JPH076764B2 - Electromagnetic induction type plate thickness measuring method and its measuring device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for measuring the plate thickness of, for example, the central portion of a flat conductive metal material in outdoor work, and a measuring apparatus therefor.

[Prior Art] It is difficult to measure the plate thickness of a central portion of a metal flat plate or a cylindrical long metal tube with a so-called mechanical measuring instrument such as a caliper. Therefore, as an example of a method that is nondestructive to the measurement of these objects to be measured and enables the thickness measurement without removing the coating layer or the rust layer formed on the surface, the electromagnetic induction method (Japanese Patent Application No. 61-169882, Japanese Patent Application No. 61-286068) have been proposed.

In the proposed method, first, the exciting coil to which a low frequency sine wave alternating current is applied is brought close to the metal conductor. The alternating magnetic field generated from the exciting coil interlinks with the metal conductor and generates an eddy current in the metal conductor. This eddy current generates a magnetic field in a direction that hinders a change in the magnetic field of the exciting coil. These two magnetic fields are superimposed on each other, and the impedance (effective inductance) of the detection coil, which is installed on the DUT side so as to face the DUT via the exciting coil and shares the iron core with the exciting coil, is changed. Let This impedance change is significant with respect to the change in the plate thickness of the metal conductor to be measured. Therefore, it is possible to obtain the plate thickness value of the measured object by measuring the induced voltage generated in the detection coil. it can.

[Problems to be Solved by the Invention] However, in the processing method of the detection signal for plate thickness measurement in the above-mentioned conventional electromagnetic induction method, a low frequency sine wave alternating current is applied to the exciting coil from the variable frequency alternating current power source. ,
In the waveform information of the output voltage that the alternating magnetic field generated from the excitation coil induces in the detection coil via the DUT side,
Amplitude value of a distorted waveform in which the harmonic component and the input fundamental wave component are superimposed due to the nonlinear effect of the magnetic circuit composed of a series of loops of excitation coil → target object → detection coil (Peek
The direct current voltage value (integral value) obtained by observing the to Peek value) with an oscilloscope or rectifying the distorted waveform was used as the detection signal.

As described above, the output voltage value is given by the sum of the fundamental wave component and the high frequency component, but as shown in the output voltage characteristic diagram of FIG. 5, the sample plate thickness is relatively thin (2 mm or less in this example). Region), the fundamental wave component increases as the sample plate thickness increases, but the harmonic component (in this example, the third, in this example, due to the nonlinear effect peculiar to the magnetic circuit such as hysteresis phenomenon and magnetic saturation).
5 and 7 harmonics) decrease on the contrary. In other words, in this region, the fundamental wave component and the higher harmonic wave component have characteristics that are opposite to each other with respect to the plate thickness change, and in the conventional method of detecting the waveform in which both are superimposed, the degree of change of the fundamental wave with respect to the plate thickness change It means that the voltage that shows a lower rate of change is detected as compared with. In the plate thickness measuring technique using the electromagnetic induction method, which is a technique handled by the present invention, it is basically advantageous to measure a signal having a greater degree of change with respect to the plate thickness change. The reasons are: (1) The accuracy of a sensor (generally a voltmeter) used for voltage measurement can be lowered, and the cost of the entire measuring device can be reduced.

(2) The present technology basically needs to be considered for outdoor use, and it is necessary to consider mixing of electromagnetic induction noise outdoors. In this case, it is considered that the larger the rate of change, the easier it is to separate from the noise, and the measurement becomes possible in an environment where the electromagnetic induction noise is larger.

As described above, in the conventional technology, the measurement is performed using the detection signal whose degree of change with respect to the plate thickness change is lower than that of the fundamental wave component, and the measurement accuracy and noise resistance characteristic of the original detection signal are fully utilized. There was a drawback that not.

The present invention has been proposed in order to solve the above-mentioned drawbacks. Instead of measuring the voltage induced in the detection coil, signal processing is performed to obtain the fundamental wave component and the harmonic wave component in the previous voltage. The purpose is to add the measurement accuracy and noise resistance that the detection signal originally has to the plate thickness measurement technology using the electromagnetic induction method.

