19 Temperature Measurement of Moving Bodies 19 .1 Introduction The method of measuring the temperature of moving bodies depends upon the character of the movement . For instance, in the flight of rockets, meteorological probes or satellites the only possible way is by wireless signal transmission . Another group of techniques, covering the temperature measurement of bodies in oscillatory or rotational movement will be discussed in detail in this chapter . As the simplest method of temperature measurement of rotating bodies uses pyrometric techniques this possibility should be the starting point of all analytical considerations . Pyrometric methods, irrespective of all the limitations resulting from the shape and state of the investigated surfaces, are best applied to surfaces which are clearly visible from outside with no obstruction in the line of sight of the pyrometer . Measuring the temperature at any particular point of a rotating body, which is not visible in this way, requires the fastening of a temperature sensor to the moving body . Its output signal is then transmitted to a stationary external measuring instrument . The majority of problems to be discussed in this chapter, concern the temperature measurement of machine parts, motors and other moving objects in a range from 20 to several hundred degrees Celsius . In this temperature range, temperature indicators, already described in Section 2 .5, are also very useful . 19 .2 Pyrometric Contactless Methods Pyrometric measurement principles are described in detail in Chapters 9 and 10 . In the temperature range mentioned above, low-temperature total radiation pyrometers and spectral and band photoelectric pyrometers may be used . It is also advisable to use two- wavelength or multi-wavelength pyrometers, especially for metallic surfaces . One of the main difficulties, which is due to the low emissivities of metallic surfaces, may be remedied in some cases by covering the investigated surface with a thin layer of black varnish or teflon (c ; t 0 .9) (Heitronics, 1994) . Temperature Measurement Second Edition L. Michalski, K. Eckersdorf, J. Kucharski, J. McGhee Copyright © 2001 John Wiley & Sons Ltd ISBNs: 0-471-86779-9 (Hardback); 0-470-84613-5 (Electronic) 388 TEMPERATURE MEASUREMENT OF MOVING BODIES Another source of errors may be any radiation from other sources, reflected by the surface whose temperature is to be measured . Here again a remedy may be either the application of radiation shields or the use of a pyrometer of properly chosen spectral sensitivity . Depending on the thermal inertia, or lag, of the pyrometer and on the rotational speed of the investigated body, the pyrometer used can indicate either the average value of the temperature or it can follow the variations in the temperature round the body's periphery . In most photoelectric pyrometers, the inertia of the associated instrument is much higher than that of the pyrometer itself . Analogue instruments or potentiometers indicate average values while the digits of digital instruments change so quickly that they cannot be read . In this last case the use a peak-picking device, as described in Chapter 12, is advised . If the access to the measured body is too difficult or not directly visible from outside, fibre optic photoelectric pyrometers are used as given in Section 10 .2 . Infrared imaging is also useful for detecting overheated points in investigated installations, which is very often, the main interest of the operator . Further details of IR imaging are given in Chapter 14 . Special dedicated pyrometers can also be used . For example for temperature measurement of the cooled blades ofa gas turbine, Land Infrared Ltd . (Amory, 1997) offers a TBTMS system (Turbine Blade Temperature Measurement System), which uses a photoelectric pyrometer equippedwith special low inertia detectors . TBTMS permits : " continuous monitoring of blade coating condition, " improvement in the conditions of operation and control of a turbine, " detection of incorrectly cooled blades, " early warning of too high temperatures . In the system, up to 32 pyrometers are engine mounted to view the rotating blades directly through a pressure proof sight glass . The optical signals, which are collected by the pyrometers, are transmitted through flexible fibre optics to a remote electronic signal conditioner with a 4-20 mA output . Even after 10 000 hrs operation, experience at various plants has shown that the optical components were still clean . A microprocessor, which can accommodate up to 32 scan signals, comprising 3000 temperature data readings per revolution around the blade array, is used to handle the sensor data associated with : " peak blade temperatures, " average blade