METHOD AND APPARATUS FOR MEASURING THE TEMPERATURE OF AN ELECTRICALLY CONDUCTIVE MATERIAL

Abstract: The invention concerns a method and an apparatus for measuring the temperature of an electrically conductive material, said method being based on the measurement of the thermal noise of the material. In the method, the fluctuation of the magnetic field generated by the random motion of the charge carriers in the conductive material is measured using a resonant circuit (3) without contact with the material (1) under measurement. The apparatus comprises a sensor (5) provided with a resonant circuit (3), the reactance element of which is placed at a distance (s) from the material (1) whose temperature is to be measured, and a preamplifier (2), said sensor (5) being mounted inside a frame (20) made of a conductive material, such as copper.

Description: METHOD AND APPARATUS FOR MEASURING THE TEMPERATURE OF AN ELECTRICALLY CONDUCTIVE MATERIAL The present invention relates to a method for measuring the temperature of an electrically conductive material as defined in the introductory part of the independent claim. The invention- also relates to an apparatus for implementing the method. The method and apparatus allow the temperature of a hot, solid or molten conductive material to be measured without a contact between the measuring sensor and the object under measurement. High temperatures of several hundreds or thousands of degrees ( β C) are mainly measured using a pyrometer or a thermoelement. The former is based on the measurement of the thermal radiation emitted by the object, the latter on the thermoelectric effect. The radiation pyrometer is a device for con- tactless temperature measurement. It is capable of measuring the temperature from a fairly long distance from the object. A drawback with the radiation pyro¬ meter e.g. in measurements in the metallurgic industry is the fact that the emissivity of the radiating object depends, in addition to its temperature, also on the radiating material. Therefore, e.g. in the measurement of the temperature of a metal surface, an oxidic recre¬ ment layer on the surface or a layer of slag on the surface of molten metal may cause significant errors that are difficult to compensate. Similarly, smoke, dust, water vapour etc. cause errors. The thermoelement is commonly used especially for measuring the temperature of molten metals. This device is used for direct measurement, i.e. the thermo¬ element is sunk into the object under measurement. The problem is the aggressivity of the circumstances. The sensors used for the measurements are generally of a disposable design and the measurements therefore relate to specific points in the objects. For continuous measu¬ rements, it is necessary to use protective tubes, and these retard the transmission of heat, increasing the measuring time lag of the element. For practical reasons, the thermoelement is difficult to use for measuring the temperature of moving hot objects, e.g. casting or rolling blooms. Therefore, thermoelements attached to the object surface have only been used for research purposes. It is known that the noise in a thermally balanced electric circuit is directly proportional to the temperature. A general formulation.of this law is given in the dissipation theorem, which, when applied to an electric resistance, yields the so-called Nyguist theorem. A device for measuring noise temperature based on this theorem has been constructed. Using this device, the temperature can be realized by using the Bolzmann constant. This principle has been employed for accurate measurements of both low and very high temperatures. In these measurements, the noise is measured from a resis¬ tor thermally anchored on the object under measurement. However, the use of this method and device for tempera¬ ture measurements on hot and particularly moving objects is prevented by the fact that the connecting leads, contacts and the resistor itself are destroyed in high temperatures. The object of the present invention is to produce a new and improved method and apparatus for measuring the temperature of electrically conductive materials, allowing contactless measurement of the temperature of a body or an object. As for the features characteristic of the invention, reference is made to claim 1 regarding the method, and to claim 8 regarding the apparatus. The method of the invention is based on the measurement of thermal noise using a resonant circuit without contact with the material under measurement, especially a hot object. In this method, the fluctua¬ tion of the magnetic field generated by the random motion of the charge carriers in a metal or other con¬ ductive material is measured from a distance from the surface of the material. The method and apparatus of the invention have the following advantages. The measurement can be per¬ formed without contact with the object under measure¬ ment. Insulating materials, e.g. air, dust, water vapour and other impurities, will not disturb the measurement significantly. Therefore, the method and the apparatus are especially applicable in