[Means for Solving the Problems] An electromagnetic induction type plate thickness measuring method and a measuring apparatus therefor according to the present invention for achieving the above-mentioned object have a predetermined low frequency sine wave and a preset constant exciting current value. In the method of exciting the exciting coil in, the method of measuring the plate thickness of the measured object from the induced voltage generated in the detection coil by the eddy current induced in the metal conductor which is the measured object,
From the frequency component of the induced voltage, the fundamental wave or the third harmonic of the input sine wave of the exciting coil, which is significant to the change in plate thickness, is detected, and the plate value of the object to be measured is measured using the output value. It is characterized by performing.

Further, in the above, both the fundamental wave and the third harmonic can be detected, and the plate thickness of the object to be measured is measured using the third harmonic output value from the upper limit of the plate thickness measurement range based on the fundamental wave output value. Is suitable for further expanding the measurement range.

An electromagnetic induction type plate thickness measuring apparatus using these methods is a measuring probe including an exciting coil and a detecting coil, and an alternating current for sending a low frequency sine wave alternating current at a preset current value in the exciting coil. A power source, a pressing mechanism section that presses the measurement probe against a metal conductor as an object to be measured with a constant force, and a detection section that detects an induced voltage generated by an eddy current induced in the metal conductor. A measuring device for measuring the plate thickness of a measurement object is provided with a detection circuit section for detecting a fundamental wave and / or a third harmonic of an input sine wave for plate thickness measurement from an output voltage induced in a detection coil. Characterize.

[Operation] An alternating magnetic field is applied to the object to be measured by an exciting coil, and an eddy current generated in the metal conductor, which is the object to be measured, produces an induced voltage of a distorted waveform due to the nonlinear effect of the magnetic circuit in the detection coil. To be This distorted waveform contains a fundamental wave and mainly odd-order harmonics as frequency components. The fundamental wave output voltage value and each harmonic output voltage value of the fundamental wave in the frequency component of such an induced voltage depend on the magnitude of the current value applied to the exciting coil (the strength of the magnetic field). A different significant difference with respect to the change (the significant difference described here means a characteristic that changes with respect to the exciting current value applied to the exciting coil. The same applies hereinafter). Especially, the higher harmonic components show almost no significant difference.

The present invention focuses on the fact that the signal components having different significant differences and the signal components that do not show significant differences become noises and result in narrowing the plate thickness measurement range of the object to be measured. By detecting only the frequency component (fundamental wave or third harmonic) that shows the most significant difference with respect to the excitation coil excitation current value), the measurement range and resolution can be improved by eliminating the frequency component that causes noise. Try. At the same time as this detection, it also acts to remove extraneous noise and reduces its adverse effects.

Embodiments Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram of an electromagnetic induction type plate thickness measuring device showing an embodiment of the present invention. In the present embodiment, a probe 1 having an exciting coil and a detecting coil, a variable frequency AC power supply 2 for supplying an exciting current to the exciting coil of the probe 1, and a detection signal for plate thickness measurement are created from an induced voltage of the detecting coil. It is composed of a detection device 3, a pressing mechanism unit 5 for pressing the probe 1 against the object to be measured 4 with a constant force, and a microcomputer 6 for obtaining a plate thickness value based on control and detection signals of the pressing mechanism unit 5. .

The probe 1 is configured as shown in the front view of FIG. 2 (a) and the schematic configuration diagram shown in the side view of (b). Excitation coil
11 is a coil for detection 12 which excites an alternating magnetic field to the DUT 4
Detects the magnetic field superposed on the magnetic field generated by the eddy current induced in the DUT 4 by the alternating magnetic field generated from the exciting coil 11 interlinking with the DUT 4 through the iron core 13. The external magnetic pole 14 is a magnetically closed circuit configured to efficiently return the superimposed magnetic field to the exciting coil 11 and the detecting coil 12. The detection coil 12 is installed on the metal conductor on the side of the DUT 4 so as to face the DUT 4 via the exciting coil 11.

Tables 1 and 2 show various amounts of the measurement probe used in the experiments described later, Table 1 shows various amounts in the probe configuration, and Table 2 shows electrical characteristics.