temperatures, " average peak values . For bodies with either rotational or oscillatory movement, Chen et al (1997) describe a pyrometric system to measure their temperatures . The radiation from the pyrometer field of view, which operates at Ae = 0 .8-1 .2 pm, is concentrated on a linear array of 20 Si photodiodes of dimensions 20x5 mm . Although all of the diodes are irradiated when the whole field of view is filled by the target surface, correct readings may also be obtained when as little as 120 of the field of view is covered . The output signals of the diodes, which are all corrected to be equal, are amplified and passed to a microprocessor controlled commutator . Only the strongest of all the signals, which corresponds to the target SLIDING CONTACT METHOD 389 temperature, is considered . The differences in readings were below ±l % for all diodes and for only one diode, being irradiated . 19 .3 Sliding Contact Method Mounting contact sensors on rotating machine parts is a popular method of temperature measurement of rotating machine parts . The acquired electrical signals are transmitted to a nearby stationary read-out instruments, using sliding contact systems . Thermocouple sensors with associated slip-rings, which is the most popular scheme as shown in Figure 19 .1, are connected into the electric circuit given in Figure 19 .2 . Inserting of slip- ring/brush arrangement into the thermoelectric circuit, following the law of the third metal as described in Chapter 3, does not alter the equivalent thermoelectric force, provided that the junction points of thermocouple conductors, 92, and of the rings and brushes, 04, are at the same temperature . However, this condition is not easily satisfied at the ring-brush interface because of the friction heat generated . Consequently the precision of the temperature measurement is rather low . Additional errors arise from parasitic thermoelectric forces, which are generated at the slip-ring/brush interface (Weiss, 1961, Chavernoz, 1966) . The common materials used for slip-ring and brush, as well as the values of the corresponding parasitic emfs are given in Table 19 .1 . Metal/meta l sets, such as silver/silver, are sometimes used . However, they are of no use for continuous measurements since they quickly wear out . To account for changes in resistances at the slip-ring/brush interface, it is advisable to use potentiometric read-out instruments . Because of the high contact resistance of sliding contacts high resistance sensors, such as thermistors or thin film Pt-RTD units, are more convenient . Thermistors with a graphite-copper set, give overall measuring errors below 1 °C . Michalski et al (1991) give more information . THERMOCOUPLE SLIP-RINGS POTENTIOMETER Acs . ___ES B ~ _ _ I I _A'___ __ Cu BRUSH ~ I Cu Figure 19 .1 Temperature measurement of a rotating body using a thermocouple and slip rings THERMOCOUPLE SLIP BRUSHES COMPENSATING POTENTIOMETER RINGS CABLES g C D B' Cu _ _ C i i D A ^ ~ Cu 4 z ' 9 3 4 4 ' ~9 0 Figure 19 .2 Equivalent electrical circuit for the arrangement in Figure 19 .1 390 TEMPERATURE MEASUREMENT OF MOVING BODIES Table 19 .1 Parasitic voltage, E, across two 6x6 nun brushes and a slip-ring at a peripheral speed of 0 .35 m/s (Baker et al, 1961) Slip-ring Brush Conta ct force (N) E (py) Silver Graphite 50 0 .3 Silver Graphite-silver 50 0 .3-2 .5 Rhodium Graphite 30 0 .8 Rhodium Graphite-silver 60 0 .5 Gold Graphite 40 0 .6 Copper oxid i sed Graphite 50 1-11 Good performance is obtained using mercury contacts . A two-terminal mercury sealed, miniature slip-ring assembly SR-2 for thermocouples by Omega Engineering (1999), has the following specifications : " dimensions : dxl = 16x20 mm, " sealed ball-bearing alignment, " speed : 0-2000 rpm in either direction, " operation in any position, " transmitted voltage : 1 pV to 120 V, " transmitted current : 1 pA to 1 A, " frequency : d .c . to 10 MHz, " ambient temperature : -25 to 70 °C . A similar assembly SRTC also produced by Omega Engineering of dimensions of, dxl = 40xl40 mm, and also mounted on sealed ball-bearings has the following specifications for different models : " maximum rotational speed : 200 to 1800 rpm, " operation in any position, " ambient temperature : -29 to 70 °C, " torque : 200 to 1000 gem, " contact resistance - static and dynamic : I mS2, " intended for thermocouples and resistance sensors, " special types for J, K, T and E thermocouple with compensating leads, " number of contacts : 2 to 8 . 