JPH03123801A - Measurement of displacement by impedance of coil - Google Patents

Abstract

PURPOSE:To accurately measure a displacement in a non-contact state without being affected by the change in an object to be measured by calculating the displacement between a coil and the measuring object from the difference in the changing amounts of impedances of three coils. CONSTITUTION:The distance(displacement) between the coils L1-L3 and the object 1 to be measured is obtained from the difference in the changing amounts of impedances Y1-Y3 of three coils L1-L3 by the calculation through the equation. In this equation, X is the displacement to be obtained and Y1-Y3 are the changing amounts of each impedance of the coils L1-L3. LnY1 is a natural logarithm of Y1 and Ln means the same on the succeeding expressions. Further, E1-E4 are constants, meaning the numerical numbers unaffected by a material quality, shape, and displacement of the object 1 to be measured. In this case, the coils L1-L3 may be connected also in parallel to an oscillation circuit 2. Then, the displacement at the time when the coils L1-L3 are moved in the same direction at the same time or when the object 1 to be measured is moved, is measured.

Description

DETAILED DESCRIPTION OF THE INVENTION (One aspect of the present invention is to apply eddy currents generated in a conductor due to changes in the magnetic field. Generally, alternating current is applied to the coils, and three coils are fixed at different distances from the conductor. When an alternating current is passed, as the distance between the conductor and the coil changes, 3
The impedance of each coil changes differently. From the difference in the amount of impedance change, calculate the distance between the coil and the object to be measured (hereinafter referred to as displacement) using the formula (■).

demand.

X=EI LnYl +E2 LnY, +E3
LnY3 +E4・---1・−■ Here, X is the displacement to be sought.

Y 1 , Y z and Y3 are the impedance changes of each of the three coils.

LnYl is the natural logarithm of Y8. The meaning of Ln is the same below.

E□, Ez, Es and B4 are constants.

As is conventionally known, when measuring displacement using eddy currents, the impedance of the coil is affected by the material and shape of the object to be measured.

Therefore, when the material or shape of the object to be measured changes during measurement, for example, when the object is a workpiece in a machine tool, it has been impossible to measure displacement using eddy currents.

The present invention is not affected by such changes in the object to be measured.
The purpose is to measure displacement with high precision without contact.

In the following, we will solve the equations that lead to the present invention.

Note that the constant used here means a numerical value that is not affected by the material, shape, or displacement of the object to be measured, or a value that can be ignored. Further, a variable means a numerical value that is affected by the material or shape of the object to be measured, or by displacement.

The relationship between displacement and impedance of each coil is approximated by an exponential function. This exponential function is shown in equations ■, ■, and ■. An example of the relationship between impedance and displacement is shown in FIG.

Z+ = At exP (BI X) + Ct
−■Z2 =A2 eXP (−B
2 X) + C2 ■ Z3 = A:s exp
(Bs
2 and B3 impedance.

X is the displacement between the coil and the object to be measured.

A□, A2, and A3 are variables influenced by the material, shape, and displacement of the object to be measured.

Bx, Bz, and B3 are variables influenced by the material and shape of the object to be measured.

C1, Cx and C3 are constants.

exp means an exponential function with e as the base.

From the above three equations, impedance 2. ．． 2° and B3
The impedance changes Y1, B2 and B are as follows:
Replace with 3.

y, =zt -cm Yx = Zt Cx B3 =Zs -Cs A2 = A20eXP (-W2 X) A3
=A30eXP (-W3X) Furthermore, A. , A2° and A3° are variables influenced by the material and shape of the object to be measured.

W1, Wz and W3 are constants.

From these, formulas (■), (2), and (2) can be replaced with the following three formulas.

Y t = A 1 exp (B I
X) B2 = A2 exp (B2 X)
Ys = As exp (-B3 This one
Examples are shown in Figures 1 and 3.

Variables A2゜ and A3゜ used in variables AI, A2 and A3 are derived from physical phenomena as follows:
. It is expressed as a function. One example is shown in FIG.

A2゜=R,. A. 'I21 A 3o=R30A 10″3″ still.

Rzo, R□, R3° and R31 are constants.

A , =AxoexP (WI Ru.

AI = AIOeXP (Ws
X) A2 = R2, A, o”' eXP (W
Z X) A3 “R36A16113” eXp
(-W3 An example of this is shown in FIG.

B2 = D20B1 + D21B 3 =
D30B 1 + B31 where, B2゜, D□, B3
° and D3□ are constants.

