Imaging of Temperature Fields of Solids 14.1 Introduction Infrared thermal imaging oftemperature fields has become an extremely versatile and popular method ofreal time temperature measu
Trang 1Imaging of Temperature Fields
of Solids
14.1 Introduction
Infrared thermal imaging oftemperature fields has become an extremely versatile and popular method ofreal time temperature measurement and thermal condition monitoring in all industrial and research applications In thenearfuture, the rapidfurther development of the method is more than guaranteed, as it will replace many existing temperature measuring methods, offering exciting new opportunities with the added most important benefit ofa two-dimensional look at problems.
The first marketed infrared imagers were based on the use of an opto-mechanical scanning system to sequentially scan the target surface The resulting series of radiated image signals is transformed into a seriesofelectrical signals in an appropriate detector In the subsequent step in their development, the opto-mechanical scanning system was replaced by a matrixofmainly photoelectric detectors However, since the detectors had to
be cooled, the device was big and heavy The majority ofcontemporary imagers is based on
an arrayofun-cooled micro-bolometers, which can operate at ambient room temperature Infrared thermal imaging systems can be divided into two main groups In surfacial systems two-dimensional imaging oftemperature fields is produced using scanning and matrix systems In the second group, which may be referred to aslinear systems, the aim is
to produce a temperature field image of continuously cooled or heated moving targets These linear systems present a certain simplificationoftwo-dimensional systems.
14.2 Surfacial Systems
Opto-mechanical scanning infrared systems Among the first commercially available infrared systems were the two-dimensional opto-mechanical scanning systems, produced since 1965 by AGEMA Infrared Systems As shown in Figure 14.1 allofthe target surface points were scanned sequentially in these systems, which were appropriately called Thermovision systems Rotating or oscillating mirrors or prisms were used in the scanning system At a scanning frequency of 25Hz, a spatial resolution of 100 to 175 measuring elements per line is achievable DetectorsofInSb or (HgTe),, (CdTe)1_Xwhich are cooled either thermoelectrically or by liquid nitrogen, ensure a temperature resolutionof 0.1°C in
ISBNs: 0-471-86779-9 (Hardback); 0-470-84613-5 (Electronic)
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IR-DETECTOR w
LENS LENS VERTICAL HORIZONTAL
ROTATING ROTATING PRISM PRISM
Figure 14.1 Thermovision scanning system
a measuring range of 30 °C The measurement ranges are between -20 and +1500 °C having a sensitivity of 0.07 to 0 1 °C and accuracy of±2% or ±2 °C The resulting series of signals was transformed in the detector into electrical signals, displayed on a monitor screen
as a visible image of the temperature field, with a reference colour scale Chosen isotherms could also be marked Microprocessor based systems vastly extended the imaging possibilities by making the whole assembly a really universal and precise measuring tool Although this whole family of Thermovision systems is no longer produced, it is still widely used by their still numerous owners In spite of their high price they were of a high quality and commensurate robustness
Matrix infrared systems Surfacial systems, based on a Focal Plane Array (FPA) matrix, are now the most popular since they do not need any moving parts in the scanning system There are two groups of FPA In the first one photon detectors based on photovoltaic (HgCdTe and InSb) or photo-conductive (PbSe, PbS) cells are mainly used In the second the detectors are either thermopiles or micro-bolometers whose operation is based on the absorption of thermal energy The signals from particular detectors, corresponding to different target points, are scanned by a contactless commutator, amplified if needed before forming a colour picture on a screen
FPA systems formed by monolithic PtSi micro-bolometers are now the most popular, since they allow the integration of many detectors in one integrated matrix of 256x256 or even 320x240 pixels Moreover, these micro-bolometer FPAs have a considerable benefit since they do not require any cooling Compensation for small ambient temperature variations is achieved by temperature sensors inside the camera The size of each FPA pixel
is about 30 ~tm square (Santa Barbara Research Center)
The ThermaCAM PM 695 by FUR Systems (2001) shown in Figure 14.2, is a good technical example of a surfacial matrix system This imager is equipped with an uncooled MicroIR solid stated detector matrix, incorporating 320x240 pixel micro-bolometers, connected by a contactless commutator The coloured target picture on the view-finder or an LCD screen, which is obtained from the matrix, is quite stationary due to the non-existent inertia of the micro-bolometers and rapid commutation of the scanning electronics At the same time a colour-temperature scale is provided for use by the operator The new electronic techniques and processor speed provides rapid operation and operates in
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Figure 14.2 Infrared ThermaCAM PM 695 system (Courtesy ofFUR Systems)
wavelength range 7.5 to 13 0 pun, thus eliminating the influence of solar radiation on the readings The system is especially useful for detecting hot points in different technical installations Digital voice recording, which allows detailed annotations for each stored image to be entered, is also included in the system
The ThermaCAM PM 695 is the first infrared system having an integrated digital camera It also includes ThermaCAM Reporter software This enables the production of automatic reports, which include all field inspection data such as thermal, visual, measurement analysis as well as voice and text The operator can store at the touch of one button both infrared and visible images
