JPH0237522B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an eddy current distance measuring method, and in particular,
Suitable for use in eddy current distance meters to detect the position of a steel plate surface, the coil and the object to be measured are based on the change in impedance that occurs in the coil when a coil carrying an alternating current is brought close to an object to be measured made of a conductor. This invention relates to an improvement of an eddy current distance measuring method for determining the distance to an object.

[Conventional technology]

For example, as disclosed in Japanese Patent Application Laid-Open No. 47-26154, electromagnetic induction is used to detect changes in impedance that occur in a coil when a coil carrying an alternating current is brought close to a conductor to be measured. An electromagnetic induction type (eddy current type) distance measuring method is known in which the distance between the object and the object to be measured is determined.
This eddy current distance measurement method is as shown in Figure 1.
When an alternating current is passed through the coil 10, the adjacent conductor 1
An eddy current 14 flows through 2, and the coil 10 is affected by the magnetic field created by this eddy current 14, and the impedance of the coil 10 changes.The magnitude of this impedance change depends on the distance h between the coil 10 and the conductor 12. The distance h is measured by measuring the impedance of the coil 10 by taking advantage of this change.

An example of a conventional eddy current distance meter that utilizes this principle is shown in FIG. This eddy current distance meter has coil 1
0 on one side, an oscillator 16 for applying an AC voltage of a constant frequency to the AC bridge 14, an amplifier 18 for amplifying the unbalanced output of the AC bridge 14, and the amplifier 1
a detector 20 for detecting the output of the detector 8; a DC amplifier 22 for amplifying the output of the detector 20;
It is composed of a linearizer 24 for converting the output of the DC amplifier 22 into an analog signal proportional to the distance, and uses the AC bridge 14 to generate an AC voltage proportional to the magnitude of change in impedance of the coil 10. After the signal is taken out and detected by a wave detector 20, a linearizer 24 converts it into an analog signal proportional to the distance.

[Problem that the invention seeks to solve]

However, as is clear from the principle shown in FIG.
It also depends on Further, when the temperature of the conductor 12 changes, the values of the electrical resistance ρ and the magnetic permeability μ also change, so the temperature change of the conductor 12 also affects the measurement.
For these reasons, conventional eddy current rangefinders
It is necessary to calibrate the linearizer 24 every time the object to be measured changes, and the measurement error increases if the object to be measured changes in temperature.

The present invention has been made to eliminate the above-mentioned conventional drawbacks, and includes changes in electrical property values of an object to be measured;
In particular, it is an object of the present invention to provide an eddy current distance measuring method that can perform highly accurate distance measurement regardless of changes in electrical property values due to temperature changes of the object to be measured.

[Means for Solving the Problems] The present invention provides an eddy current system in which the distance between the coil and the object to be measured is determined from the change in impedance that occurs in the coil when the coil passing an alternating current is brought close to the object to be measured. In the formula distance measurement method, the distance between the coil and the object to be measured is h, the electrical resistivity of the object to be measured is ρ, the relative magnetic permeability is μ _r , the angular frequency of the current flowing through the coil is ω, and the parameter q = ρ μ _r / When ω, the multiple inductances of the coil, which are uniquely determined by the shape of the coil = (Lx, Ly)
The relationship between and (h, q) is determined in advance, and then when the coil is put into the measurement state, the distance h is determined from the measured inductance using the relationship between 〓 and (h, q). Therefore, the above object is achieved by obtaining a distance measurement value that is not affected by changes in the electrical property values of the object to be measured.

[Effect]

The principle of the present invention will be explained below.

Now, as shown in Figure 3, the inner radius is a ₁ and the outer radius is
a ₂ , if we use a coil 10 whose length in the vertical direction in the figure is l, and measure the distance h between the coil 10 and a conductor 12 whose electrical resistance is ρ and whose magnetic permeability is μ, The multiple inductances of the coil 10 in such a state can be expressed by the following equation by solving the Maxwe11 equation.

