A motor and one or more generators, with their shafts mechanically coupled, used to convert an available power source to another desired frequency or voltage. The motor of the set is selected to operate from the available power supply; the generators are designed to provide the desired output.The principal advantage of a motor-generator set over other conversion systems is the flexibility offered by the use of separate machines for each function. Since a double energy conversion is involved, electrical to mechanical and back to electrical, the efficiency is lower than in most other conversion methods
Infrared technology targets industrial automation IR cameras are used in industrial-plant monitoring and more and more in industrial automation.By Andrew Wilson, Editor
English astronomer Sir William Hershel is credited with the discovery of infrared (IR) radiation in 1800. In his first experiment, Hershel subjected a liquid in a glass thermometer to different colors of the spectrum. Finding that the hottest temperature was beyond red light, Hershel christened his newly found energy "calorific rays," now known as infrared radiation.
Two centuries later, IR imagers and cameras are finding uses in applications from missile guidance tracking to plant monitoring to machine-vision automation systems. Invisible to the human eye, IR energy can be divided into the three spectral regions: near-, mid-, and far-IR, with wavelengths longer than that of visible light. Although the boundaries between these are undetermined, the wavelength ranges are approximately 0.7 to 5 µm (near-IR), 5 to 40 µm (mid-IR), and 40 to 350 µm (far-IR).
However, do not expect today's commercially available IR detectors or cameras to span such large wavelengths. Rather, they will be specified as covering more narrow bandwidths between approximately 1 and 20 µm. Many manufacturers may use the terms near, mid-, and far-IR loosely, often claiming that their 9 µm-capable camera is based on a far-IR-based sensor.
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.
Perhaps one of the largest misconceptions is that IR measures the temperature of an object. This misconception results from Plank's law, which states that all objects with a temperature above absolute zero emit IR radiation and that the higher the temperature the higher the emitted intensity. Plank's law, however, is only true for blackbody objects that have 100% absorption and maximum emitting intensity. In reality, a ratio of the emitting intensity of the object and a corresponding blackbody with the same temperature must be used. This emissivity—the measure of how a material absorbs and emits IR energy—affects how images are interpreted.
In the design of its MP50 linescan process imager, Raytek (Santa Cruz, CA, USA) incorporates a reference blackbody for continuous calibration (see Fig. 1). Targeted at continuous-sheet and web-based processes, the scanner offers a 48-line/s scan speed and is offered in a number of versions capable of capturing spectral ranges useful for examining plastics, glass, and metals. Other manufacturers offer blackbodies as accessories that can externally calibrate their cameras.
Since people cannot see IR radiation, the images captured by IR detectors and cameras must first be processed, translated, and pseudocolored into images that can be visualized. In these images, highly reflective materials may appear different from less-reflective materials, even though their temperature is the same. This is because highly reflective materials will reflect the radiation of the objects around them and therefore may appear to be "colder" than less-reflective materials of the same temperature.
In considering whether to use IR technology for any particular application, therefore, the properties of the materials being viewed must be known to properly interpret the image. In printed-circuit-board analysis, for example, the emissivity of different metals can be used to discern faults in the board. However, if the emissivity of materials is similar, it may be difficult to discern any differences in the image.
In many applications, including target tracking, this does not pose a problem. In heat-seeking missiles, for example, the difference between the emissivity of aluminium alloy used to build a rocket and the fire that emerges from its boosters is so high that discerning the two is relatively simple. In other applications, the task may be more complex.
Infrared cameras use a number of different detector types that can be broadly classified as either photon or thermal detectors. Infrared absorbed by photon-based detectors generates electrons or bandgap transitions in materials such as mercury cadmium telluride (HgCdTe; detecting IR in the 3- to 5- and 8- and 12-µm range) and indium antinomide (InSb; detecting IR in the 3- to 5-µm range). This results in a charge that can be directly measured and read out for preprocessing.
