2-D Thermographic Analysis
Adding another simultaneous geometric dimension to the linescanning concept leads to the practices of two-dimensional thermographic analysis. Two-dimensional figures are planes exhibiting only length and width but no height. The two-dimensional plane in question is the view you see with your own eyes. The thermographic equipment in question is an infrared camera comparable in size to a video camera (Figure 6-3).
|Figure 6-3: 2-D Thermographic Camera
Devices used for thermographic analysis generally fall into two broad classes. Radiometry devices are used for precise temperature measurement. The second, called viewing devices, are not designed for quantitative measurements but rather for qualitative comparisons. A viewer only tells you that one object is warmer than another, whereas radiometry tells you that one object is, for instance, 25.4 degrees warmer than the other with an accuracy in the neighborhood of 2 degrees or 2 percent.
Whereas a standard video camera responds to visible light radiating from the object in view, thermographic units responds to the object's infrared radiation. The scene through the camera's viewfinder is presented in false colors designed to convey temperature information.
Specular surfaces, especially metallic ones, reflect infrared radiation. The image of a shiny metal surface viewed through an infrared camera contains thermal information inherent to and radiated by the surface as well as thermal information about the surroundings reflected by the surface. When monitoring the temperature of a transparent object, the optical system may pick up a third source of radiation--that transmitted through from objects on the other side. Modern imagers have emissivity controls that adjust the response of the unit so that it reads accurately. (See p. 72 for emissivity tables of common materials.)
The compact size of thermographic imagers eliminates the need for tripods and other factors that limit mobility. In fact, in an industrial setting, the technician using the hand-held imager may well be reading temperatures or capturing images while walking around. Doing so can be dangerous since the user's attention is on data collection and not on environmental trip hazards. For that reason, imagers do not have the typical "eyepiece" found on a video cameras. Imagers use, instead, a 4-inch flat panel display, typically a color liquid crystal display.
The earliest thermal imaging systems featured a single detector and a spinning mirror that scanned the image coming through the lens of the camera and focused the pixels of the two-dimensional image on the detector in sequence. The electronics that captures data is synchronized with the mirror so that no thermal information is lost or garbled. One of the problems with the single detector approach is dwell time. Scanning a 120 x 120 pixel image with a spinning mirror does not give any single pixel very much time to register a reading on the detector.
The newest thermal imaging systems eliminate the need for the spinning mirror by replacing the single point detector with a solid-state detector that continuously "stares" at the entire image coming through the lens. Dwell time is no longer an issue because the scene coming through the optics maps directly on the active surface of the focal plane array detector. Using the focal plan array technology brings several benefits to the user.
The most obvious benefit is fewer moving parts in the camera. Fewer parts leads to higher reliability and almost certainly higher durability against physical abuse and other hazards of the workplace. The newer thermal imagers are smaller and lighter than their predecessors. In fact, the latest infrared imagers are of a size not much larger than the smallest of the modern hand-held video camera.
The resolution of the focal plan array--a minimum of 320 x 244 pixels--is much greater than that offered by the single detector models. A finer resolution leads directly to being able to discern smaller "hot spots" in the field of view.
The detector in either type of imager must be cooled if it is to work properly. This is analogous to looking out of a window at night. If the room lights are on, it is difficult to see clearly because there is too much visible light coming from the room itself. Turning the lights off makes it much easier to see outside. Similarly, accurately measuring the temperature can be difficult if the camera parts surrounding the detector are giving off too much infrared radiation. A cooler detector is equivalent to turning off the lights in the room.
Infrared imaging technology relies on a refrigerated detector. The earliest thermography cameras used liquefied gases to cool the detector. Certainly the technology was new and the operating refinements were crude. As you can imagine, the early units were not all that portable. The key element of contemporary radiometers is a sufficiently small, battery-operated Stirling cycle engine to keep the detectors cold.
There are two common methods for cooling the detector chip. A Stirling cycle engine provides the cryogenic cooling required by precise radiometry devices. Thermoelectric cooling provides the temperature stabilization required by a viewer. In cryogenic cooling, the detector is chilled to around -200°C, whereas temperature stabilization requires cooling the detector to somewhere near room temperature.
