JPH0219884B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention enables an eddy current displacement meter used to measure vibrational displacement and thrust displacement of a rotating body in a turbine, compressor, etc. to be able to measure over a very wide temperature range. The eddy current displacement meter will be explained. Figure 1 shows the principle block diagram of an eddy current displacement meter, which consists of a power supply 1, an oscillator 2, a bridge 3, a detection coil 4, a detection 5, a linearizer 6, an amplifier 7, and an output side 8. There is. 9 is a metal object to be measured;
Y is the distance between the object to be measured 9 and the detection coil 4. Figure 2 is an equivalent circuit diagram provided in the detection coil 4.
L is the inductance of the detection coil, R is the internal resistance of the detection coil, and C is a parallel capacitor. The eddy current type displacement meter uses the detection coil 4 as a resonant circuit, and the inductance of the detection coil 4 changes according to a change in the distance Y between the detection coil 4 and the metal object 9 to be measured. And the detection coil 4
Since the impedance |Z| of the resonant circuit changes in response to a change in the inductance of the resonant circuit, the distance Y between the detection coil 4 and the metal object 9 can be determined by measuring the impedance |Z|
is now being measured without contact. However, in the field of eddy current displacement meters, the only materials used for the detection coil 4 are ferrite, copper, silver, or other low specific electrical resistance materials. The reason for this is that when the specific electrical resistance of the detection coil 4 is increased, the precision of the resonant circuit becomes lower, making it impossible to obtain a sufficient signal change. Therefore, in order to reduce the temperature coefficient of the detection coil 4 itself when a low specific electrical resistance material is used, the change in inductance due to thermal expansion of the coil bobbin is used (the inductance increases as the diameter of the coil increases). , to offset changes in electrical resistance due to the temperature of the coil material (the electrical resistance value increases as the temperature rises).
(can be used in a temperature range of ~180℃), or a pseudo coil was installed near the detection coil to extract the difference in impedance with the detection coil, and the two coils canceled out the fluctuation due to temperature (can be used in a temperature range of -20℃ to
(Can be used in a temperature range of +100℃). By the way, in the eddy current type displacement meter, the temperature of the metal object 9 (having a large heat capacity) or the surroundings of the object changes over a certain temperature range, and this temperature change causes the detection coil (having a small heat capacity) to change temperature. .It is less than 1/100 of the heat capacity of the metal object to be measured).
The impedance was greatly affected by the temperature change of the detection coil 4 itself. Particularly in the low temperature range -20°C or lower, the effect was so great that it could not be used. The main temperature effect is a thermal shape change of the detection coil 4. That is, if the inductance of the detection coil 4 is L, the diameter of the coil is D, and the length of the coil is l, then the inductance of the detection coil is L=k・(μ _p・μ _s /4π)・(2π・D/ 2・N) ²
/l (1 equation) is given. Here, k is the Nagaoka coefficient, μ is the magnetic permeability in vacuum, μ _s is the relative magnetic permeability, and N is the number of turns of the coil. The thermal expansion coefficient of the detection coil and bobbin is (1 formula)
The inductance L changes greatly depending on the temperature change. Similarly, the change in magnetic permeability of the detection coil/bobbin is also a change in μs in equation (1), and the impedance changes depending on the temperature. Also, the capacitance between the detection coil lines is C, the effective area of the detection coil is A, and the effective distance between the detection coil lines is h.
Then, C=E _p・E _s・A/h (2 formulas), and the dielectric constant E _s changes depending on the temperature. Here, E _p represents the dielectric constant in vacuum, and Es represents the dielectric constant. Another possibility is that the electrical resistance changes due to the temperature of the detection coil itself. According to the researchers of the present invention,
It has been found that the most influential factor among the above is the change in electrical resistance due to temperature changes in the detection coil itself, and the rate of change in impedance appears to be greater than the generally known direct current temperature coefficient. This type of eddy current type displacement meter is generally used within the range where resonance occurs in order to increase sensitivity, and conventional eddy current type displacement meters are used within the compensation range even if compensation for thermal expansion or pseudo coils is compensated for. Due to these limitations, it could not be used over a wide temperature range. (-20
Operating temperature range from ℃ to +180℃). Rather than adjusting the direction and amount of impedance change due to thermal shape change to offset the overall impedance change, the present invention simply minimizes the amount of thermal shape change and reduces the specific electrical resistance value to a certain extent. It has been discovered that it is possible to obtain a signal with high fidelity over a wide temperature range by forming the detection coil with a material having a low temperature coefficient, even if the temperature coefficient is high. In Figures 1 and 2, if ω is the angular velocity of the oscillator frequency, the impedance Z of the equivalent circuit in Figure 2 is Z = 1/1/R + jωL + jωC (Equation 3) = R + jωL / 1 + jωC (R + ωL) (Equation 4) ) =R+jωL/(1-ω ² LC)+jωCR (Equation 5) =(R+jωL) {(1-ω ² LC)-jωCR}/{(1-ω
² LC))+jωCR}・{(1-ω ² LC)-jωCR} (Equation 6) = (R+jω) {L(1-ω ² CL)-CR ² / (1-ω ² LC)