Rather than generate charge or bandgap transitions directly, thermal detectors absorb the IR radiation, raising the temperature of single or multiple membrane-isolated temperature detectors on the device. Unlike photon-based detectors, thermal detectors can be operated at room temperature, although their sensitivity and response time are longer.
To create a two-dimensional IR image, camera vendors incorporate focal-plane, or staring, arrays into their cameras. These detectors are similar in concept to CCDs in that they are offered in arrays of pixels that can range from as low as 2 x 2 to 640 x 512 formats and higher, often with greater than 8 bits of dynamic range.
Incorporating a thermal detector in the form of an amorphous silicon or vanadate (YVO4) microbolometer, the Eye-R320B from Opgal (Karmiel, Israel) features a 320 x 240 FPA. With a spectral range from 8 to 12 µm, the camera also offers automatic gain correction, remote RS422 programmability, and CCIR or RS170 output. The company also offers embeddable IR camera modules that can use a number of 640 x 480-based detectors from different manufacturers.
As the wavelength of visible light is shorter than that of IR radiation, visible light can discern features within an image at higher resolution. For this reason ultraviolet (UV) radiation, which the human eye also cannot perceive, is used in to detect submicron defects in semiconductor wafers. Because the frequency of UV light is higher, the spatial resolution of the optical system is also higher, allowing greater detail to be captured.
Unfortunately, quite the opposite is true of IR radiation. With a lower frequency than visible light, IR radiation will resolve fewer line pairs/millimeter than visible light, given that all other system parameters are equal. Indeed, it is this diffraction-limited nature of optics that leads to the large pixel sizes of IR imagers. And, of course, an IR imager with 320 x 240 format and a pixel pitch of 30 µm will have a die size considerably larger than its 320 x 240 CCD counterpart with a 6-µm pixel pitch. This larger die size for any given format is another reason IR imagers are more expensive than visible imagers.
In many visible machine-vision applications, it is necessary to determine the minimum spatial resolution required by the system. And the same applies when determining whether an IR detector can be used in such an application. This is accomplished visibly by using test charts with periods of white and black lines. If, for example, the required resolution were 125 line pairs/mm, then the pitch of those line pairs would be 8 µm. From Nyquist criteria, it can be determined that the most efficient way to sample the signal is with a 4-µm pixel pitch. A smaller pitch will not add new information, and a larger pitch will result in errors.
In such optical systems the pixel pitch at the limit of resolution is given by the diffraction-limited equation
where f/# equals the focal length/aperture ratio of the lens. Thus, a pixel pitch of 2.68 µm is needed to resolve a 550-nm visible frequency at f/8. In an IR system, with a wavelength of 5 µm and the same focal length/aperture ratio, the pixel pitch required will be approximately 25 µm or nine times larger. With a 25-µm pixel pitch, the minimum number of line pairs/millimeter that can be resolved will have a 50-µm period, which equates to approximately 2 line pairs/mm with an f/1.8 lens.
Luckily, most camera manufacturers specify these parameters. The Stinger IR camera from Ircon (Niles, IL, USA), for example, is specified with an uncooled 320 x 240 FPA, spectral ranges of 5, 8, and 8 to 14 µm, a detector element size of 51 x 51 µm, and an f/1.4 lens (see Fig. 2). The company's literature states that targets as small as 0.017 in. can be measured with the camera, a fact that can be confirmed by some simple mathematics.
To increase this resolution, some manufacturers use lenses with larger numerical apertures (smaller f#s). Because glass is opaque to IR radiation, these lenses are usually fabricated from exotic materials such as zinc selenide (ZnSe) or germanium (Ge), adding to the cost of the camera. Like visible solid-state cameras, IR cameras are generally offered with both linescan and area format arrays. While linescan-based cameras are useful in IR web inspection, area-array-based cameras can capture two-dimensional images. Outputs from these cameras are also similar to visible camera and are generally standard NTSC/PAL analog formats or FireWire and USB-based or digital formats.