Returning to FIG. 1, the variable frequency AC power supply 2 generates a sine wave AC in a low frequency range or a high frequency range by a frequency oscillator 21, and a low pass filter (Low Pass Filter) 22
The harmonic component with respect to the fundamental wave of the oscillating frequency is cut by the constant current circuit 23 that always controls the current value to a constant value with respect to the load fluctuation of the variable frequency AC power supply 2 by the constant excitation preset in the excitation coil. It supplies electric current. The detection device 3 uses the detection signal excited by the detection coil in the probe 1 as an output voltage by the detection load resistor 31, and, for example, through a voltage divider 32 having a division ratio of 10: 1, a band pass filter (Band Pass Filter). ) Or a spectrum analyzer (FFT) 33 detects the fundamental wave or the third harmonic of the sinusoidal wave AC supplied by the variable frequency AC power supply 2 and detects the micro wave through the A / D converter 61. -Input as a measured output voltage value (detection signal) to the computer 6. The pressing mechanism unit 5 includes the probe 1
Pressure drive unit 51 and pressure sensor for detecting the pressure state
52 and a servo amplifier 53 for driving the pressurizing drive unit 51. The microcomputer 6 → D / A converter 62 → servo amplifier 53 → pressurizing drive unit 51 → pressure sensor 52 → A / D converter 63 The loop constitutes a one-degree-of-freedom force control system for obtaining a measured value at the time of preset pressing force in order to keep the measurement condition (contact state) of the DUT 4 and the probe 1 constant.

The method for measuring the plate thickness of the object to be measured using the measuring device having the above configuration will be described below. First, the probe 1 is pressed and the mechanical unit 5 is pressed.
To press the metal conductor, which is the object to be measured 4, with a constant force,
The contact state between the DUT 4 and the probe 1 is made to match a certain measurement condition. Subsequently, the variable frequency AC power supply 2 is used to generate a predetermined low frequency sine wave (for example, 1 to 7 Hz) which is set in advance at a constant current value which is set in advance according to the measurement conditions.
Excitation coil 11 is excited. Due to this excitation, an eddy current induced in the metal conductor which is the object to be measured induces a distorted waveform voltage in the detection coil 12 due to the non-linear effect of the magnetic circuit. This distorted waveform voltage contains a harmonic component in addition to the fundamental wave of the input sine wave. The reason why the harmonic component is detected in the detection coil while the excitation is performed with the sine wave that is the fundamental wave will be described below. The magnetic field induced by the exciting coil passes through the magnetic circuit composed of the object to be measured, the coil core, and the shield. This magnetic field strength is proportional to the current value applied to the exciting coil. In general, the magnetic permeability of the object to be measured, the core of the coil, and the shield has a hysteresis characteristic and a saturation characteristic with respect to the strength of the magnetic field passing inside. Due to the hysteresis characteristic and the saturation characteristic, the magnetic flux density in the magnetic circuit has the saturation characteristic even if the magnetic field strength changes sinusoidally. Since the voltage induced in the detection coil is proportional to the magnetic flux density, even if the applied voltage is changed to have a normal wave shape, it has a distorted waveform due to the magnetic permeability. When this distorted waveform is Fourier transformed, it is separated into a fundamental wave and a harmonic component having the same frequency as the applied current, and the harmonic component does not become zero. In summary, even if excitation is performed with the fundamental wave, the fundamental wave and the harmonic component are measured in the detection voltage due to the hysteresis characteristic and the saturation characteristic of the magnetic permeability.

As will be described later, each of these frequency components shows a significant difference in different characteristics with respect to the change in the plate thickness of the measured object under the same measurement conditions, and particularly, a higher harmonic component does not show a significant difference.
Therefore, only the signal component showing the significant difference is separated / detected by using the band pass filter (Band Pass Filter) or the spectrum analyzer (FFT) of the detector 3, and the measured object is measured by the following three measuring methods. The plate thickness of the object 4 is measured.

The first measuring method is a method in which only the fundamental wave component is extracted by the detection device 3 and the plate thickness is obtained from the plate thickness vs. output voltage value characteristic. This method has a feature that a large output voltage value can be obtained.

