is called slip. If the rotor rotated as fast as the stator magnetic field, the rotor conductor bars would appear to be standing still with respect to the rotating field. There would be no voltage induced in the rotor bars and no current would be set up to produce torque.

Induction motors are made with slip ranging from less than 5% up to 20%. A motor with a slip of 5% or less is known as a normal-slip motor. A normal-slip motor is sometimes referred to as a 'constant speed' motor because the speed changes very little from no-load to full-load conditions. A common four-pole motor with a synchronous speed of 1,800 rpm may have a no-load speed of 1,795 rpm and a full-load speed of 1,750 rpm. The rate-of-change of slip is approximately linear from 10% to 110% load, when all other factors such as temperature and voltage are held constant. Motors with slip over 5% are used for hard to start applications.

The direction of rotation of a poly-phase ac induction motor depends on the connection of the stator leads to the power lines. Interchanging any two input leads reverses rotation.

Torque and Horsepower
Torque and horsepower are two very important characteristics that determine the size of the motor for a particular application. Torque is the turning effort. For example, suppose a grinding wheel with a crank arm one-foot long takes a force of one pound to turn the wheel at steady rate. The torque required is one pound times one foot or one foot-pound. If the crank is turned twice as fast, the torque remains the same. Regardless of how fast the crank is turned, the torque is unchanged as long as the crank is turned at a steady speed.

Horsepower takes into account how fast the crank is turned. Turning the crank more rapidly takes more horsepower than turning the crank slowly. Horsepower is the rate of doing work. By definition, one horsepower equals 33,000 foot-pounds per minute. In other words, to lift a 33,000-pound load one foot in one minute would require one horsepower.

The discussion so far has only involved torque at a steady speed. More effort is required to start a load than to keep it going. An ac induction motor is built to supply the extra torque needed to overcome the inertia of starting a load. The speed-torque curve for a representative motor is shown in Figure 4 and illustrates in this example that the starting torque is 210% of rated-load torque.

Motor Losses and Loss Reduction Techniques

The only way to improve motor efficiency is to reduce motor losses. Since motor losses produce heat, reducing losses not only saves energy directly but can also reduce cooling load on a facility's air conditioning system.

Motor energy losses can be segregated into five major areas. Each area is influenced by the motor manufacturer's design and construction decisions. One design consideration, for example, is the size of the air gap between the rotor and the stator. Large air gaps tend to maximize efficiency at the expense of a lower power factor. Small air gaps slightly compromise efficiency while significantly improving power factor.

Motor losses may be grouped as fixed or variable losses. Fixed losses occur whenever the motor is energized and remain constant for any given voltage and speed. Variable losses increase with an increase in motor load. Core loss and friction windage losses are fixed. Variable losses include stator- and rotor-resistance losses and stray load losses.

Motor Economics
The principal factors in energy-saving calculations are motor efficiency, run hours (at a certain load), and the cost of electricity. When a motor runs at nearly full load for many hours at a facility with high electrical costs, the higher resulting savings will indicate the use of a 'premium efficiency' unit. In some cases, the savings may be great enough to warrant taking a perfectly serviceable older motor off-line and upgrading to a new, premium-efficiency model. For applications with less than continuous use or at lower than full loading, upgrading a working motor will usually not make sense.

Some other application considerations: The full-load speed of high-efficiency motors is usually somewhat higher than standard efficiency models. When applied to centrifugal loads such as pumps and fans, the higher speed can translate to an increased horsepower requirement, and this can offset the anticipated energy savings. The higher output speed may also increase overall output by allowing a piece of machinery to finish its job faster. Results will vary with the application. There are energy efficient motor designs that can exhibit some unusual characteristics such as lower power factor and higher locked rotor amperage; these must be considered when choosing a motor.

Adjustable Speed Drive Systems

Commercial and industrial firms today use adjustable-speed drive (ASD) systems for a variety of applications. Most common of these include standard pumps, fans, and blowers. Newer applications include hoists and cranes, conveyors, machine tools, film lines, extruders, and textile-fiber spinning machines.

Many applications have unique demands and characteristics.

Drive vendors have responded to this demand by producing a variety of drives. The combination of the many types of drives available and the abundance of applications has made the selection of the optimum drive for a given application a challenge.

New generation ASDs have evolved with advancements in solid-state electronics. ASDs can now be applied to ac motors regardless of motor horsepower or location within a facility and can be used to drive almost all types of motorized equipment, from a small fan to the largest extruder or machine tool. Commercial and industrial facilities can expect to dramatically reduce both energy consumption and operating and maintenance costs while offering improved operating conditions by using new generation electronic ASDs. The latest generation of ASDs allows ac induction motors to be just as controllable and efficient as their dc counterparts were.

Historically a variety of terms have been used to describe a system that permits a mechanical load to be driven at user-selected speeds. These terms include, but are not limited to:

Variable-Speed Drive
Variable-Frequency Drive
Adjustable-Frequency Drive
Adjustable-Speed Drive

The term variable implies a change that may or may not be under the control of the user. Adjustable is the preferred term since this refers to a change directly under control of the user. The term frequency can only be applied to drives with an ac output, while the term speed is preferred since this includes both ac and dc drives. Thus, the term most commonly accepted is Adjustable-Speed Drive (ASD).

Basic ASD Components
Most ASD units consist of three basic parts. A rectifier that converts the fixed frequency ac input voltage to dc. An inverter that switches the rectified dc voltage to an adjustable frequency ac output voltage. (The inverter may also control output current flow, if desired.) The dc link connects the rectifier to the inverter. A set of controls directs the rectifier and inverter to produce the desired ac frequency and voltage to meet the needs of the ASD system at any moment in time.

The advantages of ASDs do not stop with saving energy and improving control. ASD technology can now be applied to manufacturing equipment previously considered too expensive or uneconomical. Such applications are often unique to a particular industry and its equipment, or even to a particular facility. Cost benefits, such as those obtained from improved quality, may be desirable for each application.

ELECTRIC MOTOR CONTROLS Part 1

2009-02-06T01:42:00.000-08:00

Once the proper motor is selected, understanding the many various control devices available and

their uses and limitations becomes an important part related to reliable operation and protection of the motor and the personnel using the motor.

Motor Control Topics
There are four major motor control topics or categories to consider. Each of these has several subcategories and sometimes the subcategories overlap to some extent. Certain pieces of motor control equipment can accomplish multiple functions from each of the topics or categories.

The four categories include:

1) Starting the Motor
Disconnecting Means
Across the Line Starting
Reduced Voltage Starting

2) Motor Protection
Overcurrent Protection
Overload Protection
Other Protection (voltage, phase, etc)
Environment

3) Stopping the Motor
Coasting
Electrical Braking
Mechanical Braking

4) Motor Operational Control
Speed Control
Reversing
Jogging
Sequence Control

• An understanding of each of these areas is necessary to effectively apply motor control principles and equipment to effectively operate and protect a motor.

MOTOR STARTING

All motors must have a control device to start and stop the motor called a “motor controller”.

Motor Controller
A motor controller is the actual device that energizes and de-energizes the circuit to the motor so
that it can start and stop.

• Motor controllers may include some or all of the following motor control functions: starting, stopping, over-current protection, overload protection, reversing, speed changing, jogging, plugging, sequence control, and pilot light indication.

Controllers range from simple to complex and can provide control for one motor, groups of motors, or auxiliary equipment such as brakes, clutches, solenoids, heaters, or other signals.

Motor Starter
The starting mechanism that energizes the circuit to an induction motor is called the “starter” and must supply the motor with sufficient current to provide adequate starting torque under worst case line voltage and load conditions when the motor is energized.

• There are several different types of equipment suitable for use as “motor starters” but only two types of starting methods for induction motors:
1. Across the Line Starting
2. Reduced Voltage Starting
Across the Line Starting of Motors
Across the Line starting connects the motor windings/terminals directly to the circuit voltage “across the line” for a “full voltage start”.

• This is the simplest method of starting a
motor. (And usually the least expensive).

• Motors connected across the line are capable
of drawing full in-rush current and
developing maximum starting torque to
accelerate the load to speed in the shortest
possible time.

• All NEMA induction motors up to 200
horsepower, and many larger ones, can withstand full voltage starts. (The electric
distribution system or processing operation may not though, even if the motor will).
Across the Line Starters

Figure 26. Manual Starter
There are two different types of common “across the line” starters including
1. Manual Motor Starters
2. Magnetic Motor Starters

Manual Motor Starters
A manual motor starter is package consisting of a horsepower rated switch with one set of contacts for each phase and corresponding thermal overload devices to provide motor overload protection.

• The main advantage of a manual motor starter is lower cost than a magnetic motor starter with equivalent motor protection but less motor control capability.

• Manual motor starters are often used for smaller motors - typically fractional horsepower
motors but the National Electrical Code allows their use up to 10 Horsepower.

• Since the switch contacts remain closed if power is removed from the circuit without
operating the switch, the motor restarts when power is reapplied which can be a safety
concern.

• They do not allow the use of remote control or auxiliary control equipment like a magnetic
starter does.
Magnetic Motor Starters
A magnetic motor starter is a package consisting of a contactor capable of opening and closing a set

Figure 27. Magnetic Starter
of contacts that energize and de-energize the circuit to the motor along with additional motor
overload protection equipment.

Magnetic starters are used with larger motors (required above 10 horsepower) or where
greater motor control is desired.
• The main element of the magnetic motor starter is the contactor, a set of contacts operated by
an electromagnetic coil.

Energizing the coil causes the contacts (A) to close allowing large currents to be
initiated and interrupted by a smaller voltage control signal.

The control voltage need not be the same as the motor supply voltage and is often low
voltage allowing start/stop controls to be located remotely from the power circuit.
• Closing the Start button contact energizes the contactor coil. An auxiliary contact on the
contactor is wired to seal in the coil circuit. The contactor de-energizes if the control circuit
is interrupted, the Stop button is operated, or if power is lost.
• The overload contacts are arranged so an overload trip on any phase will cause the contactor
to open and de-energize all phases.
Reduced Voltage Starting of Motors
Reduced Voltage Starting connects the motor windings/terminals at lower than normal line voltage
during the initial starting period to reduce the inrush current when the motor starts.

• Reduced voltage starting may be required when:
S The current in-rush form the motor starting adversely affects the voltage drop on the
electrical system.
S needed to reduce the mechanical “starting shock” on drive-lines and equipment when
the motor starts.
• Reducing the voltage reduces the current in-rush to the motor and also reduces the starting
torque available when the motor starts.
• All NEMA induction motors can will accept reduced voltage starting however it may notprovide enough starting torque in some situations to drive certain specific loads.
If the driven load or the power distribution system cannot accept a full voltage start, some type of reduced voltage or "soft" starting scheme must be used.
• Typical reduced voltage starter types include:
1. Solid State (Electronic) Starters
2. Primary Resistance Starters
3. Autotransformer Starters
4. Part Winding Starters
5. Wye-Delta Starters

Reduced voltage starters can only be used where low starting torque is acceptable or a means exists to remove the load from the motor or application before it is stopped.

What is Industrial Ethernet?

2009-02-01T22:57:00.000-08:00

Industrial Ethernet Overview

The Ethernet network is a local-area network (LAN) protocol developed by Xerox Corporation in cooperation with DEC and Intel in 1976. Ethernet uses a bus or star topology, and supports data transfer rates of 10 Mbps (standard) or 100 Mbps (using the newer 100Base-T version).

The Ethernet specification served as the basis for the IEEE 802.3 standard, which specifies the physical and lower software layers. Ethernet uses the CSMA/CD access method to handle simultaneous demands. It is one of the most widely implemented LAN standards.

OSI Reference Model

Developed by International Standards Organization (ISO) and stands for Open Systems Interconnection (OSI).
It is designed to deal with connecting open systems to communicate with other systems.
It consists of seven layers: a complex structure is partitioned into a number of independent functional layers.
Each layer provides a set of services by performing some well-defined sets of functions. These services are provided by the layered-specific functional entities.
Services at a layer can only be accessed from the layer immediately above it.
Each layer uses only a well-defined set of services provided by the layer below.
Protocols operate between "peer" entities in the different end systems (peer-to-peer protocol rules)

Advantages:
More manageable -Layer N is smaller and built only on Layer (N-1).
Modularity - Different layers can be developed separately and each layer can be modified without affecting other layers as long as the interfaces with immediate layers are kept

Brief Description of model in Each Layer

Physical Layer
The physical layer is responsible for passing bits onto and receiving them from the communication channel.
This layer has no understanding of the meaning of the bits, but deals with the electrical and mechanical characteristics of the signals and signalling methods.

Data Link Layer
Data link layer is responsible for both Point-to-Point Network and Broadcast Network data transmission.
It hides characteristics of the physical layer (e.g. transmission hardware from the upper layers.
It is also responsible to convert transmitted bits into frames
It transmits the frames into an error free transmission line by adding error control and flow control.

Network Layer
Network layer is responsible for the controls of routers and subnets operation.
It also handles the formation and routing of packets from source to destination with congestion control.

Transport Layer
Transport layer is a kind of software protocol to control packets delivery, crash recovery and transmission reliability between sender and receiver.
Multiplexing between transport and network connections is possible.

Session Layer
Session layer provides dialogue control and token management.

Presentation Layer
When data is transmitted between different types of computer systems, the presentation layer negotiates and manages the way data is represented and encoded.
Essentially a 'null' layer in case where such transformations are unnecessary.

Application Layer
This top layer defines the language and syntax that programs use to communicate with other programs. For example, a program in a client workstation uses commands to request data from a program in the server.
Common functions at this layer are opening, closing, reading and writing files, transferring files and e-mail messages, executing remote jobs and obtaining directory information about network resources.

From Process Automation and Control

Motor Generator Set (MG set)

2009-01-25T22:14:00.000-08:00

Motor Generator Set are a combination of an electrical generator and an engine mounted together to form a single piece of equipment. Motor generator set is also referred to as a genset, or more commonly, a generator.

Motor generator sets are used throughout industry to provide electrical power on demand. Typical applications include emergency power backup applications for the medical industry, remote sites, or any industrial or commercial application requiring an independent or remote power source. Motor generator sets are available in most standard power delivery configurations. Power plant installations include gas turbines, diesel and gas reciprocating engines. Motor generator sets may produce alternating current power, or direct current.

Motor generator set can be customized with a wide range of options. These options include a fuel tank, a engine speed regulator and a generator voltage regulator, simple monitoring electrical devices to more advanced digital controls, wireless communication, automatic starting capabilities, and more.

A motor and one or more generators, with their shafts mechanically coupled, used to convert an available power source to another desired frequency or voltage. The motor of the set is selected to operate from the available power supply; the generators are designed to provide the desired output.

The principal advantage of a motor-generator set over other conversion systems is the flexibility offered by the use of separate machines for each function. Since a double energy conversion is involved, electrical to mechanical and back to electrical, the efficiency is lower than in most other conversion methods

Selecting motor generator set for electrical power generation does require a through understanding of power requirements, installation requirements, regulations, and cost attributes. For AC electrical generator sets, specifications will include AC power rating, and for DC specifications will include DC power, expressed in watts (W). Engine-generators are available in a wide range of power ratings. These include small portable units which can supply several hundred watts of power, up to million watt stationary or trailer-mounted units.

Calibrating HART Transmitters

2009-01-09T06:12:00.000-08:00

Abstract In order to take advantage of the digital capabilities of HART transmitters, especially for reporting process data values, it is essential that they be calibrated correctly. This paper outlines

the differences between calibrating a conventional and a HART transmitter, and gives recommendations for calibration practices.

Introduction This paper assumes that you want to make use of the digital process values that are available from a HART transmitter. However, before examining calibration requirements, we must first establish a basic understanding in two areas: calibration concepts, and the operation of a HART instrument.

Calibration Concepts The ISA Instrument Calibration Series171 defines calibration as "Determination of the experimental relationship between the quantity being measured and the output of the device which measures it; where the quantity measured is obtained through a recognized standard of measurement." There are two fundamental operations involved in calibrating any instrument:

Testing the instrument to determine its performance,
Adjusting the instrument to perform within specification

Testing the instrument requires collecting sufficient data to calculate the instrument's operating errors. This is typically accomplished by performing a multiple point test procedure that includes the following steps.

1. Using a process variable simulator that matches the input type of the instrument, set a known input to the instrument.

2. Using an accurate calibrator, read the actual (or reference) value of this input

3 Read the instrument's interpretation of the value by using an accurate calibrator to measure
the instrument output

By repeating this process for a series of different input values, you can collect sufficient data to determine the instrument's accuracy Depending upon the intended calibration goals and the error calculations desired, the test procedure may require from 5 to 2 1 input points The first test that is conducted on an instrument before any adjustments are made is called the As-Found test. If the accuracy calculations from the As-Found data are not within the specifications for the instrument, then it must be adjusted Adjustment is the process of manipulating some part of the instrument so that its input to output relationship is within specification. For conventional instruments, this may be zero and span screws. For HART instruments, this normally requires the use of a communicator to convey specific information to the instrument Often you will see the term calibrate used as a synonym for adjust.

After adjusting the instrument, a second multiple point test is required to characterize the instrument and verify that it is within specification over the defined operating range. This is called the As-Left test It is absolutely essential that the accuracy of the calibration equipment be matched to the instrument being calibrated. Years ago, a safe rule of thumb stated that the calibrator should be an order of magnitude (10 times) more accurate than the instrument being calibrated As the accuracy of field instruments increased, the recommendation dropped to a ratio of 4 to 1. Many common calibrators in use today do not even meet this ratio when compared to the rated accuracy of HART instruments.

Error Calculations
Error calculations are the principal analysis performed on the As-Found and As-Left test data. There are several different types of error calculations, most of which are defined in the publication "Process Instrumentation Terminology ". Al of the ones discussed here are usually expressed in terms of the percent of ideal span which is defined as:

% span =(reading - low range) / (high range - low range) * 100.0

The first step in the data analysis is to convert the engineering unit values for input and output into percent of span Then for each point, calculate the error, which is the deviation of the actual output from the expected output. Table 1 gives a set of example data and error calculations, while Figure 1 graphically illustrates the resulting errors.

It is important to note that the number of data points collected and the order in which they are collected both affect the types of error calculations that can be performed. The maximum error is the most common value used to evaluate an instrument's performance. If a computer program is not used to analyze the test data, it is often the only error considered and is taken to be the largest deviation from the ideal output. A more accurate value can be obtained by fitting an equation to the error data and calculating the maximum and minimum points on the curve. In practice, this should on1 y be done if there are at least five data points reasonably spaced over the instrument range. Otherwise, it is unlikely that the curve fit will give reasonable values between the data points.

By itself, the maximum error does not give a complete indication of an instrument's performance. With the availability of computer software to facilitate calculations, other error values are gaining popularity including zero error, span error, linearity error, and hysteresis error.

Zero error is defined as the error of a device when the input is at the lower range value. Span error is defined as the difference between the actual span and the ideal span, expressed as a percentage of the ideal span.

Linearity error is a measure of how close the error of the instrument over its operating range approaches a straight line. Unfortunately. there are three different methods used to calculate this, resulting in an independent linearity. a terminal based linearity, and a zero based linearity. In practice, it is best to choose one method and apply it consistently. Note that the calculation of linearity error is also greatly facilitated by a curve fit of the error data.

Hysteresis error is a measure of the dependence of the output at a given input value upon the prior history of the input. This is the most difficult error to measure since it requires great care in the collection of data, and it typically requires at least 9 data points to develop reasonable curves for the calculations. Thus a technician must collect at least five data point traversing in one direction, followed by at least four more in the opposite direction, so that each leg has five points, including the inflection point.

If any of these errors is greater than or equal to the desired accuracy for a test, then the instrument has failed and must be adjusted.

Performance over time
In order to adequately assess the long term performance of an instrument, it must be calibrated on a regular schedule You may also want to determine how often an instrument really needs to be calibrated by examining its change in performance over time. To do this, you must collect both As-Found and As-Left data. By comparing the As-Left data from one calibration to the As-
Found of the next calibration. you can determine the instrument's drift. By varying the interval between calibrations, you can determine the optimum calibration interval which allows the instrument to remain within the desired accuracy.

HART Fundamentals
HART is an acronym for Highway Addressable Remote Transducer. Originally developed by Rosemount in 1986, the HART protocol is now a mature, industry-proven technology. Because Rosemount chose to make HART an open protocol, many manufacturers incorporated it into their products. It has now become a de facto standard for field communication with instruments In 1993, to insure its continued growth and acceptance, Rosemount transferred ownership of the protocol to the HART Communication Foundation (HCF).

HART products generally fall into one of three categories: field devices, host systems, and communication support hardware. Field devices include transmitters, valves, and controllers. There are HART transmitters for almost any standard process measurement including pressure,
temperature, level, flow, and analytical (pH. ORP. density). Host systems range from small hand-held communicators to PC based maintenance management software to large scale distributed control systems. Communication support hardware includes simple single loop modems as well as an assortment of multiplexers that allow a host system to communicate with a large number of field devices. In this paper, the general term "communicator" will be used to refer to any HART host that can communicate with a field device.

HART is a transition technology that provides for the continued use of the industry standard 4 - 20 mA current loop while also introducing many of the capabilities and benefits associated with a digital field bus system. The complete technical specification is available from the HART Communication Foundation. HART follows the basic Open Systems Interconnection (OSI) reference model. The OSI model describes the structure and elements of a communication system. The HART protocol uses a reduced OS1 model, implementing only layers 1, 2 and 7. Since the details for layers I (Physical) and 2 (Link) do not directly impact calibration, they are not discussed here. Layer 7, the Application layer, consists of three classes of HART commands: Universal, Common Practice, and Device Specific. Universal commands are implemented by all HART hosts and field devices. They are primarily used by a host to identify a field device and read process data.

The Common Practice command set defines functions that are generally applicable to many field
devices. This includes items such as changing the range, selecting engineering units, and performing self tests. Although each field device implements only those Common Practice commands which are pertinent to its operation, this still provides for a reasonable level of commonality between field devices.
Device Specific commands are different for each field device. It is through these commands that unique calibration and configuration functions are implemented. For example, when configuring an instrument for operation, only temperature transmitters need to be able to change the type of
probe attached, while flow meters often need to have information about pipe sizes, calibration factors, and fluid properties. Also, the calibration procedure for a pressure transmitter isobviously different than that for a valve.

It is important to note that in most cases, proper calibration of a HART instrument requires the
use of a communicator that is capable of issuing device specific commands.

Calibrating a Conventional Instrument
For a conventional 4-20 mA instrument, a multiple point test that stimulates the input and measures the output is sufficient to characterize the overall accuracy of the transmitter. The normal calibration adjustment involves setting only the zero value and the span value, since there is effectively only one adjustable operation between the input and output as illustrated below.

This procedure is often referred to as a Zero and Span Calibration. If the relationship between the input and output range of the instrument is not linear, then you must know the transfer function before you can calculate expected outputs for each input value. Without knowing the expected output values, you cannot calculate the performance errors.

Calibrating a HART Instrument

The Parts of a HART Transmitter
For a HART instrument, a multiple point test between input and output does not provide an accurate representation of the transmitter's operation. Just like a conventional transmitter, the measurement process begins with a technology that converts a physical quantity into an electrical signal. However, the similarity ends there, lnstead of a purely mechanical or electrical path between the input and the resulting 4-20 mA output signal, a HART transmitter has a microprocessor that manipulates the input data. As shown in Figure 4, there are typically three calculation sections involved, and each of these sections may be individually tested and adjusted.

Just prior to the first box, the instrument's microprocessor measures some electrical property that is affected by the process variable of interest. The measured value may be millivolts, capacitance, reluctance, inductance, frequency, or some other property. However, before it can be used by the microprocessor, it must be transformed to a digital count by an analog to digital (AD) converter.

