tag:blogger.com,1999:blog-29188266158655194972024-03-05T02:44:53.454-08:00Industrial Automation and MechatronicsWelcome to my notebook on Industrial Automation and Mechatronics. The articles posted here are from various sources and personal notes which would serve as my compilation of the said subject. Fields of Instrumentation, Mechatronics, Industrial Automation, Interfacing, Fluid Technology, Sensorics, Process Controls, Electronics and others related to the subject matter will be posted and tackled in this site. Though these may be my personal notebook, all are welcome to scan through it.Unknownnoreply@blogger.comBlogger56125tag:blogger.com,1999:blog-2918826615865519497.post-43053020476882555352014-12-22T13:33:00.000-08:002014-12-22T13:35:12.637-08:00RTD Temperature vs Resistance Table for PT100<div dir="ltr" style="text-align: left;" trbidi="on">
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi3X_ipJsNxzVXQjg65vlLiLYV6fEjrPwyAa8hLcolvpfl1q6GGtTI1utbKKtB7aL6XsqzJC7SbagT7eCIAj5x0uodAOwyvo_hodYlWBx027IKFGZnOMoWe6ASq19mXipWypYZZV6_k9V-h/s1600/1227215082_BqLp1Dam_RTD-table-ANSI%5B1%5D.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi3X_ipJsNxzVXQjg65vlLiLYV6fEjrPwyAa8hLcolvpfl1q6GGtTI1utbKKtB7aL6XsqzJC7SbagT7eCIAj5x0uodAOwyvo_hodYlWBx027IKFGZnOMoWe6ASq19mXipWypYZZV6_k9V-h/s1600/1227215082_BqLp1Dam_RTD-table-ANSI%5B1%5D.jpg" height="640" width="496" /></a></div>
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Unknownnoreply@blogger.com1tag:blogger.com,1999:blog-2918826615865519497.post-55887374019255152572014-12-22T00:28:00.006-08:002014-12-22T00:28:55.270-08:00Proportional Valve<div dir="ltr" style="text-align: left;" trbidi="on">
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Proportional Valve Cut-Away View</div>
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Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-2918826615865519497.post-23944343493108065312014-12-21T16:30:00.001-08:002014-12-21T16:30:28.135-08:00Electromagnetic Spectrum<div dir="ltr" style="text-align: left;" trbidi="on">
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh_pMG6Zrr6STJ-Zfmqk0kNOOqnvwcdXvTpM0u-X7kSla6Pl7xDeH2VtJFnoV2SI_rgOvVjnIoCjyAXCo1BR__O5zdG-y4xbMW5FOnXgoIVkvc5ZaiX7l1TgNfZFmlGqsHLckX3s1t2ciS1/s1600/Electromagnetic+Spectrum.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh_pMG6Zrr6STJ-Zfmqk0kNOOqnvwcdXvTpM0u-X7kSla6Pl7xDeH2VtJFnoV2SI_rgOvVjnIoCjyAXCo1BR__O5zdG-y4xbMW5FOnXgoIVkvc5ZaiX7l1TgNfZFmlGqsHLckX3s1t2ciS1/s1600/Electromagnetic+Spectrum.jpg" height="160" width="320" /></a></div>
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Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-2918826615865519497.post-67890745888613792232014-12-20T22:19:00.002-08:002014-12-20T22:19:49.198-08:00Formula for Current, KiloVolt-Ampere, Kilowatts and Horsepower<div dir="ltr" style="text-align: left;" trbidi="on">
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Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-2918826615865519497.post-77811430280191781402014-12-20T22:15:00.000-08:002014-12-20T22:16:22.910-08:00What is a Current Transducer?<div dir="ltr" style="text-align: left;" trbidi="on">
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj3ygcf_b9ScDscRGD0ViBy8fuUyBxeDA3xT7r2u3UMj5dG6uVxrcL3KMqFNs934aJVqT8X9KMqTYQF__kHz55XjFFND8eJxuAmH8YXKjs96y4Q0FxSDwGv7MNLPNQDTLoOgiagqcYMGhEi/s1600/CTdiagram%5B1%5D.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img alt="" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj3ygcf_b9ScDscRGD0ViBy8fuUyBxeDA3xT7r2u3UMj5dG6uVxrcL3KMqFNs934aJVqT8X9KMqTYQF__kHz55XjFFND8eJxuAmH8YXKjs96y4Q0FxSDwGv7MNLPNQDTLoOgiagqcYMGhEi/s1600/CTdiagram%5B1%5D.jpg" height="320" title="Current Transducer" width="252" /> </a></div>
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<img alt="http://www.raztec.co.nz/images/ampere-turns.jpg" class="decoded" src="http://www.raztec.co.nz/images/ampere-turns.jpg" title="Current Transfer Function" /> </div>
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Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-2918826615865519497.post-49146662737600785772009-02-12T04:46:00.000-08:002009-02-12T04:48:41.054-08:00Motors and Drives Basics<div align="justify"><span class="fsx03" style="font-family:Verdana;"><b>Selecting the right motor and drive combination can save energy and improve performance.</b></span><br /><br /><span class="fsx01" style="font-family:Verdana;">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.<br /><br />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.<br /><br />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.</span><br /><br /><span class="fsx03" style="font-family:Verdana;"><b>AC Induction Motors</b></span><br /><br /><span class="fsx01" style="font-family:Verdana;">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.<br /><br />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.<br /><br />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.<br /><br />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.<br /><br />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.<br /><br />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.<br /><br />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.<br /><br />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:<br /><br />Number of poles RPM<br /> 2 3,600<br /> 4 1,800<br /> 6 1,200<br /> 8 900<br /> 10 720<br /> 12 600<br /><br />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.<br /><br />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.<br /><br />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.<br /><br />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.</span><br /><br /><br /><span class="fsx03" style="font-family:Verdana;"><b>Torque and Horsepower</b></span><br /><span class="fsx01" style="font-family:Verdana;">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.<br /><br />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.<br /><br /></span></div><div align="justify"><span class="fsx01" style="font-family:Verdana;">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.<br /><br /><br /></span></div><div align="justify"><span class="fsx03" style="font-family:Verdana;"><b>Motor Losses and Loss Reduction Techniques</b></span><br /><br /><span class="fsx01" style="font-family:Verdana;">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.<br /><br />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.<br /><br />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.</span><br /><br /><br /><span class="fsx03" style="font-family:Verdana;"><b>Motor Economics</b></span><br /><span class="fsx01" style="font-family:Verdana;"><b> </b>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.<br /><br /></span></div><div align="justify"><span class="fsx01" style="font-family:Verdana;">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.<br /><br /><br /></span></div><div align="justify"><span class="fsx03" style="font-family:Verdana;"><b>Adjustable Speed Drive Systems</b></span><br /><br /><span class="fsx01" style="font-family:Verdana;">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.<br /><br />Many applications have unique demands and characteristics.<br /><br />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.<br /><br />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.<br /><br />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:<br /></span></div><div align="justify"><span class="fsx01" style="font-family:Verdana;">Variable-Speed Drive<br />Variable-Frequency Drive<br />Adjustable-Frequency Drive<br />Adjustable-Speed Drive<br /><br /></span></div><span class="fsx01" style="font-family:Verdana;">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).</span><br /><br /><br /><span class="fsx03" style="font-family:Verdana;"><b>Basic ASD Components</b></span><br /><span class="fsx01" style="font-family:Verdana;">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.<br /><br />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.</span>Unknownnoreply@blogger.com1tag:blogger.com,1999:blog-2918826615865519497.post-18301092711701012342009-02-06T01:42:00.000-08:002009-02-12T04:26:54.021-08:00ELECTRIC MOTOR CONTROLS Part 1Once the proper motor is selected, understanding the many various control devices available and<br /><div style="text-align: justify;">their uses and limitations becomes an important part related to reliable operation and protection of the motor and the personnel using the motor.<br /><br />Motor Control Topics<br />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.<br /><br />The four categories include:<br /><br />1) Starting the Motor<br /> Disconnecting Means<br /> Across the Line Starting<br /> Reduced Voltage Starting<br /><br />2) Motor Protection<br />Overcurrent Protection<br />Overload Protection<br />Other Protection (voltage, phase, etc)<br />Environment<br /><br />3) Stopping the Motor<br />Coasting<br />Electrical Braking<br />Mechanical Braking<br /><br />4) Motor Operational Control<br />Speed Control<br />Reversing<br />Jogging<br />Sequence Control<br /><br />• An understanding of each of these areas is necessary to effectively apply motor control principles and equipment to effectively operate and protect a motor.<br /><br /><div style="text-align: center; font-weight: bold;">MOTOR STARTING<br /></div>All motors must have a control device to start and stop the motor called a “motor controller”.<br /><br /><span style="font-style: italic;">Motor Controller</span><br />A motor controller is the actual device that energizes and de-energizes the circuit to the motor so<br />that it can start and stop.<br /><br />• 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.<br /><br />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.<br /><br /><span style="font-style: italic;">Motor Starter</span><br />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.