² + (ωCR) ² (Equation 7) Also, |Z| is from (Equation 5) Here, R is expressed as R=r+K.r(t-20) (Equation 10), where r is the resistance value of the electrical resistance material used at 20°C, k is the temperature coefficient, and t is the temperature. Substituting (Equation 10) into (Equation 9), |Z| becomes becomes. Assuming that the specific resistance of the wire material used in the detection coil is ρ, the length is ls, and the cross-sectional area is A, it is expressed as r=ρ·l _s /A (Equation 12), as is well known. Substituting (formula 12) into (formula 11), we get becomes. By the way, the size of the detection coil is limited due to the purpose of use, and in order to increase the selectivity of the resonant circuit, in other words, in order to satisfy R<<L, the ferrite core is conventionally coated with a low specific electrical resistance material such as copper or silver. has been used. However, although these increase the sensitivity, as can be seen from equation (11), unless the internal resistance of the coil is zero, that is, r is not zero, the temperature coefficient K of the coil material has a large influence on the impedance |Z| depending on the design conditions. I understand that. Here, a conventional example in which copper is used as the coil material will be explained. Coil material Copper specific resistance ρ 1.73×10 ^-8 ohm m Temperature coefficient K 4.3×10 ^-3 Coil inductance L 2.2μH Coil resistance r 0.113ohm (at 20℃) Parallel capacitance C 620pF Diameter of wire used 0.23mm (A = π/4×0.23 ² ) Insert the length of the electric wire used l _s 0.271m into (Equation 13), and set the temperature t in (Equation 13) to -100℃ -50℃ 0℃ +20℃ +50℃ +100℃ Figure 3 shows a graph of the results calculated by a computer, with frequency MHz on the X axis and impedance |Z|kohm on the Y axis. Next, as an example of the present invention, coil material Manganin specific resistance ρ 39.8×10 ^-8 ohm m Temperature coefficient K 3×10 ^-5 Coil inductance L 2.2μH Coil resistance r 2.60ohm (at 20℃) Parallel capacitor capacitance C 620pF used Insert the wire diameter 0.23mm (A=π/4×0.23) and the length of the wire used ls 0.271m into (formula 13), and in (formula 13), set the temperature t to -100℃ -50℃ 0℃ +20℃ Figure 4 shows a graph of the results calculated by computer, with frequency MHz on the X-axis and impedance |Z|kohm on the Y-axis when +50℃ and +100℃. From the results shown in Figures 3 and 4, the influence of the impedance due to changes in the resistance of the wire at the frequency at which displacement is measured with good sensitivity (in this case, the maximum value of impedance, that is, the resonant frequency 4.3MHz) is greater than the temperature coefficient of resistance at DC. becomes larger, and from the calculation of (formula 13)
Based on the impedance at 20°C, a change in impedance from -100°C to +100°C for copper wire is 132%, and for manganese wire it is 0.6%, making the error approximately 1/220th. That is, in a wide range of temperature changes, it is possible to minimize errors due to other factors such as changes in shape, and to perform measurement with greater accuracy than by performing compensation using a pseudo coil or the like. However, in Figures 3 and 4, the detection coil,
Changes in the resonant frequency when the inductance, electrical resistance, etc. of the detection coil change due to thermal shape changes of the bobbin, etc. cannot be recognized in these figures because they are small relative to the scale width of the X-axis. Calculate the amount here. The coefficient of linear expansion of the material used for the bobbin, etc. is about 3.2×10 ^-6 , and from (Equation 1), the effect on inductance L is
It is 3.2×10 ^-6 . Also, since the resonant frequency of the resonant circuit is the point where the complex term becomes zero in equation (7), L(1-ω ² CL) = CR ² ω ² = (L-CR ² /CL ² ) ω = 2π Therefore, for an inductance change of 3.2×10 ^-6 per 1°C, the influence of the resonant frequency is 1/square root, or 3.2×10 ^-8 /°C. Further, the influence of resistance value change on the resonant frequency is the same as the resistance value temperature coefficient. i.e. 1.73 for copper wire
×10 ^-8 /°C, and it can be seen that even if there is a width of 200°C, it cannot be recognized in Figures 3 and 4. The computer calculation results are in good agreement with the actual experimental results. FIG. 5 shows an impedance measurement circuit when using the conventional detection coil 4a shown in FIG. 3, FIG. 6 shows an impedance measurement circuit when using the detection coil 4b according to the present invention shown in FIG. 4, and Table 1 shows Experimental data for the impedance measurement circuit shown in FIGS. 5 and 6, and FIG. 7 is a graph created based on Table 1. In the conventional products shown in Tables 1 to 5, the detection coil 4
When the distance Y between a and the object to be measured 9 was changed from 0 mm to 2.5 mm, the value of the DC meter 10 changed from 2.571 volts to 3.734 volts, and the range of change was 1.163 volts. On the other hand, in the present invention shown in FIG. 6, when the distance Y is similarly changed, from 6.679 volts to
The voltage changes up to 10.587 volts, and the width of change is 3.908 volts.Even with the detection coil 4b made of a material with high specific electrical resistance, the width of change in the DC voltmeter 10 is 3.980 volts, which is the width of change in the conventional DC voltmeter 10.
This is a 336% improvement compared to 1.163 volts.
In addition, in the product of the present invention, since the resistance of the detection coil 4b is increased, the resistor 11 of the resonant circuit in the conventional column shown in FIG. 5 is replaced with a capacitor 12 in the present invention shown in FIG. Coupling loss is reduced. The operating temperature range of the normally used displacement meter detector is -50°C to +150°C, the temperature range is about 200°C, and the accuracy is ±1%. 200 as shown in the calculation example in Figure 4
If a resistance material with a temperature coefficient of 3×10 ^-5 is used in a temperature range of ℃, the impedance width will be 0.6%.
Therefore, up to ±1% or 2% variation is 3.3%.
The temperature coefficient of resistance is 0.0001 or less. On the other hand, in terms of detection sensitivity, if the circuit shown in FIG. 6 is used, the precision Q of the coil must be 13 or more. From the calculation data in Table 1, R=ωL/Q=2・π・4.3・10 ⁶・2.2・10 ^-6 /13=4.
57 The specific resistance ρmax to be determined is as follows from Figure 4: ρmax=ρ・4.57/2.6=39.8・10 ^-8・4.57/2.6=70
× 10 ^-8 . The present invention comprises a detection coil for an eddy current displacement meter using a coil made of a material with a temperature coefficient of resistance of 0.0001 or less and a specific electrical resistance of 0.00000070 Ωm or less (Figs. 3 and 4). As can be seen from the figure), it is possible to obtain an extremely excellent eddy current type displacement meter with very little influence from temperature. There is no need to utilize thermal deformation of the coil or bobbin or install a pseudo coil for compensation as in other systems, and there are fewer restrictions on design.