In the first box, the microprocessor must rely upon some form of equation or table to relate the raw count value of the electrical measurement to the actual property (PV) of interest such as temperature, pressure, or flow. The principle form of this table is usually established by the manufacturer, but most HART instruments include commands to perform field adjustments. This is often referred to as a sensor trim. The output of the first box is a digital representation of the process variable. When you read the process variable using a communicator, this is the value that you see.

The second box is strictly a mathematical conversion from the process variable to the equivalent
milliamp representation. The range values of the instrument (related to the zero and span values) are used in conjunction with the transfer function to calculate this value. Although a linear transfer function is the most common, pressure transmitters often have a square root option. Other special instruments may implement common mathematical transformations or user defined break point tables. The output of the second block is a digital representation of the desired instrument output. When you read the loop current using a communicator, this is the value that you see. Many HART instruments support a command which puts the instrument into
a fixed output test mode. This overrides the normal output of the second block and substitutes a specified output value.

The third box is the output section where the calculated output value is converted to a count value that can be loaded into a digital to analog converter. This produces the actual analog electrical signal. Once again the microprocessor must rely on some internal calibration factors to get the output correct. Adjusting these factors is often referred to as a current loop trim or 4- 20 mA trim.

HART Calibration Requirements
Based on this analysis, you can see why a proper calibration procedure for a HART instrument is significantly different than for a conventional instrument. The specific calibration requirements depend upon the application.

If the application uses the digital representation of the process variable for monitoring or control, then the sensor input section must be explicitly tested and adjusted. Note that this reading is completely independent of the milliamp output, and has nothing to do with the zero or span settings. The PV as read via HART communication continues to be accurate even when it is outside the assigned output range. For example, a range 2 Rosemount 305 1 c has sensor limits of -250 to +250 inches of water. If you set the range to 0 - 100 inches of water, and then apply a pressure of 150 inches of water, the analog output will saturate at just above 20 milliamps. However, a communicator can still read the correct pressure. If the current loop output is not used (that is the transmitter is used as a digital only device), then the input section calibration is all that is required. If the application uses the milliamp output, then the output section must be explicitly tested and calibrated. Note that this calibration is independent of the input section, and again, has nothing to do with the zero and span settings.

Calibrating the Input Section
The same basic multiple point test and adjust technique is employed, but with a new definition for output. To run a test, use a calibrator to measure the applied input, but read the associated output (PV) with a communicator. Error calculations are simpler since there is always a linear relationship between the input and output, and both are recorded in the same engineering units. In general, the desired accuracy for this test will be the manufacturer's accuracy specification. If the test does not pass, then follow the manufacturer's recommended procedure for trimming the input section. This may be called a sensor trim and typically involves one or two trim points. Pressure transmitters also often have a zero trim, where the input calculation is adjusted to read exactly zero (not low range). Do not confuse a trim with any form of re-ranging or any procedure that involves using zero and span buttons.

Calibrating the Output Section
Again, the same basic multiple point test and adjust technique is employed, but with a new
definition for input. To run a test, use a communicator to put the transmitter into a fixed current
output mode. The input value for the test is the mA value that you instruct the transmitter to produce. The output value is obtained using a calibrator to measure the resulting current. This test also implies a linear relationship between the input and output, and both are recorded in the same engineering units (milliamps). The desired accuracy for this test should also reflect the manufacturer's accuracy specification.

If the test does not pass, then follow the manufacturer's recommended procedure for trimming the output section. This may be called a 4-20 mA trim, a current loop trim, or a DIA trim. The trim procedure should require two trim points close to or just outside of 4 and 20 mA. Do not confuse this with any form of re-ranging or any procedure that involves using zero and span buttons.

Testing Overall Performance
After calibrating both the Input and Output sections, a HART transmitter should operate correctly The middle block in Figure 4 only involves computations. That is why you can change the range, units, and transfer function without necessarily affecting the calibration. Notice also that even if the instrument has an unusual transfer function, it only operates in the conversion of the input value to a milliamp output value, and therefore is not involved in the testing or calibration of either the input or output sections.

If there is a desire to validate the overall performance of a HART transmitter, run a Zero and Span test just like a conventional instrument. As you will see in a moment. however, passing this test does not necessarily indicate that the transmitter is operating correctly.

Effect of Damping on Test Performance
Many HART instruments support a parameter called damping. If this is not set to zero, it can have an adverse effect on tests and adjustments. Damping induces a delay between a change in the instrument input and the detection of that change in the digital value for the instrument input reading and the corresponding instrument output value. This damping induced delay may exceed the settling time used in the test or calibration. The settling time is the amount of time the test or calibration waits between setting the input and reading the resulting output. It is advisable to adjust the instrument's damping value to zero prior to performing tests or adjustments. After calibration, be sure to return the damping constant to its required value.

Operations that are NOT Proper Calibrations
Digital Range Change
There is a common misconception that changing the range of a HART instrument by using a communicator somehow calibrates the instrument. Remember that a true calibration requires a reference standard, usually in the form of one or more pieces of calibration equipment to provide an input and measure the resulting output. Therefore, since a range change does not reference any external calibration standards, it is really a configuration change, not a calibration. Notice that in the HART transmitter block diagram (Figure 4), changing the range only affects the second block. It has no effect on the digital process variable as read by a communicator.

Zero and Span Adjustment
Using only the zero and span adjustments to calibrate a HART transmitter (the standard practice associated with conventional transmitters) often corrupts the internal digital readings. You may not have noticed this if you never use a communicator to read the range or digital process data. As shown in Figure 4, there is more than one output to consider. The digital PV and milliamp values read by a communicator are also outputs, just like the analog current loop.

Consider what happens when using the external zero and span buttons to adjust a HART instrument. Suppose that an instrument technician installs and tests a differential pressure transmitter that was set at the factory for a range of 0 to 100 inches of water. Testing the transmitter reveals that it now has a 1 inch of water zero shift. Thus with both ports vented (zero), its output is 4.16 mA instead of 4 00 mA, and when applying 100 inches of water, the output is 20.16 mA instead of 20.00 mA. To fix this he vents both ports and presses the zero button on the transmitter. The output goes to 4.00 mA, so it appears that the adjustment was successful
However, if he now checks the transmitter with a communicator, he will find that the range is 1 to 101 inches of water, and the PV is 1 inch of water instead of 0. The zero and span buttons changed the range (the second block). This is the only action that the instrument can take under these conditions since it does not know the actual value of the reference input. Only by using a digital command which conveys the reference value can the instrument make the appropriate internal adjustments.

The proper way to correct a zero shift condition is to use a zero trim. This adjusts the instrument
input block so that the digital PV agrees with the calibration standard. If you intend to use the digital process values for trending, statistical calculations, or maintenance tracking, then you should disable the external zero and span buttons and avoid using them entirely.

Loop Current Adjustment
Another observed practice among instrument technicians is to use a hand-held communicator to adjust the current loop so that an accurate input to the instrument agrees with some display device on the loop. If you are using a Rosemount model 268 communicator, this is a "current loop trim using other scale." Refer again to the zero drift example just before pressing the zero button. Suppose there is also a digital indicator in the loop that displays 0.0 at 4 mA, and 100.0 at 20 mA. During testing, it read 1.0 with both ports vented, and it read 101.0 with 100 inches of water applied. Using the communicator, the technician performs a current loop trim so that the display reads correctly at 0 and 100, essentially correcting the output to be 4 and 20 mA respectively.

While this also appears to be successful, there is a fundamental problem with this procedure. To begin with, the communicator will show that the PV still reads 1 and 101 inches of water at the test points, and the digital reading of the mA output still reads 4.16 and 20.16 mA, even though the actual output is 4 and 20 mA. The calibration problem in the input section has been hidden by introducing a compensating error in the output section, so that neither of the digital readings agrees with the calibration standards.

Conclusion
While there are many benefits to be gained by using HART transmitters, it is essential that they be calibrated using a procedure that is appropriate to their function. If the transmitter is part of an application that retrieves digital process values for monitoring or control, then the standard calibration procedures for conventional instruments are inadequate. At a minimum, the sensor input section of each instrument must be calibrated. If the application also uses the current loop output, then the output section must also be calibrated.

References
[I] HART Communication Foundation. "HART - Smart Communications Protocol Specification",
Revision 5.2, November 3, 1993.
[2] Bell System Technical Reference. PUB 4121 2, "Data Sets 202s and 202T interface
Specification", July 1976.
[3] HART Communication Foundation Pamphlet. "HART Field Communications Protocol".
[4] Holladay, Kenneth L., "Using the HART@ Protocol to Manage for Quality", ISA 1994 paper
number 94-6 17.
[5] Applied System Technologies, Inc., "User's Manual for Cornerstone Base Station", Revision
2.0.1 I , April 1995.
[6] ANSIIISA - S51 .l-1979, "Process Instrumentation Terminology"
[7] Instrument Society of America, "Instrument Calibration Series - Principles of Calibration".
1989.
[8] Instrument Society of America, "Instrument Calibration Series - Calibrating Pressure and
Temperature Instruments", 1989.

HART: The Tutorial

2009-01-09T05:50:00.000-08:00

HART® stands for:
HIGHWAY ADDRESSABLE REMOTE TRANSDUCER

HART® is an open protocol that was originally developed in the late 1980's by Fisher Rosemount to communicate with their range of Smart field devices. Over the years it has become a de facto standard for communicating with SMART devices in the Process industry. Over 100 manufacturers utilise the HART® protocol in over 560 different products, from simple temperature transmitters to gas detectors.

The HART® protocol is a powerful communication technology used to realise the full potential of digital field devices whilst preserving the traditional 4-20mA signal. The HART® protocol extends the system capabilities for two way digital communication with smart instruments.

HART® offers the best solution for smart field device communications and has the widest base of support of any field device protocol worldwide. More instruments are available with the HART® protocol than any other digital communications technology. Almost any process application can be addressed by one of the products offered by HART® instrument suppliers. Unlike other digital communication methods the HART protocol gives a unique communication solution that it is backward compatible with currently installed instrumentation. This ensures that investments in existing cabling and current control strategies remain secure into the future.

The HART® digital signal is superimposed onto the standard 4-20mA signal. It uses Bell 202 standard Frequency Shift Keying (FSK) signal to communicate at 1200 baud. The digital signal is made up of two frequencies, 1200Hz and 2200Hz, representing bits 1 and 0 respectively. Sine waves of these two frequencies are superimposed onto the analogue signal cables to give simultaneous analogue and digital communications. As the average value of the FSK signal is always zero there is no effect on the 4-20mA analogue signal. A minimum loop impedance of 230ohms is required for communication.

HART® is a master-slave protocol - this means that a field device only replies when it is spoken to. Up to two masters can connect to each HART® loop. The primary master is usually the DCS (Distributed Control System), the PLC (Programmable Logic Controller) or a PC. The secondary master can be a hand held configurator or another PC running an instrument maintenance software package. Slave devices include transmitters, actuators and controllers that respond to commands from the primary or secondary master.

The digital communication signal has a response time of approx. 2-3 updates per second without interrupting the analogue signal.

HART® Commands
The HART® protocol provides uniform and consistent communication for all field devices via the HART® command set. This includes three types of Command:

Universal
All devices using the HART® protocol must recognise and support these commands. They provide access to information useful in normal operations.

Common Practice
These provide functions implemented by many but not all HART® communication devices.

Device specific
These represent functions that are unique to each field device. They access set up and calibration information as well as information on the construction of the device.

The HART® Communication Protocol is an open standard owned by more than 100 member companies in the HART® Communication Foundation HCF. The HCF is an independent, non-profit organisation, which provides worldwide support for application of the technology and
ensures that the technology is openly available for the benefit of the industry.

Infrared technology targets industrial automation

2008-12-20T15:46:00.000-08:00

By Andrew Wilson

Infrared technology targets industrial automation IR cameras are used in industrial-plant monitoring and more and more in industrial automation.

By Andrew Wilson, Editor

English astronomer Sir William Hershel is credited with the discovery of infrared (IR) radiation in 1800. In his first experiment, Hershel subjected a liquid in a glass thermometer to different colors of the spectrum. Finding that the hottest temperature was beyond red light, Hershel christened his newly found energy "calorific rays," now known as infrared radiation.

Two centuries later, IR imagers and cameras are finding uses in applications from missile guidance tracking to plant monitoring to machine-vision automation systems. Invisible to the human eye, IR energy can be divided into the three spectral regions: near-, mid-, and far-IR, with wavelengths longer than that of visible light. Although the boundaries between these are undetermined, the wavelength ranges are approximately 0.7 to 5 µm (near-IR), 5 to 40 µm (mid-IR), and 40 to 350 µm (far-IR).

However, do not expect today's commercially available IR detectors or cameras to span such large wavelengths. Rather, they will be specified as covering more narrow bandwidths between approximately 1 and 20 µm. Many manufacturers may use the terms near, mid-, and far-IR loosely, often claiming that their 9 µm-capable camera is based on a far-IR-based sensor.

Absolute measurement

For the system developer considering an IR camera for process-monitoring applications, the choice of detector will be both manufacturer- and application-specific. Because of this, the systems integrator must gain an understanding of how and what is being measured.

FIGURE 1. Raytek incorporates a reference blackbody for continuous calibration in its MP50 linescan process imager. The scanner offers a speed of 48 lines/s speed and is available in a number of different versions for examining plastics, glass, and metals.

Perhaps one of the largest misconceptions is that IR measures the temperature of an object. This misconception results from Plank's law, which states that all objects with a temperature above absolute zero emit IR radiation and that the higher the temperature the higher the emitted intensity. Plank's law, however, is only true for blackbody objects that have 100% absorption and maximum emitting intensity. In reality, a ratio of the emitting intensity of the object and a corresponding blackbody with the same temperature must be used. This emissivity—the measure of how a material absorbs and emits IR energy—affects how images are interpreted.

In the design of its MP50 linescan process imager, Raytek (Santa Cruz, CA, USA) incorporates a reference blackbody for continuous calibration (see Fig. 1). Targeted at continuous-sheet and web-based processes, the scanner offers a 48-line/s scan speed and is offered in a number of versions capable of capturing spectral ranges useful for examining plastics, glass, and metals. Other manufacturers offer blackbodies as accessories that can externally calibrate their cameras.

Since people cannot see IR radiation, the images captured by IR detectors and cameras must first be processed, translated, and pseudocolored into images that can be visualized. In these images, highly reflective materials may appear different from less-reflective materials, even though their temperature is the same. This is because highly reflective materials will reflect the radiation of the objects around them and therefore may appear to be "colder" than less-reflective materials of the same temperature.

Material properties

In considering whether to use IR technology for any particular application, therefore, the properties of the materials being viewed must be known to properly interpret the image. In printed-circuit-board analysis, for example, the emissivity of different metals can be used to discern faults in the board. However, if the emissivity of materials is similar, it may be difficult to discern any differences in the image.

In many applications, including target tracking, this does not pose a problem. In heat-seeking missiles, for example, the difference between the emissivity of aluminium alloy used to build a rocket and the fire that emerges from its boosters is so high that discerning the two is relatively simple. In other applications, the task may be more complex.

FIGURE 2. Ircon Stinger IR camera is specified with an uncooled 320 x 240 focal plane array, spectral ranges of 5, 8, and 8 to 14 µm, a detector element size of 51 x 51 µm, and an f/1.4 lens to measure targets as small as 0.017 in.

Infrared cameras use a number of different detector types that can be broadly classified as either photon or thermal detectors. Infrared absorbed by photon-based detectors generates electrons or bandgap transitions in materials such as mercury cadmium telluride (HgCdTe; detecting IR in the 3- to 5- and 8- and 12-µm range) and indium antinomide (InSb; detecting IR in the 3- to 5-µm range). This results in a charge that can be directly measured and read out for preprocessing.

Rather than generate charge or bandgap transitions directly, thermal detectors absorb the IR radiation, raising the temperature of single or multiple membrane-isolated temperature detectors on the device. Unlike photon-based detectors, thermal detectors can be operated at room temperature, although their sensitivity and response time are longer.

To create a two-dimensional IR image, camera vendors incorporate focal-plane, or staring, arrays into their cameras. These detectors are similar in concept to CCDs in that they are offered in arrays of pixels that can range from as low as 2 x 2 to 640 x 512 formats and higher, often with greater than 8 bits of dynamic range.

Incorporating a thermal detector in the form of an amorphous silicon or vanadate (YVO₄) microbolometer, the Eye-R320B from Opgal (Karmiel, Israel) features a 320 x 240 FPA. With a spectral range from 8 to 12 µm, the camera also offers automatic gain correction, remote RS422 programmability, and CCIR or RS170 output. The company also offers embeddable IR camera modules that can use a number of 640 x 480-based detectors from different manufacturers.

Discerning features

As the wavelength of visible light is shorter than that of IR radiation, visible light can discern features within an image at higher resolution. For this reason ultraviolet (UV) radiation, which the human eye also cannot perceive, is used in to detect submicron defects in semiconductor wafers. Because the frequency of UV light is higher, the spatial resolution of the optical system is also higher, allowing greater detail to be captured.

Unfortunately, quite the opposite is true of IR radiation. With a lower frequency than visible light, IR radiation will resolve fewer line pairs/millimeter than visible light, given that all other system parameters are equal. Indeed, it is this diffraction-limited nature of optics that leads to the large pixel sizes of IR imagers. And, of course, an IR imager with 320 x 240 format and a pixel pitch of 30 µm will have a die size considerably larger than its 320 x 240 CCD counterpart with a 6-µm pixel pitch. This larger die size for any given format is another reason IR imagers are more expensive than visible imagers.

In many visible machine-vision applications, it is necessary to determine the minimum spatial resolution required by the system. And the same applies when determining whether an IR detector can be used in such an application. This is accomplished visibly by using test charts with periods of white and black lines. If, for example, the required resolution were 125 line pairs/mm, then the pitch of those line pairs would be 8 µm. From Nyquist criteria, it can be determined that the most efficient way to sample the signal is with a 4-µm pixel pitch. A smaller pitch will not add new information, and a larger pitch will result in errors.

In such optical systems the pixel pitch at the limit of resolution is given by the diffraction-limited equation

pitch = 0.6 x f/# x wavelength

where f/# equals the focal length/aperture ratio of the lens. Thus, a pixel pitch of 2.68 µm is needed to resolve a 550-nm visible frequency at f/8. In an IR system, with a wavelength of 5 µm and the same focal length/aperture ratio, the pixel pitch required will be approximately 25 µm or nine times larger. With a 25-µm pixel pitch, the minimum number of line pairs/millimeter that can be resolved will have a 50-µm period, which equates to approximately 2 line pairs/mm with an f/1.8 lens.

Luckily, most camera manufacturers specify these parameters. The Stinger IR camera from Ircon (Niles, IL, USA), for example, is specified with an uncooled 320 x 240 FPA, spectral ranges of 5, 8, and 8 to 14 µm, a detector element size of 51 x 51 µm, and an f/1.4 lens (see Fig. 2). The company's literature states that targets as small as 0.017 in. can be measured with the camera, a fact that can be confirmed by some simple mathematics.

To increase this resolution, some manufacturers use lenses with larger numerical apertures (smaller f#s). Because glass is opaque to IR radiation, these lenses are usually fabricated from exotic materials such as zinc selenide (ZnSe) or germanium (Ge), adding to the cost of the camera. Like visible solid-state cameras, IR cameras are generally offered with both linescan and area format arrays. While linescan-based cameras are useful in IR web inspection, area-array-based cameras can capture two-dimensional images. Outputs from these cameras are also similar to visible camera and are generally standard NTSC/PAL analog formats or FireWire and USB-based or digital formats.

Future developments

What has, in the past, stood in the way of acceptance of IR techniques in machine-vision systems has been the cost of IR systems compared with their visible counterparts and the lack of an easy way to combine the benefits of both wavelengths in low-cost systems. In the past few years, however, the cost of IR imaging has been lowered by the introduction of smart IR cameras that include on-board detectors, processors, embedded software, and standard interfaces. And, realizing the benefits of a combined visible/IR approach, manufacturers are now starting to introduce more sophisticated imagers that can simultaneously capturing visible and near-IR images.

Recently, Indigo Systems (Goleta, CA, USA) announced a new method for processing indium gallium arsenide (InGaAs) to enhance its short-wavelength response. The new material, VisGaAs, is a broad-spectrum substance that enables both near-IR and visible imaging on the same photodetector. According to the company, test results indicate VisGaAs can operate in a range from 0.4 to 1.7 µm. To test the detector, the company mounted a 320 x 256 FPA onto its Phoenix camera-head platform and imaged a hot soldering gun in front of a computer monitor (see Fig. 3). The results clearly show that a standard InGaAs camera can detect hardly any radiation from the CRT, while the VisGaAs-based imager can clearly detect both features.

FIGURE 3. Indigo Systems new VisGaAs material is a broad-spectrum substance that enables both near-IR and visible imaging on the same photodetector. A 320 x 256 focal plane array mounted onto its Phoenix camera head platform imaged a hot soldering gun in front of a computer monitor. The results show that a standard InGaAs camera can detect hardly any radiation from the CRT (left) while the VisGaAs-based imager can clearly detect both features (right).

Click here to enlarge image

Camera and frame grabber team up to combat SARS

To restrict the spread of severe acute respiratory syndrome (SARS), Land Instruments International (Sheffield, UK; www.landinst.com) has developed a PC-based system that detects elevated body temperatures in large numbers of people. Because individuals with SARS have a fever and above-normal skin temperature, infrared cameras can analyze and detect the viral illness.

Land Instrument's Human Body Temperature Monitoring System (HBTMS) uses the company FTI Mv Thermal Imager with an array of 160 × 120 pixels to capture a thermographic image of a human body (typically the face) at a distance of 2 to 3 m. Data captured are then compared with a 988 blackbody furnace calibration source from Isothermal Technology (Isotech, Southport, UK; www.isotech.co.uk). Permanently positioned in the field of view of the imager, this calibrated temperature reference source is set at 38°C and provides a reference area in the live image scene. The imager is then adjusted to maintain this reference area at a fixed radiance value (200).

To capture images from the FTI Mv, the camera is coupled to a Universal Interface Box (UIB), which drives the imager, images, and imager control from a PC up to 1000 m away. Video and RS422 control signals are then transmitted to the PC from the UIB. IR images are transmitted as an analog video signal via the UIB and digitized by a PC-based MV 510 frame grabber from MuTech (Billerica, MA, USA; www.mutech.com), which transfers digital data to PC memory or VGA display. Because the board offers programmable gain and offset control functionality, the incoming video signal can be adjusted for the maximum digitization range of the camera.

Once the image has been acquired, it is analyzed by Land's image-processing software that displays the images, triggers alarms via a digital output card, and records images to disk. Any pixels in this area with radiance greater than the set threshold trigger an alarm output. To highlight individuals who may have the disease, a monochrome palette is used, with any pixel on the scene with radiance levels above the threshold highlighted in red.

Click here to enlarge image

Land Instruments has developed a PC-based system around its FTI Mv Thermal Imager, a PC-based frame grabber and proprietary software (top). In operation, captured images are compared with a fixed radiance value, and pixels with radiance levels higher than this value are highlighted in red (bottom).

Photosensors...

2008-12-15T18:43:00.000-08:00

In the previous company i was connected with, a great deal of photosensors or photoswitches are used to detect workpiece presence. There were of different models and shapes. Despite their different physical appearance, almost all follow one principle of operation.

A photosensor is an electronic component that detects the presence of visible light, infrared transmission (IR), and/or ultraviolet (UV) energy. Most photosensors consist of semiconductor having a property called photoconductivity , in which the electrical conductance varies depending on the intensity of radiation striking the material.