<br /><br />• There are several different types of equipment suitable for use as “motor starters” but only two types of starting methods for induction motors:<br />1. Across the Line Starting<br />2. Reduced Voltage Starting<br /> Across the Line Starting of Motors<br /> Across the Line starting connects the motor windings/terminals directly to the circuit voltage “across the line” for a “full voltage start”.<br /><br />• This is the simplest method of starting a<br />motor. (And usually the least expensive).<br /><br />• Motors connected across the line are capable<br />of drawing full in-rush current and<br />developing maximum starting torque to<br />accelerate the load to speed in the shortest<br />possible time.<br /><br />• All NEMA induction motors up to 200<br />horsepower, and many larger ones, can withstand full voltage starts. (The electric<br />distribution system or processing operation may not though, even if the motor will).<br />Across the Line Starters<br /><br />Figure 26. Manual Starter<br />There are two different types of common “across the line” starters including<br />1. Manual Motor Starters<br />2. Magnetic Motor Starters<br /><br />Manual Motor Starters<br />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.<br /><br />• 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.<br /><br />• Manual motor starters are often used for smaller motors - typically fractional horsepower<br />motors but the National Electrical Code allows their use up to 10 Horsepower.<br /><br />• Since the switch contacts remain closed if power is removed from the circuit without<br />operating the switch, the motor restarts when power is reapplied which can be a safety<br />concern.<br /><br />• They do not allow the use of remote control or auxiliary control equipment like a magnetic<br />starter does.<br />Magnetic Motor Starters<br />A magnetic motor starter is a package consisting of a contactor capable of opening and closing a set<br /><br />Figure 27. Magnetic Starter<br />of contacts that energize and de-energize the circuit to the motor along with additional motor<br />overload protection equipment.<br /><br />Magnetic starters are used with larger motors (required above 10 horsepower) or where<br />greater motor control is desired.<br />• The main element of the magnetic motor starter is the contactor, a set of contacts operated by<br />an electromagnetic coil.<br /><br />Energizing the coil causes the contacts (A) to close allowing large currents to be<br />initiated and interrupted by a smaller voltage control signal.<br /><br />The control voltage need not be the same as the motor supply voltage and is often low<br />voltage allowing start/stop controls to be located remotely from the power circuit.<br />• Closing the Start button contact energizes the contactor coil. An auxiliary contact on the<br />contactor is wired to seal in the coil circuit. The contactor de-energizes if the control circuit<br />is interrupted, the Stop button is operated, or if power is lost.<br />• The overload contacts are arranged so an overload trip on any phase will cause the contactor<br />to open and de-energize all phases.<br />Reduced Voltage Starting of Motors<br />Reduced Voltage Starting connects the motor windings/terminals at lower than normal line voltage<br />during the initial starting period to reduce the inrush current when the motor starts.<br /><br />• Reduced voltage starting may be required when:<br />S The current in-rush form the motor starting adversely affects the voltage drop on the<br />electrical system.<br />S needed to reduce the mechanical “starting shock” on drive-lines and equipment when<br />the motor starts.<br />• Reducing the voltage reduces the current in-rush to the motor and also reduces the starting<br />torque available when the motor starts.<br />• 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.<br />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.<br />• Typical reduced voltage starter types include:<br />1. Solid State (Electronic) Starters<br />2. Primary Resistance Starters<br />3. Autotransformer Starters<br />4. Part Winding Starters<br />5. Wye-Delta Starters<br /><br />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.<br /><br /></div>Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-2918826615865519497.post-12299152652975661532009-02-01T22:57:00.000-08:002009-02-01T22:59:14.605-08:00What is Industrial Ethernet?Industrial Ethernet Overview<br /><br /><br />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).<br /><br />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.<br /><br /><br />OSI Reference Model<br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://pacontrol.com/image/OSI.gif"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 174px; height: 364px;" src="http://pacontrol.com/image/OSI.gif" alt="" border="0" /></a><br /><br />Developed by International Standards Organization (ISO) and stands for Open Systems Interconnection (OSI).<br />It is designed to deal with connecting open systems to communicate with other systems.<br />It consists of seven layers: a complex structure is partitioned into a number of independent functional layers.<br />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.<br />Services at a layer can only be accessed from the layer immediately above it.<br />Each layer uses only a well-defined set of services provided by the layer below.<br />Protocols operate between "peer" entities in the different end systems (peer-to-peer protocol rules)<br /><br />Advantages:<br />More manageable -Layer N is smaller and built only on Layer (N-1).<br />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<br /><br />Brief Description of model in Each Layer<br /><br />Physical Layer<br />The physical layer is responsible for passing bits onto and receiving them from the communication channel.<br />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.<br /><br />Data Link Layer<br />Data link layer is responsible for both Point-to-Point Network and Broadcast Network data transmission.<br />It hides characteristics of the physical layer (e.g. transmission hardware from the upper layers.<br />It is also responsible to convert transmitted bits into frames<br />It transmits the frames into an error free transmission line by adding error control and flow control.<br /><br />Network Layer<br />Network layer is responsible for the controls of routers and subnets operation.<br />It also handles the formation and routing of packets from source to destination with congestion control.<br /><br />Transport Layer<br />Transport layer is a kind of software protocol to control packets delivery, crash recovery and transmission reliability between sender and receiver.<br />Multiplexing between transport and network connections is possible.<br /><br />Session Layer<br />Session layer provides dialogue control and token management.<br /><br />Presentation Layer<br />When data is transmitted between different types of computer systems, the presentation layer negotiates and manages the way data is represented and encoded.<br />Essentially a 'null' layer in case where such transformations are unnecessary.<br /><br />Application Layer<br />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.<br />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.<br /><br />From Process Automation and ControlUnknownnoreply@blogger.com1tag:blogger.com,1999:blog-2918826615865519497.post-54436199345199201052009-01-25T22:14:00.000-08:002009-01-25T22:40:35.241-08:00Motor Generator Set (MG set)<a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg9vCxVRNBr4YR7GCHHTk3emhLxlZ3l7dNFKwCzK0mliToXWXHtnIu4f3htrcbSfmtMKNbqegRGH50_3KM-cF2l2Uw0PEi_B415ypA0i35XbzMJBDtUMfhbEI8W_avLYOuSdZJGk8QrZzht/s1600-h/untitled.JPG"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 466px; height: 226px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg9vCxVRNBr4YR7GCHHTk3emhLxlZ3l7dNFKwCzK0mliToXWXHtnIu4f3htrcbSfmtMKNbqegRGH50_3KM-cF2l2Uw0PEi_B415ypA0i35XbzMJBDtUMfhbEI8W_avLYOuSdZJGk8QrZzht/s400/untitled.JPG" alt="" id="BLOGGER_PHOTO_ID_5295488535538635634" border="0" /></a><br />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.<br /><br />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.<br /><br />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.<br /><br /><br /><p>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.</p>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<br /><br />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.Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-2918826615865519497.post-87271140760877084092009-01-09T06:12:00.000-08:002009-01-09T18:58:04.807-08:00Calibrating HART Transmitters<span style="font-weight: bold;">Abstract</span> 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<br /><div style="text-align: justify;">the differences between calibrating a conventional and a HART transmitter, and gives recommendations for calibration practices.<br /><br /><span style="font-weight: bold;">Introduction</span> 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.<br /><br /><span style="font-weight: bold;">Calibration Concepts</span> 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:<br /><br /><ul><li>Testing the instrument to determine its performance,</li><li>Adjusting the instrument to perform within specification<br /></li></ul>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.<br /><br />1. Using a process variable simulator that matches the input type of the instrument, set a known input to the instrument.<br /><br />2. Using an accurate calibrator, read the actual (or reference) value of this input<br /><br />3 Read the instrument's interpretation of the value by using an accurate calibrator to measure<br />the instrument output<br /><br />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.<br /><br />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.<br /><br /><span style="font-weight: bold;">Error Calculations</span><br />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:<br /><br /><div style="text-align: center;">% span =(reading - low range) / (high range - low range) * 100.0<br /></div><br />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.