Depending on the design of the detector, the stabilization temperature may be in the range of 20 to 30C or it may be at the Curie temperature in the range of 45-60°C. Operating at the Curie temperature offers better sensitivity to the incoming infrared radiation. In either case, the temperatures must be constant from reading to reading if the imager is to provide consistent and reproducible results. As an aside, in common industry parlance, units that rely on thermoelectric cooling are referred to as "uncooled" units relative to cryogenically cooled devices.
The Stirling Engine
In 1816, Robert Stirling developed what is called in thermodynamic terms a closed-cycle regenerable external combustion engine. This machine produces no waste exhaust gas, uses a diversity of heat sources to power it, remains quiet during operation, and has a high theoretical thermal efficiency. The Stirling cycle consists of four steps: heating at constant volume, isothermal expansion, cooling at constant volume, and isothermal compression (Figure 6-4). The device converts heat into its equivalent amount of mechanical work. In effect, one obtains shaft work by heating the engine. Fortunately, the Stirling cycle is a reversible thermodynamic process, and mechanical work can be used to produce a cooling effect.
|Figure 6-4: The Stirling Cycle
This need for cooling is a limitation only in the sense that one cannot achieve true "instant on" with an infrared imager. Cryogenic cooling requires five to nine minutes to achieve the very low temperatures required for the detector to respond. Thermoelectric cooling, on the other hand takes about one minute or less.
Other Detector Developments
Soon, researchers are expected to produce less expensive uncooled radiometric detectors with sensitivity and resolution at least as good as today's cryogenic units. One of the problems to be surmounted is the relatively low yields in the detector manufacturing process. Until the production problems find a solution, the cost of the high-performance uncooled detectors will remain on a par with cryogenic units and their Stirling engines.
There is one true "instant on" unit that depends on pyroelectric arrays for the detector elements. These imagers require absolutely no cooling but they require a constantly changing image signal. If the scene presented to the lens does not change, the camera ceases to resolve any image at all. Imagers based on the pyroelectric principle are suitable for viewer use only, not radiometry. However, it is possible to use a point detector aligned with the centerline of the image to record one temperature to represent the entire frame. Because the pyroelectric element is piezoelectric, it generates an extraneous signal in response to vibration of the camera housing. Such sensitivity requires that the units be shock mounted to damp out the vibrations.
Spatial, Temperature Resolution
There are two types of thermographic detector resolution to be distinguished. First is spatial resolution. The detector assemblies in focal plane arrays have multiple detector elements on a single detector chip that map directly to the aperture of the optical system. High spatial resolution means the camera can distinguish between two closely spaced items. Temperature resolution refers to the ability of the camera to distinguish temperature differences between two items. Temperature resolution is a function of the type of detector element; spatial resolution is a function of the number of detector elements.
|Figure 6-5: Spatial Resolution of a Thermographic Camera
Specification sheets for infrared imagers give the spatial resolution in terms of milliradians of solid angle (Figure 6-5). The milliradian value is related to the theoretical object area covered by one pixel in the instantaneous field of view. Obviously, at greater distances more of the object area maps to a single pixel and a larger area means less precise thermal information about any single element in that larger area.
Applications of Thermography
An effective predictive maintenance program implies the need to collect sometimes rather sophisticated data from productive assets located around the plant floor. A plant maintenance technician normally follows a predetermined route and visiting plant assets in a specific sequence. This makes data collection as efficient as possible. At each asset, the technician collects data from discrete sensors while working from a check list so as not to miss a reading. In the case of thermography, the data consists of one or more images of the relevant machine parts. A bare bones infrared camera is nothing more than a data collector. Raw thermal data is of little value; it is what one does with the data that makes the difference.
After the thermographic data collection is complete, the technician or the data analyst evaluates the images for evidence of thermal anomalies that indicates a need for either scheduled or immediate repair. If maintenance work is warranted, the analyst prepares a report both to justify to others the need to spend money for repairs and to retain as part of the permanent records for the given plant asset.