The most common types of photosensor are the photodiode, the bipolar phototransistor, and the photoFET (photosensitive field-effect transistor). These devices are essentially the same as the ordinary diode , bipolar transistor , and field-effect transistor , except that the packages have transparent windows that allow radiant energy to reach the junctions between the semiconductor materials inside. Bipolar and field-effect phototransistors provide amplification in addition to their sensing capabilities.

Photosensors are used in a great variety of electronic devices, circuits, and systems, including:

* fiber optic systems
* optical scanners
* wireless LAN
* automatic lighting controls
* machine vision systems
* electric eyes
* optical disk drives
* optical memory chips
* remote control devices

A Quick Guide to Thermocouples

2008-12-13T18:26:00.000-08:00

Background

Thermocouples are the most popular temperature sensors. They are cheap, interchangeable, have standard connectors and can measure a wide range of temperatures. The main limitation is accuracy, system errors of less than 1°C can be difficult to achieve.

How They Work

In 1822, an Estonian physician named Thomas Seebeck discovered (accidentally) that the junction between two metals generates a voltage which is a function of temperature. Thermocouples rely on this Seebeck effect. Although almost any two types of metal can be used to make a thermocouple, a number of standard types are used because they possess predictable output voltages and large temperature gradients.

A K type thermocouple is the most popular and uses nickel-chromium and nickel-aluminium alloys to generate voltage.Standard tables show the voltage produced by thermocouples at any given temperature, so the K type thermocouple at 300°C will produce 12.2mV. Unfortunately it is not possible to simply connect up a voltmeter to the thermocouple to measure this voltage, because the connection of the voltmeter leads will make a second, undesired thermocouple junction.

Cold Junction Compensation (CJC)

To make accurate measurements, this must be compensated for by using a technique known as cold junction compensation (CJC). In case you are wondering why connecting a voltmeter to a thermocouple does not make several additional thermocouple junctions (leads connecting to the thermocouple, leads to the meter, inside the meter etc), the law of intermediate metals states that a third metal, inserted between the two dissimilar metals of a thermocouple junction will have no effect provided that the two junctions are at the same temperature. This law is also important in the construction of thermocouple junctions. It is acceptable to make a thermocouple junction by soldering the two metals together as the solder will not affect the reading. In practice, thermocouple junctions are made by welding the two metals together (usually by capacitive discharge). This ensures that the performance is not limited by the melting point of solder.

All standard thermocouple tables allow for this second thermocouple junction by assuming that it is kept at exactly zero degrees centigrade. Traditionally this was done with a carefully constructed ice bath (hence the term 'cold' junction compensation). Maintaining a ice bath is not practical for most measurement applications, so instead the actual temperature at the point of connection of the thermocouple wires to the measuring instrument is recorded.

Typically cold junction temperature is sensed by a precision thermistor in good thermal contact with the input connectors of the measuring instrument. This second temperature reading, along with the reading from the thermocouple itself is used by the measuring instrument to calculate the true temperature at the thermocouple tip. For less critical applications, the CJC is performed by a semiconductor temperature sensor. By combining the signal from this semiconductor with the signal from the thermocouple, the correct reading can be obtained without the need or expense to record two temperatures. Understanding of cold junction compensation is important; any error in the measurement of cold junction temperature will lead to the same error in the measured temperature from the thermocouple tip.

Linearisation

As well as dealing with CJC, the measuring instrument must also allow for the fact that the thermocouple output is non linear. The relationship between temperature and output voltage is a complex polynomial equation (5^th to 9^th order depending on thermocouple type). Analogue methods of linearisation are used in low cost themocouple meters. High accuracy instruments store thermocouple tables in computer memory to eliminate this source of error.

Thermocouple Selection

Thermocouples are available either as bare wire 'bead' thermocouples which offer low cost and fast response times, or built into probes. A wide variety of probes are available, suitable for different measuring applications (industrial, scientific, food temperature, medical research etc). One word of warning: when selecting probes take care to ensure they have the correct type of connector. The two common types of connector are 'standard' with round pins and 'miniature' with flat pins, this causes some confusion as 'miniature' connectors are more popular than 'standard' types.

Types

When choosing a thermocouple consideration should be given to both the thermocouple type, insulation and probe construction. All of these will have an effect on the measurable temperature range, accuracy and reliability of the readings. Listed below is a subjective guide to thermocouple types.

When selecting thermocouple types, ensure that your measuring equipment does not limit the range of temperatures that can be measured. Note that thermocouples with low sensitivity (B, R and S) have a correspondingly lower resolution. The table below summarises the useful operating limits for the various thermocouple types which are described in more detail in the following paragraphs.

Table 1. Range of Temperatures for Each Thermocouple Type

Thermocouple Type	Overall Range (°C)	0.1°C Resolution	0.025°C Resolution
B	100..1800	1030..1800	-
E	-270..790	-240..790	-140..790
J	-210..1050	-210..1050	-120..1050
K	-270..1370	-220..1370	-20..1150
N	-260..1300	-210..1300	340..1260
R	-50..1760	330..1760	-
S	-50..1760	250..1760	-
T	-270..400	-230..400	-20..400

Type K (Chromel / Alumel)

Type K is the 'general purpose' thermocouple. It is low cost and, owing to its popularity, it is available in a wide variety of probes. Thermocouples are available in the -200°C to +1200°C range. Sensitivity is approx 41uV/°C. Use type K unless you have a good reason not to.

Type E (Chromel / Constantan)

Type E has a high output (68uV/°C) which makes it well suited to low temperature (cryogenic) use. Another property is that it is non-magnetic.

Type J (Iron / Constantan)

Limited range (-40 to +750°C) makes type J less popular than type K. The main application is with old equipment that cannot accept 'modern' thermocouples. J types should not be used above 760°C as an abrupt magnetic transformation will cause permanent decalibration.

Type N (Nicrosil / Nisil)

High stability and resistance to high temperature oxidation makes type N suitable for high temperature measurements without the cost of platinum (B,R,S) types. Designed to be an 'improved' type K, it is becoming more popular.

Thermocouple types B, R and S are all 'noble' metal thermocouples and exhibit similar characteristics. They are the most stable of all thermocouples, but due to their low sensitivity (approx 10uV/⁰C) they are usually only used for high temperature measurement (>300°C).

Type B (Platinum / Rhodium)

Suited for high temperature measurements up to 1800°C. Unusually type B thermocouples (due to the shape of their temperature / voltage curve) give the same output at 0°C and 42°C. This makes them useless below 50°C.

Type R (Platinum / Rhodium)

Suited for high temperature measurements up to 1600°C. Low sensitivity (10uV/°C) and high cost makes them unsuitable for general purpose use.

Type S (Platinum / Rhodium)

Suited for high temperature measurements up to 1600°C. Low sensitivity (10uV/vC) and high cost makes them unsuitable for general purpose use. Due to its high stability type S is used as the standard of calibration for the melting point of gold (1064.43°C).

Precautions and Considerations for Using Thermocouples

Most measurement problems and errors with thermocouples are due to a lack of understanding of how thermocouples work. Thermocouples can suffer from ageing and accuracy may vary consequently especially after prolonged exposure to temperatures at the extremities of their useful operating range. Listed below are some of the more common problems and pitfalls to be aware of.

Connection problems

Many measurement errors are caused by unintentional thermocouple junctions. Remember that any junction of two different metals will cause a junction. If you need to increase the length of the leads from your thermocouple, you must use the correct type of thermocouple extension wire (eg type K for type K thermocouples). Using any other type of wire will introduce a thermocouple junction. Any connectors used must be made of the correct thermocouple material and correct polarity must be observed.

Lead Resistance

To minimise thermal shunting and improve response times, thermocouples are made of thin wire (in the case of platinum types cost is also a consideration). This can cause the thermocouple to have a high resistance which can make it sensitive to noise and can also cause errors due to the input impedance of the measuring instrument. A typical exposed junction thermocouple with 32AWG wire (0.25mm diameter) will have a resistance of about 15 ohms / meter. If thermocouples with thin leads or long cables are needed, it is worth keeping the thermocouple leads short and then using thermocouple extension wire (which is much thicker, so has a lower resistance) to run between the thermocouple and measuring instrument. It is always a good precaution to measure the resistance of your thermocouple before use.

Decalibration

Decalibration is the process of unintentionally altering the makeup of thermocouple wire. The usual cause is the diffusion of atmospheric particles into the metal at the extremes of operating temperature. Another cause is impurities and chemicals from the insulation diffusing into the thermocouple wire. If operating at high temperatures, check the specifications of the probe insulation.

Noise

The output from a thermocouple is a small signal, so it is prone to electrical noise pick up. Most measuring instruments reject any common mode noise (signals that are the same on both wires) so noise can be minimised by twisting the cable together to help ensure both wires pick up the same noise signal. Additionally, an integrating analog to digital converter can be used to helps average out any remaining noise. If operating in an extremely noisy environment, (such as near a large motor) it is worthwhile considering using a screened extension cable. If noise pickup is suspected first switch off all suspect equipment and see if the reading changes.

Common Mode Voltage

Although thermocouple signal are very small, much larger voltages often exist at the input to the measuring instrument. These voltages can be caused either by inductive pick up (a problem when testing the temperature of motor windings and transformers) or by 'earthed' junctions. A typical example of an 'earthed' junction would be measuring the temperature of a hot water pipe with a non insulated thermocouple. If there are any poor earth connections a few volts may exist between the pipe and the earth of the measuring instrument. These signals are again common mode (the same in both thermocouple wires) so will not cause a problem with most instruments provided they are not too large.

Thermal Shunting

All thermocouples have some mass. Heating this mass takes energy so will affect the temperature you are trying to measure. Consider for example measuring the temperature of liquid in a test tube: there are two potential problems. The first is that heat energy will travel up the thermocouple wire and dissipate to the atmosphere so reducing the temperature of the liquid around the wires. A similar problem can occur if the thermocouple is not sufficiently immersed in the liquid, due to the cooler ambient air temperature on the wires, thermal conduction may cause the thermocouple junction to be a different temperature to the liquid itself. In the above example a thermocouple with thinner wires may help, as it will cause a steeper gradient of temperature along the thermocouple wire at the junction between the liquid and ambient air. If thermocouples with thin wires are used, consideration must be paid to lead resistance. The use of a thermocouple with thin wires connected to much thicker thermocouple extension wire often offers the best compromise.

(From AZOM)

What are Level Controllers?

2008-11-30T02:11:00.000-08:00

Level controllers monitor, regulate, and control liquid or solid levels in a process. There are three basic types of control functions that level controllers can use. Limit control works by interrupting power through a load circuit when the level exceeds or falls below the limit set point. A limit controller can protect equipment and people when it is correctly installed with its own power supply, power lines, switch and sensor. Advanced or non-linear control includes dead-time compensation, lead/lag, adaptive gain, neural networks, and fuzzy logic. Level controllers can be used for either liquid or powder or other dry material applications.

Linear level controllers can take many different styles. Feedforward control offers direct control or compensation from the reference signal. It may be open loop or in conjunction with PID control. Proportional, integral, and derivative (PID) control is an intelligent I/O module or program instruction, which provides automatic closed-loop operation of process control loops. Proportional plus integral (PI) control has the error signal integrated and is used for eliminating steady state or offset errors. It may also be called automatic reset/bias/offset control.

Proportional plus derivative (PD) control has the error signal differentiated to get the rate of change. This type of control is used to increase controller speed of response, but can be noisy and make the system less stable. In proportional (P) control, the control signal is proportional to the error between the reference and feedback signals.

Level controllers differ in terms of specifications, user interface, and features. Specifications include the number of inputs, control outputs and control feedback loops. Control loops may be linked to improve control performance and/or stability. The control output is usually analog current, voltage or a switched output. These controllers can have discrete or TTL I/O as well and can handle high power switching needs. The user interface for level controllers may be analog, digital or computer controlled. Displays for level controllers can be analog meters, digital numerical readouts, or video display terminals. Another possible type of display is a strip chart or circle chart. When connecting to a computer host, level controllers can use the standard serial, parallel or SCSI interfaces or can be networkable via Ethernet, CANBus or a number of other network protocols. Features that are sometimes optional for level controllers include sensor excitation current or voltage, built-in alarms or indicators and washdown or waterproof ratings. Other features can include programmable setpoints, autotune or self-tuning functions and signal computation functions or filters.

What are Level Sensors?

2008-11-30T02:09:00.001-08:00

Level sensors are used to detect liquid or powder levels, or interfaces between liquids. These level measurements can be either continuous or point values represented with various output options. Continuous level sensors are devices that measure level within a specified range and give output of a continuous reading of level. Point level sensors devices mark a specific level, generally used as high alarm or switch.

Multiple point sensors can be integrated together to give a stepped version of continuous level. These level sensors can be either plain sensors with some sort of electrical output or else can be more sophisticated instruments that have displays and sometimes computer output options. The measuring range is probably the most important specification to examine when choosing a level sensor. Field adjustability is a nice feature to have for tuning the instrument after installation.

Depending on the needs of the application, level sensing devices can be mounted a few different ways. These sensors can be mounted on the top, bottom or side of the container holding the substance to be measured. Among the technologies for measuring level are air bubbler technology, capacitive or RF admittance, differential pressure, electrical conductivity or resistivity, mechanical or magnetic floats, optical units, pressure membrane, radar or microwave, radio frequency, rotation paddle, ultrasonic or sonic and vibration or tuning fork technology. Analog outputs level sensors can be current or voltage signals. Also possible is a pulse or frequency. Another option is to have an alarm output or a change in state of switches. Computer signal outputs that are possible are usually serial or parallel. Level sensors can have displays that are analog, digital or video displays. Control for the devices can be analog with switches, dials and potentiometers; digital with menus, keypads and buttons; or controlled by a computer.

What is Statistical Process Control SPC?

2008-11-29T19:07:00.001-08:00

Statistical process control (SPC) is a method for achieving quality control in manufacturing processes. It is a set of methods using statistical tools such as mean, variance and others, to detect whether the process observed is under control.

Statistical process control was pioneered by Walter A. Shewhart and taken up by W. Edwards Deming with significant effect by the Americans during World War II to improve industrial production. Deming was also instrumental in introducing SPC methods to Japanese industry after that war. Dr. Shewhart created the basis for the control chart and the concept of a state of statistical control by carefully designed experiments. While Dr. Shewhart drew from pure mathematical statistical theories, he understood data from physical processes never produce a "normal distribution curve" (a Gaussian distribution, also commonly referred to as a "bell curve"). He discovered that observed variation in manufacturing data did not always behave the same way as data in nature (Brownian motion of particles).
Dr. Shewhart concluded that while every process displays variation, some processes display controlled variation that is natural to the process, while others display uncontrolled variation that is not present in the process causal system at all times.

Classical Quality control was achieved by observing important properties of the finished product and accept/reject the finished product. As opposed to this technique, statistical process control uses statistical tools to observe the performance of the production line to predict significant deviations that may result in rejected products.

The underlying assumption in the SPC method is that any production process will produce products whose properties vary slightly from their designed values, even when the production line is running normally, and these variances can be analyzed statistically to control the process. For example, a breakfast cereal packaging line may be designed to fill each cereal box with 500 grams of product, but some boxes will have slightly more than 500 grams, and some will have slightly less, producing a distribution of net weights. If the production process itself changes (for example, the machines doing the manufacture begin to wear) this distribution can shift or spread out. For example, as its cams and pulleys wear out, the cereal filling machine may start putting more cereal into each box than it was designed to. If this change is allowed to continue unchecked, product may be produced that fall outside the tolerances of the manufacturer or consumer, causing product to be rejected.

By using statistical tools, the operator of the production line can discover that a significant change has been made to the production line, by wear and tear or other means, and correct the problem - or even stop production - before producing product outside specifications. An example of such a statistical tool would be the Shewhart control chart, and the operator in the aforementioned example plotting the net weight in the Shewhart chart.

What is Industrial Automation?

2008-11-29T19:00:00.000-08:00

Automation (ancient Greek: = self dictated), roboticization or industrial automation or numerical control is the use of control systems such as computers to control industrial machinery and processes, replacing human operators. In the scope of industrialization, it is a step beyond mechanization. Whereas mechanization provided human operators with machinery to assist them with the physical requirements of work, automation greatly reduces the need for human sensory and mental requirements as well.

Currently, for manufacturing companies, the purpose of automation has shifted from increasing productivity and reducing costs, to broader issues, such as increasing quality and flexibility in the manufacturing process.

The old focus on using automation simply to increase productivity and reduce costs was seen to be short-sighted, because it is also necessary to provide a skilled workforce who can make repairs and manage the machinery.

Moreover, the initial costs of automation were high and often could not be recovered by the time entirely new manufacturing processes replaced the old. (Japan's "robot junkyards" were once world famous in the manufacturing industry.)

Automation is now often applied primarily to increase quality in the manufacturing process, where automation can increase quality substantially. For example, automobile and truck pistons used to be installed into engines manually. This is rapidly being transitioned to automated machine installation, because the error rate for manual installment was around 1-1.5%, but has been reduced to 0.00001% with automation. Hazardous operations, such as oil refining, the manufacturing of industrial chemicals, and all forms of metal working, were always early contenders for automation.

Another major shift in automation is the increased emphasis on flexibility and convertibility in the manufacturing process. Manufacturers are increasingly demanding the ability to easily switch from manufacturing Product A to manufacturing Product B without having to completely rebuild the production lines.

What is an LVDT and other information...

2008-11-28T05:31:00.000-08:00

What is an LVDT?

The letters LVDT are an acronym for Linear Variable Differential Transformer, a common type of electromechanical transducer that can convert the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical signal. LVDT linear position sensors are readily available that can measure movements as small as a few millionths of an inch up to several inches, but are also capable of measuring positions up to ±20 inches (±0.5 m).

The figure shows the components of a typical LVDT. The transformer's internal structure consists of a primary winding centered between a pair of identically wound secondary windings, symmetrically spaced about the primary. The coils are wound on a one-piece hollow form of thermally stable glass reinforced polymer, encapsulated against moisture, wrapped in a high permeability magnetic shield, and then secured in a cylindrical stainless steel housing. This coil assembly is usually the stationary element of the position sensor.

The moving element of an LVDT is a separate tubular armature of magnetically permeable material called the core, which is free to move axially within the coil's hollow bore, and mechanically coupled to the object whose position is being measured. This bore is typically large enough to provide substantial radial clearance between the core and bore, with no physical contact between it and the coil.

In operation, the LVDT's primary winding is energized by alternating current of appropriate amplitude and frequency, known as the primary excitation. The LVDT's electrical output signal is the differential AC voltage between the two secondary windings, which varies with the axial position of the core within the LVDT coil. Usually this AC output voltage is converted by suitable electronic circuitry to high level DC voltage or current that is more convenient to use.

Why use an LVDT?

LVDTs have certain significant features and benefits, most of which derive from its fundamental physical principles of operation or from the materials and techniques used in its construction.

Friction-Free Operation

One of the most important features of an LVDT is its friction-free operation. In normal use, there is no mechanical contact between the LVDT's core and coil assembly, so there is no rubbing, dragging or other source of friction. This feature is particularly useful in materials testing, vibration displacement measurements, and high resolution dimensional gaging systems.

Infinite Resolution

Since an LVDT operates on electromagnetic coupling principles in a friction-free structure, it can measure infinitesimally small changes in core position. This infinite resolution capability is limited only by the noise in an LVDT signal conditioner and the output display's resolution. These same factors also give an LVDT its outstanding repeatability.

Unlimited Mechanical Life

Because there is normally no contact between the LVDT's core and coil structure, no parts can rub together or wear out. This means that an LVDT features unlimited mechanical life. This factor is especially important in high reliability applications such as aircraft, satellites and space vehicles, and nuclear installations. It is also highly desirable in many industrial process control and factory automation systems.

Overtravel Damage Resistant

The internal bore of most LVDTs is open at both ends. In the event of unanticipated overtravel, the core is able to pass completely through the sensor coil assembly without causing damage. This invulnerability to position input overload makes an LVDT the ideal sensor for applications like extensometers that are attached to tensile test samples in destructive materials testing apparatus.

Single Axis Sensitivity

An LVDT responds to motion of the core along the coil's axis, but is generally insensitive to cross-axis motion of the core or to its radial position. Thus, an LVDT can usually function without adverse effect in applications involving misaligned or floating moving members, and in cases where the core doesn't travel in a precisely straight line.

Separable Coil And Core

Because the only interaction between an LVDT's core and coil is magnetic coupling, the coil assembly can be isolated from the core by inserting a non-magnetic tube between the core and the bore. By doing so, a pressurized fluid can be contained within the tube, in which the core is free to move, while the coil assembly is unpressurized. This feature is often utilized in LVDTs used for spool position feedback in hydraulic proportional and/or servo valves.

Environmentally Robust

The materials and construction techniques used in assembling an LVDT result in a rugged, durable sensor that is robust to a variety of environmental conditions. Bonding of the windings is followed by epoxy encapsulation into the case, resulting in superior moisture and humidity resistance, as well as the capability to take substantial shock loads and high vibration levels in all axes. And the internal high-permeability magnetic shield minimizes the effects of external AC fields.

Both the case and core are made of corrosion resistant metals, with the case also acting as a supplemental magnetic shield. And for those applications where the sensor must withstand exposure to flammable or corrosive vapors and liquids, or operate in pressurized fluid, the case and coil assembly can be hermetically sealed using a variety of welding processes.

Ordinary LVDTs can operate over a very wide temperature range, but, if required, they can be produced to operate down to cryogenic temperatures, or, using special materials, operate at the elevated temperatures and radiation levels found in many nuclear reactors.

Null Point Repeatability

The location of an LVDT's intrinsic null point is extremely stable and repeatable, even over its very wide operating temperature range. This makes an LVDT perform well as a null position sensor in closed-loop control systems and high-performance servo balance instruments.

Fast Dynamic Response

The absence of friction during ordinary operation permits an LVDT to respond very fast to changes in core position. The dynamic response of an LVDT sensor itself is limited only by the inertial effects of the core's slight mass. More often, the response of an LVDT sensing system is determined by characteristics of the signal conditioner.

Absolute Output

An LVDT is an absolute output device, as opposed to an incremental output device. This means that in the event of loss of power, the position data being sent from the LVDT will not be lost. When the measuring system is restarted, the LVDT's output value will be the same as it was before the power failure occurred.

How does an LVDT work?

This figure illustrates what happens when the LVDT's core is in different axial positions. The LVDT's primary winding, P, is energized by a constant amplitude AC source. The magnetic flux thus developed is coupled by the core to the adjacent secondary windings, S1 and S2 . If the core is located midway between S1 and S2 , equal flux is coupled to each secondary so the voltages, E1 and E2 , induced in windings S1 and S2 respectively, are equal. At this reference midway core position, known as the null point, the differential voltage output, (E1 - E2 ), is essentially zero.

If the core is moved closer to S1 than to S2 , more flux is coupled to S1 and less to S2 , so the induced voltage E1 is increased while E2 is decreased, resulting in the differential voltage (E1 - E2). Conversely, if the core is moved closer to S2 , more flux is coupled to S2 and less to S1 , so E2 is increased as E1 is decreased, resulting in the differential voltage (E2 - E1 ).

The top graph shows how the magnitude of the differential output voltage, EOUT, varies with core position. The value of EOUT at maximum core displacement from null depends upon the amplitude of the primary excitation voltage and the sensitivity factor of the particular LVDT, but is typically several volts RMS. The phase angle of this AC output voltage, EOUT, referenced to the primary excitation voltage, stays constant until the center of the core passes the null point, where the phase angle changes abruptly by 180 degrees, as shown in the middle graph.