19 .4 Inductive Circuits Inductive circuits, which were introduced by Keinath (1934), can be used to avoid any problems arising from the application of sliding contacts, especially at higher rotational speed . Signal transmission from a thermocouple, rotating together with the body under measurement, can occur as in the axial field arrangement of Figure 19 .3(a) or radial field scheme of Figure 19 .3(b) . In the axial case, the current in the axial coil of Figure 19 .3(a) WIRELESS SYSTEMS 391 (a) AXIALMAGNETIC (b) RADIAL MAGNETIC FIELD FIELD ROTATING SENDER COIL t-T HALL GENERATOR STATIONARY RECEIVER COIL Figure 19 .3 Inductive signal transmission from rotating bodies which is proportional to measured temperature, generates a constant magnetic field, whose strength may be measured by a Hall generator . The radial field, applied by the rotating coil in Figure 19 .3(b) induces an alternating voltage in the stationary receiver coil, whose value is proportional to the measured temperature . Both of these methods are not precise, because the transmitted signal depends upon the air gap dimension, the core saturation and upon the rotational speed of the shaft in the radial arrangement . Inductive compensating circuits may also be used with the above methods . In the axial arrangement at the moment of measurement, the magnetic field, excited by the rotating sender coil, is compensated by a contrary directed magnetic field induced by a stationary coil . The state of full compensation, which corresponds to zero magnetic field strength in the air gap, is detected by a special probe, most commonly a Hall generator in the axial field arrangement . In the radial field arrangement a flat coil, connected to an oscilloscope, is used as a zero detector . Both of these arrangements, which have to be adjusted manually, can only be used for spot measurements . A similar but automatic system, described by Weiss (1961), can be used for continuous measurements with an error below about ±2 °C . More information on inductive circuits can be found in Michalski et al (1991) . 19 .5 Wireless Systems In the temperature measurement of rotating or moving bodies transmission of the measuring signals is frequently only possible with wireless systems . The whole combination of sensor, transmitter, batteries and antenna are attached to the moving body, while the signal receiver and the recorder are stationary . The main problems in the design of this type of rotating transmitting arrangement, are their necessarily small dimensions and their robustness against the very large acceleration forces, which can even amount to 300 N . Thermocouple and thermistor sensors are used exclusively with IC amplifiers and transmitters . A typical arrangement whose principle is described by Adler (1971) is shown in Figure 19 .4 . It uses a thermistor sensor, whose temperature dependent variations of resistance, R T , change the frequency of a relaxation oscillator in accordance with RTC+K (19 .1) where R T is the thermistor resistance, C is the capacitance, and K is a constant . 392 TEMPERATURE MEASUREMENT OF MOVING BODIES 160 kHz STATIONARY POWER SUPPLY FEEDING COIL _L, .~_~______ ROTATING RECEIVER COIL ANTENNA THERMISTOR Rr LEE RELAXATION OSCILLATOR H . F . GENERATOR Figure 19 .4 Wireless system of signal transmissions from moving bodies with a thermistor sensor The output signal of the oscillator which modulates the 90 MHz carrier frequency of a HF generator coupled to the antenna, is linearised in the receiver . In the temperature range from 0 to 150 °C, the measuring error of the system which is about ±0 .5 °C, is insensitive to supply voltage variations of about ±25 % . The batteries, which can operate at ambient temperatures up to 150 °C, last for about 50 to 200h of continuous operation . They contribute to a total volume of transmitter and batteries of some few cubic centimetres . As the battery lifetime is limited, it is advisable to power the system using the inductive feeding scheme shown in Figure 19 .4 at higher acceleration . A stationary sender coil generates an alternating magnetic field of 160 kHz which induces a high frequency signal in the receiver- coil mounted with the rotating transmitter . Subsequently the induced high frequency signal is rectified and stabilised . One sender-coil can consecutively supply several rotating measuring arrangements with the distance between both coils even up to 25 mm . If thermocouple sensors are used, transistorised FET converters are most commonly used . These operate at a frequency of 2 to 4 kHz, with output signals, which supply an integrated high frequency amplifier, of frequency about 100 MHz . The system operates on the basis of amplitude modulation and its output stage on frequency modulation . Adler (1971) points out that such a system gives a lower noise level than systems with double frequency modulation . Some disturbances and additional frequency modulation may occur owing to variations in the distance between the rotating transmitter and stationary metal masses, which can be caused by changing parasitic capacitances . Transmitter shielding with the application of an additional amplifier separating the antenna from the environment, are suitable precautions to reduce these effects . 