In the second measuring method, the detecting device 3 detects the binary values of the fundamental wave and the third harmonic of the low frequency sine wave alternating current applied to the exciting coil 11 from the variable frequency alternating current power source 2, and first detects the first value.
The plate value of the object to be measured is measured by the fundamental wave output value in the same manner as in the above method, and then the third harmonic output value generated by the condition of the exciting current value is determined from the plate thickness measurement range upper limit value by the fundamental wave output value. It is used to supplementally measure the plate thickness of the object to be measured. This method has a feature that the plate thickness measurement range can be expanded as a whole.

The third measuring method is a method of applying a low exciting current value to the exciting coil under a measuring condition of a small exciting current value as compared with the measuring condition using a relatively large exciting current value in the above-described first and second methods. Detects the third harmonic of frequency range sine wave AC,
This is a method of measuring the plate thickness of the object to be measured with low power by the output value of the third harmonic.

The operation of the embodiment configured as described above will be described based on the results of experiments conducted under various conditions.

FIGS. 3 (a), (b), (c), and (d) show the plate thickness as an object to be measured by the electromagnetic induction type plate thickness measuring device of the present invention.
Waveform and frequency spectrum of input and output signals under the measurement conditions of 0.5 (mm) iron flat plate (JIS standard, SS-41) with low frequency sine wave AC 7 (Hz) and exciting current 10 (mA) Is shown. (A) is an input waveform of a low frequency sine wave AC 7 (Hz) applied to the excitation coil in the probe from the variable frequency AC power source, and (B) is a frequency spectrum of the input waveform. Also, (C)
Is the output waveform detected by the detection coil in the probe, and (d) is the frequency spectrum of the output waveform. 4 (a), (b), (c), and (d) show only the excitation current value condition in the measurement conditions shown in FIG.
70 (mA), (a) shows the input waveform, (b) shows the frequency spectrum of the input waveform, (c) shows the output waveform,
(D) shows the frequency spectrum of the output waveform. As is clear from (d) of FIGS. 3 and 4,
Due to the non-linear effect of the magnetic circuit formed by the measurement probe and the DUT, the output signal should contain harmonic components even under each exciting current value condition from low exciting current value (10mA) to high exciting current value (70mA). I understand.

FIGS. 5 to 10 show the output voltage characteristics with respect to the change in the plate thickness of the object to be measured using the measuring device shown in FIG.
The material to be measured is a flat iron plate (JIS standard, SS-41), and the plate thickness is 0.5 to 8 (mm), 0.5 (mm) increments, and the number is 16.

Fig. 5 shows the frequency component of the detection signal induced in the detection coil when a test frequency of 7 (Hz) and an excitation current of 50 (mA) were applied to the excitation coil, and a bandwidth of 726 (mHz) using a spectrum analyzer. 4) shows the output voltage obtained by normalizing the fundamental frequency and the odd harmonic output value in () with the maximum output value of each frequency component. In the figure, the solid line with a circle shows the fundamental frequency, the broken line with a triangle shows the third harmonic, the one-dot chain line with a square shows the fifth harmonic, and the two-dot chain line with a cross shows the seventh harmonic. From this figure, when the excitation current value is relatively large, only the fundamental frequency indicated by the solid line with a circle indicates that it can be used to measure the plate thickness of the object to be measured. On the other hand, in the case of odd harmonics under a relatively large excitation current value (50mA), the tendency is not constant with respect to the plate thickness, so the measurement method using a single output value for each harmonic component There is no possibility.

Figure 6 shows the test frequency of 1 (Hz), the exciting current of 10 (mA), and the spectrum analyzer bandwidth of 72.6 (mHz) under the measurement conditions. The output voltage normalized by the maximum output value of is shown. The solid line with a circle is the fundamental frequency, and the broken line with a triangle is the third frequency.
It is the next harmonic. From the figure, it can be seen that the third harmonic output voltage value can widely measure the plate thickness change of the object under measurement as compared with the fundamental frequency output value under a relatively small exciting current value condition (10 mA). In addition, since the bandwidth is extremely narrowed by FFT, external noise is almost cut, and highly accurate measurement becomes possible. The results of a more detailed examination of the facts described in FIGS. 5 and 6 are shown in FIGS.