This 180 degree phase shift can be used to determine the direction of the core from the null point by means of appropriate circuitry. This is shown in the bottom graph, where the polarity of the output signal represents the core's positional relationship to the null point. The figure shows also that the output of an LVDT is very linear over its specified range of core motion, but that the sensor can be used over an extended range with some reduction in output linearity. The output characteristics of an LVDT vary with different positions of the core. Full range output is a large signal, typically a volt or more, and often requires no amplification. Note that an LVDT continues to operate beyond 100% of full range, but with degraded linearity.

From Macro Sensors website

What are Temperature Transmitters?

2008-11-26T02:10:00.000-08:00

Temperature measurement using modern scientific thermometers and temperature scales goes back at least as far as the early 18th century, when Gabriel Fahrenheit adapted a thermometer (switching to mercury) and a scale both developed by Ole Christensen Røemer. Fahrenheit's scale is still in use, alongside the Celsius scale and the Kelvin scale.

Many methods have been developed for measuring temperature. Most of these rely on measuring some physical property of a working material that varies with temperature. One of the most common devices for measuring temperature is the glass thermometer. This consists of a glass tube filled with mercury or some other liquid, which acts as the working fluid. Temperature increases cause the fluid to expand, so the temperature can be determined by measuring the volume of the fluid. Such thermometers are usually calibrated, so that one can read the temperature, simply by observing the level of the fluid in the thermometer. Another type of thermometer that is not really used much in practice, but is important from a theoretical standpoint is the gas thermometer.

Temperature transmitters, RTD, convert the RTD resistance measurement to a current signal, eliminating the problems inherent in RTD signal transmission via lead resistance. Errors in RTD circuits (especially two and three wire RTDs) are often caused by the added resistance of the leadwire between the sensor and the instrument. Transmitter input, specifications, user interfaces, features, sensor connections, and environment are all important parameters to consider when searching for temperature transmitters, RTD.

Transmitter input specifications to take into consideration when selecting temperature transmitters, RTD include reference materials, reference resistance, other inputs, and sensed temperature. Choices for reference material include platinum, nickel or nickel alloys, and copper. Platinum is the most common metal used for RTDs - for measurement integrity platinum is the element of choice. Nickel and nickel alloys are very commonly used metal. They are economical but not as accurate as platinum. Copper is occasionally used as an RTD element. Its low resistivity forces the element to be longer than a platinum element. Good linearity and economical. Upper temperature range typically less than 150 degrees Celsius. Gold and Silver are other options available for RTD probes - however their low resistivity and higher costs make them fairly rare, Tungsten has high resistivity but is usually reserved for high temperature work. When matching probes with instruments - the reference resistance of the RTD probe must be known. The most standard options available include 10 ohms, 100 ohms, 120 ohms, 200 ohms, 400 ohms, 500 ohms, and 1000 ohms. Other inputs include analog voltage, analog current, and resistance input. The temperature range to be sensed and transmitted is important to consider.

Important transmitter specifications to consider when searching for temperature transmitters, RTD, include mounting and output. Mounting styles include thermohead or thermowell mounting, DIN rail mounting, and board or cabinet mounting. Common outputs include analog current, analog voltage, and relay or switch output. User interface choices include analog front panel, digital front panel, and computer interface. Computer communications choices include serial and parallel interfaces. Common features for temperature transmitters, RTD, include intrinsically safe, digital or analog display, and waterproof or sealed. Sensor connections include terminal blocks, lead wires, screw clamps or lugs, and plug or quick connect. An important environmental parameter to consider when selecting temperature transmitters, RTD, is the operating temperature.

Magnetic Flowmeters

2008-11-17T06:30:00.000-08:00

Surfing and reading articles in the world wide web, I recently came up to this article about flowmeters by The OMEGA website. OMEGA is a well known industry in providing instrumentation solutions to various manufacturing firms. As I 100 percent trusted this company with regards to instrumenation and controls, I would like to post here the said article. I have encountered various brands and models of flowmeters and all follow one principle in operation. Read on to feed to your brain.

INTRODUCTION
Magnetic flowmeters are low pressure drop, volumetric, liquid flow measuring devices. The low maintenance design–with no moving parts, high accuracy, linear analog outputs, insensitivity to specific gravity, viscosity, pressure and temperature, and the ability to measure a wide range of difficult-to meter fluids (such as corrosives, slurries and sludges)–differentiates this type of metering system from other flowmeters. Two basic styles of magnetic flowmeter are currently available from OMEGA Engineering:

1) Wafer-style, where highest accuracy (up to +0.5% of reading) measurements are required; and
2) Insertion-style, for greater economy and particularly for larger pipe sizes.

All OMEGA® magnetic flowmeters employ the state-of-the-art dc pulsed magnetic field system. The following discussion details the principle of operation, as well as the advantages, of dc pulsed type magnetic flowmeters.

PRINCIPLE OF OPERATION
Faraday’s Law
The operation of a magnetic flowmeter is based upon Faraday’s Law, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field is proportional to the velocity of that conductor.

Faraday’s Formula:
E is proportional to V x B x D
where:
E = The voltage generated in a conductor
V = The velocity of the conductor
B = The magnetic field strength
D = The length of the conductor
To apply this principle to flow measurement with a magnetic flowmeter, it is necessary first to state that the fluid being measured must be electrically conductive for the Faraday principle to apply.

As applied to the design of magnetic flowmeters, Faraday’s Law indicates that signal voltage (E) is dependent on the average liquid velocity (V) the magnetic field strength (B) and the length of the conductor (D) (which in this instance is the distance between the electrodes).
In the case of wafer-style magnetic flowmeters, a magnetic field is established throughout the entire cross-section of the flow tube (Figure 1). If this magnetic field is considered as the measuring element of the magnetic flowmeter, it can be seen that the measuring element is exposed to the hydraulic conditions throughout the entire cross-section of the flowmeter. With insertion-style flowmeters, the magnetic field radiates outward from the inserted probe (Figure 2).

Figure 1: In-line magnetic flowmeter operating principle

Figure 2: Insertion-type flowmeter operating principle

MAGMETER SELECTION
The characteristics of the fluid to be metered, the liquid flow parameters, and the environment of the meter are the determining factors in the selection of a particular type of flowmeter.

Conductivity
Electrical conductivity is simply a way of expressing the ability of a liquid to conduct electricity. Just as copper wire is a better conductor than tin, some liquids are better conductors than others. However, of even greater importance is the fact that some liquids have little or no electrical conductivity (such as hydrocarbons and many nonaqueous solutions, which lack sufficient conductivity for use with magmeters). Conversely, most aqueous solutions are well suited for use with a magmeter. Depending on the individual flowmeter, the liquid conductivity must be above the minimum requirements specified. The conductivity of the liquid can change throughout process operations without adversely affecting meter performance, as long as it is homogeneous and does not drop below the minimum conductivity threshold. Several factors should be taken into consideration concerning liquids to be metered using magnetic flowmeters. Some of these are:
1. All water does not have the same conductivity. Water varies greatly in conductivity due to various ions present. The conductivity of “tap water” in Maine might be very different from that of “tap water” in Chicago.
2. Chemical and pharmaceutical companies often use deionized or distilled water, or other solutions which are not conductive enough for use with magnetic flowmeters.
3. Electrical conductivity is a function of temperature. However, conductivity does not vary in any set pattern for all liquids as temperature changes. Therefore, the temperature of the liquid being considered should always be known.
4. Electrical conductivity is a function of concentration. Therefore, the concentration of the solution should always be provided. However, avoid what normally is a logical assumption, such as: That electrical conductivity increases as concentration increases. This is true up to a point in some solutions, but then reverses. For example, the electrical conductivity of aqueous solutions of acetic acid increases as concentration rises up to 20%, but then shows a decrease with increased concentration to the extent that, at some concentration above 99%, it falls below the minimum requirement.
Acid/Caustics
The chemical composition of the liquid slurry to be metered will be a determining factor in selecting the flowmeter with the proper design and construction. Operating experience is the best guide to selection of liner and electrode materials, especially in industrial applications, because, in many cases, a process liquid or slurry will be called by a generic name, even though it may contain other substances which affect its corrosion characteristics. Commonly available corrosion guides may also prove helpful in selecting the proper materials of construction.

Velocity
The maximum (full scale) liquid velocity must be within the specified flow range of the meter for proper operation. The velocity through the flowhead can be controlled by properly sizing the meter. It isn’t necessary that the flowhead be the same line size, as long as such sizing does not conflict with other system design parameters. Although the meter will increase hydraulic head loss when sized smaller than the line size (because the meter is both obstructionless and of short lay length), the amount of increase in head loss is negligible in most applications. The amount of head loss increase can be further limited by using concentric reducers and expanders at the pipe size transitions. As a rule of thumb, meters should be sized no smaller than one-half of the line size. Because of the wide rangeability of magnetic flowmeters, it is almost never necessary to oversize a meter to handle future flow requirements. When future flow requirements are known to be significantly higher than start-up flow rates, it is imperative that the initial flows be sufficiently high and that the pipeline remain full under normal flow conditions.

Abrasive Slurries
Mildly abrasive slurries can be handled by magnetic flowmeters without problems, provided consideration is given to the abrasiveness of the solids and the concentration of the solids in the slurry. The abrasiveness of a slurry will affect the selection of the construction materials and the use of protective orifices. Abrasive slurries should be metered at 6 ft/sec or less in order to minimize flowmeter abrasion damage. Velocities should not be allowed to fall much below 4 ft/sec, since any solids will tend to settle out. An ideal slurry installation would have the meter in a vertical position. This would assure uniform distribution of the solids and avoid having solids settle in the flow tube during no-flow periods. Consideration should also be given to use of a protective orifice on the upstream end of a wafer-style magnetic flowmeter to prevent excessive erosion of the liner. This is especially true since Tefzel liner have excellent chemical resistance, but poor resistance to abrasion. In lined or non-conductive piping systems, the upstream protective orifice can also serve as a grounding ring.

Sludges and Grease-Bearing Liquids
Sludges and grease-bearing liquids should be operated at higher velocities, about 6 ft/sec minimum, in order to reduce the coating tendencies of the material.

Viscosity
Viscosity does not directly affect the operation of magnetic flowmeters, but, in highly viscous fluids, the size should be kept as large as possible to avoid excessive pressure drop across the meter.

Temperature
The liquid’s temperature is generally not a problem, providing it remains within the mechanism’s operating limits. The only other temperature considerations would be in the case of liquids with low conductivities (below around 3 micromhos per centimeter) which are subject to wide temperature excursions. Since most liquids exhibit a positive temperature coefficient of conductivity, the liquid’s minimum conductivity must be determined at the lower temperature extreme.

Advantages of the DC Pulse Style
From the principles of operation, it can be seen that a magnetic flowmeter relies on the voltage generated by the flow of a conductive liquid through its magnetic field for a direct indication of the velocity of the liquid or slurry being metered. The integrity of this low-level voltage signal (typically measured in hundreds of microvolts) must be preserved so as to maintain the high accuracy specification of magnetic flowmeters in industrial environments. The superiority of the dc pulse over the traditional ac magnetic meters in preserving signal integrity can be demonstrated as follows:

Quadrature
Some magnetic flowmeters employ alternating current to excite the magnetic field coils which generate the magnetic field of the flowmeter (ac magnetic flowmeters). As a result, the direction of the magnetic field alternates at line frequency, i.e., 50 to 60 times per second. If a loop of conductive wire is located in a magnetic field, a voltage will be generated in that loop of wire. From physics, we can determine that this voltage is 90° out of phase with respect to the primary magnetic field. The magnitude of this error signal is a function of the number of turns in the loop, and the change in magnetic flux per unit time. In a magnetic flowmeter, the electrode wires and the path through the conductive liquid between the electrodes represent a single turn loop. The flow-dependent voltage is in phase with the changing magnetic field; however, flow independent voltage is also generated, which is 90°out of phase with the changing magnetic field. The flow-independent voltage is therefore an error voltage which is 90° out of phase with the desired signal. This error voltage is often referred to as quadrature. In order to minimize the amount of quadrature generated, the electrode wires must be arranged so that they are parallel with the lines of flux of the magnetic field. In ac field magmeters, because the magnetic field alternates continuously at line frequency, quadrature is significant. It is necessary to employ phase sensitive circuitry to detect and reject quadrature. It is this circuitry which makes the ac magnetic meter highly sensitive to coating on the electrodes. Since coatings cause a phase shift in the voltage signal, phase-sensitive circuitry leads to rejection of the true voltage flow signal, thus leading to error. Since dc pulse magmeters are not sensitive to phase shift and require no phase-sensitive circuitry, coatings on the electrodes have a very limited effect on flowmeter performance.

Wiring
In ac magnetic flowmeters, the signal generated by flow through the meter is at line frequency. This makes these meters susceptible to noise pickup from any ac lines. Therefore, complicated wiring systems are typically required to isolate the ac flowmeter signal lines from both its own and from any other nearby power lines, in order to preserve signal integrity. In comparison, dc pulse magmeters have a pulse frequency much lower (typically 5 to 10% of ac line frequency) than ac meters. This lower frequency eliminates noise pickup from nearby ac lines, allowing power and signal lines to be run in the same conduit and thus simplifying wiring. Wiring is further simplified by the use of integral signal conditioners to provide voltage and current outputs. No separate wiring to the signal conditioners is required.

Power
By design, ac magnetic flowmeters typically have high power requirements, owing to the fact that the magnetic field is constantly being powered. Because of the pulsed nature of the dc pulse magmeter, power is supplied intermittently to the magnetic field coil. This greatly reduces both power requirements and heating of the electronic circuitry, extending the life of the instrument.

Figure 3: Vertical installation of inline meter

Auto-Zero
In traditional ac magnetic flowmeters, it is necessary after installation of the meter to “null” or “zero” the unit. This is accomplished by manual adjustment which requires that the flowmeter be filled with process liquid in a no-flow condition. Any signal present under full pipe, no-flow conditions is considered to be an error signal. The ac field magmeter is therefore “nulled” to eliminate the impact of these error signals. In the case of OMEGA® FMG-400 Series Magmeters, automatic zeroing circuitry has been included to eliminate the need for manual zeroing. When the magnetic field strength is zero between pulses, the voltage output from the electrodes is measured. If any voltage is measured during this period, it is considered extraneous noise in the system and is subtracted from the signal voltage generated when the magnetic field is on. This feature insures high accuracy, even in electrically noisy industrial environments.

Installation
OMEGA® magnetic flowmeters are designed for easy installation. FMG-400 Series Magmeters are ideal substitutes for the flanged spool type meter, which are heavier and significantly more expensive. The thin wafer style of the FMG-400 Series allows them to be slipped between standard flanges, without the need to cut away pipe to make room for the meter. Furthermore, the low weight of the meter means that, in many cases, no additional pipe supports are required after meter installation. Recommended piping configurations include the installation of by-pass piping, cleanout tees and isolation valves around the flowmeter (Figures 3 and 4). With insertion-style magmeters, even greater reductions in weight and cost have been achieved. Installation is accomplished by threading the piping system into the tee fitting supplied with the meter, or by drilling a tap into the line to accept the fitting that comes with the meter. Prior to installation of the meter, the following recommendations and items of general information should be considered. First, before installing a magmeter, it is important to consider location. Stray electromagnetic or electrostatic fields of high intensity may cause disturbances in normal operation. For this reason, it is desirable to locate the meter away from large electric motors, transformers, communications equipment, etc., whenever possible. Second, for proper and accurate operation, it is necessary that the flowmeter be installed so that the pipe will be full of the process liquid under all operating conditions. When the meter is only partially filled, even though the electrodes are covered, an inaccurate measurement will result. Third, for magnetic flowmeters, grounding is required to eliminate stray current and voltage which may be transmitted through the piping system, through the process liquid, or can arise by induction from electromagnetic fields in the same area as the magmeter. Grounding is achieved by connecting the piping system and the flowmeter to a proper earth ground system. Unfortunately, this is not always done properly, resulting in unsatisfactory meter performance. In conductive piping systems, a “third wire” safety ground to the power supply and a conductive path between the meter and the piping flanges are typically all that is required. In non-conductive or lined piping systems, a protective grounding orifice must be supplied to provide access to the potential of the liquid being metered. Dedicated or sophisticated grounding systems are not normally required. Detailed information concerning proper flowmeter grounding is provided with the owner’s manual that comes with each flowmeter. Finally, the position of the flowtube in relation to other devices in the system is also important in assuring system accuracy. Tees, elbows, valves, etc., should be placed at least 10 upstream and 5 downstream pipe diameters away from the meter to minimize any obstructions or flow disturbances.

Figure 4: Horizontal installation of in-line meter

Technical texts courtesy of Omega.

Modbus TCP Overview according to Intellicom Innovation

2008-10-31T06:41:00.000-07:00

Modbus TCP - An introduction
MODBUS® TCP/IP IS an Internet protocol. The fact that TCP/IP is the transport protocol of the Internet automatically means that MODBUS® TCP/IP can be used over the Internet! Among other things it was designed to reach this goal, and as part of this goal the MODBUS® protocol specification has been submitted to the Internet Engineering Task Force (IETF). In practical terms, this means that a MODBUS® TCP/IP device installed in Europe can be addressed over the Internet from the USA from anywhere else in the world.
The implications for a vendor of equipment or an end-user are endless.

Performing maintenance and repair on remote devices from the office using a PC and browser reduce support costs and improve customer service.
Logging onto a plant's control system from home allows the maintenance engineer to maximize his plant's uptime and reduce the number of times that he is called out from home.
Managing geographically distributed systems becomes easy using commercially available internet/intranet technologies.

MODBUS® TCP/IP has became an industry de facto standard because of its openness, simplicity, low cost development, and minimum hardware required to support it.
At this moment there are more than 200 MODBUS® TCP/IP devices available in the market. It is used to exchange information between devices, monitor and program them. It is also used to manage distributed I/Os, being the preferred protocol by the manufacturers of this type of devices.

Combining a versatile, scaleable, and ubiquitous physical network (Ethernet) with a universal networking standard (TCP/IP) and a vendor-neutral data representation (MODBUS® ) gives a truly open, accessible network for exchange of process data.

The protocol - Modbus TCP
Modbus/TCP basically embeds a Modbus frame into a TCP frame in a simple manner. This is a connection-oriented transaction which means every query expects a response.

This query/response technique fits well with the master/slave nature of Modbus, adding to the deterministic advantage that Switched Ethernet offers industrial users. The use of OPEN Modbus within the TCP frame provides a totally scaleable solution from ten nodes to ten thousand nodes without the risk of compromise that other multicast techniques would give.

Performance from a MODBUS TCP/IP system
The performance basically depends on the network and the hardware. If you are running MODBUS® TCP/IP over the Internet, you won't get better than typical Internet response times. However, for communicating for debug and maintenance purposes, this may be perfectly adequate and save you from having to catch a plane or go to site on a Sunday morning!

For a high-performance Intranet with high-speed Ethernet switches to guarantee performance, the situation is completely different.

In theory MODBUS® TCP/IP carries data at up to 250/(250+70+70) or about 60% efficiency when transferring registers in bulk, and since 10 Base T Ethernet carries about 1.25 Mbytes/sec raw, the theoretical throughput is:
1.25M / 2 * 60% = 360000 registers per second and the 100 Base T speed is 10 x greater.

This assumes that you are using devices that can service Ethernet as fast as bandwidth is available.
Practical tests carried out by Schneider Automation using a MOMENTUMTM Ethernet PLC with Ethernet I/O demonstrated that up to 4000 I/O bases could be scanned per second, each I/O base having up to 16 12-bit analog I/O or 32 discrete I/O. Four bases could be updated in one millisecond. While this is below the theoretical limit calculated above, it must be remembered that the tested device was running with a lowly 80186 CPU running at 50Mhertz with an effective computing power of 3 MIPS (compared to the 700 MIPS of a 500MHz Pentium). Also, these results are nevertheless faster than the proprietary I/O scan methods used to date.

As low-end CPU's get cheaper, Momentum-type devices will chase the theoretical limit, although they'll never reach it because the limit will be continually pushed further away with 1 Gigabit Ethernet, 10 Gigabit Ethernet, etc. This is in contrast to other field-buses which are inherently stuck at one speed.

How can existing MODBUS devices communicate over MODBUS TCP/IP?
MODBUS® TCP/IP is simply MODBUS® protocol with a TCP wrapper. It is therefore extremely simple for existing MODBUS® devices to communicate over MODBUS® TCP/IP. To do this a gateway device is required to convert MODBUS protocol to MODBUS TCP/IP.

Recommended Modbus products:

Modbus web server Gateway (Supports Ethernet, GSM, GPRS, modem)

Features
• Connects Modbus RTU products to SCADA or PLC’s over Ethernet
• Supports Ethernet, Internet, LAN, GSM/GPRS, Modbus TCP, email, FTP etc.

Modbus RTU – Modbus TCP Gateway

Features
• Connects Modbus RTU products to SCADA or PLC’s over Ethernet
• Transparent Gateway between Modbus RTU and Modbus TCP.

Serial Server

Features
• Connects Modbus RTU/ASCII products to any Windows based software
• Transparent connection
• 2 ports (1 RS485 + 1 RS232)

Modbus I/O over Internet or GSM/GPRS

Features
• Modbus I/O: 4 DI, 4 DO, 2AI, 1AO, “ RTD (Temperature)
• Can be accessed through Internet and Mobile phones

IAM Forum Site Now Online

2008-10-29T02:12:00.000-07:00

Friends, I've some good news! The Industrial Automation and Mechatronics Forum site is now up and running. We now have a portal where we can interact. This would be great to find and chat up your colleagues, mentors and friends. The site can be accessed through the link at the navigation bar at the address http://forum.iamechatronics.com.

Remember, this is ours.

the Admin

Notes on Pyrometer Calibration Procedure

2008-10-23T05:18:00.001-07:00

From the desk of Edwin B. Mariano (Hotmill Systems Engineer of Global Steel Philippines Inc.), who is a good friend of mine, generated a step by step procedure on how to calibrate the pyrometers.

Below is a note on the blackbody furnace and proposed standard procedure in the calibration of "Infrared Thermometer" otherwise known as "Pyrometers" used in rolling mills.

Every object radiates thermal energy at temperatures above absolute zero. Measuring the temperature of an object using optical pyrometer is based on the principle that the thermal radiation from the object being measured is a function of its temperature. For any particular temperature and wavelength, the energy radiated by a surface is directly proportional to the spectral emissivity of the object. Emissivity is the value associated with the surface's ability to get of heat by radiating thermal energy, and different substances have different emissivities. The value of a substance's spectral emissivity is a number in the range from 0 to 1.0, which is the ratio of the energy radiated by object's surface to the energy radiated by a perfect blackbody at the same temperature.

The primary issue when using infrared thermometer sensors is that a real object does not behave like a perfect radiator.

Having calibrated an infrared thermometer on a perfect radiator (blackbody source), the ability to measure temperature of a real object relates directly to how well the object's emissivity is known. An object with an emissivity of 0.8 emits only 80 percent of the energy of the blackbody, so unless one accounts for an object's actual emissivity, the indicated temperature reading will be lower than the object's actual temperature. Most radiation thermometers provide for emissivity adjustment.

The instrument manufacturer calibrates an infrared thermometer (pyrometer) by aiming it a blackbody source, which is designed specifically for testing and calibrating infrared thermometers. Blackbody sources resemble a furnace with an opening to view a surface or cavity heated and controlled to a selected temperature.

There are basically two types of infrared calibration sources, the Hot Plate Blackbody and the Cavity Type Blackbody source. But, let us concentrate on the cavity type blackbody source as this is available within the author's reach.