<br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg49tkz00PAE4dCsWKiYyDSAY-T6xfQXP2zNIElRpIHdD8dH3hA5MnWV8RS7PQ5TZw8aqYmBDJhXEsSjk9cPlnFAuF7wqkQL7h-SuVEqX3DUKhhjFI59k_cPvaBAdKqWVnvO9m_lwOHQ5Dc/s1600-h/HART+CAL+1.JPG"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 368px; height: 393px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg49tkz00PAE4dCsWKiYyDSAY-T6xfQXP2zNIElRpIHdD8dH3hA5MnWV8RS7PQ5TZw8aqYmBDJhXEsSjk9cPlnFAuF7wqkQL7h-SuVEqX3DUKhhjFI59k_cPvaBAdKqWVnvO9m_lwOHQ5Dc/s320/HART+CAL+1.JPG" alt="" id="BLOGGER_PHOTO_ID_5289490899543361618" border="0" /></a><br />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.<br /><br />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.<br /><br /><span style="font-style: italic;">Zero error</span> 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.<br /><br /><span style="font-style: italic;">Linearity error</span> 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.<br /><br /><span style="font-style: italic;">Hysteresis error</span> 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.<br /><br />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.<br /><br /><span style="font-weight: bold;">Performance over time</span><br />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-<br />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.<br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgYJ1zYDiXkoZn-_5RskIlaSz3NLHN1a0e8OiZ117RViaxbHomTA7Kka03jadonG5-A6b9MiHQvwRVsqclgjuL8Scu4xUeRL5BWtwqeEIgFTg2W_4tr2jXHknFBYA2Ce8hoMggv4g4jtppj/s1600-h/HART+CAL+4.JPG"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 363px; height: 221px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgYJ1zYDiXkoZn-_5RskIlaSz3NLHN1a0e8OiZ117RViaxbHomTA7Kka03jadonG5-A6b9MiHQvwRVsqclgjuL8Scu4xUeRL5BWtwqeEIgFTg2W_4tr2jXHknFBYA2Ce8hoMggv4g4jtppj/s320/HART+CAL+4.JPG" alt="" id="BLOGGER_PHOTO_ID_5289492064502350626" border="0" /></a><br /><span style="font-weight: bold;">HART Fundamentals<br /></span>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).<br /><br />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,<br />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.<br /><br />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.<br /><br />The Common Practice command set defines functions that are generally applicable to many field<br />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.<br />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<br />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.<br /><br />It is important to note that in most cases, proper calibration of a HART instrument requires the<br />use of a communicator that is capable of issuing device specific commands.<br /><br /><span style="font-weight: bold;">Calibrating a Conventional Instrument</span><br />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.<br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjOr22e5t_CRhcM54mAIrBPnSFp32p5-yohPzGyDOmD08HjtnwZ2oJ8K-zZkU9EHnsPzTSrP_EStw0gj2zuyAILFOtoNCd0z5ThxdODSCtILIWA4-EMMttGm_0JfUMCb2NhwGgWBr3uScXP/s1600-h/HART+CAL+3.JPG"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 320px; height: 168px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjOr22e5t_CRhcM54mAIrBPnSFp32p5-yohPzGyDOmD08HjtnwZ2oJ8K-zZkU9EHnsPzTSrP_EStw0gj2zuyAILFOtoNCd0z5ThxdODSCtILIWA4-EMMttGm_0JfUMCb2NhwGgWBr3uScXP/s320/HART+CAL+3.JPG" alt="" id="BLOGGER_PHOTO_ID_5289493041117691346" border="0" /></a>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.<br /><br /><span style="font-weight: bold;">Calibrating a HART Instrument</span><br /><br /><span style="font-style: italic;">The Parts of a HART Transmitter</span><br />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.<br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg1cbcLDwZdySHayVPSyHuytczmHh3mQ4vxwTjTMyEGzfMubyDRYs8czAwlSowN5CpWIuSNle7RWbwdzhTBIJO3xWj3XBRsCl1-_FcbpiKoAhHoLVdJvOkQk8qzhzWiFqINOkJ6BSfvfaqj/s1600-h/HART+CAL+2.JPG"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 367px; height: 213px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg1cbcLDwZdySHayVPSyHuytczmHh3mQ4vxwTjTMyEGzfMubyDRYs8czAwlSowN5CpWIuSNle7RWbwdzhTBIJO3xWj3XBRsCl1-_FcbpiKoAhHoLVdJvOkQk8qzhzWiFqINOkJ6BSfvfaqj/s320/HART+CAL+2.JPG" alt="" id="BLOGGER_PHOTO_ID_5289493299876909122" border="0" /></a>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.<br /><br />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.<br /><br />The second box is strictly a mathematical conversion from the process variable to the equivalent<br />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<br />a fixed output test mode. This overrides the normal output of the second block and substitutes a specified output value.<br /><br />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.<br /><br /><span style="font-style: italic;">HART Calibration Requirements</span><br />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.<br /><br />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.<br /><br /><span style="font-style: italic;">Calibrating the Input Section</span><br />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.<br /><br /><span style="font-style: italic;">Calibrating the Output Section</span><br />Again, the same basic multiple point test and adjust technique is employed, but with a new<br />definition for input. To run a test, use a communicator to put the transmitter into a fixed current<br />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.<br /><br />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.<br /><br /><span style="font-style: italic;">Testing Overall Performance</span><br />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.<br /><br />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.<br /><br /><span style="font-style: italic;">Effect of Damping on Test Performance</span><br />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.<br /><br /><span style="font-style: italic;">Operations that are NOT Proper Calibrations</span><br />Digital Range Change<br />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.<br /><br />Zero and Span Adjustment<br />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.<br /><br />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<br />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.<br /><br />The proper way to correct a zero shift condition is to use a zero trim. This adjusts the instrument<br />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.<br /><br />Loop Current Adjustment<br />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.<br /><br />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.<br /><br />Conclusion<br />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.<br /><br />References<br />[I] HART Communication Foundation. "HART - Smart Communications Protocol Specification",<br />Revision 5.2, November 3, 1993.<br />[2] Bell System Technical Reference. PUB 4121 2, "Data Sets 202s and 202T interface<br />Specification", July 1976.<br />[3] HART Communication Foundation Pamphlet. "HART Field Communications Protocol".<br />[4] Holladay, Kenneth L., "Using the HART@ Protocol to Manage for Quality", ISA 1994 paper<br />number 94-6 17.<br />[5] Applied System Technologies, Inc., "User's Manual for Cornerstone Base Station", Revision<br />2.0.1 I , April 1995.<br />[6] ANSIIISA - S51 .l-1979, "Process Instrumentation Terminology"<br />[7] Instrument Society of America, "Instrument Calibration Series - Principles of Calibration".<br />1989.<br />[8] Instrument Society of America, "Instrument Calibration Series - Calibrating Pressure and<br />Temperature Instruments", 1989.</div>Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-2918826615865519497.post-66793103952323205992009-01-09T05:50:00.000-08:002009-01-09T06:12:05.145-08:00HART: The TutorialHART® stands for:<br /><span style="font-weight: bold;">HIGHWAY ADDRESSABLE REMOTE TRANSDUCER</span><br /><br /><div style="text-align: justify;">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.<br /><br />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.<br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhbiC25Nk76zwU3I_JbgCMvnKskkuvVdRO_bD12fbNWKmi5U7XJBHv_8lM5ukLawVVhFEt0wfX4IKKkBJZLnEQtUczGOhlHP-iZovohUbE3LmSSQpf7yrJbGF4EmOkzUO0ztruKngRuNK3e/s1600-h/HART1.JPG"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 320px; height: 145px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhbiC25Nk76zwU3I_JbgCMvnKskkuvVdRO_bD12fbNWKmi5U7XJBHv_8lM5ukLawVVhFEt0wfX4IKKkBJZLnEQtUczGOhlHP-iZovohUbE3LmSSQpf7yrJbGF4EmOkzUO0ztruKngRuNK3e/s320/HART1.JPG" alt="" id="BLOGGER_PHOTO_ID_5289296168679800306" border="0" /></a><br />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.<br /><br />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.<br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhgPgcjGay1rjHJG1lqBYXW62TjTJLYGh4Rretn6WRMOkRw1LIjiE9-wsul-29iSMGPFXrw3j8cAkmFXU8fp1Z0YKJjMYUwjzWqhwe-SHXL_tPVgn1cYVed6OO77KYQ_gPJWEgwO9yABdvJ/s1600-h/HART2.JPG"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 320px; height: 193px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhgPgcjGay1rjHJG1lqBYXW62TjTJLYGh4Rretn6WRMOkRw1LIjiE9-wsul-29iSMGPFXrw3j8cAkmFXU8fp1Z0YKJjMYUwjzWqhwe-SHXL_tPVgn1cYVed6OO77KYQ_gPJWEgwO9yABdvJ/s320/HART2.JPG" alt="" id="BLOGGER_PHOTO_ID_5289296167878511746" border="0" /></a><br />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.<br /><br />The digital communication signal has a response time of approx. 2-3 updates per second without interrupting the analogue signal.<br /><br />HART® Commands<br />The HART® protocol provides uniform and consistent communication for all field devices via the HART® command set. This includes three types of Command:<br /><span style="font-weight: bold;"><br />Universal</span><br />All devices using the HART® protocol must recognise and support these commands. They provide access to information useful in normal operations.<br /><span style="font-weight: bold;"><br />Common Practice</span><br />These provide functions implemented by many but not all HART® communication devices.