〓/L ₀ =1−∫ ^a2 ／ _a1 ada∫ ^a2 ／ _a1 a′da′∫ ^∞ ／ ₀ J ₁ (a
ζ) J ₁ (a′ζ)W/ζ ² (e ^-h 〓−e ^-(h+l) 〓) ² dζ/∫
^a2 ／ _a1 ada∫ ^a2 ／ _a1 a′da′∫ ^∞ ／ ₀ J ₁ (aζ)J ₁ (a′ζ
)(l/ζ−1/ζ ² +1/ζ ² e ^-l )dζ……(1) =F(h, q)……(2) Here, W={(jμ ₀ a ₀ ² / q) ^1/2 −ζ} / {jμ ₀ a ₀ ² /q) ^1/2
+ζ}......(3) q=ρμ _r /ω......(4) In addition, L ₀ is the inductance of the coil in vacuum that is not close to the conductor 12, and μ _r is the inductance of the coil in vacuum that is not close to the conductor 12. The relative magnetic permeability (μ/μ ₀ , μ ₀ : vacuum permeability), ω is the angular frequency of the current flowing in the coil.

Therefore, the distance h and the parameter q expressed by the above equation (4) are expressed by the following equation.

(h, q)=F ^-1 (〓/L ₀ )=F ^-1 (Lx/Lo, Ly/Lo) ......(5) Here, F ^-1 is the inverse function of F, and , F is defined as a transformation to obtain the complex inductance 〓 from distance h and parameter q according to equation (1) above, then
F ^-1 is the inverse transformation of F.

Equation (1) above indicates that the distance h and the parameter q each independently contribute to the impedance of the coil 10, and the inverse transformation F ^-1 of Equation (1) expressed by Equation (5) is , if the real and imaginary parts of the impedance of the coil 10 are known, the parameter q, that is, the conductor 12
The distance h independent of the electrical resistance ρ and relative permeability μ _r
It is clear that it is possible to measure

Since it is difficult to use the elementary function formula method to directly obtain the inverse transformation F ^-1 from the above equation (1), specifically, for example, the inverse transformation F ^-1 can be realized by the following method. It is possible to do so.

That is, for example, let x=Lx/Lo, y=Ly/Lo, and the distance Expand h with respect to x and y as shown in the following equation.

h=H(x,y)= _o 〓 ^i,j=0 Hijx ^j y ⁱ ………(6) On the other hand, according to the above equation (1), the distance to be measured h,
In the range of parameter q, (h, q) and (x, y)
Calculate the relationship between For example, distance h=8~13
mm, product of electrical resistance ρ and magnetic permeability ρμ = 0.95 to 10.8
(×10 ^-5 Ωm) for a range of 20×15 points, a mesh figure as shown in FIG. 4 can be obtained. This fourth
In the figure, the vertical curve group shows the trajectory when the distance h is constant and the product ρμ changes, and the horizontal curve group shows the trajectory when the product ρμ is constant and the distance h changes. This shows the trajectory when Therefore, if the coefficient Hij is determined so that the above equation (6) matches this figure, the inverse transformation F ^-1 required by the equation (6)
becomes.

Another way to specifically obtain the inverse transformation F ^-1 is to use equation (1) above, keep the distance h constant,
The vertical curve group shown in FIG. 4, which is created by changing the parameter q, is expressed by the following equation.

f (x, y, h) 〓 ^i,j,k fijkx ⁱ y ^j h ^k (i, j, k=0.1...n) = 〓 ^k (〓 ^i,j fijkx ⁱ y ^j ) h ^k = 0... ...(7) The coefficients in equation (7) are
Define fijk. Since x and y are determined by measurement, equation (7) becomes an n-dimensional equation regarding distance h, and the operation of finding the root corresponds to inverse transformation F ^-1 .

Specifically, this inverse transformation F ^-1 is: the inner radius a ₁ = 30 mm, the outer radius a ₂ = 32 mm, and the length l = 5 mm of the coil 10.
If the distance h = 8 to 13 mm, the object to be measured is steel and the temperature changes from 0 to 600°C, then i =
When 0, 1, j=0, 1, 2, and k=0, 1, 2, 3, the following equation is obtained.

{fojk}=0.303012, 0.215586, 0.00274301, 0.001039
26 −0.931536, −0.384401, −0.00823891, −0.00201839 −0.931536, −0.384401, −0.00823891, −0.00201839 0.467187, 0.192914, 0.00405447, 0.001 01132……(8
) {f1jk}=1000 0000 0000 ………(9) In this case, the inverse transformation F ^-1 is Σfijkx ⁱ y ^j (k=0,
1, 2, 3) as the coefficients of h ^k to find the roots of a cubic equation. Specifically, this cubic equation is as shown in the following equation.