The cavity type blackbody source consists of a blind hole in a cylinder or sphere where the temperature of teh cavity is controlled by a temperature controller, using a thermocouple probe. The cavity type blackbody source has a higher emissivity compared to a hot plate blackbody source. The emissivity of a cavity type blackbody source is typically 0.98 or higher which makes them ideal for precise calibration tasks.

When using a blackbody calibration source, we need to follow certain ground rules:

The infrared thermometer should be aimed perpendicular to the target area of the blackbody unit. If aimed at an angle, reflected infrared radiation energy can impair calibration accuracy.
The field of view area of the thermometer at a selected distance should be smaller than the target area of the blackbody unit.
Do not bring the infrared thermometer too close to the target area of the blackbody especially at high temperatures. The radiated heat from the blackbody can not only impair the calibration accuracy but also can potentially damage the unit under test.
Always aim the infrared thermometer to the center of the target area.
When changing the temperature setpoint on the blackbody unit, make sure the unit is fully stabilized to the new temperature setting before making any calibration test. Usually going up in temperature takes less time than going down in temperature.
Do not unplug a blackbody calibration source while it hot. There are usually built-in fans to remove the heat even when the power switch is off. Let the internal fan continue to run until the unit is cooled down.

User should be guided on recommended operating conditions. These factors, along with proper maintenance, will allow safe and satisfactory operation of the furnace.

Safety is very important when working with furnaces. Follow safety procedures for working around the furnace. Controllers are used to control temperatures to constant value ( the operators sets the required temperature; when achieved the temperature is maintained on this value until next intervention of the operator) or to control temperature according to simple program (the operator sets the temperature build-up speed).

Step 1.
• Set-up and mount the pyrometer’s sensor and controller. When blackbody furnace has been properly prepared and ready for operation. Operating suggestions: Manual intervention if batch calibration for auto mode, refer to controller operating manual ramp programming.

Step 2.
• Power up the blackbody furnace to heat up 400 degrees Celsius at rate of 3 degrees Celsius per minute. Hold at 400 degrees Celsius for two hours or more (preferably overnight). Let furnace to heat up until set point 400 degrees Celsius is stabilized at this set point.

Step 3.
• While the furnace is building up its temperature, do preliminary observation on the controller before turning on the power, check if the indicator/processor analog meter pointer is in zero position. If not, with a small screwdriver, gently turn the advancement screw as necessary to zero the pointer at the left side of the scale.

Step 4.
• Operator should follow a projected heating curve for the duration of heating at the desired temperature for calibration. Let furnace heat up until each REF. # set point and stabilized at this set point.Step
• Size = Pre-determined value for temperature change.
• Ramp = Select the pre-determined rate changes to the next value.
• Increment using numerical keypad (increment and decrement).

Step 5.
• Power on the controller, and allow 20 minutes warm-up time.

Step 6.
• Cover the objective lens with a solid, opaque object that will completely block any external radiation.

Step 7.
• The Response Time control must be in the FAST position. If a Peak Picker option is installed, toggle the Peak Picker switch to DIRECT.

Step 8.
• Depress the momentary toggle switch label CALIBRATE.
• If your instrument has and analog meter, observe if the meter reads within the read CAL. ZONE. If the meter reads outside the CAL. ZONE, make some adjustment.
• If your instrument has a digital readout, depressing the Calibrate switch should produce a reading of 500 +/- 10 digits, regardless of model. If it does not, do some calibration.
• Normal calibration level: Pointer to center of CAL position on analog display; numerical reading of 500 +/- 10 digits on digital display.

Note: Refer to manufacturer’s infrared thermometer manual for Calibration Check.

Step 9.
• Without any calibration adjustment to be undertaken, take reading on the projected temperatures for upscale and then downscale reading. This is to determine possible error. Set the Emissivity/E-slope adjustment to the highest value of the controller.

Step 10.
• If calibration is to be done for several pyrometer set or batch, repeat steps3 to 9 and note the reading taken for each set of upscale and downscale reading.

Step 11.
• With each set of the reading taken for upscale and the downscale, determine if there are noticeable errors. If reading is linear and within the projected temperature range. No further adjustment is needed, the pyrometer set is calibrated.

Step 12.
• If error is present, calibrate the pyrometer set. By setting the blackbody furnace temperature to the lowest range of the controller then calibrate the controller to zero.

Step 13.
• Do step 12 to other pyrometer set with determined error.

Step 14.
• Increase the blackbody furnace temperature setting to the highest range of the controller then calibrate the controller for this setting.

Step 15.
• Do step 14 to other pyrometer set with determined error.

Step 16.
• Check upscale reading against projected temperature of the pyrometer then set and take note of its reading. If this batch calibration, do the same to the other pyrometer sets.

Step 17.
• Check the downscale reading against the projected temperature of the pyrometer set and take note of its reading. If this is a batch calibration do the same to other pyrometer sets.

Steps 18.
• If there are still discrepancies on the linearity, repeat step 12 to 17. If there are none, calibration is done.

Step 19.
• Cool down blackbody slowly. (Refer to the cooling curve below).

Note: During operation, one of the most important requirements for accurate temperature measurements is the Emissivity/E-slope adjustment to effectively tune circuits measurement to the characteristics of the process material. To get a true measure of temperature, we must set the Emissivity/E-slope control in the system to match Emissivity of the material being measured.

The BSIAM Studes...

2008-10-23T01:06:00.000-07:00

The BSIAM students in one of their discussions with Professor Joel Barrera on one of their Mechatronics subjects.

Listening...

Young, Wacky but very promising.

Industrial Motor Control Room of IACET, MSU-IIT

2008-10-22T23:52:00.000-07:00

The Industrial Motor Control Room of the Industrial Automation and Controls Engineering Technology, Mindanao State University - Iligan Institute of Technology is the center of the education for Motors Circuits, Building Wiring, and Sequence Control and other related subjects for students. Trainings are conducted here for such fields either in lecture and/or practical application. Conventional hard-wired industrial motor control circuits are taken up like the simple start-stop circuit, dynamic braking circuit, forward-reverse control and extending up to controls like elevator systems with safety interlocking and to other fully systemized motor control operations. This is done by using magnetic contactors, relays, timers and other electromechanical devices.

In the advancement of the lessons, the student/practicioner will apply the said circuits learned using the conventional electromechanical circuits to using PLC as the processing unit for control. More advanced industrial circuits will be taken up using advance PLC programming. This will be done in actual practice and not just in lecture method.

The training offered here is of world-class quality for instructors and professors are trained in various known institutions worldwide.

For more information, contact Professor Cesar Gabo of IACET, MSU-IIT for more details. He is in-charge for these trainings. Feel free to contact him.

Level Measurement Techniques

2008-10-21T09:11:00.000-07:00

Someone may ask you if you can measure level with using a pressure meter, particularly a differential pressure meter, you should answer yes. Since pressure and level is somewhat related if we would deeply dig into their relationship.

Answering the question in how to do it, see the picture below. The bottom area of the vessel is connected to the high pressure side of the differential pressure meter or transmitter.
Differential Pressure = H x D
The diffferential pressure is applied to the high pressure side of the transmitter and is calibrated.

How is D.P. transmitter applied to a close tank?
In close tank the bottom of the tank is connected to the high pressure side of the transmitter and top of the tank in connected to L.P. side of the transmitter. In this way the vessel pressure is balanced.

How is D.P. transmitter applied to an open tank?
On an open tank level measurement the L.P. side is vented to atmosphere. Whatever pressure acts is on the H.P. side which is a measure of level.

SPAN = (X) ()
ZERO SUPPRESSION = (Y) (Specific Gravity)

Also, how can a differential pressure transmitter be applied to a close and open tank with Dry Leg to measure level?

Considering the formula below;

Span = (X) (GL)
Hw at minimum level - (Z)(GS) + (Y)(GL)
Hw at maximum level = ( Z ) ( Gs ) + (X + Y) ( GL)
Where:
GL= Specific gravity of tank liquid
Gs = Specific gravity of seal liquid
Hw = Equivalent head of water
X, Y & Z are shown below

Example:
Open tank with:

X =300 inches

Y =50 inches

Z =10 inches

GL= 0.8

Gs= 0.9

Span = (300) (0.8) = 240 inches

Solving,
Hw at minimum level = ( 10 ) ( 0.9 ) + ( 50 ) ( 0.8 ) = 49 inches
Hw at maximum level = (10 ) ( 0.9 ) + ( 50 + 300 ) ( 0.8 ) = 289 inches
Calibrated range = 49 to 289 inches head of water

Level Measurement Part 1

2008-10-21T08:47:00.000-07:00

(A part of Instrumentation Basics...)

We will be discussing about level measurement which is very vital part in plant automation. Under the industrial category of instrumentation, it is very important to measure levels of fluid for open and closed loop systems.

There are two ways to measure level, the Direct Method and the Indirect Method.

For direct level measurement, we can use the Bob and tape and Sight glass methods

A bob weight and measuring tape provide the most simple and direct method of measuring liquid level. See picture at the right for the graphical presentation.

One can also use a sight glass. This consists of a graduated glass tube mounted on the side of the vessel. As the level of the liquid in the vessel change, so does the level of the liquid in the glass tube.

For the indirect level measurement, One can use pressure gauges.

This is the simplest method, for pressure gauge is located at the zero level of the liquid in the vessel. Any rise in the level causes an increase of pressure which can be measured by any gauge.

Another way is through the use of a purge system. In this method a pipe is installed vertically with the open end at zero level. The other end of the pipe is connected to a regulated air supply and to a pressure gauge. To make a level measurement the air supply is adjusted so that pressure is slightly higher than the pressure due to height of the liquid. This is accomplished by regulating the air pressure until bubbles cab be seen slowly leaving the open end of the pipe.

The air pressure to the bubbler pipe is minutely in Excess of the liquid pressure in the vessel, so that Air pressure indicated is a measure of the level in The tank.

The method above is suitable for open tank applications. When a liquid is in a pressure vessel, the liquid column pressure can't be used unless the vessel pressure is balanced out. This is done through the use of different pressure meters.

Through differential pressure meters, one can also measure the level of liquid inside the tank.Connections are made at the vessel top and bottom, and to the two columns of the D.P. meter. The top connection is made to the L.P. column of the transmitter and the bottom to H.P. column of the transmitter. The difference in pressure in the vessel is balanced out, since it is fed to both the column of the meter. The difference in pressure deducted by the meter will be due only to the changing, level of the liquid.

Another way is through the displacer type level measurement.The leveltrol is one of the most common instruments used measuring level in closed tanks. This instrument works of Archimedes principle. The displacer in immersed in the liquid due to which there is loss of weight depending on the specified gravity of the liquid. This displacer hangs freely on a knife transmitted to the pneumatic or electronic counterpart at the other end.

How to's of Instrument Calibration

2008-10-20T04:59:00.000-07:00

(a part of Instrumentation Basics...)

How do you identify an orifice in the pipe line?
An orifice tab is welded on the orifice plate which extends outer of the line giving an indication of the orifice plate.

Why is the orifice tab provided?
The orifice tab is provided due to the following reasons.
• Indication of an orifice plate in a line.
• The orifice diameter is marked on it.
• The material of the orifice plate.
• The tag no. of the orifice plate.
• The mark the inlet of an orifice.

Advantages and Disadvantages of Orifice Plates
Advantages of orifice plates include:
• High differential pressure generated
• Exhaustive data available
• Low purchase price and installation cost
• Easy replacement

What is Bernoulli's theorem and where it is applicable?
Bernoulli's theorem states the "total energy of a liquid flowing from one point to another remains constant." It is applicable for non compressible liquids.

How do you identify the H. P. side or inlet of an orifice plate in line?
The marking is always done H. P. side of the orifice tab which gives an indication of the H. P. side.

How do you calibrate a D. P. transmitter?
The following steps are to be taken which calibrating:

Adjust zero of the Tx'r.
Static pressure test : Give equal pressure on both sides of the transmitter. Zero should not shift. If it is shifting carry out static alignment.
Vacuum test: Apply equal vacuum to both the sides. The zero should not shift.
Calibration Procedure: * Give 20 psi air or 24Vdc supply to the transmitter.
* Vent the L.P. side to atmosphere.
* Connect output of the Instrument to a standard test gauge or Multimeter and adjust zero.
Apply required pressure to high pressure side of the transmitter and adjust the span.
Adjust zero again if necessary.

What is the seal liquid used for filling impulse lines on crude and viscous liquid ?
Glycol.

How do you carry out piping for a Different pressure flow transmitter on liquids, gas and steam services? Why ?
Liquid lines : On liquid lines the transmitter is mounted below the orifice plate because liquids have a property of self draining.

Gas Service : On gas service the transmitter is mounted above the orifice plate because Gases have a property of self venting and secondly condensate formation.

Steam Service : On steam service the transmitter is mounted below the orifice plate with condensate pots. The pots should be at the same level.
Previous Lesson
Next Lesson

Calibration of Pyrometers using Black Body

2008-10-17T09:10:00.000-07:00

According to wikipedia, a Pyrometer is any non-contacting device that intercepts and measures thermal radiation. This measure is used to determine temperature, often of the object's surface.

The word pyrometer comes from the Greek word for fire, "πυρ", and meter, meaning to measure. Pyrometer was originally coined to denote a device capable of measuring temperatures of objects above incandescence (i.e. objects bright to the human eye).

Pyrometers are used to 'read' radiated heat in steel industries, ceramic manufacturing and other applications requiring high temperature from a few hundreds to thousands of degrees Celsius of temperature. Most pyrometers come in a set of a sensor and a controller. The controller may come readouts of analog or digital.

I have here some pictures while in the process of calibrating pyrometers using a black body.

A brand new digital pyrometer from Ircon (Modline 3 Series). The plastic bag encasing the said meter is removed. This unit is already calibrated by its manufacterer and is delivered with a calibration certificate but we are testing this one just making sure if their calibration standards are in line with the user's standards.

^^^ At the back of the meter, one can see eight screws securing the back panel cover. Remove it using a flat screwdriver. Various connectors can be seen like analog and relay outputs, connectors to receive and drive the sensor head, power input and others as well.

^^^ Connect the wires from the cable from the sensor head using a small flat screwdriver. Don't worry on which wire to connect to the slot. If one bought a multi core cable designated for use for this pyrometer (contact the manufacturer for the said cable specifications), color coding are used as based on the ones printed below the connector as to what color of the wire it is should be paired with. Lastly connect the power line as indicated in the second pic. Do not turn on yet the said unit.

^^^ Mount the sensor head on a stable stand or platform in front of the black body calibrator set to a certain temperature within the range of the pyrometer. What one can see here is that the said sensor is mounted on a stable but adjustable stand with variable x and y axes so one can adjust the unit without any external 'disturbance'. Connect the cable from the controller/indicator to the sensor.

^^^ Remove the eypiece cover and see to it that the sensor head is exactly aligned with the black body by sighting through it. One can see a small circle as a guide. A little deviation from the said alignment would result in significant error in its reading so be very careful.

^^^Turn on the the unit by plugging it to a convenience outlet or other power source which can provide a 90-230 volts AC source. Let it stand for 30 to 60 minutes before officially taking readings to allow the electronics circuits to stabilize. The second pic is showing a calibration specialist taking a reading from the black body using a digital infrared thermometer in par with the pyrometer to ensure a standardized read out.

If through the test process, the meter is not in line with the standards, adjustments would be made such as zeroing, span, and gain through potentiometers located inside the sensor head and the controller.

A whole bunch of digital and analog pyrometers waiting to be test read and calibrated.

P&ID PFD Symbols

2008-10-13T06:55:00.000-07:00

What I have to offer here are General Instrument and Function Symbols readily downloadable for everyone. This is very useful and a must for the instrumentation engineers and practitioners. In all instrumentation exams that I have taken, a major part of the said activity comprises of the identification of the said symbols and analyzing process diagrams.

So what is P&ID?
A Piping and Instrumentation Diagram - P&ID, is a schematic illustration of functional relationship of piping, instrumentation and system equipment components
It is also a big plus for practitioners of other the fields of engineering.

and what is PFD?
A Process Flow Diagram - PFD - (or System Flow Diagram - SFD) shows the relationships between the major components in the system. PFD also tabulate process design values for the components in different operating modes, typical minimum, normal and maximum. A PFD does not show minor components, piping systems, piping ratings and designations.

Here are the files:

P&ID PFD Symbols 1
P&ID PFD Symbols 2
P&ID PFD Symbols 3
P&ID PFD Symbols 4

Open Collector Outputs Explained

2008-10-13T05:58:00.000-07:00

Discussed below will be about open collector outputs which is a very common method of outputting signals from industrial process instruments to actuators or loads. It provides flexible voltage output required for various kinds of actuators and loads instead of only one output designed. This is very useful and very convenient and yet provide a very good protection in your process instrumentation systems.

Though the example below use +GF+ Signet instruments which also use open collector outputs, the theory also applies on almost all instruments and PLCs which uses the type of outputs as said.

So, read on...

Open Collector Output

Many +GF+ SIGNET instruments and sensors feature “Open Collector Outputs” for purposes of signal transmission, alarming, control signal output, etc. Although such outputs allow for a lot of wiring flexibility, care must be taken not to destroy the circuits via incorrect polarity, over-voltage, transients or current overload. Below is an explanation of proper wiring and dimensioning of related circuit components. Please note that the following recommendations may or may not apply to other manufacturer’s equipment.

1. Function

Open Collector (“OC”) outputs are low powered, solid state switches. Although the term “Open Collector” stipulates the use of bipolar transistors (NPN-type or PNP-type) as a switch, nowadays Field Effect Transistors (FET or MOSFET) are used. Unlike electromechnical switches (e.g. pushbuttons or dry contact relays) these OC switches are very fast, use little power, and do not wear. However, OC’s are also more limited in terms of voltage and current rating as well as being polarized (i.e. they have a “plus” and “minus” terminal and thus DC only switching capability). They are less tolerant to overload abuse than electromechanical devices. Usually these switches have higher resistance and voltage drop.

2. Sensor Wiring

A typical example of the need for high speed switching capability is the OC frequency output of +GF+ SIGNET flow sensors like 3-2536 or 3-2540. Signal frequencies can reach several hundred pulses per second while voltage and current requirements are small enough, allowing the use of a transistor switch. For each output pulse this switch connects the signal output to the negative supply or ground terminal of the sensor and is therefore an “NPN” style output. +GF+ SIGNET does not produce sensors with PNP style outputs (which connect the signal output internally to the positive supply terminal).

Most indicating instruments or control system inputs require a signal voltage of 0 to 5V (TTL or CMOS logic levels) or 0 to 24V. Therefore, Open Collector output circuits must be complemented with a “Pull-Up-Resistor” to function properly.

Please see the following example diagram for wiring with a PLC input:

Please note that the voltage connected to the positive sensor supply (V+) must correspond to the required high-level PLC input voltage (i.e. if the high-input voltage of the PLC is 24V, then the pull-up must be supplied with 24V). If the input is “TTL-Level” or “CMOS-Level”, that means 5V for high level, then the pull-up should not be connected with a supply higher than 5V.

+GF+ SIGNET instruments already have the pull-up-resistor and the sensor power supply built into the instrument. No external pull-up-resistors are required.

3. Instrument Output Wiring

Open collector control and alarm outputs on +GF+ SIGNET instruments (i.e. ProcessPro® or ProPoint™ series) are electrically isolated from the instrument’s power supply. That means these can be used in the above mentioned NPN configuration as well as in PNP configuration, if required. Below are a few sample circuits:

• PLC Wiring “NPN” style

• PLC Wiring “PNP” style

• Alarm circuit or alarm lamp wiring to a single +GF+ SIGNET instrument

• Alarm circuit or alarm lamp wiring to serve multiple +GF+ SIGNET instruments

4. Voltage and Current Limitation

As mentioned before, the supply voltage in the OC output circuit MUST be limited to the specified maximum OC voltage (see operating manual for specific instrument). The use of a quality regulated 5V, 12V or 24V (depending on the application) power supply is recommended. Depending on the stability of the main power line and the location, it may be necessary to use additional overvoltage protection components or lightning arrestors.

Similarly, the current through the Open Collector switch must be limited. Typical OC outputs allow only for 10 to 50mA switch current (please consult manual). Exceeding this current limit can burn out the OC output components immediately. Please see the following section on how to dimension the loads.

5. Load and Pull-Up/Down Resistor Considerations

By utilizing basic arithmetic and Ohm’s law, one can determine the safe limits of load resistance. When the OC switch is closed, almost the entire supply voltage is applied to the load, (i.e. the pull-up or pull-down resistor, the alarm horn input, a potential power relay coil or annunciator lamp). The resulting current through the load and through the OC switch, as well, can be calculated as:

(Current) = (Supply Voltage)/(Load Resistance)

• Example 1:

The supply voltage is 24V and a pull-up-resistor of 10kΩ is used.

Current is 24/10,000 = 2.4mA

(If the OC current rating is 10mA, then in this example, it would be considered safe.)

• Example 2:

The supply voltage is 12V and a horn with a resistance of 100Ω is used

Current is 12/100 = 120mA

(Even if the OC current rating is 50mA, this load will damage the instrument)

6. Transient Protection

There are several “difficult” load cases that must be considered:

• Inductive loads

These can be power relay or other solenoids, motors, alarm horn coils, etc. Such loads generate very high voltage spikes when the OC switch is turned off. If such a load is unavoidable, the use of transient suppression components or RC Filters (or snubbers) wired parallel to the load is required.

This is critical, as a single transient pulse may destroy the output.

• Capacitive loads

This type of load should be rare but can occur if the load contains an internal power supply/regulator that is fed from the OC output circuit. In such a case, it must be assured that the in-rush current does not exceed the OC current rating.

• Incandescent lamps

Such lamps have a very high start-up current until the filament glows and the current settles to the specified value. The use of incandescent lamps on an OC output is not recommended. An LED type annunciator should be used instead.

7. “Active High” and “Active Low” Setting

Depending on the desired function of the circuit attached to the OC output, it may be necessary to have the OC output switch turned “on” or “off” when the criteria for the activation of this output are met.

By default, +GF+ SIGNET instruments are set to operate in “active low” mode. This means when the user-defined condition for the activation is met (e.g. exceeding of an alarm limit) the OC switch is turned “on”. If wired as standard “NPN-style” output (see previous page) the logic level of the attached control system or PLC input consequently becomes “low” logic level.

If a high input logic level is required for activation, it can be accomplished in two

ways:

• to wire the OC output “PNP” style as described in the previous page

• to change the OC output function to “active high” in the menu system of the instrument. Most +GF+ SIGNET instruments allow for this option.

8. Fail-Safe Behavior

No matter what the setting, all OC outputs of +GF+ SIGNET instruments turn off when the instrument loses power. This must be taken into account when evaluating system failure consequences. If the system layout requires a “closed” or “on” condition for the output in case of power loss, a mechanical dry contact relay (NC contacts) must be used instead of the OC output.

Hydraulics Basics Tutorial

2008-10-10T09:52:00.000-07:00

Hydraulic Systems are an essential part of various industrial system processes. It is required for transfer of various work loads from one area to another thru different types of cylinders and valves actuation. Though can do work for heavy loads many times more than their pneumatic system counterpart, it is not suitable for 'clean' environment with products such as food and semiconductors production due to the use of liquid as their means of transferring power. To further have an understanding of the hydraulic fluid power technology, below is link to a powerpoint presentation to aid students and practitioners have a thorough knowledge and understanding of the subject. Click the link below to start downloading.

Hydraulics Tutorial PPT Presentation

The IACET Department of MSU-IIT

2008-10-07T20:37:00.000-07:00

Some of our mentors from the Industrial Automation and Controls Engineering Technolology and Bachelor of Science in Industrial Automation and Mechatronics. They are experts in the field of fluid technology, motor controls, plant automation, instrumentation, process controls, PLC programming, mechatronic technology, electronics circuits and others related in the field. Two other professors are not shown here due to some reasons.