industrial measurements, e.g. for the measurement of the temperature of hot casting and rolling blooms. Continuous temperature measurement on moving blooms is also possible. The method also permits the measurement of the temperature of molten metals through an insulating layer of facing material. In closely controlled laboratory condidtionε, the measurement can be performed with a very high ac- curacy, so that the method can be used to obtain refer¬ ence temperatures for the calibration of thermoelements or pyrometers. In the following, the invention is described in detail by referring to the appended drawings, in which Fig. 1 shows a block diagram representing an embodiment of the apparatus of the invention; Fig. 2 represents the noise model of the apparatus of fig. 1; Fig. 3 shows a perspective view of an embodiment of the apparatus of the invention, partially sectioned; Fig. 4 shows a block diagram representing another em¬ bodiment of the apparatus of the invention together with a measuring unit; Fig. 5 illustrates the results obtained with the ap¬ paratus of the invention in experimental measurements on a steel object. On the basis of the Nyquist theorem, an elec¬ tric resistor in a state of thermal balance relative to its environment can supply a noise energy of k s T per band unit into a resistor at the absolute zero tempera- ture. This applies to low frequencies only, because at high frequencies the thermal energy is discharged into the environment in the form of quanta and the power transfer per band unit becomes frequency-dependent. This phenomenon only occurs at very high frequencies, so that, as far as the method and apparatus of the in¬ vention are concerned, the power is evenly distributed over different frequencies. Put simply, the Nyquist theorem means that every electric loss is associated with a thermal noise voltage, the magnitude of which depends on the temperature T, the magnitude of the re¬ sistance R and the width B N of the noise band associ¬ ated with the measurement. Correspondingly, the gen¬ eration of the noise can be described as an equivalent current generator connected in parallel with the resis- tance. In other words <eg > = 4k B TB N R (1) <i£ > = 4k B TB N /R (2) where k B is the Bolzmann constant (1.38 x io~= 3 /K) . When the energy associated with the quantum hf (h being the Planck constant and f the quantum frequency) ap¬ proaches the thermal energy k B T, equations (1) and (2) are no longer valid. In the apparatus of the invention in figs. 1 and 2, the losses of the object 1 under measurement are applied to the input of the amplifier 2 through a par¬ tially open resonant circuit 3. In this case, the re- sonant circuit 3 uses a coil 4. The inductance of the resonant circuit is L*., and its wasteful resistances are reduced to a parallel resistance R and a σorre- sponding noise current generator i . The parallel resis¬ tance Rsc represents the equivalent losses of the object 1 under measurement in the resonant circuit 3 , and i» c the current noise resulting from the losses. Since the conductive object 1 also influences the distribution of the external field, the resonant circuit inductance L depends on the distance ε between the coil 4 and the object 1. By using static protectors the inductance of the coil 4 can be regarded as only changing, consisting of a constant part L , and the change in the resonant frequency f 0 ( ω o) is represented by a variable induc¬ tance Lse. The amplifier noise in fig. 2 . is represented by current and voltage noise generators i-ret and e fet , respectively, connected to the output. The load capacity of the amplifier is represented by a resistance R*. The temperatures T x , Tic and * represent the temperature of the object under measurement, the coil noise temperature and the amplifier noise temperature, respectively. All capacitances of the circuit are included in the capaci- tor C, which is assumed to be independent of the dis¬ tance ε between the object 1 and the sensor 5, which in its simplest form consists of a coil 4 and an amplifier 5. It is to be noted that the effects of other losses in the coil 4 have not been considered in the model. After the amplifier 2, the noise can be detect¬ ed by means of a suitable detector or measuring unit 6 either directly or by mixing the noise with low frequen¬ cies and detecting it by means of a suitable detector circuit. In an embodiment of the method and apparatus of the invention, the noise is mixed down with a fre¬ quency corresponding to the resonant frequency of the resonant circuit 3. Fig. 4 illustrates such an apparatus. The measurement unit 6, which corresponds to the detector unit 6 in fig. 1, comprises a high-pass filter 7, a voltage-controlled oscillator 8, two mixers 9 and 10, high-pass filters 11 and 12, an adder 13, a r.m.s converter 14, a voltage amplifier 15 and current amplifiers or integrators 16 and 17. In addition, the sensor 5 is provided with a summing amplifier 18 and, if required, a coupling capacitor 19. In the measurement circuit 6, high-pass filter 7 is connected to mixers 9 and 10. The output terminals 8a