<br /><span style="font-weight: bold;"><br />Device specific</span><br />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.<br /><br />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<br />ensures that the technology is openly available for the benefit of the industry.<br /></div>Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-2918826615865519497.post-16057050480469710612008-12-20T15:46:00.000-08:002008-12-20T15:49:11.224-08:00Infrared technology targets industrial automation<h2 class="author"><span style="font-size:100%;">By Andrew Wilson</span></h2> <p><em>Infrared technology targets industrial automation IR cameras are used in industrial-plant monitoring and more and more in industrial automation.</em></p>By Andrew Wilson, Editor <p>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. </p> <p>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). </p> <p>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. </p> <h3>Absolute measurement</h3> <p>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. </p> <div style="font-style: italic;"><div style="text-align: center;"> <img src="http://images.pennnet.com/articles/vsd/thm/th_137978.jpg" style="border: medium none ;" alt="" /> <br /></div>FIGURE 1. <em>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.</em> </div> <p>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. </p> <p>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.</p> <p>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. </p> <h3>Material properties</h3> <p>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. </p> <p>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. </p> <div style="font-style: italic;"><div style="text-align: center;"> <img src="http://images.pennnet.com/articles/vsd/thm/th_137979.jpg" style="border: medium none ;" alt="" /> <br /></div>FIGURE 2. <em>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.</em> </div> <p>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. </p> <p>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. </p> <p>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. </p> <p>Incorporating a thermal detector in the form of an amorphous silicon or vanadate (YVO<sub>4</sub>) 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. </p> <h3>Discerning features</h3> <p>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. </p> <p>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. </p> <p>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. </p> <p>In such optical systems the pixel pitch at the limit of resolution is given by the diffraction-limited equation </p> <div align="center"><strong>pitch = 0.6 x f/# x wavelength </strong></div> <p>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. </p> <p>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. </p> <p>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.</p> <h3>Future developments</h3> <p>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. </p> <p>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. </p> <div style="font-style: italic;"><div style="text-align: center;"> <img src="http://images.pennnet.com/articles/vsd/thm/th_137980.jpg" style="border: medium none ; width: 325px;" alt="" onclick="OpenLargeWindow(137980,400,174,'vsd');" /> <br /></div>FIGURE 3. <em>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).</em> <div> <a href="http://www.vision-systems.com/articles/enlarge_image.html?siteId=vsd&id=137980" onclick="OpenLargeWindow(137980,400,174,'vsd'); return false" style="font-family: Arial,Helvetica; font-size: smaller; color: rgb(0, 0, 128);">Click here to enlarge image</a> </div> </div> <hr /> <h3>Camera and frame grabber team up to combat SARS</h3> <p>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. </p> <p>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). </p> <p>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. </p> <p>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.</p> <div style="font-style: italic;"> <img src="http://images.pennnet.com/articles/vsd/thm/th_137981.gif" style="border: medium none ; width: 325px;" alt="" onclick="OpenLargeWindow(137981,496,333,'vsd');" /> <div> <a href="http://www.vision-systems.com/articles/enlarge_image.html?siteId=vsd&id=137981" onclick="OpenLargeWindow(137981,496,333,'vsd'); return false" style="font-family: Arial,Helvetica; font-size: smaller; color: rgb(0, 0, 128);">Click here to enlarge image</a> </div> </div> <div style="font-style: italic;"><div style="text-align: left;"> <img src="http://images.pennnet.com/articles/vsd/thm/th_137982.jpg" style="border: medium none ; width: 325px;" alt="" onclick="OpenLargeWindow(137982,400,260,'vsd');" /> </div><div> <a href="http://www.vision-systems.com/articles/enlarge_image.html?siteId=vsd&id=137982" onclick="OpenLargeWindow(137982,400,260,'vsd'); return false" style="font-family: Arial,Helvetica; font-size: smaller; color: rgb(0, 0, 128);">Click here to enlarge image</a> </div> </div> <img src="http://images.pennnet.com/articles/vsd/thm/th_137983.jpg" style="border: medium none ; width: 325px;" alt="" onclick="OpenLargeWindow(137983,400,256,'vsd');" /> <br /><em>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).</em>Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-2918826615865519497.post-14223542080146182412008-12-15T18:43:00.000-08:002008-12-15T18:51:26.547-08:00Photosensors...<div style="text-align: justify;"><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://img.alibaba.com/photo/12033964/Riko_Tq18_Easy_Fixed_Photo_Sensor.jpg"><img style="margin: 0pt 0pt 10px 10px; float: right; cursor: pointer; width: 381px; height: 251px;" src="http://img.alibaba.com/photo/12033964/Riko_Tq18_Easy_Fixed_Photo_Sensor.jpg" alt="" border="0" /></a><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://www.foxcontrols.co.za/images/photosensor.jpg"><img style="margin: 0pt 0pt 10px 10px; float: right; cursor: pointer; width: 172px; height: 152px;" src="http://www.foxcontrols.co.za/images/photosensor.jpg" alt="" border="0" /></a><br /><br /><br /><br /><br /><br /><br /><br /><br />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.<br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://www.skillbank.co.uk/unnwebpage/images/photosensor.jpg"><img style="margin: 0pt 0pt 10px 10px; float: right; cursor: pointer; width: 285px; height: 217px;" src="http://www.skillbank.co.uk/unnwebpage/images/photosensor.jpg" alt="" border="0" /></a><br />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.<br /><br />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.<br /><br />Photosensors are used in a great variety of electronic devices, circuits, and systems, including:<br /><br /> * fiber optic systems<br /> * optical scanners<br /> * wireless LAN<br /> * automatic lighting controls<br /> * machine vision systems<br /> * electric eyes<br /> * optical disk drives<br /> * optical memory chips<br /> * remote control devices<br /></div>Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-2918826615865519497.post-84824170136981762672008-12-13T18:26:00.000-08:002008-12-13T18:31:04.149-08:00A Quick Guide to Thermocouples<table style="width: 100%; border-collapse: collapse; text-align: left; margin-left: 0px; margin-right: 0px;" width="100%" border="0" cellpadding="0" cellspacing="0"><tbody><tr><td style="padding: 0cm 5.4pt;" valign="top"><h2><span style="font-size:100%;">Background<a name="_Background"></a></span></h2> </td> </tr> <tr> <td style="padding: 0cm 5.4pt;" valign="top"><div style="text-align: justify;"> </div><p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p> </td> </tr> <tr> <td style="padding: 0cm 5.4pt;" valign="top"> <h2><span style="font-size:100%;">How T<a name="_How_They_Work"></a>hey Work</span></h2> </td> </tr> <tr> <td style="padding: 0cm 5.4pt;" valign="top"><div style="text-align: justify;"> </div><p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p> <h2><span style="font-size:100%;">Cold Junction Compensation (CJC)<a name="_Cold_Junction_Compensation"></a></span></h2> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p> <h2><span style="font-size:100%;">Linearisation<a name="_Linearisation"></a></span></h2> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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<sup>th</sup> to 9<sup>th</sup> 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.</span></p> <h2><span style="font-size:100%;">Thermo<a name="_Thermocouple_Selection"></a>couple Selection</span></h2> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p> <h2><span style="font-size:100%;">Types<a name="_Types"></a></span></h2> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p> <p class="MsoNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><b>Table 1. </b>Range of Temperatures for Each Thermocouple Type</span></p> <div align="center"> <table style="border: 0.25pt solid windowtext; width: 99%;" width="99%" border="0" cellpadding="0" cellspacing="3"> <tbody><tr> <td style="padding: 2.25pt; background: black none repeat scroll 0% 0%; width: 33.74%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="33%"> <p class="TableHeader" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">Thermocouple Type<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; background: black none repeat scroll 0% 0%; width: 21.06%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="21%"> <p class="TableHeader" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">Overall Range (°C)<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; background: black none repeat scroll 0% 0%; width: 21.04%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="21%"> <p class="TableHeader" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">0.1°C Resolution<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; background: black none repeat scroll 0% 0%; width: 20.22%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="20%"> <p class="TableHeader" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">0.025°C