Awards and Achievements

The various projects made by the students of the said department.

One of rooms equipped with OEM trainers and simulators.

With such great training acquired by the students from this world class department, The industrial automation and controls engineering technology department boasts of a 99% employment rate locally and abroad within 1 year time since graduation. Non-employed students are due to reasons as continuing their studies to related fields or because of personal matters.

Elpac Power Supply Repair

2008-10-06T07:34:00.000-07:00

I was given a job order to repair a switch mode power supply made by ELPAC Power Systems of Santa Ana, California, USA just a while ago. This P.S. is used to provide 5VDC at 30 Amps capacity to power digital control cards. 15VDC and 28VDC are also provided by this power supply unit at about 4 Amps capacity.

Usually, the cause of trouble for this kind of switch-mode power supplies is just calibration and adjustment of the various potentiometers or commonly called 'pots' by electronic technicians for the overload protection, over-voltage protection and under-voltage protection. But i was tweaking the said supply unit for almost an hour, and it just keeps on shutting down enabling its protection.

There was no choice but to open up the unit and check its internals. I traced the circuit from the primary power side to the secondary power side to its feedback and pulse width modulation circuits. All are okay (capacitors, SCRs, resistors, transistors, diodes, etc). The only thing that I haven't checked yet is its transformer. I tried tapping the said transformer with the tip of a screwdriver while the unit is power upped, and just like magic, it released it desired outputs. I tried tapping the transformer again and the unit died down.

There is only one thing to do. REBAKE the transformer, not rewind because it is just intermittent and could be the coils inside the said transformer might be loose due to time and vibration. I sent it to the Electrical Repair Shop Department of the company where I am currently connected with because they are the experts when it comes to inductors. I requested the said job order and put the ELPAC Power Supply into its pending status.

We'll just wait for the transformer and make any final observations when it is done. For the meantime, i'll just do another job load.

What are the Primary Elements used for Flow Measurement?

2008-10-03T09:10:00.001-07:00

The primary elements used for flow measurement are:

* Orifice Plate
* Venturi Tube
* Pitot Tube
* Annubars
* Flow Nozzle
* Weir and Flumes

What are the Different Types of Orifice Plates?

The different types of orifice plates are:
* Concentric
* Segmental
* Eccentric
* Quadrant Edge

Concentric:
The Concentric Orifice Plate is used for ideal liquid as well as gases and steam service. This orifice plate beta ratio fall between of 0.15 to 0.75 for liquids and 0.20 to 0.70 for gases and steam. Best results occur between value of 0.4 and 0.6. Beta ratio means ratio of the orifice bore to the internal pipe diameters. (45 degrees beveled edges are often used to minimize friction resistance to flowing fluid.)

Eccentric:
The Eccentric Orifice Plate has a hole eccentric. Use full of measuring liquids containing solids, oil containing water and wet steam. Eccentric plates can be used either flange or vena contracta taps, but the taps must be at 180 degrees or 90 degrees to the eccentric opening.

Eccentric orifices have the bore offset from the center to minimize problems in services of solids-containing materials.

Segmental:
The Segmental Orifice Plates has the hole in the form of a segment of circle. This is used for colloidal or slurry flow measurement. For best accuracy, the tap location should 180 degrees from the center of the tangency.

Segment orifices provide another version of plates useful for solids-containing materials.

Quadrant Edge:

It is commonly used in Europe and are particularly useful for pipe sizes less than two (2) inches.

Quadrant edges orifices produce a relatively constant coefficient of discharge for services with low Reynolds numbers in the range of 100,000 down to 5,000.

Previous Lesson

Next Lesson

Instrumentation Basics Part 1

2008-10-02T08:26:00.000-07:00

What is Instrumentation Engineering?
It is the branch of engineering which deals with the measurement, monitoring, display, etc. of the several of energy exchanges which takes place during process operations. "In short Instrumentation is the study of Instrument."

Instrument:
Instrument is a device which is used to measure, monitor, display, etc. of a process variable.

So what are the process variables?
The process variables are:

Flow
Pressure
Temperature
Level
Quality i.e. %O2, CO2, PH, etc.

Define all the process variables and state their unit of measurement?
Flow: Any fluid or liquid moving from one place to another is called flow and it is defined as volume per unit of time at specified temperature and pressure conditions, generally measured by positive-displacement or rate meters.
Units: kg/hr, liter/minute, gallon/minute, m3/hr, Nm3/hr. (Gases)

Pressure: It is defined as force per unit area. P=F/A
Units: bar, Pascal, kg/cm2, lb/in2

Level: The height of the water column, liquid, and powder, etc. at the desired measurement of height between minimum level points to maximum level point is called level. The measurement principle is head pressure method.
Units: meters, mm, cm, percentage.

Temperature: It is the degree of hotness or coldness of a body is called temperature.
Units: Degree Centigrade, Degree Fahrenheit, Degree Kelvin, Degree Rankin

Quality: It deals with analysis. (pH, %CO2, %O2, Conductivity, Viscosity

Next Lesson

Basic Elements of a Pneumatic System

2008-09-30T22:13:00.000-07:00

A – Compressor: a pump which compresses air, raising it to a higher pressure, and delivers it to the pneumatic system (sometimes, can also be used to generate a vacuum).

B – Check valve: one-way valve that allows pressurized air to enter the pneumatic system, but prevents backflow (and loss of pressure) into the compressor when it is stopped.

C – Accumulator: stores compressed air, preventing surges in pressure and relieving the duty cycle of the compressor.

D – Directional valve: controls the flow of pressurized air from the source to the selected port. Some valves permit free exhaust from the port not selected. These valves can be actuated either manually or electrically (the valves typically provided in the FIRST kits use dual solenoids to change the direction of the valve, based on input signals from the control system).

E – Actuator: converts energy stored in the compressed air into mechanical motion. A linear piston is shown. Alternate tools include rotary actuators, air tools, expanding bladders, etc.

What is Mechatronics?

2008-09-27T02:48:00.000-07:00

Back in the early 1970s, Japanese industry coined the word Mechatronics in relation to the development of the world's first industrial robots. Robotics has since come to be regarded as a generic term, and yet it forms only a subset of Mechatronics, which still lacks recognition in some quarters as an indigenous term in our technological vocabulary. 'Mechatronics ' is a natural choice for explaining a process that seeks, from the outset, to generate definitive engineering system solutions, which are inextricably bound by those integrating technologies associated with the inveterate mechanical, electronic and computer based disciplines.

First and foremost Mechatronics should be seen to represent technology integration and not merely a combination of the primary disciplines. In fact, the 'fusion' of mechanical, electronic and computer based structures into a complete Mechatronics 'product' can only achieve its desired functionality through a process of systematic integration of all inherent disciplines involved right through from the conceptual stages. Invoking only mechanical electronic or computer-based entities would not in itself provide the complete system solution. In reality, Mechatronics opens up enormous technological possibilities, as already evidenced by the appearance of sophisticated products like ever-smaller camcorders and compact disc players. These would never have been plausible by adopting a traditional single disciplinary or combinational approach. By definition, then, Mechatronics is not a subject, science or technology per se - it is instead to be regarded as a philosophy - a fundamental way of looking at and doing things, and by its very nature requires a unified approach to its delivery.

The traditional western approach has relied on single discipline identities and evolutionary solutions based on bolt-on technology. On the other hand, Mechatronics solutions require the use of integrated teams of personnel working towards a common goal. Thus the Mechatronics engineer identifies with systems thinking, and a philosophy that lies behind it all. A Mechatronics 'product' derived through systematic, rather than piecemeal processing. It, therefore, seeks to optimize an 'engineered' solution rather than compromise it. Mechatronics philosophy adequately describes the process by which it is achieved. This insight quite naturally lends itself to the concept of 'total quality', something that western industrialized nations have only in the last decade or so come to aspire to. But for Mechatronics, quality is already implied by the way in which system based solutions are to be sought, and the methodologies used for achieving it. It is hoped that industry and commerce will similarly come to respect and aspire to Mechatronics for what it stands for - total synergy.

What do Mechatronics Engineers do?

Mechatronics combines mechanical, electrical and software engineering in the design, development and control of diverse systems used in a range of industries including manufacturing, medicine and the service industries. Examples of mechatronic systems include aircraft, dishwashers, motor vehicles, automated manufacturing plants, medical and surgical devices and systems, robots of all types, many toys, artificial organs and many others. Mechatronics engineers are therefore involved in almost every possible industry at levels from applications development to manufacturing to advanced research.

Where do Mechatronics Engineers work?

Graduates with a Mechatronics degree can take up careers in a wide spectrum of industries including robotics, aerospace, chemical, defence and automotive and manufacturing where complex software plays a major role, as well as in businesses that require extensive computer support, such as banking and commerce. Contributions can be made to these industries in a variety of roles including design engineer, software engineer, project planner, product designer and project manager.

Some Common Pneumatic Circuits

2008-09-08T05:25:00.000-07:00

Controlling Cylinder Speed

Five Ported Four Way

**Description:**
4-Way	4 Flow Directions
2 - Position	Actuated/at rest
Normally Passing Classification	Not Applicable
Push Button, Spring Return	Operator Type
5 Ports	1, 2, 3, 4, and 5

Each cylinder port has its own exhaust

Standard labels apply --- ISO standard shown above, may also be:

Sandwich Speed Control

NOTE:

Dotted line indicates an assembly --- a modular combination of devices; in this case a valve and a speed control sandwich.
Metering devices are needle valves and serve to restrict only the exhaust --- supply is full flow.
Modular design allows control from the valve (often the cylinder is inaccessible or in a protected cell).
Only by restricting both exhaust flow paths can we extend and retract at the same speed.

Controlling Cylinder Force

NOTE:

Dotted line indicates an assembly---a modular combination of devices; in this case a valve and a double regulator sandwich.
Observe that the addition of the sandwich regulator has altered the valve flow paths. IMPORTANT!
Modular design allows control from the valve's location.
A direct acting multi-purpose valve can have supply and even different pressures at ports other than 1 or P---achieving different functions.

Vacuum Cap

Objective: Vacuum cup picks up part/part blow off

NOTE:

Valve in "12" condition has vacuum at the cup and regulated air fills a small volume chamber.
Valve in "14" condition has vacuum blocked and a puff of air breaks the vacuum lock and blows off the part. Force is adjustable. Volume chamber sized for volume required by the circuit.
Valve selected to maintain position in the event of power failure --- double solenoid detent also only requires momentary pulse of electricity.
Single valve solution/saves energy/fail safe/adjustable.

Cylinder Deceleration

Objective: Cylinder Decel Circuit or Varying Clamp Force

NOTE:

Cylinder extends and retracts with signal to main valve.
Pressure to main valve controlled by selector valve.
Cylinder can extend, then, with higher pressure selected, clamp with greater force.
Varying selected pressures can decelerate/accelerate the cylinder.

Three Position Valve

OBSERVE:

Cylinder will not stop until the back pressure rises high enough to balance the forces on the piston --- analysis follow.

NOTE:

Air is trapped in the cylinder. Any leakage (fittings, piston seal, rod seal, valve) will allow the cylinder to move or drift.
When air is exhausted by a lockout or dump valve, air will be trapped in cylinder.
If load is vertical --- any cylinder lines' inadvertent exhaust will cause the load to drop unexpectedly.
During start up --- the all ports blocked center valve does not allow the air to pressurize the cylinder. First stroke could be at high speed due to potential lack of air at either end of cylinder.
Disconnecting any air lines for maintenance may cause unexpected rapid movement of the cylinder --- even if OSHA lock out has been correctly actuated.

These actions may occur when least expected

Three Position Valve 2

NOTE:

Check valves stop and hold cylinder in mid position.
Regulators balance pressures.
In case of electrical failure, valve defaults to mid position, and the check valves stop and maintain cylinder position.
In case of pneumatic supply failure, valve defaults to mid position, and the check valves stop and maintain cylinder position.
During start up, first cycle does not cause rapid cylinder motion.
Be aware, however, air will be trapped even if a lock out valve is opened upstream of this valve.

Strain Gauge Configuration Types

2008-09-04T04:52:00.000-07:00

Wheatstone Bridges and Strain Gauges

All strain-gauge configurations are based on the concept of a Wheatstone bridge. A Wheatstone bridge is a network of four resistive legs. One or more of these legs can be active sensing elements. Figure 1-1 shows a Wheatstone bridge circuit diagram.

Figure 1-1. Basic Wheatstone Bridge Circuit Diagram

The Wheatstone bridge is the electrical equivalent of two parallel voltage divider circuits. R₁ and R₂ compose one voltage divider circuit, and R₄ and R₃ compose the second voltage divider circuit. The output of a Wheatstone bridge is measured between the middle nodes of the two voltage dividers.

A physical phenomena, such as a change in strain applied to a specimen or a temperature shift, changes the resistance of the sensing elements in the Wheatstone bridge. The Wheatstone bridge configuration is used to help measure the small variations in resistance that the sensing elements produce corresponding to a physical change in the specimen.

Strain-gauge configurations are arranged as Wheatstone bridges. The gauge is the collection of all of the active elements of the Wheatstone bridge. There are three types of strain-gauge configurations: quarter-bridge, half-bridge, and full-bridge. The number of active element legs in the Wheatstone bridge determines the kind of bridge configuration. Refer to Table 1-1 to see how many active elements are in each configuration.

Each of these three configurations is subdivided into multiple configuration types. The orientation of the active elements and the kind of strain measured determines the configuration type. National Instruments (NI) supports seven various configuration types in software. However, with custom software scaling all Wheatstone bridge configuration types can be used with any NI hardware product that supports your gauge configuration type.

The supported strain gauge configuration types measure axial strain, bending strain, or both. While some similar configuration types can be used to measure torsional strain, NI’s software scaling does not support these configuration types. It is possible to use NI products to measure torsional strain, but to properly scale these configuration types you must create a custom scale in Measurement & Automation Explorer (MAX) or perform scaling in your software application.

This document discusses all of the mechanical, electrical, and scaling considerations of each strain-gauge configuration type supported by NI.

Acronyms, Formulas, and Variable Definitions

In the figures and equations in this document, the acronyms, formulas, and variables are defined as:

e is the measured strain (+e is tensile strain and -e is compressive strain).

e_S is the simulated strain.

GF is the Gauge Factor, which should be specified by the gauge manufacturer.

R_g is the nominal gauge resistance, which should be specified by the gauge manufacturer.

R_L is the lead resistance. If lead lengths are long, R_L can significantly impact measurement accuracy.

R_s is the shunt calibration resistor value.

U is the ratio of expected signal voltage to excitation voltage with the shunt calibration circuit engaged. Parameter U appears in the equations for simulated strain and is defined by the following equation:

n is the Poisson’s ratio, defined as the negative ratio of transverse strain to axial strain (longitudinal) strain.

V_CH is the measured signal’s voltage.

V_EX is the excitation voltage.

V_r is the voltage ratio that is used in the voltage to strain conversion equations and is defined by the following equation:

Software Scaling and Equations

Once you have acquired the voltage signal V_CH , you can scale this voltage to the appropriate strain units in software. This is done automatically for you in traditional NI-DAQ using the strain virtual channel and in NI-DAQmx using a strain task or strain channel. You can also scale the voltages manually in your application using the voltage to strain conversion equations provided in this document for each configuration type.

Finally, there are voltage to strain conversion functions included in LabVIEW, traditional NI-DAQ, and NI-DAQmx. In LabVIEW, the conversion function, Convert Strain Gauge Reading.vi, is in the Data Acquisition»Signal Conditioning subpalette. The prototypes for the NI-DAQ functions, Strain_Convert and Strain_Buf_Convert, are in the header file convert.h for C/C++, and convert.bas for Visual Basic. In LabVIEW 7.x or earlier, refer to the Traditional NI-DAQ User Manual and the LabVIEW Measurements Manual for more information. In LabVIEW 8.0 or later, refer to the Traditional NI-DAQ User Manual and the Strain Gauges topic in the LabVIEW Help.

The names given the strain-gauge types in the following sections directly correspond to bridge selections in MAX and the LabVIEW Convert Strain Gauge Reading VI.

Quarter-Bridge Type I

This section provides information for the quarter-bridge strain-gauge configuration type I. The quarter-bridge type I measures either axial or bending strain.

Figure 1-2. Quarter-Bridge Type I Measuring Axial and Bending Strain

A quarter-bridge type I has the following characteristics:

• A single active strain-gauge element is mounted in the principle direction of axial or bending strain.

• A passive quarter-bridge completion resistor (dummy resistor) is required in addition to half-bridge completion.

• Temperature variation in specimen decreases the accuracy of the measurements.

• Sensitivity at 1000 me is ~ 0.5 mV_out/ V_EX input.

Figure 1-3. Quarter-Bridge Type I Circuit Diagram

The following symbols apply to the circuit diagram and equations:

• R₁ and R₂ are half-bridge completion resistors.

• R₃ is the quarter-bridge completion resistor (dummy resistor).

• R₄ is the active strain-gauge element measuring tensile strain (+e).

To convert voltage readings to strain units use the following equation:

To simulate the effect on strain of applying a shunt resistor across R₃, use the following equation:

Quarter-Bridge Type II

This section provides information for the quarter-bridge strain-gauge configuration type II. The quarter-bridge type II measures either axial or bending strain.

Figure 1-4. Quarter-Bridge Type II Measuring Axial and Bending Strain

A quarter-bridge type II has the following characteristics:

• One active strain-gauge element and one passive, temperature-sensing quarter-bridge element (dummy gauge). The active element is mounted in the direction of axial or bending strain. The dummy gauge is mounted in close thermal contact with the strain specimen but not bonded to the specimen, and is usually mounted transverse (perpendicular) to the principle axis of strain.

• This configuration is often confused with the half-bridge type I configuration, with the difference being that in the half-bridge type I configuration the R₃ element is active and bonded to the strain specimen to measure the effect of Poisson’s ratio.

• Completion resistors provide half bridge completion.

• Compensates for temperature.

• Sensitivity at 1000 me is ~ 0.5 mV_out/ V_EX input.

Figure 1-5. Quarter-Bridge Type II Circuit Diagram

The following symbols apply to the circuit diagram and equations:

• R₁ and R₂ are a half-bridge completion resistors.

• R₃ is the quarter-bridge temperature-sensing element (dummy gauge).

• R₄ is the active strain-gauge element measuring tensile strain (+e).

To convert voltage readings to strain units use the following equation:

To simulate the effect on strain of applying a shunt resistor across R₃, use the following equation:

Half-Bridge I

This section provides information for the half-bridge strain-gauge configuration type I. The half-bridge type I measures either axial or bending strain.

Figure 1-6. Half-bridge Type I Measuring Axial and Bending Strain

A half-bridge type I has the following characteristics:

• Two active strain-gauge elements. One is mounted in the direction of axial strain, the other acts as a Poisson gauge and is mounted transverse (perpendicular) to the principal axis of strain.

• Completion resistors provide half bridge completion.

• Sensitive to both axial and bending strain.

• Compensates for temperature

• Compensates for the aggregate effect on the principle strain measurement due to the Poisson’s ratio of the specimen material.

• Sensitivity at 1000 me is ~ 0.65 mV_out/ V_EX input.

Figure 1-7. Half-Bridge Type I Circuit Diagram

The following symbols apply to the circuit diagram and equations:

• R₁ and R₂ are half-bridge completion resistors.

• R₃ is the active strain-gauge element measuring compression from Poisson effect (–ne).

• R₄ is the active strain-gauge element measuring tensile strain (+e).

To convert voltage readings to strain units use the following equation:

To simulate the effect on strain of applying a shunt resistor across R₃, use the following equation:

Half-Bridge II

This section provides information for the half-bridge strain-gauge configuration type II. The half-bridge type II only measures bending strain.

Figure 1-8. Half-Bridge Type II Rejecting Axial and Measuring Bending Strain

A half-bridge type II configuration has the following characteristics:

• Two active strain-gauge elements. One is mounted in the direction of bending strain on one side of the strain specimen (top), the other is mounted in the direction of bending strain on the opposite side (bottom).

• Completion resistors provide half bridge completion.

• Sensitive to bending strain.

• Rejects axial strain.

• Compensates for temperature.

• Sensitivity at 1000 me is ~ 1 mV_out/ V_EX input.

Figure 1-9. Half-Bridge Type II Circuit Diagram

The following symbols apply to the circuit diagram and equations:

• R₁ and R₂ are half-bridge completion resistors.

• R₃ is the active strain-gauge element measuring compressive strain (–e).

• R₄ is the active strain-gauge element measuring tensile strain (+e).

To convert voltage readings to strain units use the following equation:

To simulate the effect on strain of applying a shunt resistor across R₃, use the following equation:

Full-Bridge I

This section provides information for the full-bridge strain-gauge configuration type I. The full-bridge type I only measures bending strain.

Figure 1-10. Full-Bridge Type I Rejecting Axial and Measuring Bending Strain

A full-bridge type I configuration has the following characteristics:

• Four active strain-gauge elements. Two are mounted in the direction of bending strain on one side of the strain specimen (top), the other two are mounted in the direction of bending strain on the opposite side (bottom).

• Highly sensitive to bending strain.

• Rejects axial strain.

• Compensates for temperature.

• Compensates for lead resistance.

• Sensitivity at 1000 me is ~ 2.0 mV_out / V_EX input.

Figure 1-11. Full-Bridge Type I Circuit Diagram

The following symbols apply to the circuit diagram and equations:

• R₁ is an active strain-gauge element measuring compressive strain (–e).

• R₂ is an active strain-gauge element measuring tensile strain (+e).

• R₃ is an active strain-gauge element measuring compressive strain (–e).

• R₄ is an active strain-gauge element measuring tensile strain (+e).

To convert voltage readings to strain units use the following equation:

To simulate the effect on strain of applying a shunt resistor across R₃, use the following equation:

Full-Bridge II

This section provides information for the full-bridge type II strain-gauge configuration. The full-bridge type II only measures bending strain.

Figure 1-12. Full-Bridge Type II Rejecting Axial and Measuring Bending Strain

A full-bridge type II configuration has the following characteristics:

• Four active strain-gauge elements. Two are mounted in the direction of bending strain with one on one side of the strain specimen (top), the other on the opposite side (bottom). The other two act together as a Poisson gauge and are mounted transverse (perpendicular) to the principal axis of strain with one on one side of the strain specimen (top), the other on the opposite side (bottom).

• Rejects axial strain.

• Compensates for temperature.

• Compensates for the aggregate effect on the principle strain measurement due to the Poisson’s ratio of the specimen material.

• Compensates for lead resistance.

• Sensitivity at 1000 me is ~ 1.3 mV_out / V_EX input.

Figure 1-13. Full-Bridge Type II Circuit Diagram

The following symbols apply to the circuit diagram and equations:

• R₁ is an active strain-gauge element measuring compressive Poisson effect (–ne).

• R₂ is an active strain-gauge element measuring tensile Poisson effect (+ne).

• R₃ is an active strain-gauge element measuring compressive strain (–e).

• R₄ is an active strain-gauge element measuring tensile strain (+e).

To convert voltage readings to strain units use the following equation:

To simulate the effect on strain of applying a shunt resistor across R₃, use the following equation:

Full-Bridge III

This section provides information for the full-bridge strain-gauge configuration type III. The full-bridge type III only measures axial strain.

Figure 1-14. Full-Bridge Type III Measuring Axial and Rejecting Bending Strain

A full-bridge type III configuration has the following characteristics:

• Four active strain-gauge elements. Two are mounted in the direction of axial strain with one on one side of the strain specimen (top), the other on the opposite side (bottom). The other two act together as a Poisson gauge and are mounted transverse (perpendicular) to the principal axis of strain with one on one side of the strain specimen (top), the other on the opposite side (bottom).