and 8b of the voltage-controlled oscillator 8, pro¬ viding cosine and sine signals respectively, are con¬ nected to mixers 10 and 9, respectively. The outputs of the mixers 9, 10 are connected to high-pass filters 11, 12, and the outputs of theεe are connected to the in¬ puts of the adder 13, whose output is connected to the r.m.s converter 14, whose output provides the measure¬ ment signal U«» β . Output 8b of the voltage-controlled oscillator 8 is connected via amplifier 15 to the second input 18b of the summing amplifier 18. Output 8a of the voltage-controlled oscillator 8 is connected via the coupling capacitor 19 to the input of the amplifier 2 in the sensor 5. The output of amplifier 2 is connected to the first input 18a of the summing amplifier 18. The output of mixer 9 is connected via current amplifier 16 to the second input of amplifier 15, allowing control of the gain. The output of mixer 10 is connected via current amplifier 17 to the input of the voltage-con- trolled oscillator 8. In the apparatus illustrated by fig. 4, the measured noise is mixed down with a frequency corre¬ sponding to the resonant frequency - β = 27rfo of the resonant circuit 3. Due to interference, the mixed signal has to be filtered with a high-pass filter 7 with a response of Hm(j-). Taking the resonant circuit response and the filter 11,12 after the mixer 9,10 into account, the variance of the output voltage can be expressed as s i where the equivalent temperature T. of the resonant circuit can be expressed using the component tempera¬ tures as follows: τ e - E < -Ir + -k- + * kv > < 4 > The load resistance R of the resonant circuit is ob¬ tained from the equation. R-~- = (1/R . + 1/Rjc + l/R-c) (5) In equation (3) , ^ α iε the natural frequency R/L of the coil. The noise temperature of the amplifier is T_e for an optimal input impedance R So . Both T * and R £o are constants characteristic of the amplifier. When the resonant frequency • 0 and total im¬ pedance R of the resonant circuit 3 are measured, the temperature T x of the object 1 can be determined unam¬ biguously from equations (3) - (5) . If the coupling to the object 1 is very good R « R x , then T e « T * .. If it is additionally assumed that the amplifier has a low noise temperature, i.e. T.. << Tx, and that the bandwidth of the high-pass filter 11,12 after the mixer 9,10 is clearly larger than that of the resonant circuit 3, then equation (3) will be reduced to the following simple form <U a o> = A a k B T«/C (6) where A is the overall gain before detection and C is the capacitance of the resonant circuit. It can be shown that better results are achieved if a filter bandwidth H m (jω) close to the resonant circuit bandwidth iε se¬ lected, which means in practice that it is necessary to measure the impedance R and resonant frequency ^ 0 of the resonant circuit and to calculate T * . on the basis of these resultε. The essential point about the equations pre¬ sented above is that the detected voltage <U=> iε an unambiguouε function <U 0 2 > = f(T x ,R, ωo ) of the object temperature T x and the loaded impedance R and resonant frequency ^0 of the coil 4; in other words, object temperature T κ = g (<U 0 a >, R, - ) . Correction can be effected either by using equations (3) and (5) or by measuring the output voltage for different temperatures and from different distances and applying the results to a suitable mathematical model. The object temperature T x is determined by measuring the variance of the amplified voltage. The variance (inaccuracy of the temperature measurement) of the measured variance σ 2 depends on the measuring time : and the equivalent noise bandwidth B e in accordance with the following equation: Var [σ] = -§σ a --- (7) Thus, the relative resolution of the tempera¬ ture measurement is e = _/\_ T/T = (B e τ)-3. If the reso¬ nant circuit is tuned to a frequency of about 1 MHz, then it is possible to achieve a 100 kHz noise band, so that a 10-second meaεuring time leads to a resolution of 10 -3 . The final impedance is determined from the inaccuracy of the impedance measurement, the changes in the noise temperatures of the amplifier and the open coil and from posεible interference. In the apparatuε εhown in fig. 4, the reεonant circuit 3 receiveε a current via capacitance 19 from output 8a of the voltage-controlled oscillator 8, and a voltage from output 8b via the variable amplifier 15. When the resonant circuit iε purely real, the output signal of the summing amplifier 18 has a phase shift of 90° relative to the cosine signal at the oscillator output 8a, and thus the phase detector 10 output becomes zero and the output of the integrator 17 controlling the voltage-controlled oscillator remains unchanged. If the voltage induced in the resonant circuit by the current supplied through output 8a of the voltage- controlled oscillator does not cancel the voltage added to it in the summing amplifier 18, then the output of phase detector 9 will not become zero either, and thus the output of the integrator 16 controlling the gain of amplifier 15 will change " until balance is reached. The