JP2009033720A - Metal state detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve detection precision in a metal state detector. <P>SOLUTION: The metal state detector has: a first oscillation circuit 2 causing phase shift in oscillation waves according to a change in the inductance of a first detection coil L1; a second oscillation circuit 3 causing phase shift in oscillation waves according to a change in the inductance of a second detection coil L2; and a detection circuit 4 for detecting the phase shift in the oscillation waves outputted from the first oscillation circuit 2 and that in the oscillation waves outputted from the second oscillation circuit 3 to obtain a difference between them. The first and second detection coils L1, L2 have: a core for composing a closed magnetic circuit with the surface of a specimen to limit detection regions and detection directions on the surface of the specimen; and a coil wound around the core. The first and second detection coils L1, L2 are arranged to cross each other in a plan view in overlapping detection regions to detect a change in permeability in directions crossing each other. The first and second oscillation circuit 2, 3 are driven alternately to avoid interference. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a metal state detection device that detects a metal state, and more particularly, to a metal state detection device suitable for state detection of metal stress, strain, material, fatigue, damage, defect, and the like.

  2. Description of the Related Art A metal state detection device that detects a metal state based on a change in magnetic characteristics of the metal is known. For example, a magnetostrictive strain sensor that detects the strain of a subject using the inverse effect of magnetostriction and a magnetostrictive torque sensor that detects the torque of a rotating shaft have been put into practical use. The inverse effect of magnetostriction is a magnetic strain phenomenon in which when the surface of the specimen (magnetostrictive film) is distorted, the permeability increases in the tensile direction but decreases in the compression direction. The type strain sensor includes a detection coil that detects a change in permeability on the surface of the subject as a change in inductance (see, for example, Patent Document 1).

Magnetostrictive torque sensors are classified into types that do not impart magnetic anisotropy to the rotating shaft (for example, see Patent Document 2) and types that impart magnetic anisotropy to the rotating shaft (for example, see Patent Documents 3 and 4). can do. For example, the latter gives magnetic anisotropy of + 45 ° and −45 ° to two outer peripheral regions of the rotating shaft, respectively, and arranges a pair of detection coils facing each outer peripheral region. It is comprised so that the differential voltage which arises between may be output. That is, when torque is applied to the rotating shaft, the magnetic permeability of each outer peripheral region changes inversely due to the inverse effect of magnetostriction, so that a differential voltage is generated between the detection coils, and an output proportional to the torque is obtained.
JP-A-8-271359 JP 2001-133337 A JP 7-83769 A Japanese Patent Laid-Open No. 11-37863

  However, in the conventional metal state detection device, a slight differential voltage generated between the detection coils is detected using a bridge circuit or the like, and this differential voltage is amplified in multiple stages by an amplifier. It was difficult to detect with high accuracy.

In view of the above circumstances, the metal state detection device of the present invention created for the purpose of solving these problems is a metal state detection device for detecting a metal state, and is a first direction on the surface of the metal. The first detection coil is arranged to detect a change in permeability of the metal and detects the change in permeability as an inductance change, and is arranged to detect a change in permeability in the second direction on the metal surface. A first detection circuit that oscillates autonomously at a predetermined reference frequency and causes a phase shift in the oscillation wave in accordance with the inductance change of the first detection coil, and a predetermined A second oscillation circuit that autonomously oscillates at a reference frequency of the second and generates a phase shift in the oscillation wave in accordance with a change in inductance of the second detection coil, and a first oscillation circuit And a detection circuit for detecting a phase shift of the oscillation wave output from the second oscillation circuit and a phase shift of the oscillation wave output from the second oscillation circuit, and obtaining a difference between the two, the first detection coil and the second detection coil are: In order to limit the detection region and the detection direction on the metal surface, it is configured to include a core that forms a closed magnetic circuit between the metal surface and a coil wound around the core, and is superposed on each other. In the detection area, in order to detect the permeability change in the direction crossing each other, they are arranged in a cross shape in plan view, and the first oscillation circuit and the second oscillation circuit are driven alternately to avoid mutual interference. It is characterized by that. In this way, the state of the metal can be detected with high accuracy. That is, in the oscillation wave output from the oscillation circuit as described above, the metal state (permeability change) clearly appears as a phase shift (including frequency change and period change), and the phase in the oscillation wave Since the deviation is not easily affected by noise, the state of the metal can be detected with high accuracy based on the phase deviation of the oscillation wave. In addition, the first detection coil and the second detection coil limit the detection region and the detection direction on the metal surface, and detect the magnetic permeability change in the direction intersecting each other in the detection region overlapping with each other. It is possible to suppress the occurrence of an error due to the deviation between the detection area of the coil and the detection area of the second detection coil, and further improve the detection accuracy. Furthermore, since the first oscillation circuit and the second oscillation circuit are driven alternately, even if the detection areas are overlapped, the mutual interference of the magnetic fields generated by the respective detection coils is prevented, and the detection accuracy is reduced due to the mutual interference. Can be avoided.
The detection circuit counts a plurality of oscillation waves output from the first oscillation circuit, performs an oscillation wave count process for determining whether or not the count number has reached a predetermined number N, and outputs the oscillation wave count. Based on the time required for processing, a first direction permeability detecting means for detecting a change in permeability in the first direction and a plurality of oscillation waves output from the second oscillation circuit are counted, and the count number is a predetermined number. Oscillating wave counting processing for determining whether or not N has been reached, and based on the time required for the oscillating wave counting processing, second direction magnetic permeability detecting means for detecting a change in magnetic permeability in the second direction; Difference detection means for obtaining a difference between the magnetic permeability in one direction and the magnetic permeability in the second direction is provided. In this way, the detection accuracy of the metal state detection device can be further improved. That is, the phase shift in the oscillating wave is accumulated by the number of oscillating waves, and the accumulated phase shift is measured as time, so that highly accurate detection can be performed using an inexpensive digital circuit. Moreover, since the resolution is determined by the counter speed for time measurement and does not depend on the reference frequency of the oscillation circuit, it is possible to detect the metal state with high resolution while optimizing the reference frequency of the oscillation circuit according to the detection target. it can.