【table】

【table】 [Brief explanation of drawings]

The drawings are for explaining embodiments of the invention, and FIG. 1 is a block diagram of the principle of an eddy current displacement meter, and FIG. 2 is an equivalent circuit diagram of a resonant circuit. Third
The figure shows the relationship between oscillation frequency and impedance when copper is used for the detection coil, Figure 4 is the same diagram as above when manganin is used for the detection coil, Figure 5 is a conventional impedance measurement circuit diagram, and Figure 6 is FIG. 7, which is an impedance measurement circuit diagram of the product of the present invention, is a graph comparing the conventional product and the product of the present invention. 1...power supply, 2...oscillator, 3...bridge, 4,4
a, 4b...detection coil, 5...detection, 6...linearizer, 7...amplifier, 8...output side, 9...measured object, 10...DC meter, 11...resistance, 12...capacitor, L...detection coil inductance, R...internal resistance of the detection coil, C...parallel capacitor for resonance.

Claims (1)

[Claims] 1. The detection coil is a resonant circuit, and the inductance of the detection coil changes according to a change in the distance between the detection coil and a metal object to be measured, and the impedance of the resonant circuit changes in response to a change in the inductance of the detection coil. , an eddy current displacement meter configured to non-contactly measure the distance between the detection coil and the metal object by measuring the impedance of the resonant circuit, the temperature coefficient of resistance is 0.0001.
An eddy current non-contact displacement meter characterized in that a detection coil is made of a material having a specific electrical resistance of 0.00000070 Ωm or less.