outputs of both integrators 16, 17 will only become zero when the resonant circuit 3 is fed with a reso¬ nant-frequency signal and if the voltage fed into ampli¬ fier 18 cancels the voltage obtained from the resonant circuit. Thus, the output of integrator 16 becomes proportional to the real part R of the resonant-circuit impedance during resonance, whereas the output of in¬ tegrator 15 becomes directly proportional to the reso¬ nant frequency 0 . After the mixers, the signals as- sociated with the measurement of the impedance R and resonant frequency ^ 0 have a low frequency and they have been filtered by the high-pass filters 11 and 12. The noise signals obtained after these filters from both mixers are proportional to the temperature T x of the object under measurement. The signals are summed by the summing amplifier 13, and the root-mean-square value Uinnβ of the noise is detected by the r.m.s. converter 14. If necessary, the measuring unit 6 can be provided with one or more absorbing circuits tuned to radio transmitter frequencies close to the resonant frequency. In this way it is possible to avoid the interference caused by these radio stationε. The detected voltage U obtained from the mea- εuring unit 6 can be transferred via an A/D converter to a data processing unit, such as a microcomputer, and used e.g. for process control or supervision. Fig. 3 illustrates an embodiment of the appa¬ ratus of the invention, especially the senεor 5, in perεpective and partly εectioned. The εenεor 5 comprises a coil 4, in this case a winding 3b provided with a ferrite core 3a and fitted inside a frame 3c made of a suitable material, e.g. teflon. The coil 4 is placed at distance ε from the material whoεe temperature x is to be measured, aε described above. Moreover, the sensor comprises a preamplifier 2 and posεibly a mixer ampli- fier 18 (fig. 4) . The preamplifier 2 iε built on a circuit board 2a placed in the immediate vicinity of the coil 4. The ~ sensor is mounted inside a static screening, in this case a frame 20 made of a conductive material, such as copper. The frame 20 is provided with a cooling means 21, which preferably consists of a tubular element wound in a spiral around that end of the frame 20 where the coil 3 iε located. A suitable coolant is circulated through the cooling means during meaεurement. The frame 20 is provided with a shield plate 22 or equivalent placed between the coil 4 and the material under measurement. The shield iε made of a heat-resistant and neutral material, e.g. sapphire. In principle, the coil 4 of the sensor 5 iε εo deεigned aε to make it aε inεenεitive aε poεεible to homogeneous magnetic fields, so that it will only sense the mag¬ netic field fluctuations occurring on the surface of the object under measurement. Fig. 5 shows a graph representing the output Uo of the r.m.s. converter 14 when measuring the tem¬ perature of a steel object being heated. The left-hand vertical axis of the graph represents the square <U 0 => of the r.m.s. value of the noise voltage, repreεenting the measured temperature (K) in accordance with formula (3) . The right-hand vertical axis represents the imped¬ ance of the coil 4 at the resonant frequency. The hori¬ zontal axis represents the temperature (K) of the steel object. The results presented have been normalized by multiplying them with a constant. The r.m.s. value of the noise is linearly dependent on the temperature of the steel object. Therefore, the temperature has been obtained in these measurements by using the formula T x = Uo 2 /R + B, where A and B are constants. It can be concluded from the measurement reεultε that the εquare <Uo > of the r.m.s. value of the noise voltage follows the surface temperature T> of steel quite well. The accuracy of the apparatus can be further improved by increasing the measuring bandwidth, which can be achiev¬ ed by increasing the resonant frequency. At high fre¬ quencies, the amplifier noise temperature can be reduced by cooling the preamplifier aε well. The apparatus of the invention for measuring surface temperatures can be so realized that the measur¬ ing accuracy is below ±3K. A very high accuracy will be achieved by replacing the copper winding of the coil 4 with a superconductor winding, preferably made of cera- mic materials. Superconductor windings made of ceramic materials are already usable at the temperature (+77 °K) of liquid nitrogen. In the above description, the reactance in the resonant circuit 3 consists of an inductance, but it is obvious that thiε can aε well be replaced with a capa¬ citance. In this case, the impedance of the object under measurement is adapted to a level suited for a low-noise preamplifier by means of a capacitance tuned into reso¬ nance, e.g. a capacitor, instead of an inductance, e.g. a coil. The measurement is performed fully analogously with the method described above. In the above, the invention has been described by referring to one of its preferred embodiments, but it is obvious that it can..be varied in many ways within the scope of the idea of the invention as defined in the following claims.