What has, in the past, stood in the way of acceptance of IR techniques in machine-vision systems has been the cost of IR systems compared with their visible counterparts and the lack of an easy way to combine the benefits of both wavelengths in low-cost systems. In the past few years, however, the cost of IR imaging has been lowered by the introduction of smart IR cameras that include on-board detectors, processors, embedded software, and standard interfaces. And, realizing the benefits of a combined visible/IR approach, manufacturers are now starting to introduce more sophisticated imagers that can simultaneously capturing visible and near-IR images.
Recently, Indigo Systems (Goleta, CA, USA) announced a new method for processing indium gallium arsenide (InGaAs) to enhance its short-wavelength response. The new material, VisGaAs, is a broad-spectrum substance that enables both near-IR and visible imaging on the same photodetector. According to the company, test results indicate VisGaAs can operate in a range from 0.4 to 1.7 µm. To test the detector, the company mounted a 320 x 256 FPA onto its Phoenix camera-head platform and imaged a hot soldering gun in front of a computer monitor (see Fig. 3). The results clearly show that a standard InGaAs camera can detect hardly any radiation from the CRT, while the VisGaAs-based imager can clearly detect both features.
To restrict the spread of severe acute respiratory syndrome (SARS), Land Instruments International (Sheffield, UK; www.landinst.com) has developed a PC-based system that detects elevated body temperatures in large numbers of people. Because individuals with SARS have a fever and above-normal skin temperature, infrared cameras can analyze and detect the viral illness.
Land Instrument's Human Body Temperature Monitoring System (HBTMS) uses the company FTI Mv Thermal Imager with an array of 160 × 120 pixels to capture a thermographic image of a human body (typically the face) at a distance of 2 to 3 m. Data captured are then compared with a 988 blackbody furnace calibration source from Isothermal Technology (Isotech, Southport, UK; www.isotech.co.uk). Permanently positioned in the field of view of the imager, this calibrated temperature reference source is set at 38°C and provides a reference area in the live image scene. The imager is then adjusted to maintain this reference area at a fixed radiance value (200).
To capture images from the FTI Mv, the camera is coupled to a Universal Interface Box (UIB), which drives the imager, images, and imager control from a PC up to 1000 m away. Video and RS422 control signals are then transmitted to the PC from the UIB. IR images are transmitted as an analog video signal via the UIB and digitized by a PC-based MV 510 frame grabber from MuTech (Billerica, MA, USA; www.mutech.com), which transfers digital data to PC memory or VGA display. Because the board offers programmable gain and offset control functionality, the incoming video signal can be adjusted for the maximum digitization range of the camera.
Once the image has been acquired, it is analyzed by Land's image-processing software that displays the images, triggers alarms via a digital output card, and records images to disk. Any pixels in this area with radiance greater than the set threshold trigger an alarm output. To highlight individuals who may have the disease, a monochrome palette is used, with any pixel on the scene with radiance levels above the threshold highlighted in red.
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.
In 1822, an Estonian physician named Thomas Seebeck discovered (accidentally) that the junction between two metals generates a voltage which is a function of temperature. Thermocouples rely on this Seebeck effect. Although almost any two types of metal can be used to make a thermocouple, a number of standard types are used because they possess predictable output voltages and large temperature gradients.
A K type thermocouple is the most popular and uses nickel-chromium and nickel-aluminium alloys to generate voltage.Standard tables show the voltage produced by thermocouples at any given temperature, so the K type thermocouple at 300°C will produce 12.2mV. Unfortunately it is not possible to simply connect up a voltmeter to the thermocouple to measure this voltage, because the connection of the voltmeter leads will make a second, undesired thermocouple junction.