Framing the Image
A certain level of skill and experience aided by common sense is a prime requisite in gathering and analyzing thermographic data. The technician using the imager in the field must be aware of reflections of irrelevant heat sources that appear to be coming from the object being scanned. Physically moving the imager to the left or right could make a dramatic difference in the apparent temperature of the object. The difference is caused by the reflections of shop floor lighting fixtures, sunlight through windows, and other extraneous sources.
As with taking snapshots with a single lens reflex camera, framing the scene is somewhat of an art. If the object is in the path of the heated or cooled air issuing from an HVAC system, the readings will be skewed. It may make more sense to return after sundown or to shut down the HVAC system to gather meaningful data. Analysts need common sense, as well. Modern cameras offer resolutions of a fraction of a degree.
Data Analysis Tools
Some thermal imagers work in conjunction with on-board microprocessors and specialized software that give the user the ability to quickly prepare diagnostic reports. In fact, downloading the field data to a desktop PC frees the technician to continue gathering data while the analyst prepares a report. But, report preparation is not the only enhancement to standard infrared picture.
Some thermal imagers simplify work for the user by making it possible to annotate thermal images with voice messages stored digitally with the digital image itself. Some units automate the process of setting the controls to permit the camera to capture the best, most information-rich thermal image as long as the view is in clear focus.
Watching a part "age" may be of great value to the plant maintenance department in predicting the expected failure date for a machine component. Being able to trend thermal data as a function of time is one way to watch the aging process. The software package that processes the thermal data from the camera can produce a graph of the time-series temperature data corresponding to the same point in the infrared image of the plant asset. The analyst simply moves a spot meter to the part of the image to be trended and the temperature data for that spot links to a spreadsheet cell.
These enhancements to the basic thermographic technology continue to make the IR imagers easier to use. With a few hours of training, even a novice can generate excellent thermal scans that capture all the thermal information present in a given scene.
Electrical wiring involves many discrete physical connections between cables and various connectors, and between connectors and mounting studs on equipment. The hallmark of a high-quality electrical connection is very low electrical resistance between the items joined by the connection. Continued electrical efficiency depends on this low contact resistance.
Passing a current through an electrical resistor of any sort dissipates some of the electrical power. The dissipated power manifests itself as heat. If the quality of the connection degrades, it becomes, in effect, an energy dissipating device as its electrical resistance increases. With increased resistance, the connector or joint exhibits a phenomenon called ohmic heating. Electricians and maintenance technicians use the thermographic camera to locate these hot spots in electrical panels and wiring. The heated electrical components appear as bright spots on a thermogram of the electrical panel.
Three-phase electrical equipment connects to the power supply through three wires. The current through each wire of the circuit should be equal in magnitude. However, it is possible to have an unbalance in the phases. In this case, the current in one of the phases differs significantly from the others. Consequently, there exists a temperature difference among the three connections. Thermographic cameras can illustrate this imbalance quite easily and dramatically. Consider, for a moment, the ease with which a thermographer can inspect overhead electrical connections or pole-mounted transformers from a remote, safe place on the ground.
Thermography also finds use in inspecting the building envelope. It can locate sections of wall that have insufficient insulation. It can also spot differences in temperatures that indicate air leaks around window and door frames. Thermal imaging is useful for inspecting roofs as well.
If a defect in the outermost roof membrane admits rain water that gets trapped between the layers making up the roof, the thermal conductivity of the waterlogged section of roof is greater than that of the surrounding areas. Because the thermal conductivity differs, so does the temperature of the outer roof membrane. An infrared camera can easily detect such roof problems. A thermal scan of the roof and a can of spray paint is all that is needed to identify possible roof defects for a roofing contractor to repair.
Because thermography is a non-contact measurement method, it makes possible the inspection of mechanical systems and components in real time without shutting down the underlying production line.
Energy constitutes a major cost in most manufacturing plants. Every wasted BTU represents a drain on plant profitability. Thermography lends itself to eliminating the energy loss related to excessive steam consumption and defective steam traps. If steam is leaking through a steam trap, it heats the downstream condensate return piping. The heated section of piping is clearly visible to an infrared imager.