• Compensates for temperature.

• Rejects bending strain.

• Compensates for the aggregate effect on the principle strain measurement due to the Poisson’s ratio of the specimen material.

• Compensates for lead resistance.

• Sensitivity at 1000 me is ~ 1.3 mV_out / V_EX input.

Figure 1-15. Full-Bridge Type III Circuit Diagram

The following symbols apply to the circuit diagram and equations:

• R₁ is an active strain-gauge element measuring compressive Poisson effect (–ne).

• R₂ is an active strain-gauge element measuring tensile strain (+e).

• R₃ is an active strain-gauge element measuring compressive Poisson effect (–ne).

• R₄ is an active strain-gauge element measuring the tensile strain (+e).

To convert voltage readings to strain units use the following equation:

To simulate the effect on strain of applying a shunt resistor across R₃, use the following equation:

Courtesy of National Instruments

Strain and Strain Gauges

2008-09-04T04:49:00.001-07:00

What Is Strain?

Strain is the amount of deformation of a body due to an applied force. More specifically, strain (e) is defined as the fractional change in length, as shown in Figure 1 below.

Figure 1. Definition of Strain

Strain can be positive (tensile) or negative (compressive). Although dimensionless, strain is sometimes expressed in units such as in./in. or mm/mm. In practice, the magnitude of measured strain is very small. Therefore, strain is often expressed as microstrain (me), which is e x 10^-6.

When a bar is strained with a uniaxial force, as in Figure 1, a phenomenon known as Poisson Strain causes the girth of the bar, D, to contract in the transverse, or perpendicular, direction. The magnitude of this transverse contraction is a material property indicated by its Poisson's Ratio. The Poisson's Ratio n of a material is defined as the negative ratio of the strain in the transverse direction (perpendicular to the force) to the strain in the axial direction (parallel to the force), or n = e_T/e. Poisson's Ratio for steel, for example, ranges from 0.25 to 0.3.

The Strain Gauge

While there are several methods of measuring strain, the most common is with a strain gauge, a device whose electrical resistance varies in proportion to the amount of strain in the device. The most widely used gauge is the bonded metallic strain gauge.

The metallic strain gauge consists of a very fine wire or, more commonly, metallic foil arranged in a grid pattern. The grid pattern maximizes the amount of metallic wire or foil subject to strain in the parallel direction (Figure 2). The cross sectional area of the grid is minimized to reduce the effect of shear strain and Poisson Strain. The grid is bonded to a thin backing, called the carrier, which is attached directly to the test specimen. Therefore, the strain experienced by the test specimen is transferred directly to the strain gauge, which responds with a linear change in electrical resistance. Strain gauges are available commercially with nominal resistance values from 30 to 3000 Ω, with 120, 350, and 1000 Ω being the most common values.

Figure 2. Bonded Metallic Strain Gauge

It is very important that the strain gauge be properly mounted onto the test specimen so that the strain is accurately transferred from the test specimen, through the adhesive and strain gauge backing, to the foil itself.

A fundamental parameter of the strain gauge is its sensitivity to strain, expressed quantitatively as the gauge factor (GF). Gauge factor is defined as the ratio of fractional change in electrical resistance to the fractional change in length (strain):

The Gauge Factor for metallic strain gauges is typically around 2.

Strain Gauge Measurement

In practice, the strain measurements rarely involve quantities larger than a few millistrain(e x 10^-3). Therefore, to measure the strain requires accurate measurement of very small changes in resistance. For example, suppose a test specimen undergoes a strain of 500 me. A strain gauge with a gauge factor of 2 will exhibit a change in electrical resistance of only 2 (500 x 10^-6) = 0.1%. For a 120 W gauge, this is a change of only 0.12 W.

To measure such small changes in resistance, strain gauges are almost always used in a bridge configuration with a voltage excitation source. The general Wheatstone bridge, illustrated below, consists of four resistive arms with an excitation voltage, V_EX, that is applied across the bridge.

Figure 3. Wheatstone Bridge

The output voltage of the bridge, V_O, will be equal to:

From this equation, it is apparent that when R₁/R₂ = R₄/R₃, the voltage output V_O will be zero. Under these conditions, the bridge is said to be balanced. Any change in resistance in any arm of the bridge will result in a nonzero output voltage.

Therefore, if we replace R₄ in Figure 3 with an active strain gauge, any changes in the strain gauge resistance will unbalance the bridge and produce a nonzero output voltage. If the nominal resistance of the strain gauge is designated as R_G, then the strain-induced change in resistance, DR, can be expressed as DR = R_G·GF·e. Assuming that R₁ = R₂ and R₃ = R_G, the bridge equation above can be rewritten to express V_O/V_EX as a function of strain (see Figure 4). Note the presence of the 1/(1+GF·e/2) term that indicates the nonlinearity of the quarter-bridge output with respect to strain.

Figure 4. Quarter-Bridge Circuit

Ideally, we would like the resistance of the strain gauge to change only in response to applied strain. However, strain gauge material, as well as the specimen material to which the gauge is applied, will also respond to changes in temperature. Strain gauge manufacturers attempt to minimize sensitivity to temperature by processing the gauge material to compensate for the thermal expansion of the specimen material for which the gauge is intended. While compensated gauges reduce the thermal sensitivity, they do not totally remove it.

By using two strain gauges in the bridge, the effect of temperature can be further minimized. For example, Figure 5 illustrates a strain gauge configuration where one gauge is active (R_G + DR), and a second gauge is placed transverse to the applied strain. Therefore, the strain has little effect on the second gauge, called the dummy gauge. However, any changes in temperature will affect both gauges in the same way. Because the temperature changes are identical in the two gauges, the ratio of their resistance does not change, the voltage V_O does not change, and the effects of the temperature change are minimized.

Figure 5. Use of Dummy Gauge to Eliminate Temperature Effects

The sensitivity of the bridge to strain can be doubled by making both gauges active in a half-bridge configuration. For example, Figure 6 illustrates a bending beam application with one bridge mounted in tension (R_G + DR) and the other mounted in compression (R_G - DR). This half-bridge configuration, whose circuit diagram is also illustrated in Figure 6, yields an output voltage that is linear and approximately doubles the output of the quarter-bridge circuit.

Figure 6. Half-Bridge Circuit

Finally, you can further increase the sensitivity of the circuit by making all four of the arms of the bridge active strain gauges in a full-bridge configuration. The full-bridge circuit is shown in Figure 7.

Figure 7. Full-Bridge Circuit

The equations given here for the Wheatstone bridge circuits assume an initially balanced bridge that generates zero output when no strain is applied. In practice however, resistance tolerances and strain induced by gauge application will generate some initial offset voltage. This initial offset voltage is typically handled in two ways. First, you can use a special offset-nulling, or balancing, circuit to adjust the resistance in the bridge to rebalance the bridge to zero output. Alternatively, you can measure the initial unstrained output of the circuit and compensate in software.

The equations given above for quarter, half, and full-bridge strain gauge configurations assume that the lead wire resistance is negligible. While ignoring the lead resistances may be beneficial to understanding the basics of strain gauge measurements, doing so in practice can be a major source of error. For example, consider the 2-wire connection of a strain gauge shown in Figure 8a. Suppose each lead wire connected to the strain gauge is 15 m long with lead resistance R_L equal to 1 W. Therefore, the lead resistance adds 2 W of resistance to that arm of the bridge. Besides adding an offset error, the lead resistance also desensitizes the output of the bridge.

You can compensate for this error by measuring the lead resistance R_L and accounting for it in the strain calculations. However, a more difficult problem arises from changes in the lead resistance due to temperature fluctuations. Given typical temperature coefficients for copper wire, a slight change in temperature can generate a measurement error of several me.

Using a 3-wire connection can eliminate the effects of variable lead wire resistance because the lead resistances affect adjacent legs of the bridge. As seen in Figure 8b, changes in lead wire resistance, R_L2, do not change the ratio of the bridge legs R₃ and R_G. Therefore, any changes in resistance due to temperature cancel each other.

Figure 8. 2-Wire and 3-Wire Connections of Quarter-Bridge Circuit

Signal Conditioning for Strain Gauges

Strain gauge measurement involves sensing extremely small changes in resistance. Therefore, proper selection and use of the bridge, signal conditioning, wiring, and data acquisition components are required for reliable measurements. To ensure accurate strain measurements, it is important to consider the following:

Bridge completion
Excitation
Remote sensing
Amplification
Filtering
Offset
Shunt calibration

Bridge Completion – Unless you are using a full-bridge strain gauge sensor with four active gauges, you will need to complete the bridge with reference resistors. Therefore, strain gauge signal conditioners typically provide half-bridge completion networks consisting of high-precision reference resistors. Figure 9 shows the wiring of a half-bridge strain gauge circuit to a conditioner with completion resistors R₁ and R₂.

Figure 9. Connection of Half-Bridge Strain Gauge Circuit

Excitation – Strain gauge signal conditioners typically provide a constant voltage source to power the bridge. While there is no standard voltage level that is recognized industry wide, excitation voltage levels of around 3 and 10 V are common. While a higher excitation voltage generates a proportionately higher output voltage, the higher voltage can also cause larger errors because of self-heating.

Remote Sensing – If the strain gauge circuit is located a distance away from the signal conditioner and excitation source, a possible source of error is voltage drop caused by resistance in the wires connecting the excitation voltage to the bridge. Therefore, some signal conditioners include a feature called remote sensing to compensate for this error. Remote sense wires are connected to the point where the excitation voltage wires connect to the bridge circuit. The extra sense wires serve to regulate the excitation supply through negative feedback amplifiers to compensate for lead losses and deliver the needed voltage at the bridge.

Amplification – The output of strain gauges and bridges is relatively small. In practice, most strain gauge bridges and strain-based transducers will output less than 10 mV/V (10 mV of output per volt of excitation voltage). With 10 V excitation, the output signal will be 100 mV. Therefore, strain gauge signal conditioners usually include amplifiers to boost the signal level to increase measurement resolution and improve signal-to-noise ratios.

Filtering – Strain gauges are often located in electrically noisy environments. It is therefore essential to be able to eliminate noise that can couple to strain gauges. Lowpass filters, when used in conjunction with strain gauges, can remove high-frequency noise prevalent in most environmental settings.

Offset Nulling – When a bridge is installed, it is very unlikely that the bridge will output exactly zero volts when no strain is applied. Slight variations in resistance among the bridge arms and lead resistance will generate some nonzero initial offset voltage. Offset nulling can be performed by either hardware or software:

1. Software Compensation – With this method, you take an initial measurement before strain input is applied, and use this offset to compensate subsequent measurements. This method is simple, fast, and requires no manual adjustments. The disadvantage of the software compensation method is that the offset of the bridge is not removed. If the offset is large enough, it limits the amplifier gain you can apply to the output voltage, thus limiting the dynamic range of the measurement.

2. Offset-Nulling Circuit – The second balancing method uses an adjustable resistance, a potentiometer, to physically adjust the output of the bridge to zero. By varying the resistance of potentiometer, you can control the level of the bridge output and set the initial output to zero volts.

Shunt Calibration – The normal procedure to verify the output of a strain gauge measurement system relative to some predetermined mechanical input or strain is called shunt calibration. Shunt calibration involves simulating the input of strain by changing the resistance of an arm in the bridge by some known amount. This is accomplished by shunting, or connecting, a large resistor of known value across one arm of the bridge, creating a known DR. The output of the bridge can then be measured and compared to the expected voltage value. The results are used to correct span errors in the entire measurement path, or to simply verify general operation to gain confidence in the setup.

Strain and Strain Gauges

2008-09-04T04:49:00.000-07:00

What Is Strain?

Strain is the amount of deformation of a body due to an applied force. More specifically, strain (e) is defined as the fractional change in length, as shown in Figure 1 below.

Figure 1. Definition of Strain

The Strain Gauge

Figure 2. Bonded Metallic Strain Gauge

The Gauge Factor for metallic strain gauges is typically around 2.

Strain Gauge Measurement

Figure 3. Wheatstone Bridge

The output voltage of the bridge, V_O, will be equal to:

Figure 4. Quarter-Bridge Circuit

Figure 5. Use of Dummy Gauge to Eliminate Temperature Effects

Figure 6. Half-Bridge Circuit

Figure 7. Full-Bridge Circuit

Figure 8. 2-Wire and 3-Wire Connections of Quarter-Bridge Circuit

Signal Conditioning for Strain Gauges

Bridge completion
Excitation
Remote sensing
Amplification
Filtering
Offset
Shunt calibration

Figure 9. Connection of Half-Bridge Strain Gauge Circuit

Flowmeter Types and their Principles

2008-08-23T07:01:00.000-07:00

INTRODUCTION

Measuring the flow of liquids is a critical need in many industrial plants. In some operations, the ability to conduct accurate flow measurements is so important that it can make the difference between making a profit or taking a loss. In other cases, inaccurate flow measurements or failure to take measurements can cause serious (or even disastrous) results.

With most liquid flow measurement instruments, the flow rate is determined inferentially by measuring the liquid's velocity or the change in kinetic energy. Velocity depends on the pressure differential that is forcing the liquid through a pipe or conduit. Because the pipe's cross-sectional area is known and remains constant, the average velocity is an indication of the flow rate. The basic relationship for determining the liquid's flow rate in such cases is:

Q = V x A

where

Q = liquid flow through the pipe

V = average velocity of the flow

A = cross-sectional area of the pipe

Other factors that affect liquid flow rate include the liquid's viscosity and density, and the friction of the liquid in contact with the pipe.

Direct measurements of liquid flows can be made with positive-displacement flowmeters. These units divide the liquid into specific increments and move it on. The total flow is an accumulation of the measured increments, which can be counted by mechanical or electronic techniques.

Reynolds Numbers

The performance of flowmeters is also influenced by a dimensionless unit called the Reynolds Number. It is defined as the ratio of the liquid's inertial forces to its drag forces.

Figure 1: Laminar and turbulent flow are two types normally encountered in liquid flow Measurement operations. Most applications involve turbulent flow, with R values above 3000. Viscous liquids usually exhibit laminar flow, with R values below 2000. The transition zone between the two levels may be either laminar or turbulent.

The equation is:

R = 3160 x Q x Gt
D x h

where:

R = Reynolds number

Q = liquid's flow rate, gpm

Gt = liquid's specific gravity

D = inside pipe diameter, in.

h = liquid's viscosity, cp

The flow rate and the specific gravity are inertia forces, and the pipe diameter and viscosity are drag forces. The pipe diameter and the specific gravity remain constant for most liquid applications. At very low velocities or high viscosities, R is low, and the liquid flows in smooth layers with the highest velocity at the center of the pipe and low velocities at the pipe wall where the viscous forces restrain it. This type of flow is called laminar flow. R values are below approximately 2000. A characteristic of laminar flow is the parabolic shape of its velocity profile, Fig. 1.

However, most applications involve turbulent flow, with R values above 3000. Turbulent flow occurs at high velocities or low viscosities. The flow breaks up into turbulent eddies that flow through the pipe with the same average velocity. Fluid velocity is less significant, and the velocity profile is much more uniform in shape. A transition zone exists between turbulent and laminar flows. Depending on the piping configuration and other installation conditions, the flow may be either turbulent or laminar in this zone.

FLOWMETER TYPES

Differential Pressure	Positive Displacement	Velocity	Mass	Open-Channel
Orifice Plate Venturi Tube Flow Tube Flow Nozzle Pitot Tube Elbow Tap Target Variable-Area(Rotameter)	Reciprocating Piston Oval Gear Nutating Disk Rotary Vane	Turbine Vortex Shedding Swirl Conada Effect & Momentum Exchange Electromagnetic Ultrasonic, Doppler Ultrasonic, Transit-Time	Coriolis Thermal	Weir Flume

Numerous types of flowmeters are available for closed-piping systems. In general, the equipment can be classified as differential pressure, positive displacement, velocity, and mass meters. Differential pressure devices (also known as head meters) include orifices, venturi tubes, flow tubes, flow nozzles, pitot tubes, elbow-tap meters, target meters, and variable-area meters, Fig. 2.

Positive displacement meters include piston, oval-gear, nutating-disk, and rotary-vane types. Velocity meters consist of turbine, vortex shedding, electromagnetic, and sonic designs. Mass meters include Coriolis and thermal types. The measurement of liquid flows in open channels generally involves weirs and flumes.

Space limitations prevent a detailed discussion of all the liquid flowmeters available today. However, summary characteristics of common devices are shown in Table 1. Brief descriptions follow.

Differential Pressure Meters

The use of differential pressure as an inferred measurement of a liquid's rate of flow is well known. Differential pressure flowmeters are, by far, the most common units in use today. Estimates are that over 50 percent of all liquid flow measurement applications use this type of unit.

The basic operating principle of differential pressure flowmeters is based on the premise that the pressure drop across the meter is proportional to the square of the flow rate. The flow rate is obtained by measuring the pressure differential and extracting the square root.

Differential pressure flowmeters, like most flowmeters, have a primary and secondary element. The primary element causes a change in kinetic energy, which creates the differential pressure in the pipe. The unit must be properly matched to the pipe size, flow conditions, and the liquid's properties. And, the measurement accuracy of the element must be good over a reasonable range. The secondary element measures the differential pressure and provides the signal or read-out that is converted to the actual flow value.

Orifices are the most popular liquid flowmeters in use today. An orifice is simply a flat piece of metal with a specific-sized hole bored in it. Most orifices are of the concentric type, but eccentric, conical (quadrant), and segmental designs are also available.

In practice, the orifice plate is installed in the pipe between two flanges. Acting as the primary device, the orifice constricts the flow of liquid to produce a differential pressure across the plate. Pressure taps on either side of the plate are used to detect the difference. Major advantages of orifices are that they have no moving parts and their cost does not increase significantly with pipe size.

Conical and quadrant orifices are relatively new. The units were developed primarily to measure liquids with low Reynolds numbers. Essentially constant flow coefficients can be maintained at R values below 5000. Conical orifice plates have an upstream bevel, the depth and angle of which must be calculated and machined for each application.

The segmental wedge is a variation of the segmental orifice. It is a restriction orifice primarily designed to measure the flow of liquids containing solids. The unit has the ability to measure flows at low Reynolds numbers and still maintain the desired square-root relationship. Its design is simple, and there is only one critical dimension the wedge gap. Pressure drop through the unit is only about half that of conventional orifices.

Integral wedge assemblies combine the wedge element and pressure taps into a one-piece pipe coupling bolted to a conventional pressure transmitter. No special piping or fittings are needed to install the device in a pipeline.

Metering accuracy of all orifice flowmeters depends on the installation conditions, the orifice area ratio, and the physical properties of the liquid being measured.

Venturi tubes have the advantage of being able to handle large flow volumes at low pressure drops. A venturi tube is essentially a section of pipe with a tapered entrance and a straight throat. As liquid passes through the throat, its velocity increases, causing a pressure differential between the inlet and outlet regions.

The flowmeters have no moving parts. They can be installed in large diameter pipes using flanged, welded or threaded-end fittings. Four or more pressure taps are usually installed with the unit to average the measured pressure. Venturi tubes can be used with most liquids, including those having a high solids content.

Flow tubes are somewhat similar to venturi tubes except that they do not have the entrance cone. They have a tapered throat, but the exit is elongated and smooth. The distance between the front face and the tip is approximately one-half the pipe diameter. Pressure taps are located about one-half pipe diameter downstream and one pipe diameter upstream.

Flow Nozzles, at high velocities, can handle approximately 60 percent greater liquid flow than orifice plates having the same pressure drop. Liquids with suspended solids can also be metered. However, use of the units is not recommended for highly viscous liquids or those containing large amounts of sticky solids.

Pitot tubes sense two pressures simultaneously, impact and static. The impact unit consists of a tube with one end bent at right angles toward the flow direction. The static tube's end is closed, but a small slot is located in the side of the unit. The tubes can be mounted separately in a pipe or combined in a single casing.

Pitot tubes are generally installed by welding a coupling on a pipe and inserting the probe through the coupling. Use of most pitot tubes is limited to single point measurements. The units are susceptible to plugging by foreign material in the liquid. Advantages of pitot tubes are low cost, absence of moving parts, easy installation, and minimum pressure drop.

Elbow tap meters operate on the principle that when liquid travels in a circular path, centrifugal force is exerted along the outer edges. Thus, when liquid flows through a pipe elbow, the force on the elbow's interior surface is proportional to the density of the liquid times the square of its velocity. In addition, the force is inversely proportional to the elbow's radius.

Any 90 deg. pipe elbow can serve as a liquid flowmeter. All that is required is the placement of two small holes in the elbow's midpoint (45 deg. point) for piezometer taps. Pressure-sensing lines can be attached to the taps by using any convenient method. The difference in pressure on the outside and inside walls, caused by centrifugal force, can be measured with a differential pressure transducer. Figure 2 shows a typical installation.

Pressure measurements are obtained by placing taps at 45- degree angles on opposite sides of the elbow. The size of each of the two taps should not exceed one-eighth of the pipe diameter. Flow is calculated according to the following formula:

W = 244 [SQ.ROOT SIGN] rhD³p

where W = flow in pounds per hour

r = elbow radius (inches)

D = elbow diameter (inches)

h = differential pressure (inches H₂0)

p = density in lbs/ft³

Target meters sense and measure forces caused by liquid impacting on a target or drag-disk suspended in the liquid stream. A direct indication of the liquid flow rate is achieved by measuring the force exerted on the target. In its simplest form, the meter consists only of a hinged, swinging plate that moves outward, along with the liquid stream. In such cases, the device serves as a flow indicator.

A more sophisticated version uses a precision, low-level force transducer sensing element. The force of the target caused by the liquid flow is sensed by a strain gage. The output signal from the gage is indicative of the flow rate. Target meters are useful for measuring flows of dirty or corrosive liquids.

Variable-area meters, often called rotameters, consist essentially of a tapered tube and a float, Fig. 3. Although classified as differential pressure units, they are, in reality, constant differential pressure devices. Flanged-end fittings provide an easy means for installing them in pipes. When there is no liquid flow, the float rests freely at the bottom of the tube. As liquid enters the bottom of the tube, the float begins to rise. The float is selected so as to have a density higher than that of the fluid and the position of the float varies directly with the flow rate. Its exact position is at the point where the differential pressure between the upper and lower surfaces balance the weight of the float.

Because the flow rate can be read directly on a scale mounted next to the tube, no secondary flow-reading devices are necessary. However, if desired, automatic sensing devices can be used to sense the float's level and transmit a flow signal. Rotameter tubes are manufactured from glass, metal, or plastic. Tube diameters vary from 1/4 to greater than 6 in.

Positive-Displacement Meters

Operation of these units consists of separating liquids into accurately measured increments and moving them on. Each segment is counted by a connecting register. Because every increment represents a discrete volume, positive-displacement units are popular for automatic batching and accounting applications. Positive-displacement meters are good candidates for measuring the flows of viscous liquids or for use where a simple mechanical meter system is needed.

Reciprocating piston meters are of the single and multiple-piston types. The specific choice depends on the range of flow rates required in the particular application. Piston meters can be used to handle a wide variety of liquids. A magnetically driven, oscillating piston meter is shown in Fig. 4. Liquid never comes in contact with gears or other parts that might clog or corrode.

Oval-gear meters have two rotating, oval-shaped gears with synchronized, close fitting teeth. A fixed quantity of liquid passes through the meter for each revolution. Shaft rotation can be monitored to obtain specific flow rates.