Resolution<o:p></o:p></span></span></p> </td> </tr> <tr> <td style="padding: 2.25pt; width: 33.74%;" width="33%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">B<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; width: 21.06%;" width="21%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">100..1800<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; width: 21.04%;" width="21%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">1030..1800<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; width: 20.22%;" width="20%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">-<o:p></o:p></span></span></p> </td> </tr> <tr> <td style="padding: 2.25pt; background: rgb(224, 224, 224) none repeat scroll 0% 0%; width: 33.74%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="33%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">E<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; background: rgb(224, 224, 224) none repeat scroll 0% 0%; width: 21.06%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="21%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">-270..790<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; background: rgb(224, 224, 224) none repeat scroll 0% 0%; width: 21.04%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="21%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">-240..790<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; background: rgb(224, 224, 224) none repeat scroll 0% 0%; width: 20.22%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="20%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">-140..790<o:p></o:p></span></span></p> </td> </tr> <tr> <td style="padding: 2.25pt; width: 33.74%;" width="33%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">J<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; width: 21.06%;" width="21%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">-210..1050<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; width: 21.04%;" width="21%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">-210..1050<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; width: 20.22%;" width="20%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">-120..1050<o:p></o:p></span></span></p> </td> </tr> <tr> <td style="padding: 2.25pt; background: rgb(224, 224, 224) none repeat scroll 0% 0%; width: 33.74%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="33%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">K<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; background: rgb(224, 224, 224) none repeat scroll 0% 0%; width: 21.06%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="21%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">-270..1370<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; background: rgb(224, 224, 224) none repeat scroll 0% 0%; width: 21.04%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="21%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">-220..1370<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; background: rgb(224, 224, 224) none repeat scroll 0% 0%; width: 20.22%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="20%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">-20..1150<o:p></o:p></span></span></p> </td> </tr> <tr> <td style="padding: 2.25pt; width: 33.74%;" width="33%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">N<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; width: 21.06%;" width="21%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">-260..1300<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; width: 21.04%;" width="21%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">-210..1300<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; width: 20.22%;" width="20%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">340..1260<o:p></o:p></span></span></p> </td> </tr> <tr> <td style="padding: 2.25pt; background: rgb(224, 224, 224) none repeat scroll 0% 0%; width: 33.74%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="33%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">R<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; background: rgb(224, 224, 224) none repeat scroll 0% 0%; width: 21.06%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="21%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">-50..1760<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; background: rgb(224, 224, 224) none repeat scroll 0% 0%; width: 21.04%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="21%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">330..1760<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; background: rgb(224, 224, 224) none repeat scroll 0% 0%; width: 20.22%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="20%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">-<o:p></o:p></span></span></p> </td> </tr> <tr> <td style="padding: 2.25pt; width: 33.74%;" width="33%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">S<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; width: 21.06%;" width="21%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">-50..1760<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; width: 21.04%;" width="21%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">250..1760<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; width: 20.22%;" width="20%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">-<o:p></o:p></span></span></p> </td> </tr> <tr> <td style="padding: 2.25pt; background: rgb(224, 224, 224) none repeat scroll 0% 0%; width: 33.74%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="33%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">T<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; background: rgb(224, 224, 224) none repeat scroll 0% 0%; width: 21.06%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="21%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">-270..400<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; background: rgb(224, 224, 224) none repeat scroll 0% 0%; width: 21.04%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="21%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">-230..400<o:p></o:p></span></span></p> </td> <td style="padding: 2.25pt; background: rgb(224, 224, 224) none repeat scroll 0% 0%; width: 20.22%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;" width="20%"> <p class="TableNormal" style="text-align: center;" align="center"><span style="font-size:100%;"><span style="font-size: 8pt;">-20..400<o:p></o:p></span></span></p> </td> </tr> </tbody></table> </div> <h2><span style="font-size:100%;">Type K (Chromel / Alumel)<a name="_Type_K_(Chromel"></a></span></h2> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p> <h2><span style="font-size:100%;">Type E (Chromel / Constantan)<a name="_Type_E_(Chromel"></a></span></h2> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p> <h2><span style="font-size:100%;">Type J (Iron / Constantan)<a name="_Type_J_(Iron"></a></span></h2> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p> <h2><span style="font-size:100%;">Type N (Nicrosil / Nisil)<a name="_Type_N_(Nicrosil"></a></span></h2> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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/<sup>0</sup>C) they are usually only used for high temperature measurement (>300°C).</span></p> <h2><span style="font-size:100%;">Type B (Platinum / Rhodium)<a name="_Type_B_(Platinum"></a></span></h2> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p> <h2><span style="font-size:100%;">Type R (Platinum / Rhodium)<a name="_Type_R_(Platinum"></a></span></h2> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">Suited for high temperature measurements up to 1600°C. Low sensitivity (10uV/°C) and high cost makes them unsuitable for general purpose use.</span></p> <h2><span style="font-size:100%;">Type S (Platinum / Rhodium)<a name="_Type_S_(Platinum"></a></span></h2> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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).</span></p> <h2><span style="font-size:100%;">Precautions and Considerations for Using Thermocouples<a name="_Precautions_and_Considerations"></a></span></h2> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">Most measurement problems and errors with thermocouples are due to a lack of understanding of how thermocouples work. T<span style="">hermocouples can suffer from ageing and accuracy may vary consequently especially after prolonged exposure to temperatures at the extremities of their useful operating range</span>. Listed below are some of the more common problems and pitfalls to be aware of.</span></p> <h2><span style="font-size:100%;">Connection<a name="_Connection_problems"></a> problems</span></h2> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p> <h2><span style="font-size:100%;">Lea<a name="_Lead_Resistance"></a>d Resistance</span></h2> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p> <h2><span style="font-size:100%;">Decalibration<a name="_Decalibration"></a></span></h2> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p> <h2><span style="font-size:100%;">Noise<a name="_Noise"></a></span></h2> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p> <h2><span style="font-size:100%;">Common Mode Voltage<a name="_Common_Mode_Voltage"></a></span></h2> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p> <h2><span style="font-size:100%;">Thermal Shunting<a name="_Thermal_Shunting"></a></span></h2> <p style="text-align: justify;" class="MsoNormal"><span style="font-size:100%;">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.</span></p><p style="text-align: justify;" class="MsoNormal"><span style="font-size:78%;">(From AZOM</span>)<br /></p></td></tr></tbody></table>Unknownnoreply@blogger.com1tag:blogger.com,1999:blog-2918826615865519497.post-37296401403504899082008-11-30T02:11:00.000-08:002008-11-30T02:13:46.210-08:00What are Level Controllers?