Heat loss to the surrounding environment is a function of temperature of the inside temperature. The heat loss increases nonlinearly with increased temperature because radiant losses can easily exceed convective and conductive losses at higher temperatures. For example, the refractory block installed inside of a kiln, boiler, or furnace is intended to minimize heat loss to the environment. Thermography can quickly locate any refractory defects. Another application for the technology is a blast furnace, with its massive amount of refractory.
The location of the blockage in a plugged or frozen product transfer line can sometimes be detected with thermography. If the level indicator on a storage tank fails, thermography can reveal the level of the inventory in the tank.
Thermography finds further use in the inspection of concrete bridge decks and other paved surfaces. The defects in question are voids and delamination in and among the various layers of paving materials. The air or water contained within the interlaminar spaces of the pavement slab affects its overall thermal conductivity. The IR imager can detect these defects.
Painted surfaces become multilayer composites when a bridge or storage tank has been repainted numerous times during its service life. Here, too, the possibility of hidden rust, blistering, cracking, and other delamination defects between adjacent paint layers make objective visual inspection difficult. A technique called transient thermography returns objectivity to the evaluation of a potentially costly repainting project.
Transient thermography entails using a pulse of thermal energy, supplied by heat lamps, hot air blowers, engine exhaust, or some similar source of energy, to heat the surface from behind for a short period of time. Because the imager detects temperature differentials of less than a degree, voids and delaminations become readily apparent.
Forestry departments use thermography to monitor the scope and range of forest fires to most efficiently deploy the valuable, limited, urgently needed resources of manpower and fire-retardant chemicals.
Corporate research and development rely on radiometric thermography, as well. Auto makers use the technology to optimize the performance of windshield defrosting systems and rear window defoggers. Semiconductor manufacturers use it to analyze operational failures in computer chips.
Enter the Microprocessor
Microprocessors and the software behind thermography units are important to the versatility of the technology. Digital control and high-speed communication links give thermographic devices the interconnectivity and signal processing expected in a digital manufacturing environment. For example, the linescanner output can be subdivided into several segments or zones, each corresponding to a portion of the width of the moving web. Each of these zones can provide individual 4-20 mA control and alarm signals to the process machinery.
Because the thermal data is digitized, it is easy to store the optimum thermograph for use as a standard of comparison. This standard thermal image--the golden image-- is used to simplify process setup, a feature especially valuable when changing products in the processing line.
Contemporary thermographic units use 12-bit dynamic range architecture. This is the practical minimum if radiometry is to capture all the thermal information that the scene contains. It allows the analyst to position a set of crosshairs on a single pixel and determine the precise temperature that it represents.
The microprocessor also makes interpreting the thermogram easier. The analyst can spread the color palette across the full range of temperatures represented by the thermogram. For instance, when inspecting a roof in July, the temperature of every point in the scene is high and the difference in temperature between sound areas and defects is relative small, perhaps 20 degrees. On the other hand, production process may well mean that parts of the scene are 250 degrees (or more) warmer than the background objects. In either case, the analyst can spread 256 colors across the 20-degree and the 250-degree range in the scene to generate a usable picture.
||References and Further Reading
||Applications of Thermal Imaging, S.G. Burnay, T.L. Williams, and C.H. Jones (editor), Adam Higler, 1988.
||Infrared Thermography, (Microwave Technology, Vol. 5), G. Gaussorgues and S. Chomet (translator), Chapman & Hall, 1994.
||An International Conference on Thermal Infrared Sensing for Diagnostics and Control, (Thermosene Vii), Andronicos G. Kantsios (editor), SPIE, 1985.
||Nondestructive Evaluation of Materials by Infrared Thermography, Xavier P.V. Maldague, Springer Verlag, 1993.
||Practical Applications of Infrared Thermal Sensing and Imaging Equipment (Tutorial Texts in Optical Engineering, Vol. 13), Herbert Kaplan, SPIE, 1993
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