19 .6 Friction Sensors and `Quasi-Contactless' Method Surface temperature measurement of smooth metallic cylinders, which is typically necessary in paper making machines, plastics processing and also in the rubber and textile industries, can be measured by friction sensors pressed to the surface of slowly rotating cylinders . These are usually bow-band or convex-band thermocouples, which are illustrated in Figures 16.15 and 16 .17 . Measuring errors caused by heat conduction, are compensated by an amount of generated friction heat depending on the peripheral speed, the applied force FRICTION SENSORS AND `QUASI-CONTACTLESS' METHOD 393 and the surface state . Figure 19 .5, shows the true cylinder temperature, 9 t , and the sensor temperature, 3r, as a function of cylinder peripheral speed for different contact loads P, for a bow-band sensor . With a sufficiently high contact force it is seen that, at one given peripheral speed (point A), compensation of both influences happens at one and only one measured temperature . Therefore, it is rather difficult to base any correction of the readings on these phenomena (Kriiger, 1959) . Omega Engineering (1999) produces friction temperature sensors, for temperature measurement of rotating cylinders, equipped with roll-bearings rolling on the cylinder surface . Such a construction ensures that the contact force of the sensor is constant . An interesting development of contact sensors, which is similar in design to a band- thermocouple and produced by Heitronics GmbH (1994), is the so called emissivity converter . An elastic teflon band, when pressed to the rotating cylinder surface, adopts the surface temperature . The pyrometer, which is directed at the other side of the band, measures the temperature at a constant and well defined emissivity, c= 0 .95, which is set at the indicating instrument . Application of the device is permissible up to about 250 °C and up to peripheral speeds of 10 m/s . Thelow friction coefficient of teflon eliminates the generation of friction heat . The quasi-contactless method, described in Section 16 .4, which allows more precise measurements, is typically used in the temperature measurement of the surfaces of rotating cylinders . A thermocouple, or RTD encapsulated in a thin plate placed near the investigated surface, is heated by radiation, convection and conduction . To reduce its heat losses to the environment, either thermal insulation, as in Figure 19 .6(a), or concentration mirror, as in Figure 19 .6(b), are used . The measuring errors depend upon the distance, h, from the surface, as in Figure 19 .6(a), and the peripheral speed and temperature of the surface . This dependence, at ,9 t = 120 °C, is shown in Figure 19 .7 . The errors, which increase with increasing air gap as a result of the temperature drop across the surface air film, also increase with increasing peripheral speed, as then more cool air is sucked into the air-gap . Similar dependence of errors on air-gap and peripheral speed are also exhibited by those sensors with a concentrating mirror depicted in Figure 19 .6(b) . For the temperature measurement of moving cylindrical and flat surfaces, Weichert (1976), describes the use of thermovirbulators, which intensify the convective heat transfer between the surface and the sensor using a blower . This method is also described by Fothergill (1975) . 75 3 =f(V1 - P=500g A If - (V) Cr 70 's T =f(V) P=250 9 a f 65 0 1 2 3 4 5 6 7 PERIPHERAL SPEED V, mls Figure 19 .5 True cylinder temperature, 9c, and temperature, .9r, indicated by a bow-band friction sensor versus peripheral cylindrical speed . A is the point of error compensation . L wry- " -, i i movie OTHER METHODS 395 large, investment in a system for the early detection of overheated points is beneficial . Thus the avoidance of such losses is achieved by elimination of their sources, which is due to faulty or inadequate operation of the cooling system in the majority of cases . While the estimation of the average rotor winding temperature is possible by the measurement of its resistance, the detection of hot points can only be done using a fluorescent fibre optic thermometer (Wickersheim and Sun, 1985) . In the system described above (Mannik and Brown, 1992), the two types of fluorescent compounds investigated were Gd 2 0 2 S,0 .5 %Eu and Y 2 0 2 S,1 %Eu, which were mixed with a binder and deposited on the investigated rotor area . The compound is irradiated through a light guide, as shown in Figure 