Nutating-disk meters have a moveable disk mounted on a concentric sphere located in a spherical side-walled chamber. The pressure of the liquid passing through the measuring chamber causes the disk to rock in a circulating path without rotating about its own axis. It is the only moving part in the measuring chamber.

A pin extending perpendicularly from the disk is connected to a mechanical counter that monitors the disk's rocking motions. Each cycle is proportional to a specific quantity of flow. As is true with all positive-displacement meters, viscosity variations below a given threshold will affect measuring accuracies. Many sizes and capacities are available. The units can be made from a wide selection of construction materials.

Rotary-vane meters are available in several designs, but they all operate on the same principle. The basic unit consists of an equally divided, rotating impeller (containing two or more compartments) mounted inside the meter's housing. The impeller is in continuous contact with the casing. A fixed volume of liquid is swept to the meter's outlet from each compartment as the impeller rotates. The revolutions of the impeller are counted and registered in volumetric units.

Helix flowmeters consist of two radically pitched helical rotors geared together, with a small clearance between the rotors and the casing. The two rotors displace liquid axially from one end of the chamber to the other.

Velocity Meters

These instruments operate linearly with respect to the volume flow rate. Because there is no square-root relationship (as with differential pressure devices), their rangeability is greater. Velocity meters have minimum sensitivity to viscosity changes when used at Reynolds numbers above 10,000. Most velocity-type meter housings are equipped with flanges or fittings to permit them to be connected directly into pipelines.

Turbine meters have found widespread use for accurate liquid measurement applications. The unit consists of a multiple-bladed rotor mounted with a pipe, perpendicular to the liquid flow. The rotor spins as the liquid passes through the blades. The rotational speed is a direct function of flow rate and can be sensed by magnetic pick-up, photoelectric cell, or gears. Electrical pulses can be counted and totalized, Fig. 5.

The number of electrical pulses counted for a given period of time is directly proportional to flow volume. A tachometer can be added to measure the turbine's rotational speed and to determine the liquid flow rate. Turbine meters, when properly specified and installed, have good accuracy, particularly with low-viscosity liquids.

A major concern with turbine meters is bearing wear. A "bearingless" design has been developed to avoid this problem. Liquid entering the meter travels through the spiraling vanes of a stator that imparts rotation to the liquid stream. The stream acts on a sphere, causing it to orbit in the space between the first stator and a similarly spiraled second stator. The orbiting movement of the sphere is detected electronically. The frequency of the resulting pulse output is proportional to flow rate.

Vortex meters make use of a natural phenomenon that occurs when a liquid flows around a bluff object. Eddies or vortices are shed alternately downstream of the object. The frequency of the vortex shedding is directly proportional to the velocity of the liquid flowing through the meter, Fig. 6.

The three major components of the flowmeter are a bluff body strut-mounted across the flowmeter bore, a sensor to detect the presence of the vortex and to generate an electrical impulse, and a signal amplification and conditioning transmitter whose output is proportional to the flow rate, Fig. 7. The meter is equally suitable for flow rate or flow totalization measurements. Use for slurries or high viscosity liquids is not recommended.

Swirl Meter The principle of operation of a swirl flowmeter is as follows. The meter body has a set of blades welded at the inlet, called the swirler. These blades impart a tangential velocity (or swirl) to liquids, gases or vapors. The fluid is then accelerated by a reduction in the meter body bore. A piezoelectric sensor is located in the center of the meter at the point of maximum fluid velocity. Flow is then decelerated as it approaches the meter outlet by an increase in meter body bore. A deswirler is welded to the meter body near the outlet. This deswirler eliminates the tangential velocity imparted to the fluid at the inlet so that other instrumentation downstream of the meter will not be affected by its operation.

A swirl flowmeter consists of the flowmeter body and an electronics housing (the electronics can be remote mounted for safety or convenience). Swirlmeters are only available with flanged meter bodies. A wafer option is not available, as there is for vortex meters. Swirlmeters use the same sensors and electronics as vortex meters, only the meter bodies differ in design. Swirlmeters are most cost effective with stainless steel construction, although Hastelloy is also offered. Swirlmeters are available in sizes from ½ to 16 inches, and have options for ANSI 150, 300 or 600 mating flanges.

The primary rotation caused by the swirler has at its core a low-pressure zone. The low-pressure zone is thrown into a secondary rotation proportional to flow rate. This rotating low-pressure zone can be likened to a helical coil. At low flow rates, the low-pressure swirls are farther apart (the helical coil is stretched out). At higher flows, the low-pressure swirls are closer together (the coil is compressed). Areas of slightly higher pressure separate the low-pressure swirls. The sensor will deflect (to the left and then to the right) as a pressure swirl passes from one side to other. The alternating deflection of the sensor produces a sine wave voltage output, similar to the output shown in the vortex section. The frequency of this output voltage is the same frequency as the rotating low-pressure zones, and is therefore proportional to the volumetric flow rate.

The swirl flowmeter factor does not exhibit the same deviation at high Reynolds numbers as does the vortex meter factor. This has been verified by testing on water and air at independent facilities. For this reason, a swirlmeter factor determined by water calibration is universally valid for all fluids. The swirlmeter, based on the water calibration, has a published accuracy of 0.5 percent of rate for liquids, gases or steam.

As with the vortex meter, swirlmeter rangeability is fixed by the size of the meter and the fluid properties. The sensor requires a minimum strength pressure pulse to be able to distinguish the flow signal from hydraulic noise. While turbulent flow is required at all times, the swirlmeter does not have the same limitation on Reynolds number, as does the vortex meter and can generally measure lower flows. On the other hand, the swirlmeter body presents more of an obstruction to flow than does the vortex meter, and creates higher permanent head losses under similar conditions. For this reason, the swirlmeter does not measure flow rates as high as the vortex meter. The swirlmeter, like the vortex meter, averages 10:1 turndowns or higher on liquids, and 20:1 or higher on gases and vapors. However, the flow range of a 2-inch swirlmeter will be different than the flow range of a 2-inch vortex meter under the same operating conditions. Just as for a vortex meter, you select the swirlmeter size to achieve a desired flow range given the process conditions, and never to match the process piping. Swirlmeters have to be downsized (using a 2-inch meter for 3-inch process piping) less frequently than vortex meters. Free computer software is available from manufacturers that make sizing swirl flowmeters quick and easy.

Swirl Flowmeter Application Information

Swirlmeters cost about 50 percent more than the same size vortex meter because of the added complexity and welding requirements. The rule of thumb is to use the vortex meter whenever possible for cost, and use the swirlmeter for:

Tight piping situations.
More viscous liquids (8 cp < μ <>
Lower flow measurement capability.
Higher accuracy on gases and steam.
When downsizing to install a vortex meter is not feasible (head loss).
These meters are compatible with low viscosity (<>

Swirlmeters are less affected by as many real world parameters as orifice plates and turbine meters, and are less sensitive to piping effects than vortex meters. Swirlmeters require just 3 diameters of straight pipe upstream (regardless of bends, valves, etc.), and either 1 or 3 diameters of straight pipe downstream (the latter only required when a control valve is downstream of the meter).

Like vortex meters, they measure velocity and infer actual volumetric flow rate from the known geometry of the meter body. Swirlmeters are generally used with flow computers to measure flow in standard volumetric or mass units (along with external pressure and/or temperature measurements). The flow computer is not necessary if the fluid density is constant. A pressure tap is provided on the meter body for making the pressure measurement (using an external sensor). The temperature element should be located downstream of the meter if needed.

The swirlmeter does not measure to true zero flow. There is a flow cut-off point below which the meter output is automatically clamped at zero (4 mA for analog output). For most applications, this limitation does not pose a problem, as the swirlmeter has good low flow capability. However, this can be a draw back for applications where flows during start-up or shutdown operations, or other upset conditions, can be greatly different than under normal operating conditions. Users may need an indication of flow under such upsets, even if they do not need to measure flow accurately, making use of the swirlmeter questionable.

Swirlmeters may also be questionable for some batching applications, especially if the pipe does not remain full between batches. The meter will not register flow as the fluid accelerates from zero to the cut-off value, and as the fluid decelerates back to zero at the end of the batch. Swirlmeters are unidirectional and will not measure or subtract any backflow from the batch total. This may create significant measurement errors, depending on the system dynamics, and the size of the batch.

There may be a potential problem installing swirlmeters on existing processes where the flow range to be measured is completely unknown. Many times, the instrument engineer makes an educated guess on flow range. A swirlmeter sized for the wrong flow range, or wrong process conditions, may need to be replaced by a different size meter entirely. Other devices, like magnetic flowmeters, orifice plates and turbine meters, are more forgiving, and can be easily adapted to fit the actual process conditions after installation.

Measuring gas flows when the process pressure is low (low-density gases) is less of a problem for swirlmeters than for vortex meters. Low-density gases can be measured with a swirlmeter, however rangeability may be less than the 20:1 mentioned previously, and extreme care must be taken in selecting the correct size meter.

Measurement of multi-phase flow has lower accuracy than for single-phase fluids. The meter will measure the flow of all phases present and report it as all liquid or gas (depending on how the meter is configured). The secondary phase should be removed, if feasible, before the meter for the highest accuracy. Any secondary phase should be homogeneously dispersed and should not have any potential for sticking to or coating the meter. There is no evidence to suggest that a swirlmeter has any advantage over a vortex meter in regards to measuring multi-phase flow.

Pressure drop must also be considered when selecting a swirlmeter. Flashing and cavitation have an adverse affect on meter accuracy, and can damage the meter itself. It was previously stated that a swirlmeter produces higher head loss than a vortex meter under the same conditions, up to five times higher. However, this is not a fair comparison. In real practice, the head loss for the swirlmeter is about the same as for the vortex meter, because you generally use a larger size swirlmeter than vortex meter to handle the same application.

Fluids that tend to form coatings are bad applications for swirlmeters.

Installation Recommendations

One of the main advantages of the swirlmeter is its insensitivity to piping effects. It is an excellent meter for tight piping situations.

Swirlmeters can be installed vertically, horizontally or at any angle. Allow liquids to flow against gravity to keep the pipe full. When the liquid is moving with gravity, elevate the downstream piping above the meter installation level to maintain a full pipe. Install the meter to avoid standing liquid when the pipe is empty. Also plan for the installation so as to avoid formation of gas bubbles in liquid flow. Check valves may be used when installing a vortex meter to keep it full of liquid when there is no active flow in the process.

Mating flanges on the process piping must be of the same nominal size as on the flowmeter. Flanges with a smooth bore, similar to weld neck flanges, are preferred. Do not use reducing flanges. Most performance specifications are based upon using Schedule 40 or Schedule 80 mating pipe. The mating pipe should be of good quality, and have an internal surface free from mill scale, pits, holes, reaming scores, bumps, etc., for a distance of 4 diameters upstream and 2 diameters downstream of the meter. The bores of the adjacent piping, meter and gaskets should be aligned to prevent steps.

Control valves should be placed no closer than 3 pipe diameters upstream or 3 pipe diameters downstream of the meter.

The sensor used in the swirlmeter can be replaced in the field, but does require process shutdown. The meter should be installed with blocking valves, or in a bypass line, if process shutdown for maintenance poses a problem.

Excessive pipe vibration or process noise can affect measurement accuracy. Mechanical pipe vibration can be eliminated by placing proper piping supports on either side of the meter, or by rotating the meter in the process piping so that the sensor is located in a plane different than the vibration. Process noise (from chattering valves, steam traps, pumps, etc.) is hydraulically connected to the meter by the fluid. The swirlmeter uses the same electronics with digital signal processing, as the vortex meter, to eliminate the adverse effects of vibration and noise, without sacrificing rangeability.

Conada Effect Flowmeter & Momentum Exchange Flowmeter. While vortex shedding flowmeters are the most recognized types of oscillating flow measurement devices, less well known are meters based on the Coanda Effect, and the phenomenon known as momentum exchange.

The Coanda Effect is named after aerodynamicist Henri-Marie Coanda, who discovered that a free jet emerging from a nozzle or conduit will follow a nearby surface and attach to it. Fluid flowing through the meter body bends toward and attaches to a sidewall. A portion of the flow is diverted through a feedback passage, however, and pushes the stream toward a sidewall on the opposite side of the meter body, which also has a feedback passage through which a portion of the flow is diverted. The fluid from this feedback passage pushes the stream back toward the sidewall to which it was initially attached, and the self-initiating, self-sustaining process is repeated. A sensor, located in one of the two feedback passages, detects the presence and absence of flow. The frequency of the pulse signals is linear with volumetric flow rate.

The momentum exchange flowmeter is similar to the Coanda model, but relies on a different mechanism to create oscillations. Unlike the Coanda meter, the momentum exchange meter does not have sidewalls. The shape of the meter body creates a main flow that passes through the nozzle and towards one side of the meter body or the other. This creates a flow pulse in a feedback passage, exerting a force on the main jet and deflecting it so it exerts a force on the fluid in the opposite passage. The pattern repeats continuously, creating a self-sustaining oscillation. Like the Coanda meter, it has a sensor in one of the feedback passages that detects the pulsing of fluids.

While most oscillating flowmeters -- including the Coanda fluidic flowmeter -- require turbulent flows to function, the momentum exchange meter does not, enabling its use with high-viscosity fluids.

Like other types of oscillating flowmeters, benefits include minimum maintenance, high stability and relatively inexpensive purchase cost. However, they are limited to use on pipes four inches in diameter or less. Use of larger pipes would create too few pulses per gallon for accurate measurement.

Electromagnetic meters can handle most liquids and slurries, providing that the material being metered is electrically conductive. Major components are the flow tube (primary element), Fig. 8. The flow tube mounts directly in the pipe. Pressure drop across the meter is the same as it is through an equivalent length of pipe because there are no moving parts or obstructions to the flow. The voltmeter can be attached directly to the flow tube or can be mounted remotely and connected to it by a shielded cable.

Electromagnetic flowmeters operate on Faraday's law of electromagnetic induction that states that a voltage will be induced when a conductor moves through a magnetic field. The liquid serves as the conductor; the magnetic field is created by energized coils outside the flow tube, Fig. 9. The amount of voltage produced is directly proportional to the flow rate. Two electrodes mounted in the pipe wall detect the voltage, which is measured by the secondary element.

Electromagnetic flowmeters have major advantages: They can measure difficult and corrosive liquids and slurries; and they can measure forward as well as reverse flow with equal accuracy. Disadvantages of earlier designs were high power consumption, and the need to obtain a full pipe and no flow to initially set the meter to zero. Recent improvements have eliminated these problems. Pulse-type excitation techniques have reduced power consumption, because excitation occurs only half the time in the unit. Zero settings are no longer required.

Ultrasonic flowmeters can be divided into Doppler meters and time-of-travel (or transit) meters.

Doppler meters measure the frequency shifts caused by liquid flow. Two transducers(one to transmit and the other to receive signal) are mounted in a case attached to one side of the pipe. A signal of known frequency is sent into the liquid to be measured. Solids, bubbles, or any discontinuity in the liquid, cause the pulse to be reflected to the receiver element, Fig. 10. Because the liquid causing the reflection is moving, the frequency of the returned pulse is shifted. The frequency shift is proportional to the liquid's velocity.

A portable Doppler meter capable of being operated on AC power or from a rechargeable power pack has recently been developed. The sensing heads are simply clamped to the outside of the pipe, and the instrument is ready to be used. Total weight, including the case, is 22 lb. A set of 4 to 20 millampere output terminals permits the unit to be connected to a strip chart recorder or other remote device.

Because solids particles or entrained gases are required for measurement, Doppler meters are not appropriate for clean liquids. In general, Doppler flowmeters are less accurate than TOF flowmeters, however, they are less expensive.

Time-of-travel(Transit-Time) meters have transducers mounted on each side of the pipe. The configuration is such that the sound waves traveling between the devices are at a 45 deg. angle to the direction of liquid flow. The speed of the signal traveling between the transducers increases or decreases with the direction of transmission and the velocity of the liquid being measured. A time-differential relationship proportional to the flow can be obtained by transmitting the signal alternately in both directions.

A limitation of time-of-travel meters is that the liquids being measured must be relatively free of entrained gas or solids to minimize signal scattering and absorption.

Mass Flowmeters

The continuing need for more accurate flow measurements in mass-related processes (chemical reactions, heat transfer, etc.) has resulted in the development of mass flowmeters. Various designs are available, but the one most commonly used for liquid flow applications is the Coriolis meter. Its operation is based on the natural phenomenon called the Coriolis force, hence the name.

Coriolis meters are true mass meters that measure the mass rate of flow directly as opposed to volumetric flow. Because mass does not change, the meter is linear without having to be adjusted for variations in liquid properties. It also eliminates the need to compensate for changing temperature and pressure conditions. The meter is especially useful for measuring liquids whose viscosity varies with velocity at given temperatures and pressures.

Coriolis meters are also available in various designs. A popular unit consists of a U-shaped flow tube enclosed in a sensor housing connected to an electronics unit. The sensing unit can be installed directly into any process. The electronics unit can be located up to 500 feet from the sensor.

Inside the sensor housing, the U-shaped flow tube is vibrated at its natural frequency by a magnetic device located at the bend of the tube. The vibration is similar to that of a tuning fork, covering less than 0.1 in. and completing a full cycle about 80 times/sec. As the liquid flows through the tube, it is forced to take on the vertical movement of the tube, Fig. 11. When the tube is moving upward during half of its cycle, the liquid flowing into the meter resists being forced up by pushing down on the tube.

Having been forced upward, the liquid flowing out of the meter resists having its vertical motion decreased by pushing up on the tube. This action causes the tube to twist. When the tube is moving downward during the second half of its vibration cycle, it twists in the opposite direction. The ammount of twist is directly proportional to the mass flow rate of the liquid flowing through the tube. Magnetic sensors located on each side of the flow tube measure the tube velocities, which change as the tube twists. The sensors feed this information to the electronics unit, where it is processed and converted to a voltage proportional to mass flow rate. The meter has a wide range of applications from adhesives and coatings to liquid nitrogen.

This meter has extremely high accuracy but can also cause a high pressure drop.

Thermal-type mass flowmeters have traditionally been used for gas measurements, but designs for liquid flow measurements are available. These mass meters also operate independent of density, pressure, and viscosity. Thermal meters use a heated sensing element isolated from the fluid flow path. The flow stream conducts heat from the sensing element. The conducted heat is directly proportional to the mass flow rate. The sensor never comes into direct contact with the liquid, Fig. 12. Through pre-existing built-in calibrations, the temperature differential is translated to mass flow. The accuracy of the thermal mass flow device is dependent on the reliability of the calibrations of the actual process gas or liquid and variations in the temperature, pressure, flow rate, heat capacity and viscosity of the fluid. The electronics package includes the flow analyzer, temperature compensator, and a signal conditioner that provides a linear output directly proportional to mass flow.

Open Channel Meters

The "open channel" refers to any conduit in which liquid flows with a free surface. Included are tunnels, nonpressurized sewers, partially filled pipes, canals, streams, and rivers. Of the many techniques available for monitoring open-channel flows, depth-related methods are the most common. These techniques presume that the instantaneous flow rate may be determined from a measurement of the water depth, or head. Weirs and flumes are the oldest and most widely used primary devices for measuring open-channel flows.

Weirs operate on the principle that an obstruction in a channel will cause water to back up, creating a high level (head) behind the barrier. The head is a function of flow velocity, and, therefore, the flow rate through the device. Weirs consist of vertical plates with sharp crests. The top of the plate can be straight or notched. Weirs are classified in accordance with the shape of the notch. The basic types are V-notch, rectangular, and trapezoidal.

Flumes are generally used when head loss must be kept to a minimum, or if the flowing liquid contains large amounts of suspended solids. Flumes are to open channels what venturi tubes are to closed pipes. Popular flumes are the Parshall and Palmer-Bowlus designs.

The Parshall flume consists of a converging upstream section, a throat, and a diverging downstream section. Flume walls are vertical and the floor of the throat is inclined downward. Head loss through Parshall flumes is lower than for other types of open-channel flow measuring devices. High flow velocities help make the flume self-cleaning. Flow can be measured accurately under a wide range of conditions.

Palmer-Bowlus flumes have a trapezoidal throat of uniform cross section and a length about equal to the diameter of the pipe in which it is installed. It is comparable to a Parshall flume in accuracy and in ability to pass debris without cleaning. A principal advantage is the comparative ease with which it can be installed in existing circular conduits, because a rectangular approach section is not required.

Discharge through weirs and flumes is a function of level, so level measurement techniques must be used with the equipment to determine flow rates. Staff gages and float-operated units are the simplest devices used for this purpose. Various electronic sensing, totalizing, and recording systems are also available.

A more recent development consists of using ultrasonic pulses to measure liquid levels. Measurements are made by sending sound pulses from a sensor to the surface of the liquid, and timing the echo return. Linearizing circuitry converts the height of the liquid into flow rate. A strip chart recorder logs the flow rate, and a digital totalizer registers the total gallons. Another recently introduced microprocessor-based system uses either ultrasonic or float sensors. A key-pad with an interactive liquid crystal display simplifies programming, control, and calibration tasks.

Courtesy of Seil Enterprises Co.

Load Cell Troubleshooting Tips

2008-08-22T01:57:00.000-07:00

The following four tests can be used to assist in troubleshooting load cells. A precision power supply and multimeter are necessary.

Test 1: Mechanical Inspection

Each load cell should be inspected for physical damage. A thorough inspection of the entire length of any cables should be done. Nicked or abraded cables can short out a load cell. Inspect for distortion or cracks on all metal surfaces. Flexure surfaces, if any, should be parallel to each other and be perpendicular to both end surfaces. If the load cell has been physically deformed (bent, stretched, or compressed from original configuration) it is non-repairable. Consult Measurement Specialists for load cell application information.

Test 2: Zero Balance (electrical output with no load)

Changes in the zero balance are usually caused by residual stress in the sensing area. Residual stresses result from overloading the cell, and /or from millions of cycles.

With a millivoltmeter, measure the load cell's output under a 'no load' condition, (no weight on the load cell). The output of a trimmed cell should typically be within +/- .1% of rated output. Always consult published specifications to obtain the correct values.

EXAMPLE: If the typical load cell output =2 mV/V and the typical power supply output =10V, then the full scale output would be =20 mV. Typical zero balance =+/-.1% of rated output. The zero reading should then be within +/- .02 mV.

If the output is greater than the zero balance tolerance, the cell is damaged but may still be useable. When the zero reading exceeds the zero balance tolerance but is less than 50% of the full scale output, the load cell may be electronically 're-zeroed' if the electronic readout with the cell allows it; however, there is no guarantee that the load cell will remain linear. If a load cell has more than 50% full scale zero balance shift, it may require 're-gauging'.

Test 3: Bridge Resistance

Out of tolerance bridge resistance values are usually caused by the failure of a compensating element. Another cause is a broken or burned bridge wire. These failures commonly result from electrical transients or lightening strikes.

TO TEST FOR BRIDGE RESISTANCE: Measure the resistance across each pair of input and output leads. The input and output resistance is typically 350 +/- 3.5 ohms, however, there are many variations, depending on each load cell's published specifications. Consult the published specifications of the subject load cell for proper values and tolerances. If these readings are 'out of spec', the load cell requires repair.

Test 4: Resistance to Ground

Current flow from the load cell's electronics to the load cell body is usually caused by water contamination within the load cell or its cable.

TO TEST FOR LEAKAGE: Connect all of the input, output, sense and ground leads together and measure the resistance between the load cell body and the leads with a megohmmeter. Make sure good contact is made with the metal load cell sensor. The reading should be at least 5000 Megohms. If the load cell fails this test, retest without the ground wire. If the load cell passes this test, a problem may exist with the load cell cable. If the test fails the load cell requires repair.

- Courtesy of Measurement Specialists