FIG. 7 shows the output voltage characteristic of the fundamental frequency output value when the exciting current is changed among the measurement conditions shown in FIG. From this figure, the output voltage characteristic with respect to the plate thickness has a dependency on the exciting current value, and when using the fundamental frequency output value, when trying to measure the plate thickness up to 5 (mm) 50
It can be seen that a large exciting current of (mA) or more is required and a large exciting current value is required as compared with the third harmonic wave shown in FIG.

FIG. 8 shows the output voltage characteristic of the third harmonic output value (3 Hz) when the exciting current value is changed among the measurement conditions shown in FIG. From this figure, the output voltage characteristic with respect to the plate thickness has a dependency on the exciting current value, and the third harmonic output value tends to be significant with respect to the change in the plate thickness of the DUT at a relatively small exciting current value. I understand that there is.

Fig. 9 shows the measurement conditions of test frequency 7 (Hz) and exciting current value 70 (mA). The solid line with ○ mark is based on the conventional detection method, and the output voltage induced in the detection coil is shown. In the low frequency range sine wave alternating current applied to the excitation coil at the fundamental wave, the distortion output waveform including the harmonic component of the fundamental wave
Peek To Peek value is measured with an oscilloscope. The broken line with triangles is the spectrum analyzer (FFT)
Low frequency range sine wave AC fundamental wave 7 (Hz), bandwidth 72
It shows the output value at 6 (mHz). The horizontal axis is the plate thickness of the DUT, and the vertical axis is the output voltage normalized by each output value. From the figure, compared with the conventional method, the saturation point of the output voltage with respect to the change in the thickness of the DUT is delayed, and the slope of the output voltage with respect to the change in the thickness can be increased (improved resolution).
I understand. Here, the reason why the output voltage with respect to the thick plate of the sample differs depending on the applied current value will be described below. If the plate thickness of the object to be measured is constant, the magnetic field generated by the exciting coil becomes stronger as the current value applied to the exciting coil becomes larger. Since the magnetic circuit formed by the object to be measured, the coil core, and the shield is constant, the demagnetizing field (the magnetic field in the direction opposite to the magnetic field induced by the exciting coil) due to the eddy current in this exciting circuit is constant. . Therefore, when the value of the current applied to the exciting coil becomes large, the magnetic field interlinking with the detecting coil becomes strong and the output voltage induced in the detecting coil also becomes large. Since this phenomenon occurs for each plate thickness, as shown in FIGS. 7 and 8, the detected voltage increases for each plate thickness as the applied current value increases for each plate thickness.

Fig. 10 shows that, under the measurement conditions of test frequency 5 (Hz) and exciting current 70 (mA), the solid line with a circle is the fundamental frequency, the broken line with a triangle is the third harmonic, and the spectrum analyzer bandwidth 72
It shows the output voltage at 6 (mHz). From the figure, it can be seen that the output voltage of the fundamental wave is saturated (4 mm) with respect to the change in the thickness of the DUT, but the output value of the third harmonic becomes the basic value by giving a relatively large exciting current value. Since there is a tendency to show a significant difference with respect to the change in the plate thickness of the measured object from near the saturation point of the frequency, the output value of the fundamental frequency and the third harmonic number (measurement method by a relatively large exciting current value) and the third harmonic It can be seen that the measurable range can be wide compared with the output value of (measurement method using a relatively small exciting current value).

In FIGS. 8 and 10, the reason why the third harmonic shows a particularly significant difference is due to waveform distortion caused by a nonlinear effect such as hysteresis phenomenon or magnetic saturation of the magnetic substance, and it is considered that it differs depending on the sample. However, these are explained by Fourier analysis or the like.