Yu = A + oexp [-Wt XI ex
p[-BIXkoY2=R20AIOR2”
exp [WZ XI”eXp [(Dzo
Bx +D21)XIY3” R3oA*oR3”
eXP [-W3 XI・exp [-(D
30B1 +D31)XI 1 in the above three formulas
Take the logarithm of both sides, eliminate variables A1゜ and B,
When X is determined, the above-mentioned formula (■) is determined. However, the coefficients E1, E, , B3, and B4 in equation (2) are constants determined by the following equation.

E 1 = (Rz, B20- R21D30)/
A I , Ax , As , B over M-■
Substituting the formulas for x and Byu into 2〇, ■ and ■, we get
The following formula is found: B2 = CDs. -Rst)/M
−[Phase]E3 = (Rzt
B2゜)/M --■ E 4 = (D zoLnRso R2tLnR
s.

+ R3iLnR20D 36LnR2o) /
M■ In addition, M = (R21D s.-Rs 1D z.)W□+
(R31Ds.) WZ + (B2゜-R2t) Wx +D2゜Ds* D2□D3゜+RsxDz□-R2
□D31■ Constants E 1 , Ex , Es and B4 used in formula ■
are the above constants I) 2o, B21, D 3o, D 3
1. It can be determined from R2o, R2□, R3゜, R31, W□, WZ and W3, and its constants D2゜, D2□
, B3゜, D3□, R2゜, R21, R3゜, R3□,
W t , W 2 and W3 can be determined by experiment.

However, when implementing this method with an electronic circuit, the constant D2
0. I) zt・D,,,B3. , Rto, R21s
R3o・R3□, W 1. Without determining W z and W3, Es , B2 . It is possible to determine B3 and B4 directly. That is, since E ,,B2) B3 and B4
El, Ex, B3 and B4 are determined by creating four simultaneous equations with unknowns and solving the equations.

FIG. 6 is one embodiment of the invention. In FIG. 6, the main circuit consists of the following three parts.

1) Three coils, an oscillation circuit that applies alternating current to the coils, and impedance changes of each coilff1Y1
．． Arithmetic circuit that calculates Yz and Y3.

2) Constants E,, E! used in formula ■. , E, and B
Circuit to set 4.

3) An arithmetic circuit that calculates displacement according to formula (■) and outputs the calculation result.

Each of the oscillation circuit, buffer amplifier, and arithmetic circuit is constructed by combining known circuits.

Although the circuit shown in FIG. 6 is an analog circuit, it is also easy to convert the signal extracted by the buffer amplifier into a digital signal and perform digital calculations thereafter.

(2) The formula used in claim 2 is shown in formula (2). This formula (2) is derived from formula (2) by relaxing the physical conditions.

X=Fx Ln (Yt/Yz)+F2
Ln(F2/F3)+Fs
"""""'-■Here, Fi, Fz and F3 are constants.

Below, we will explain how to derive equation (■).

Among the constants used in formulas ■, [phase], ■, ■, and @, constants R2□ and R3□ take values physically close to 1. For example Rx1=0.9841, and R31
: 0.9838. From this, Rz s =
Assuming that R3□=1, this becomes the formula ■, [phase], ■, ■
By substituting for and @, the formula ■ can be found.

However, the coefficients F□, F2, and F3 in equation (2) are constants determined by the following equation.

F 1 = (D2゜-D3゜) / NF2 = (D2゜-1) / NF3 = (D 2oLnR3o LnR3゜+L
nRto-D 3oLnR20)/ NN” (D3
0 Dzo) Wl + (1-D3°) W2 + (D2°-1) W3 + D 20 D 31- D z t D 30 +
D z □-D 31 Shown in Figure 8.

Constants Fw, Fz, and F3 of formula ■ are constants E□ of formula ■,
E2. In the same way as E3 and F4 were determined, the amount of change in impedance of the coil can be determined and calculated from the known displacement.

Effects of the Invention When conventional methods are used, the impedance of the coil caused by eddy currents varies depending on the material or area of the object to be measured, in addition to displacement.

An example of this is shown in Figure 7. Therefore, in cases where the material or shape of the object to be measured changes during measurement, such as in machine tools, it is difficult to accurately measure displacement without contact. It was possible.

The present invention is capable of measuring displacement despite such changes in the object to be measured. The following is an example of the results when the impedance is measured by changing the object to be measured to steel material, the end of the steel material, aluminum alloy material, and brass material, and the displacement is calculated using 2〇 and ■.