<a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://www.lavallab.com/images/level-controller-hi7873.jpg"><img style="margin: 0pt 0pt 10px 10px; float: right; cursor: pointer; width: 308px; height: 366px;" src="http://www.lavallab.com/images/level-controller-hi7873.jpg" alt="" border="0" /></a>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.<br /><div style="text-align: justify;"><br />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.<br /><br />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.<br /><br />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.<br /><br /></div>Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-2918826615865519497.post-9446833413135642602008-11-30T02:09:00.001-08:002008-11-30T02:10:42.791-08:00What are Level Sensors?<a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://www.what-is-net.info/image/what-are-level-sensors.jpg"><img style="margin: 0pt 10px 10px 0pt; float: left; cursor: pointer; width: 155px; height: 134px;" src="http://www.what-is-net.info/image/what-are-level-sensors.jpg" alt="" border="0" /></a>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.<br /><div style="text-align: justify;"><br />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.<br /><br />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.<br /><br /></div>Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-2918826615865519497.post-57429920472939775762008-11-29T19:07:00.001-08:002008-11-29T19:07:39.493-08:00What is Statistical Process Control SPC?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.<br /><br />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).<br />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.<br /><br />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.<br /><br />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.<br /><br />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.Unknownnoreply@blogger.com1tag:blogger.com,1999:blog-2918826615865519497.post-30875286390916797282008-11-29T19:00:00.000-08:002008-11-29T19:04:11.441-08:00What is Industrial Automation?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.<br /><br />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.<br /><br />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.<br /><br />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.)<br /><br />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.<br /><br />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.Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-2918826615865519497.post-81859731249257714622008-11-28T05:31:00.000-08:002008-11-28T05:40:16.651-08:00What is an LVDT and other information...<div class="tahoma11" style="margin: 11px; text-align: justify;"> <p><span style="font-weight: bold;">What is an LVDT?</span><br /></p><p>The letters <b>LVDT</b> are an acronym for <b>Linear Variable Differential Transformer</b>, 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). </p></div><div style="text-align: justify;"> </div><p style="text-align: center;"><img src="http://www.macrosensors.com.cn/images/lvdt-diagram.jpg" alt="Structure of a typical LVDT" width="400" height="371" /></p><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="tahoma11" style="margin-left: 11px; margin-right: 10px; text-align: justify;">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. </div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="tahoma11" style="margin-left: 11px; margin-right: 10px; text-align: justify;">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. </div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div style="text-align: justify;"> </div><p style="text-align: justify;">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.</p><p style="text-align: justify;"><span style="font-weight: bold;">Why use an LVDT?</span></p><div class="tahoma11" style="margin: 11px; text-align: justify;">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. </div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="blue" style="margin-left: 11px; text-align: justify;">Friction-Free Operation</div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="tahoma11" style="margin-left: 11px; margin-right: 11px; text-align: justify;">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. </div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="blue" style="margin-left: 11px; margin-right: 11px; text-align: justify;">Infinite Resolution </div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="tahoma11" style="margin-left: 11px; margin-right: 11px; text-align: justify;">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. </div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="blue" style="margin-left: 11px; margin-right: 11px; text-align: justify;">Unlimited Mechanical Life</div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="tahoma11" style="margin-left: 11px; margin-right: 11px; text-align: justify;">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. </div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="blue" style="margin-left: 11px; margin-right: 11px; text-align: justify;">Overtravel Damage Resistant </div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="tahoma11" style="margin-left: 11px; text-align: justify;">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. </div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="blue" style="margin-left: 11px; margin-right: 11px; text-align: justify;">Single Axis Sensitivity </div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="tahoma11" style="margin-left: 11px; margin-right: 11px; text-align: justify;">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. </div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="blue" style="margin-left: 11px; margin-right: 11px; text-align: justify;">Separable Coil And Core </div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="tahoma11" style="margin-left: 11px; margin-right: 11px; text-align: justify;"> 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.</div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="blue" style="margin-left: 11px; margin-right: 11px; text-align: justify;">Environmentally Robust </div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="tahoma11" style="margin-left: 11px; margin-right: 11px; text-align: justify;"> 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. </div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="tahoma11" style="margin-left: 11px; margin-right: 11px; text-align: justify;"> 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.</div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="tahoma11" style="margin-left: 11px; margin-right: 11px; text-align: justify;"> 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. </div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="blue" style="margin-left: 11px; margin-right: 11px; text-align: justify;"> Null Point Repeatability</div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="tahoma11" style="margin-left: 11px; margin-right: 11px; text-align: justify;"> 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. </div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="blue" style="margin-left: 11px; margin-right: 11px; text-align: justify;"> Fast Dynamic Response</div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="tahoma11" style="margin-left: 11px; margin-right: 11px; text-align: justify;"> 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. </div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="blue" style="margin-left: 11px; margin-right: 11px; text-align: justify;"> Absolute Output </div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div style="text-align: justify;"> </div><p style="text-align: justify;">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.</p><p style="text-align: justify;"><span style="font-weight: bold;">How does an LVDT work?</span></p><div class="tahoma11" style="margin: 11px; text-align: justify;"> <p>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.</p> </div><div style="text-align: justify;"> </div><p style="text-align: center;"><img src="http://www.macrosensors.com.cn/images/chartwiring.jpg" alt="LVDT Core Position Diagram" width="420" /> </p><p style="text-align: justify;"> </p><div class="tahoma11" style="margin: 11px; text-align: justify;"> 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 ).</div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div class="tahoma11" style="margin: 11px; text-align: justify;"> 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.</div><div style="text-align: justify;"> </div><p style="text-align: justify;"> </p><div style="text-align: justify;"> </div><p style="text-align: justify;">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.</p><p style="text-align: justify;"><span style="font-size:85%;"><span style="font-style: italic;">From Macro Sensors website</span></span><br /></p>Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-2918826615865519497.post-57610798577825336102008-11-26T02:10:00.000-08:002008-11-26T02:12:15.593-08:00What are Temperature Transmitters?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.<br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://www.emersonprocess.com/Rosemount/products/temperature/images/644rotating.gif"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 350px; height: 350px;" src="http://www.emersonprocess.com/Rosemount/products/temperature/images/644rotating.gif" alt="" border="0" /></a><br />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.<br /><br />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.<br /><br />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.<br /><br />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.Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-2918826615865519497.post-81907301216810596762008-11-17T06:30:00.000-08:002008-11-17T06:45:02.878-08:00Magnetic FlowmetersSurfing and reading articles in the world wide web, I recently came up to this article about flowmeters by The OMEGA website.<span style="font-weight: bold;"><span style="font-weight: bold;"> </span></span>OMEGA is a well known industry in providing instrumentation solutions to various manufacturing firms.