To make accurate measurements, this must be compensated for by using a technique known as cold junction compensation (CJC). In case you are wondering why connecting a voltmeter to a thermocouple does not make several additional thermocouple junctions (leads connecting to the thermocouple, leads to the meter, inside the meter etc), the law of intermediate metals states that a third metal, inserted between the two dissimilar metals of a thermocouple junction will have no effect provided that the two junctions are at the same temperature. This law is also important in the construction of thermocouple junctions. It is acceptable to make a thermocouple junction by soldering the two metals together as the solder will not affect the reading. In practice, thermocouple junctions are made by welding the two metals together (usually by capacitive discharge). This ensures that the performance is not limited by the melting point of solder.
All standard thermocouple tables allow for this second thermocouple junction by assuming that it is kept at exactly zero degrees centigrade. Traditionally this was done with a carefully constructed ice bath (hence the term 'cold' junction compensation). Maintaining a ice bath is not practical for most measurement applications, so instead the actual temperature at the point of connection of the thermocouple wires to the measuring instrument is recorded.
Typically cold junction temperature is sensed by a precision thermistor in good thermal contact with the input connectors of the measuring instrument. This second temperature reading, along with the reading from the thermocouple itself is used by the measuring instrument to calculate the true temperature at the thermocouple tip. For less critical applications, the CJC is performed by a semiconductor temperature sensor. By combining the signal from this semiconductor with the signal from the thermocouple, the correct reading can be obtained without the need or expense to record two temperatures. Understanding of cold junction compensation is important; any error in the measurement of cold junction temperature will lead to the same error in the measured temperature from the thermocouple tip.
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 (5th to 9th 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.
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.
When choosing a thermocouple consideration should be given to both the thermocouple type, insulation and probe construction. All of these will have an effect on the measurable temperature range, accuracy and reliability of the readings. Listed below is a subjective guide to thermocouple types.
When selecting thermocouple types, ensure that your measuring equipment does not limit the range of temperatures that can be measured. Note that thermocouples with low sensitivity (B, R and S) have a correspondingly lower resolution. The table below summarises the useful operating limits for the various thermocouple types which are described in more detail in the following paragraphs.
Table 1. Range of Temperatures for Each Thermocouple Type
Type K is the 'general purpose' thermocouple. It is low cost and, owing to its popularity, it is available in a wide variety of probes. Thermocouples are available in the -200°C to +1200°C range. Sensitivity is approx 41uV/°C. Use type K unless you have a good reason not to.
Type E has a high output (68uV/°C) which makes it well suited to low temperature (cryogenic) use. Another property is that it is non-magnetic.
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.
High stability and resistance to high temperature oxidation makes type N suitable for high temperature measurements without the cost of platinum (B,R,S) types. Designed to be an 'improved' type K, it is becoming more popular.
Thermocouple types B, R and S are all 'noble' metal thermocouples and exhibit similar characteristics. They are the most stable of all thermocouples, but due to their low sensitivity (approx 10uV/0C) they are usually only used for high temperature measurement (>300°C).
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.
Suited for high temperature measurements up to 1600°C. Low sensitivity (10uV/°C) and high cost makes them unsuitable for general purpose use.
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).
Most measurement problems and errors with thermocouples are due to a lack of understanding of how thermocouples work. Thermocouples can suffer from ageing and accuracy may vary consequently especially after prolonged exposure to temperatures at the extremities of their useful operating range. Listed below are some of the more common problems and pitfalls to be aware of.
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.
To minimise thermal shunting and improve response times, thermocouples are made of thin wire (in the case of platinum types cost is also a consideration). This can cause the thermocouple to have a high resistance which can make it sensitive to noise and can also cause errors due to the input impedance of the measuring instrument. A typical exposed junction thermocouple with 32AWG wire (0.25mm diameter) will have a resistance of about 15 ohms / meter. If thermocouples with thin leads or long cables are needed, it is worth keeping the thermocouple leads short and then using thermocouple extension wire (which is much thicker, so has a lower resistance) to run between the thermocouple and measuring instrument. It is always a good precaution to measure the resistance of your thermocouple before use.
Decalibration 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.
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.
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.
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.