The technical data ofThermaCAM PM 695 are as follows:
" field of view : 24°x 18° at a minimum distance of 0.5 m,
" detector: uncooled micro-bolometer MicroIR,
" spectral range: 7.5-13 pm,
" colour image in viewfinder, LCD screen or video output,
" temperature range: -40 to 120 °C or 0 to 500 °C with options up to 1500 °C or 2000 °C,
" accuracy: ±2 %ofrange or ±2 °C,
" possible correction oftarget emissivity,
" measurement functions:
- a cross-point which is movable over entire image,
- horizontal or vertical temperature profile with movable spot measurement,
- isotherms,
- automatic reading ofmax, min and average values in the view field,
- difference temperature relative to a reference temperature,
" weight with battery 2.4kg, size 220xl33xl4Omm,
" remote operation via RS 232 data output
Light infrared cameras Some producers offer these for applications, where they can replace much more expensive thermal imaging systems They are used for observing and recording thermal images ofstationary objects An example of such a camera is digiCAM-IR by Ircon Inc (1999) This camera is equipped with an array of uncooled thermopile detectors, giving
an image scan time below 1 5 s Image display is on 4.0 inch active matrix colour LCD
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screen, with RS232 communication and an accuracy within 2 °lo of reading A "Hot Spot Mode", which is unique to this camera, allows automatic production of an image concentrating on the hot spot of the image Four buttons are used by the operator to manoeuvre the cursor over the image displaying the corresponding temperature values The spectral range is 8 to 12 gm Up to 140 images can be stored on smart card The camera, which has an overall weight of 2.0 kg including the lens and battery, has a physical size of 240x 100x 130 mm
14.3 Linear Systems
Two-dimensional colour pictures of temperature fields of continuously heated moving charges is easily possible using the FPA matrix systems described in Section 14.2 However
it is much simpler to apply what are calledlinear systems. There is no need for electronic scanning of the second dimension since this is replaced by the charge movement as shown
in Figure 14.3
The Landscan Infrared Linescan System by Land Infrared (1997) is a representative example of this technique The system consists of a sensor head, performing the temperature measurement, and a dedicated Landscan software program for a PC or a dedicated LPU 1 microcomputer Ethernet capabilities, which are provided using the LPU 2E model of this system, can provide data over a network to a process control computer The sensing head in the six different models of the camera, has a Si or Ge detector, which is scanned by a rotating mirror to produce the line scanned image The scanning occurs along a line, perpendicular to the charge movement
For different models the technical data of the system are:
" temperature range: 70-350 °C to 800-1400 °C,
" wavelength range: 1 pm to 3-5 pm,
" scan angle: 60 °C,
" emissivity adjustable from 0.2 to 1 0 by internal switch or by 4 to 20 mA isolated input,
distance ratio 11d: 120/1 or 30011,
LINE SCANNER
SCANNING ANGLE
MOVING CHARGE SCANNING
LINE
Figure 14.3 Principle oflinear scanning of moving charges
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" detector time constant: 6 to 25 ps,
" dimensions : 430x230x165 mm,
" scan speed: 5 to 25 scans/s, having 10 000 points in one cycle,
" accuracy : ±5 °C,
" additional laser aiming,
output : up to 14 analogue outputs 0/4-20 mA
There are four basic display modes:
1 Two-dimensional colour picture (map) with a colour temperature scale
2 Temperature profile across the moving charge
3 Zones ofgiven temperature
4 Three-dimensional, colour temperature relief
As in the Land Landscan and Ircon ScanlR II systems, a pixel line with an output signal commutator could be used instead of one detector and a rotating mirror for scanning
14.4 Applications
The range of applications of thermal imaging systems is extremely large including the analysis of thermal problems, monitoring operation of industrial installations, investigation and optimisation of different prototypes and all types of research
Some typical applications ofsurfacial systems are:
" Monitoring and detection of hot points of moving or rotating equipment like bearings, gears, clutches, shafts, chains, transporters, pumps and blowers among others
" Detection of
- hot points in electrical installations such as in fuses, contactors, power networks, transformers, motors, isolators and cables
- overheated points in the thermal insulation of different furnaces, dryers, pipelines and boilers
- heat leaks in central heating installations and cold storage plants
- insufficient thermal insulation and moisture in buildings
- overheating in store houses, grain silos or dumps
" Temperature imaging of electronic circuits
" Tracing of steel reinforcements in concrete structures after their preliminary induction or resistance heating
" Checking the heating of car tyres while running
" Medical and veterinary diagnostics
" Surveying the temperature fields ofland and waters from the air
Linear System can be used for the following continuously heated charges:
" tape and plate rolling,
" heating in continuous furnaces,
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" rod and wire drawing,
" continuous casting,
" continuous induction heating,
" glass heat treatment, hardening and annealing,
" float glass manufacture,
" galvanising
14.5 References
FLIR Systems (1999) Catalog,ThermaCam PM 695
FUR News, (1999)Summer 1999
IRCON Inc (1999) Catalog,digiCam-IR
Land Infrared (1997)IndustrialNon-contact Temperature Measurement