From the above results, the fundamental wave output voltage value in the frequency component of the distortion output voltage in which the higher harmonic wave of the fundamental wave induced in the detection coil is superimposed and each harmonic wave output voltage value of the fundamental wave are applied to the exciting coil. The output voltage characteristic changes according to the plate thickness of the object to be measured according to the magnitude of the exciting current value (magnetic field strength). This is because when the test frequency is kept constant, the output voltage characteristics with respect to the change in the plate thickness of the object to be measured show the dependence on the exciting current. In the measurement conditions, when the excitation current value is relatively large, the fundamental wave output voltage value of the distortion output voltage frequency component shows a large significant difference with respect to the plate thickness change of the DUT, and the excitation current value is relatively small under the measurement conditions. In the case of, the third harmonic output voltage value tends to show a significant difference with respect to the change in the plate thickness of the measured object as compared with the fundamental wave output voltage value. That is,
Magnitude of current value applied to excitation coil (magnetic field strength)
The frequency component of the distortion output voltage induced by the detection coil depends on the change in the thickness of the object to be measured. Since it is a detection signal in which the (component) is superimposed, the signal component showing a significant difference of different characteristics and the signal component showing no significant difference become noise, resulting in narrowing the plate thickness measurement range of the object to be measured. Further, the saturation point at which the significant difference is not shown has a thicker plate thickness value in the case of the third harmonic than the fundamental wave.

Therefore, in the measurement method of the present embodiment, only one frequency component (fundamental wave or third harmonic) showing a significant difference is separated and detected to eliminate the factor that narrows the plate thickness measurement range, and Improve the resolution. As a result of performing the filtering as described above, external noise such as commercial power supply noise is removed, and the influence thereof is reduced. In addition, the second measurement method makes it possible to further expand the measurement range. Further, in the third measuring method, the third harmonic wave is used to enable the plate thickness of the object to be measured to be measured even at a relatively low exciting current value.

[Effects of the Invention] As described above, when the electromagnetic induction type plate thickness measuring method and the measuring apparatus of the present invention are used, the plate thickness of the rusted metal conductor in outdoor work, which is the greatest feature of the prior art. The measurement range can be expanded and the resolution can be improved while taking advantage of the fact that measurement can be performed without surface treatment, or highly accurate measurement that is not affected by external noise can be performed. It is possible to reduce the size of the power supply and realize a sensor system with excellent portability by making the power supply smaller and lighter.

That is, first, the frequency component of the distortion output waveform generated in the detection coil due to the non-linear effect of the magnetic circuit at the preset low-frequency sine wave and excitation current value is filtered or Fourier-analyzed by the detection circuit unit to obtain the excitation coil. A measurement method using a fundamental wave output voltage value that shows a significant difference in the plate thickness change of the object to be measured according to the exciting current value applied to. Second
The frequency component of the distortion output waveform generated in the detection coil due to the non-linear effect of the magnetic circuit at the preset excitation current value is filtered or Fourier-analyzed by the detection circuit, and the excitation current value applied to the excitation coil is adjusted according to the excitation current value. Using the two values of the fundamental wave output value and the third harmonic output value that show a significant difference in the plate thickness change of the measured object, first measure the plate thickness of the measured object with the fundamental wave output value, and then the fundamental wave output A measurement method for supplementally measuring the plate thickness of the object to be measured from the upper limit of the plate thickness measurement range based on the value using the third harmonic output value generated under the conditions of the exciting current value. By these two measuring methods, the measuring range can be expanded and the resolution can be improved, or the influence of external electromagnetic noise can be reduced. Thirdly, the distortion waveform generated in the detection coil due to the non-linear effect of the magnetic circuit in the above-mentioned two measuring methods or the conventional electromagnetic induction method (Japanese Patent Application Nos. 61-169882 and 61-286068). Amplitude value (Peek To Pe
ek) or the third harmonic showing a significant difference with respect to the change in the plate thickness of the DUT under a small exciting current value condition as compared with the measuring method using the DC voltage value (integral value) obtained by rectifying. With the measurement method of measuring the plate thickness of the DUT using the output voltage value, it is possible to measure the plate thickness of the DUT with low power, which has the advantage of enabling a sensor system with excellent portability. it can.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an electromagnetic induction type plate thickness measuring apparatus of the present invention, and FIGS. 2 (a) and 2 (b) are probes used in the electromagnetic induction type plate thickness measuring apparatus of the present invention. Figure 3, (a), (b), (c), and (d) show the input signal waveform and frequency spectrum and output signal waveform and frequency of the measuring probe at a relatively small excitation current value (10 mA). Figure showing spectrum, Fig. 4 (a), (b),
(C) and (d) are diagrams showing the input signal waveform and the frequency spectrum and the output signal waveform and the frequency spectrum of the measuring probe at a relatively large exciting current value (70 mA).
The figure shows the output voltage characteristic diagram of the fundamental wave (7Hz) and each odd harmonic (21Hz, 35Hz, 49Hz) against the change of the thickness of the DUT under the condition of relatively large exciting current value (50mA). Output voltage characteristic diagram of the fundamental wave (1Hz) and the third harmonic (3Hz) with respect to the change in the thickness of the DUT at a relatively small exciting current value (10mA). Fig. 7 shows the DUT at the fundamental wave (1Hz). Output voltage characteristic diagram of each excitation current value with respect to plate thickness change, Fig. 8 is an output voltage characteristic diagram of each excitation current value with respect to plate thickness change of the measured object at the 3rd harmonic (3Hz), and Fig. 9 is the measured value. Comparison of output voltage characteristics by the conventional electromagnetic induction method (Japanese Patent Application No. 61-286068) and fundamental wave output voltage characteristics of the present invention against changes in the plate thickness of an object. Based on the output voltage characteristics of the fundamental wave (5Hz) and the third harmonic (15Hz), measurement using two values of each output value It is an explanatory view of a method. 1 ... probe, 2 ... variable frequency power supply, 3 ... detector, 4 ... measurement object, 5 ... pressing mechanism part, 6 ... microcomputer, 11 ... excitation coil, 12 ... detection coil.