<span style="font-weight: bold;"><span style="font-weight: bold;"> </span></span>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.<br /><div style="text-align: justify;"><span style="display: block; font-weight: bold;" id="formatbar_Buttons"><span class="" style="display: block;" id="formatbar_JustifyFull" title="Justify Full" onmouseover="ButtonHoverOn(this);" onmouseout="ButtonHoverOff(this);" onmouseup="" onmousedown="CheckFormatting(event);FormatbarButton('richeditorframe', this, 13);ButtonMouseDown(this);"><img src="http://www.blogger.com/img/blank.gif" alt="Justify Full" class="gl_align_full" border="0" /></span></span><br /><span style="font-weight: bold; font-style: italic;">INTRODUCTION</span><br />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:<br /><br />1) Wafer-style, where highest accuracy (up to +0.5% of reading) measurements are required; and<br />2) Insertion-style, for greater economy and particularly for larger pipe sizes.<br /><br />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.<br /><br /><span style="font-style: italic; font-weight: bold;">PRINCIPLE OF OPERATION</span><br /><span style="font-style: italic;">Faraday’s Law</span><br />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.<br /><br />Faraday’s Formula:<br />E is proportional to V x B x D<br />where:<br />E = The voltage generated in a conductor<br />V = The velocity of the conductor<br />B = The magnetic field strength<br />D = The length of the conductor<br />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.<br /><br />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).<br />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).<br /><br /><br /><div style="text-align: center;"><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjxESFD75YQikswryCS2tV2KRLu3sTxdVOgajvSECWqyaUlCsK9YkIChhH82sfmqIpIa-N8KFgc52NydkgFwJa3_op1TNwnEOvBbOyvx8Yi6BAlOV9pkXxrt2P4znfAHopdle8k_jQi1MOn/s1600-h/In+Line+Magnetic+Flowmeter+Operating+Principle.JPG"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 320px; height: 220px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjxESFD75YQikswryCS2tV2KRLu3sTxdVOgajvSECWqyaUlCsK9YkIChhH82sfmqIpIa-N8KFgc52NydkgFwJa3_op1TNwnEOvBbOyvx8Yi6BAlOV9pkXxrt2P4znfAHopdle8k_jQi1MOn/s320/In+Line+Magnetic+Flowmeter+Operating+Principle.JPG" alt="" id="BLOGGER_PHOTO_ID_5269636175380882930" border="0" /></a>Figure 1: In-line magnetic flowmeter operating principle<br /></div><br /><div style="text-align: center;"><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiLpHiC3kuh_TIZJTQr1bkvVy0-hkrc3LdxsUYJRxTWFQ_YPwwQRdNd4tASpIGoAhJ_-zeOFlhrwC4CHcoIOgfbnN3-86ALp_EYCt00I8SbTGT9PVRHEy1hmsc-CmTg7PM-yPLUN0MA4EWz/s1600-h/Insertion+Type+Flowmeter+Operating+Principle.JPG"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 320px; height: 226px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiLpHiC3kuh_TIZJTQr1bkvVy0-hkrc3LdxsUYJRxTWFQ_YPwwQRdNd4tASpIGoAhJ_-zeOFlhrwC4CHcoIOgfbnN3-86ALp_EYCt00I8SbTGT9PVRHEy1hmsc-CmTg7PM-yPLUN0MA4EWz/s320/Insertion+Type+Flowmeter+Operating+Principle.JPG" alt="" id="BLOGGER_PHOTO_ID_5269636174982893394" border="0" /></a>Figure 2: Insertion-type flowmeter operating principle<br /></div><br /><span style="font-weight: bold;">MAGMETER SELECTION</span><br />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.<br /><br /><span style="font-style: italic;">Conductivity</span><br />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:<br />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.<br />2. Chemical and pharmaceutical companies often use deionized or distilled water, or other solutions which are not conductive enough for use with magnetic flowmeters.<br />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.<br />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.<br /><span style="font-style: italic;">Acid/Caustics</span><br />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.<br /><span style="font-style: italic;"><br />Velocity</span><br />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.<br /><br /><span style="font-style: italic;">Abrasive Slurries</span><br />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.<br /><br /><span style="font-style: italic;">Sludges and Grease-Bearing Liquids</span><br />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.<br /><br /><span style="font-style: italic;">Viscosity</span><br />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.<br /><br /><span style="font-style: italic;">Temperature</span><br />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.<br /><br /><span style="font-weight: bold;">Advantages of the DC Pulse Style</span><br />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:<br /><br /><span style="font-style: italic;">Quadrature</span><br />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.<br /><br /><span style="font-style: italic;">Wiring</span><br />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.<br /><br /><span style="font-style: italic;">Power</span><br />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.<br /><br /><div style="text-align: center;"><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjUWIExcJ7N74z1yfdNAG3p3T50hU76-UXruI0M9L0JDfTa1K4nnGI05XRubjOo49S1sMtRJqMfy-5wrSoKMHka_pQKBL1jI5Piz-NuP8iqz2BPh-89wzPs2E6_y1-AK5yFf5S9wPy-ga3q/s1600-h/Vertical+Installation+of+In+Line+Flowmeter.JPG"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 208px; height: 320px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjUWIExcJ7N74z1yfdNAG3p3T50hU76-UXruI0M9L0JDfTa1K4nnGI05XRubjOo49S1sMtRJqMfy-5wrSoKMHka_pQKBL1jI5Piz-NuP8iqz2BPh-89wzPs2E6_y1-AK5yFf5S9wPy-ga3q/s320/Vertical+Installation+of+In+Line+Flowmeter.JPG" alt="" id="BLOGGER_PHOTO_ID_5269636183325429202" border="0" /></a>Figure 3: Vertical installation of inline meter<br /></div><br /><span style="font-style: italic;">Auto-Zero</span><br />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.<br /><br /><span style="font-style: italic;">Installation</span><br />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.<br /><br /><div style="text-align: center;"><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjWgGOLMsuYEYc4cHT8h5I3Rzbasi4oyxhp08Kcc8UDaS__zAIYnShhmgQElawh_Kjoxj10q1T1IveqAXeWI2A3SCIYy9B1EDl8SdKTUPzu-z48dgLb5k-rG2q5LNdwq8WQI3CoZkaDOStH/s1600-h/Horizontal+Installation+of+In+Line+Flowmeter.JPG"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 320px; height: 201px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjWgGOLMsuYEYc4cHT8h5I3Rzbasi4oyxhp08Kcc8UDaS__zAIYnShhmgQElawh_Kjoxj10q1T1IveqAXeWI2A3SCIYy9B1EDl8SdKTUPzu-z48dgLb5k-rG2q5LNdwq8WQI3CoZkaDOStH/s320/Horizontal+Installation+of+In+Line+Flowmeter.JPG" alt="" id="BLOGGER_PHOTO_ID_5269636167771692338" border="0" /></a>Figure 4: Horizontal installation of in-line meter<br /><span style="font-size:78%;"><br /></span><div style="text-align: justify;"><span style="font-size:78%;">Technical texts courtesy of Omega.</span><br /></div><div style="text-align: justify;"><br /></div></div></div>Unknownnoreply@blogger.com2tag:blogger.com,1999:blog-2918826615865519497.post-50138568102662081202008-10-31T06:41:00.000-07:002008-10-31T06:48:13.024-07:00Modbus TCP Overview according to Intellicom Innovation<strong>Modbus TCP - An introduction</strong><br /> 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.<br /> The implications for a vendor of equipment or an end-user are endless.<br /> <ul> <li>Performing maintenance and repair on remote devices from the office using a PC and browser reduce support costs and improve customer service.<p> </p></li><li>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.<p> </p></li><li>Managing geographically distributed systems becomes easy using commercially available internet/intranet technologies. </li> </ul> 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.<br /> 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. <p> 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.</p> <p> <strong>The protocol - Modbus TCP</strong><br />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.</p> <p> 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.</p> <p> </p> <center><img src="http://www.intellicom.se/images/res_modbus_tcp_pic_1.gif" alt="" border="0" height="113" width="500" /></center> <p> <strong>Performance from a MODBUS TCP/IP system</strong><br />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!</p> <p> For a high-performance Intranet with high-speed Ethernet switches to guarantee performance, the situation is completely different.</p> <p> 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:<br /> 1.25M / 2 * 60% = 360000 registers per second and the 100 Base T speed is 10 x greater.</p> <p> This assumes that you are using devices that can service Ethernet as fast as bandwidth is available.<br />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.</p> <p> 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.</p> <p> <strong>How can existing MODBUS devices communicate over MODBUS TCP/IP?</strong><br />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.</p> <strong id="title">Recommended Modbus products:</strong> <table border="0" cellpadding="0" cellspacing="0" width="517"> <tbody><tr> <td colspan="2" height="22" width="517"> <table border="0" cellpadding="0" cellspacing="0" width="517"> <tbody><tr> <td width="5"><img src="http://www.intellicom.se/images/product_group_top_left.gif" height="22" width="5" /></td> <td background="images/product_group_top.gif"><strong id="ProductGroup">Modbus web server Gateway (Supports Ethernet, GSM, GPRS, modem)</strong></td> <td width="5"><img src="http://www.intellicom.se/images/product_group_top_right.gif" height="22" width="5" /></td> </tr> </tbody></table> </td> </tr> <tr><td bgcolor="#255092" height="2"><img src="http://www.intellicom.se/images/transparent.gif" height="1" width="1" /></td></tr> <tr> <td> <center><img src="http://www.intellicom.se/images/modbus_banner1.png" height="208" width="460" /></center> <strong>Features</strong><br /> • Connects Modbus RTU products to SCADA or PLC’s over Ethernet<br /> • Supports Ethernet, Internet, LAN, GSM/GPRS, Modbus TCP, email, FTP etc.