( 〓 ^i,j fij ₃ x ⁱ y ^j ) h ³ + ( 〓 ^i,j fij ₂ x ⁱ y ^j ) h ² + ( 〓 ^i,j fij ₁ x ⁱ y ^j ) h + 〓 ^i,j fij ₀ = 0 (10) (i=0, 1, j=0, 1, 2) Therefore, since there is always one real root, its value becomes the distance h.

The present invention can be concretely implemented as follows.

That is, first, the shape of the coil 10 to be used (inner radius a ₁ , outer radius a ₂ , length l), the range of distance h to be measured, and the range of changes in electrical resistance ρ and magnetic permeability μ of the object to be measured. Correspondingly, calculate the equation (1) above, and perform the inverse transformation using one of the methods for realizing the inverse transformation F ^-1 described above.
Find F ^-1 .

Next, the impedance ₀ of the coil 10 is measured in a state where the coil 10 is not close to the conductor 12.

Further, the impedance of the coil 10 at that time is sequentially measured with the coil 10 in the measurement state (close to the object to be measured).

Next, x and y are calculated using the following equations.

x+jy=〓/L ₀ = {〓−Real(〓 ₀ )}/{〓 ₀ −Real(
〓 ₀ )}......(11) Furthermore, the conversion process shown by the following equation is performed, and the result is used as the distance measurement value.

h=F ^-1 (x, y) ......(12) [Example] Below, with reference to the drawings, an example of an eddy current distance meter employing the eddy current distance measuring method according to the present invention will be described in detail. explain.

In this embodiment, as shown in FIG. 5, an AC bridge 14 including a coil 10 on one side, the AC bridge 1
an oscillator 1 that supplies an alternating current voltage with a constant frequency to 4;
6. A synchronous phase detector 32 that detects the unbalanced output of the AC bridge 14 into an in-phase component and a quadrature component, an in-phase component amplifier 34 that amplifies the in-phase component, a quadrature component amplifier 36 that amplifies the quadrature component, and the in-phase component. An analog-digital converter 38 converts the analog signals output from the amplifier 34 and the quadrature component amplifier 36 into digital signals, and the impedance of the coil 10 is determined from the in-phase component and the quadrature component according to the output of the analog-digital converter 38. The impedance measuring device 30 consists of an arithmetic processing circuit 40 that calculates and outputs ), an inductance conversion processing circuit 44 that converts the inductance into
an inverse conversion processing circuit 46 that inversely converts the output of
and an arithmetic processing unit 42 comprising a distance measurement value output circuit 48 for outputting the output of the distance measurement value as a distance measurement value.

The action will be explained below. First, the oscillator 16 of the impedance measuring device 30 supplies the AC bridge 14 with an AC voltage of a constant frequency (angular frequency ω). The coil 10 is one side of an AC bridge 14, and the AC bridge 14 outputs an unbalanced output (AC voltage) according to a change in the impedance of the coil 10. This unbalanced output is detected by the synchronous phase detector 32 into a DC voltage including an in-phase component and a quadrature component. These two voltage values are amplified by an in-phase component amplifier 34 and a quadrature component amplifier 36, respectively, and then sent to an analog-to-digital converter 38.
The signal is input to the processing circuit 40, converted into a digital signal, and input to the arithmetic processing circuit 40. The arithmetic processing circuit 40 calculates and outputs the value of the impedance of the coil 10 from the in-phase component and the quadrature component. The impedance of the output of the arithmetic processing circuit 40 is input to the inductance conversion processing circuit 44 of the arithmetic processing device 42, and is
It is converted into an inductance (x+jy) by equation (11) and input to the inverse conversion processing circuit 46. The inverse transformation processing circuit 46 inversely transforms x+jy using the above-mentioned equation (12) to obtain the distance h. This distance h is output from the distance measurement value output circuit 48 as a distance measurement value.