Claims (4)

[Claims] 1. An exciting coil is excited with a predetermined low frequency sine wave and a preset constant exciting current value, and is generated in the detecting coil by an eddy current induced in a metal conductor as an object to be measured. In the method of measuring the plate thickness of the object to be measured from the induced voltage, the fundamental frequency component of the input sine wave frequency applied to the exciting coil is detected from the frequency component of the induced voltage, and the object to be measured is determined by the fundamental frequency output value. An electromagnetic induction type plate thickness measuring method characterized by measuring a plate thickness. 2. An exciting coil is excited with a predetermined low frequency sine wave and a preset constant exciting current value, and is generated in the detecting coil by an eddy current induced in a metal conductor as an object to be measured. In the method for measuring the plate thickness of the object to be measured from the induced voltage, the fundamental wave and the third harmonic component of the input sine wave applied to the exciting coil are detected from the frequency component of the induced voltage, and the fundamental wave output value is first detected. To measure the plate thickness of the measured object, and then from the upper limit of the measured thickness range of the measured object based on the fundamental wave output value, measure the plate thickness of the measured object using the third harmonic output value. An electromagnetic induction type plate thickness measuring method characterized by the above. 3. An exciting coil is excited with a predetermined low-frequency sine wave and a preset constant exciting current value, and is generated in the detecting coil by an eddy current induced in a metal conductor as an object to be measured. In the method of measuring the plate thickness of the object to be measured from the induced voltage, the third harmonic component of the input sine wave applied to the exciting coil is detected from the frequency component of the induced voltage, and measured by the third harmonic output value. An electromagnetic induction type plate thickness measuring method characterized by measuring the plate thickness of an object. 4. A measuring probe comprising an exciting coil and a detecting coil, an AC power source for sending a low frequency sine wave AC at a preset current value to the exciting coil, and the measuring probe. Measurement for measuring the plate thickness of an object to be measured, which includes a pressing mechanism section for pressing a metal conductor as a measurement object with a constant force and a detection section for detecting an induced voltage generated by an eddy current induced in the metal conductor An electromagnetic induction type plate thickness measurement, characterized in that the device is provided with a detection circuit section for detecting an input sine wave fundamental wave and / or third harmonic wave for plate thickness measurement from an output voltage induced in a detection coil. apparatus.