19 .8, by the ultraviolet radiation of a nitrogen laser generating energy pulses of over 150 pJ at a repetition frequency of 20 Hz . The fluorescent emission detector system consists of a second light guide, with a filter of Ae = 514 run, which is connected to a photo-multiplier . The scheme allows measurement of the 1/e decay time, which is a function of measured temperature, with values of 50 ps at 60 °C to 2 bus at 150 °C . A microprocessor was used for data logging and conditioning, as well as for synchronisation and triggering of the laser pulses . During the measurements, a photodiode was excited by a marker placed on the other side of the rotating disk, releasing stroboscopic pulses . The microprocessor was linked to a PC to determine which points on the circumference should be investigated as well as their succession . Up to 24 measuring points round the circumference were possible . Further investigations concerned the durability of the lens materials and of the binder in the hydrogen cooled generator atmosphere, in temperature range from 20 to 150'C . Polyurethane resin was found to be the best binder for the fluorescent compounds . The achieved precision of rotor temperature measurements, was about+_ 2 °C . A similar type of a fluorescent fibre optic thermometer for temperature measurement of the rotor of a gas turbine is described by Noel et al . (1992) . The fluorescent compounds used were YV04,Eu, Y203,Eu and YAG,Tb, covering the temperature range of 450 to 1300 °C . After grinding these compounds, to be excited by ultraviolet radiation, were sprayed at or deposited on the investigated surfaces by an electron gun in a layer thickness of 5 to 35 pin . The main problem in operating this system appeared to be the poor durability of the deposited photo-luminescent layers . More details are given by Noel et al (1992) . TIGHT LIGHT GUIDE PHOTOMULTIPLIER PASSAGE LIGHT GUIDES GENERATOR MICRO- _ ~ - PROCESSOR LASER ~ _ - LIGHT GUIDES STATOR ROTOR FLUORESCENT I STATOR MATERIAL Figure 19 .8 Fibre optic thermometer for temperature measurement of a turbo-generator rotor (Mannik and Brown, 1992) 396 TEMPERATURE MEASUREMENT OF MOVING BODIES 19 .8 References Adler, A . (1971) Transmission des signaux 6lectriques des jauges de contrainte et thermocouples par radio-t616mesure . Mesures et Controle Industr ., 36(1/2), . 72-77 . Amory, D . (1997) Turbine Blade Temperature Measurement, Proc . TEMPMEKO 96, 6th International Symposium on Temperature and Thermal Measurements in Industry and Science, Levrotto and Bella, Torino, 401-406 . Baker, H .D ., Ryder, E .A . and Baker, N .H . (1961) Temperature Measurement in Engineering, 2, John Wiley and Sons, New York . Braun, F . (1981) Messung von Oberflachentemperaturen mit Farbindikatoren an bewegten Korpem, Messen and Prufen, 17(9), 574-577 . Chavemoz, R . (1966) Transmetteur des signaux 6lectriques issues de pieces en rotation . Mesures, Regulation, Automatisme, 31(1), 73-76 . Chen, F ., Zhao, G and Zhao, H . (1997) A radiation pyrometer with silicon photodiode array for wobbling target . Proc . TEMPMEKO 96, 6th International Symposium on Temperature and Thermal Measurements in Industry and Science, Levrotto and Bella, Torino, 407-412 . Fothergill, R . (1975) Non-contact temperature measurement using forced air convection . Temperature Measurement, Conference Series No . 26, Institute of Physics, London, 1975, . 409-414 . Heitronics GmbH (1994) Catalog . KT15D . Keinath, G . (1934) Induktive Temperaturmessung . ATM, No . 1, V215-2 . Kruger, H . : (1959) Messung and Regelung der Oberflachentemperatur umlaufender Walzen VDI-Z, 101(9), 343-346 . Mannik, L . and Brown, S .K . (1992) Electrical industry applications of fiber optic thermometry : measurement of generator rotor temperatures . Temperature : Its Measurement and Control in Science and Industry, 6(2), . American Institute of Physics, New York, 1243-1248 . Michalski, L ., Eckersdorf, K and McGhee ; J . (1991) Temperature Measurement (1st ed) . John Wiley and Sons, Chichester . Noel, B .W ., Turley, W .D ., Lewis, W ., Tobin, K .W . and Beshears, D .L . (1992) Phosphor thermometry on turbine-engine blades and vanes . Temperature : Its Measurement and Control in Science and Industry, 6(2), American Institute of Physics, New York, 1249-1254 . Omega Engineering Inc . (1999), The Temperature Handbook . Weichert, L . (Editor) (1976) Temperaturmessung in der Technik, Grundlagen and Praxis, Lexica Verlag, Grafenau . Weiss, H . (1961) Ein Messgerat fur die Temperaturmessung mit Thermoelementen auf sehr schnell umlaufenden Maschinen . ETZ, 13(13), 353-357 . Wickersheim, K .A and Sun, M .H . (1985) Phosphors and Fiber Optics Remove Doubt from Difficult Temperature Measurements, Res . Dev . 27, 114 .