Claims: CLAIMS 1. Method for measuring the temperature of an electrically conductive material, based on the measure- ment of the thermal noise of the material, c h a r¬ a c t e r i z e d in that the fluctuation of the mag¬ netic field generated by the random motion of the charge carriers in the conductive material is measured using a resonant circuit (3) without contact with the material (1) under measurement. 2. Method according to claim 1, c h a r a c¬ t e r i z e d in " that the measured high-frequency noise is mixed down with low frequencies and detected. 3. Method according to claim 2, c h a r a c- t e r i z e d in that the mixing is performed at the frequency of the resonant circuit (3) . 4. Method according to claim 2 or 3, c h a r¬ a c t e r i z e d in that the mixed noise signal (<Uo a >) is detected, the resonant frequency ( ωo ) and resistance (R) of the resonant circuit (3) are measured and the temperature (T x ) of the object under measurement is calculated from these results. 5. Method according to claim 4, c h a r a c¬ t e r i z e d in that the resistance (R) of the reso- nant circuit (3) is measured by supplying the circuit with a current (I) and a voltage (U) of inverted phase relative to the current, that the frequency and the current amplitude are so adjusted that the signals generated by the current and voltage will cancel each other, so that in the balanced state the resonant fre¬ quency (ω 0 ) iε obtained directly from the frequency and the voltage/current ratio (U/I) iε directly proportion¬ al to the resistance (R) of the resonant circuit (3) . 6. Method according to claim 3 or 4, c h a r- a c t e r i z e d in that, after the mixing, in order to remove the lower frequencies so aε to eliminate interference, the signal is filtered using a filtering bandwidth at least roughly corresponding to the band¬ width of the resonant circuit (3) . 7. Apparatus for measuring the temperature of an electrically conductive material, the temperature being determined from low-frequency thermal noise meas¬ ured by the apparatus, c h a r a c t e r i z e d in that the apparatus comprises - a sensor (5) provided with a resonant circuit (3) , the reactance element of which is placed at a distance (s) from the material (1) whose temperature is to be mea¬ sured; - a preamplifier " (2) ; said senεor (5) being mounted inside a frame (20) made of a conductive material, such as copper. 8. Apparatus according to claim 7, c h a r¬ a c t e r i z e d in that the frame is provided with a cooling means (21) . 9. Apparatus according to claim 7 or 8, c h a r a c t e r i z e d in that the frame (20) iε provided with a shield plate (22) or equivalent placed between the reactance element and the material under measurement, the shield being made of a heat-resistant, neutral material, e.g. sapphire. 10. Apparatus according to claim 7, c h a r- a c t e r i z e d in that the reactance element is a coil (3) consisting of a superconductor winding, prefer¬ ably made of a ceramic superconductor. 11. Apparatus according to any one of the preceding claims, c h a r a c t e r i z e d in that the apparatus additionally comprises a measuring unit (6) including at least a mixer (9, 10) and a detector circuit (14) , by means of which the measured noise is mixed down with low frequencies. 12. Apparatus according to claim 11, c h a r- a c t e r i z e d in that the apparatus additionally compriseε a summing amplifier (18) fitted in the sensor (5) , and in the measuring unit (6) a high-pass filter (7), a voltage-controlled oscillator (8) with outputs (8a, 8b) for cosine and sine signals, two mixers (8, 9), high-pasε filters (11, 12), an adder (13), a r.m.s converter (14), a voltage amplifier (15) and two in- tegratorε (16, 17) , εaid high-pass filter (7) being connected to the mixers (9, 10), the outputs (8a, 8b) of the voltage-controlled oscillator (8) to the mixers (9, 10) respectively, the outputs of the mixers (9, 10) to the high-paεε filterε (11, 12) and the outputε of the high-paεε filterε (11, 12) to the inputs of the adder (13) , the output of which is connected to the r.m.s. converter, " whose output provides a measurement signal (U-e-m.) proportional to the temperature of the object under measurement, and in which apparatus the output (8b) of the voltage-controlled oscillator (8) is connected via amplifier (15) to the second input (18b) of the summing amplifier (18) , the output (8a) of the voltage-controlled oscillator (8) is connected (19) to the input of the amplifier (2) in the sensor (5) and the output of amplifier (2) is connected to the first input (18a) of the summing amplifier (18) , and in which the output of mixer (9) is connected via current ampli¬ fier (16) to the second input of amplifier (15) and the output of mixer (10) is connected via current amplifier (17) to the input of the voltage-controlled oscillator (8).

Inventors: HOLAPPA LAURI, SEPPAE HEIKKI, HOLAPPA, LAURI, SEPPAE, HEIKKI
Applicants: VALTION TEKNILLINEN, VALTION TEKNILLINEN TUTKIMUSKESKUS
Application year: 1990
Main classification: G01K7
IPC codes: G01K7