Now, attach the coil to the micrometer stand with a steel plate as the target, and set the distance between the coil and the steel plate to 8 to 13 mm.
When changing the impedance of the coil at intervals of 0.25 mm within the range of 0.25 mm and plotting it on the inductance complex plane, the measurement points shown by the horizontal circles in FIG. 4 are obtained. The distance h is determined by performing the aforementioned inverse transformation F ^-1 on these points. That is, when the inductance is measured, the position of one point on FIG. 4 is determined, and the distance h is determined by finding which curve of the group of vertical curves that point is on. FIG. 6 shows the relationship between the measured values obtained by such operations and the micrometer readings. As is clear from the figure, it shows good linearity and the difference from the micrometer reading is within +5 μm, indicating that very high measurement accuracy is obtained.

Next, while keeping the distance between the steel plate and the coil constant at 10.2 mm, the temperature of the steel plate was varied at 10°C intervals in the range of 20 to 150°C, and the change in coil impedance was measured and plotted on the inductance complex plane. Then,
Measurement points as indicated by ● marks in FIG. 4 mentioned above are obtained. For these points, the distance h is found by similarly performing the operation of inverse transformation F ^-1 . This result is shown by solid line A in FIG. (Measurement points in Figure 7 are at 5°C intervals). In FIG. 7, a solid line B indicates a change in distance output measured using a conventional eddy current distance meter under the same conditions. In the case of the conventional method, the electrical resistance ρ and magnetic permeability μ of the steel plate change due to the temperature change of the steel plate, and therefore, as shown by the ● mark in Fig. 4,
Because the inductance of the coil changes, the output changes even if the distance is constant. As can be seen from this example, in the conventional method, when the temperature of the steel plate changes by 150°C, the output changes by as much as 300 μm, whereas in the method of the present invention, the output does not change by ±5 μm.
It can be seen that it is also very good against temperature changes of the measuring object.

In the above embodiment, the distance measurement value h was determined by the inverse transformation F ^-1 corresponding to the above-mentioned equation (1), but the method for determining the distance measurement value is not limited to this, and for example, experimental It is also possible to calculate the

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to perform highly accurate distance measurement regardless of changes in the electrical property values of the object to be measured, especially changes in the electrical property values due to changes in the temperature of the object to be measured. Therefore, it has an excellent effect of being able to measure the distance and profile of objects (such as steel materials) whose temperature changes are large with high accuracy.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing the principle of the eddy current distance measuring method, FIG. 2 is a block diagram showing the configuration of a conventional eddy current distance meter, and FIG. 3 is a diagram illustrating the principle of the present invention. , a perspective view showing the arrangement of the conductor and the coil, FIG. 4 is a diagram showing changes in coil inductance depending on distance, the product of electrical resistance and magnetic permeability, and FIG. 5 is a diagram showing the eddy current according to the present invention. FIG. 6 is a block diagram showing the configuration of an embodiment of an eddy current distance meter in which the equation distance measuring method is adopted; FIG. Similarly, the figure is a diagram showing the temperature characteristics of distance measurement values in the embodiment and the conventional example. 10... Coil, 12... Conductor (object to be measured),
14... AC bridge, 16... Oscillator, 30...
... Impedance measuring instrument, 32 ... Synchronous phase detector, 34 ... In-phase component amplifier, 36 ... Quadrature component amplifier, 38 ... Analog-digital converter, 4
0... Arithmetic processing circuit, 42... Arithmetic processing unit, 4
4... Inductance conversion processing circuit, 46... Inverse conversion processing circuit, 48... Distance measurement value output circuit.

Claims (1)

[Scope of Claims] 1. An eddy current distance measuring method in which the distance between the coil and the object to be measured is determined from a change in impedance that occurs in the coil when the coil passing an alternating current is brought close to the object to be measured, comprising: The distance between the coil and the object to be measured is h, the electrical resistivity of the object to be measured is ρ, the relative permeability is μ _r , the angular frequency of the current flowing through the coil is ω, and the parameter q = ρμ _r /ω
When, the complex inductance of the coil = (Lx,
Find the relationship between Ly) and (h, q) in advance. Next, when the coil is in the measurement state, the distance h can be calculated from the measured inductance using the relationship between 〓 and (h, q). An eddy current distance measuring method characterized in that by calculating, a distance measurement value that is not affected by changes in electrical property values of an object to be measured is obtained.