[Brief explanation of the drawing]

FIG. 1 shows the displacement X, the relative position of the object to be measured 1 and the coils L, , L, , Ln. However, coil Lt, L
It is not necessary that z and Ln are arranged vertically in a single line. It is also possible to connect the coils L□, L2, and Ln in parallel to the oscillation circuit 2. An object of the present invention is to measure the displacement X when the coils Ls, L2, and Ln move in the same direction at the same time, or when the object to be measured 1 moves. Fig. 2 Fig. 2 shows an example of the relationship between the impedances Zl, F2, F3 of each coil and displacement when an alternating current is passed through the three coils. FIG. 3 shows that the variables A i , A x and A3 used in ②, ■ and ■ are expressed by exponential functions of displacement. FIG. 4 FIG. 4 shows that the variables A2° and A36 are expressed as functions of powers of A8°. FIG. 5 shows that the variables B2 and B used in the fifth diagram (■, ■, and ■) are expressed by a linear equation of B1. FIG. 6 is a block diagram of a circuit as an example of implementing the present invention.
Illustrated in the diagram. Variable resistor r, 2) r3 determines the constant ct, C2) C3 of equations (2), (2), and (2), respectively. A fixed resistor r0 is provided to measure the current flowing through the coils L□, Lz, and B3. Amplifier 3.4.5.6.7.8.9 has coils L□, L
2) B3 and resistors r0, s, rz, r3 (') are high input impedance buffer amplifiers that read the voltage. The output of the buffer amplifier 3 for the variable resistor r1 is subtracted from the output of the buffer amplifier 4 for the coil L□ by the subtraction circuit 10, and the value is divided by the output of the buffer amplifier 9 for the resistor r0 by the division circuit 13. The result is Yl of formula ■
corresponds to Similarly, a circuit for obtaining Yl and Y3 is configured. Conversion circuits 16, 17, and 18 are respectively divided by division circuits 13.
14.15 output, that is, the amount of change in impedance Y
1. Change the signals of Yl and Y from AC to DC. Each DC signal is input to the arithmetic circuit 19. The potentiometers 20, 21, 22, and 23 are respectively connected to the coefficient Es −B2. B3 and B4 are set, and their outputs are input to the arithmetic circuit 19. Power supply 24 supplies DC electricity to potentiometers 20.21.22.23. The arithmetic circuit 19 is comprised of a known four arithmetic arithmetic circuit and a logarithm arithmetic circuit, and executes the arithmetic operation of equation (2). Terminal 25 is an output terminal of arithmetic circuit 19. FIG. 7 FIG. 6 shows that the impedance of one coil varies depending on the material and shape of the object to be measured. Fig. 8 Measures the impedance of three coils using steel material, the end of the steel material, aluminum alloy, and brass as objects to be measured. Fig. 8 shows the relationship between the displacement calculated by formulas ■ and ■ and the displacement at actual temperature. This shows that formula (1) satisfies the purpose of the present invention, and that formula (2) satisfies the purpose of the present invention to a limited extent. ■' Calculated value according to formula ■ ■' Calculated value according to formula ■ 1 - - 1 DUT 2 - Oscillator 3.4.5, 6.7, 8.9 - Buffer amplifier 10.1
1.12 - - Subtraction circuit 13.14.15 - - Division circuit 16.17.18 - - AC/DC conversion circuit 19 -
Arithmetic circuit for formula ■ 20.21.22.23 - Potentiometer 24
~ DC power supply 25 Output terminal 26 Curve 27 when steel is the object to be measured
Curve 28 when the end of #4 material is the object to be measured
Curve 29 when aluminum alloy is used as the object to be measured
Curve AI - value A of variable A1 when brass is the object to be measured. - Value A2 of variable A1゜ --- Value of variable A2 A2゜ - Value of variable A2゜ A3 Value of variable A3 A3゜ -- Value of variable A3゜ B □ ~ Value of variable B1 Value of variable B2 Value of variable B3 Magnetic field coil L □ Coil 2 Coil L3 Fixed resistance constant 01, variable resistance constant 02, variable resistance constant C3, impedance of variable resistance displacement coil L1, impedance of coil Lx, impedance diagram of coil L3.

Claims (2)

[Claims] (1) An alternating current is passed through three coils placed at different distances from the object to be measured, and the amount of change in impedance of each of the three coils due to the eddy current generated in the object to be measured is calculated. A method of calculating and finding the distance between an object and a coil. (2) A method of determining the distance between the object to be measured and the coil using a simpler calculation formula that can be derived from the calculation method described in claim 1.