<a href="http://www.intellicom.se/eg_webscada_modbus.shtml?17" id="link"><strong></strong></a><br /> </td> </tr> <tr><td bgcolor="#255092" height="2"><img src="http://www.intellicom.se/images/transparent.gif" height="1" width="1" /></td></tr> </tbody> </table> <br /> <table border="0" cellpadding="0" cellspacing="0" width="517"> <tbody><tr> <td colspan="2" height="22" width="517"> <table border="0" cellpadding="0" cellspacing="0" width="517"> <tbody><tr> <td width="5"><img src="http://www.intellicom.se/images/product_group_top_left.gif" height="22" width="5" /></td> <td background="images/product_group_top.gif"><strong id="ProductGroup">Modbus RTU – Modbus TCP Gateway</strong></td> <td width="5"><img src="http://www.intellicom.se/images/product_group_top_right.gif" height="22" width="5" /></td> </tr> </tbody></table> </td> </tr> <tr><td bgcolor="#255092" height="2"><img src="http://www.intellicom.se/images/transparent.gif" height="1" width="1" /></td></tr> <tr> <td> <center><img src="http://www.intellicom.se/images/modbus_banner2.png" height="170" width="460" /></center> <strong>Features</strong><br /> • Connects Modbus RTU products to SCADA or PLC’s over Ethernet<br /> • Transparent Gateway between Modbus RTU and Modbus TCP<span style="text-decoration: underline;"><span style="font-weight: bold;">.</span></span><a href="http://www.intellicom.se/eg_modbus_rtu_tcp.shtml?2" id="link"><strong></strong></a><br /> </td> </tr> <tr><td bgcolor="#255092" height="2"><img src="http://www.intellicom.se/images/transparent.gif" height="1" width="1" /></td></tr> </tbody> </table> <br /> <table border="0" cellpadding="0" cellspacing="0" width="517"> <tbody><tr> <td colspan="2" height="22" width="517"> <table border="0" cellpadding="0" cellspacing="0" width="517"> <tbody><tr> <td width="5"><img src="http://www.intellicom.se/images/product_group_top_left.gif" height="22" width="5" /></td> <td background="images/product_group_top.gif"><strong id="ProductGroup">Serial Server</strong></td> <td width="5"><img src="http://www.intellicom.se/images/product_group_top_right.gif" height="22" width="5" /></td> </tr> </tbody></table> </td> </tr> <tr><td bgcolor="#255092" height="2"><img src="http://www.intellicom.se/images/transparent.gif" height="1" width="1" /></td></tr> <tr> <td> <center><img src="http://www.intellicom.se/images/modbus_banner3.png" height="210" width="469" /></center> <strong>Features</strong><br /> • Connects Modbus RTU/ASCII products to any Windows based software<br /> • Transparent connection<br /> • 2 ports (1 RS485 + 1 RS232) <br /> </td> </tr> <tr><td bgcolor="#255092" height="2"><img src="http://www.intellicom.se/images/transparent.gif" height="1" width="1" /></td></tr> </tbody> </table> <br /> <table border="0" cellpadding="0" cellspacing="0" width="517"> <tbody><tr> <td colspan="2" height="22" width="517"> <table border="0" cellpadding="0" cellspacing="0" width="517"> <tbody><tr> <td width="5"><img src="http://www.intellicom.se/images/product_group_top_left.gif" height="22" width="5" /></td> <td background="images/product_group_top.gif"><strong id="ProductGroup">Modbus I/O over Internet or GSM/GPRS</strong></td> <td width="5"><img src="http://www.intellicom.se/images/product_group_top_right.gif" height="22" width="5" /></td> </tr> </tbody></table> </td> </tr> <tr><td bgcolor="#255092" height="2"><img src="http://www.intellicom.se/images/transparent.gif" height="1" width="1" /></td></tr> <tr> <td> <center><img src="http://www.intellicom.se/images/modbus_banner4.png" height="232" width="342" /></center> <strong>Features</strong><br /> • Modbus I/O: 4 DI, 4 DO, 2AI, 1AO, “ RTD (Temperature)<br /> • Can be accessed through Internet and Mobile phones</td></tr></tbody> </table>Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-2918826615865519497.post-77701216962864043432008-10-29T02:12:00.000-07:002008-10-29T05:55:32.459-07:00IAM Forum Site Now OnlineFriends, 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.<br /><br />Remember, this is ours.<br /><br />the AdminUnknownnoreply@blogger.com0tag:blogger.com,1999:blog-2918826615865519497.post-41654079854058835472008-10-23T05:18:00.001-07:002008-10-25T09:31:35.108-07:00Notes on Pyrometer Calibration ProcedureFrom 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.<br /><br />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.<br /><br />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.<br /><br />The primary issue when using infrared thermometer sensors is that a real object does not behave like a perfect radiator.<br /><br />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.<br /><br />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.<br /><br />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.<br /><br />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.<br /><br />When using a blackbody calibration source, we need to follow certain ground rules:<br /><ul><li>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.</li><li>The field of view area of the thermometer at a selected distance should be smaller than the target area of the blackbody unit.</li><li>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.</li><li>Always aim the infrared thermometer to the center of the target area.</li><li>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.</li><li>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.</li></ul>User should be guided on recommended operating conditions. These factors, along with proper maintenance, will allow safe and satisfactory operation of the furnace.<br /><br />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).<br /><br />Step 1.<br />• 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.<br /><br />Step 2.<br />• 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.<br /><br />Step 3.<br />• 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.<br /><br />Step 4.<br />• 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<br />• Size = Pre-determined value for temperature change.<br />• Ramp = Select the pre-determined rate changes to the next value.<br />• Increment using numerical keypad (increment and decrement).<br /><br />Step 5.<br />• Power on the controller, and allow 20 minutes warm-up time.<br /><br />Step 6.<br />• Cover the objective lens with a solid, opaque object that will completely block any external radiation.<br /><br />Step 7.<br />• The Response Time control must be in the FAST position. If a Peak Picker option is installed, toggle the Peak Picker switch to DIRECT.<br /><br />Step 8.<br />• Depress the momentary toggle switch label CALIBRATE.<br />• 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.<br />• 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.<br />• Normal calibration level: Pointer to center of CAL position on analog display; numerical reading of 500 +/- 10 digits on digital display.<br /><br />Note: Refer to manufacturer’s infrared thermometer manual for Calibration Check.<br /><br />Step 9.<br />• 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.<br /><br />Step 10.<br />• 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.<br /><br />Step 11.<br />• 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.<br /><br />Step 12.<br />• 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.<br /><br />Step 13.<br />• Do step 12 to other pyrometer set with determined error.<br /><br />Step 14.<br />• Increase the blackbody furnace temperature setting to the highest range of the controller then calibrate the controller for this setting.<br /><br />Step 15.<br />• Do step 14 to other pyrometer set with determined error.<br /><br />Step 16.<br />• 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.<br /><br />Step 17.<br />• 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.<br /><br />Steps 18.<br />• If there are still discrepancies on the linearity, repeat step 12 to 17. If there are none, calibration is done.<br /><br />Step 19.<br />• Cool down blackbody slowly. (Refer to the cooling curve below).<br /><br />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.Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-2918826615865519497.post-87629622106002513122008-10-23T01:06:00.000-07:002008-10-24T06:23:27.300-07:00The BSIAM Studes...The BSIAM students in one of their discussions with Professor Joel Barrera on one of their Mechatronics subjects.<br /><br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://i106.photobucket.com/albums/m273/d4rkhowl/10222008005.jpg"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 551px; height: 413px;" src="http://i106.photobucket.com/albums/m273/d4rkhowl/10222008005.jpg" alt="" border="0" /></a><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://i106.photobucket.com/albums/m273/d4rkhowl/10222008004.jpg"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 656px; height: 491px;" src="http://i106.photobucket.com/albums/m273/d4rkhowl/10222008004.jpg" alt="" border="0" /></a><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://i106.photobucket.com/albums/m273/d4rkhowl/10222008003.jpg"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 567px; height: 425px;" src="http://i106.photobucket.com/albums/m273/d4rkhowl/10222008003.jpg" alt="" border="0" /></a><br /><div style="text-align: center;"><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://i106.photobucket.com/albums/m273/d4rkhowl/10222008002.jpg"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 668px; height: 501px;" src="http://i106.photobucket.com/albums/m273/d4rkhowl/10222008002.jpg" alt="" border="0" /></a>Listening...<br /></div><br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://i106.photobucket.com/albums/m273/d4rkhowl/10222008001.jpg"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 604px; height: 452px;" src="http://i106.photobucket.com/albums/m273/d4rkhowl/10222008001.jpg" alt="" border="0" /></a><br /><div style="text-align: center;">Young, Wacky but very promising.<br /></div>Unknownnoreply@blogger.com0