  Next, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a block diagram showing a configuration of a magnetostrictive strain sensor according to an embodiment of the present invention. The magnetostrictive strain sensor 1 shown in this figure detects the strain of the subject H by utilizing the inverse effect of magnetostriction generated on the subject surface, and includes a first detection coil L1, a second detection coil L2, a first detection coil L2, and a second detection coil L2. A single oscillation circuit 2, a second oscillation circuit 3, and a detection circuit 4 are provided.

  The first detection coil L1 is arranged to detect a magnetic permeability change in a first direction (for example, a detection target direction) on the surface of the subject, and detects the magnetic permeability change as a change in inductance. The second detection coil L2 is arranged to detect a magnetic permeability change in the second direction (for example, a direction orthogonal to the detection target direction) on the subject surface, and detects the magnetic permeability change as a change in inductance. To do.

  The detection coils L1 and L2 include a core formed of a high magnetic permeability material and a coil wound around the core in order to limit the detection region and the detection direction on the surface of the subject. . Specifically, a coil is wound around U-shaped cores 2a and 3a made of ferrite, and both ends of the U-shaped cores 2a and 3a are brought close to or in contact with the surface of the object. A closed magnetic circuit is formed between the two. This makes it possible to form magnetic paths in the first direction and the second direction in a limited region on the surface of the subject, and to detect a change in permeability in the magnetic path.

  The first oscillation circuit 2 is configured to autonomously oscillate at a predetermined reference frequency and to cause a phase shift in the oscillation wave in accordance with an inductance change of the first detection coil L1. Further, the second oscillation circuit 3 is configured to autonomously oscillate at a predetermined reference frequency and to cause a phase shift in the oscillation wave in accordance with an inductance change of the second detection coil L2. For example, if the detection coils L1 and L2 are arranged in the feedback circuit of the Schmitt oscillation circuit, it is possible to configure the oscillation circuits 2 and 3 in which a phase shift occurs in the oscillation wave according to the inductance change of the detection coils L1 and L2.

  The Schmitt oscillation circuit is an oscillation circuit that uses the hysteresis characteristics of the Schmitt trigger circuit included in the Schmitt inverter INV. For example, the Schmitt inverter INV, the capacitor C connected to the input side of the Schmitt inverter INV, and the Schmitt inverter INV A feedback circuit that feeds back the output to the input side of the Schmitt inverter INV and a resistance element interposed in the feedback circuit are configured.

In the Schmitt oscillation circuit in the initial state, since no charge is accumulated in the capacitor C, the voltage across the capacitor C is 0V. At this time, the Schmitt inverter INV has an output H level (5 V) because the input side voltage Vin is equal to or lower than _VL . When the output side voltage Vout of the Schmitt inverter INV is 5V, a current flows to the input side of the Schmitt inverter INV via the feedback circuit 2a, so that electric charges are gradually accumulated in the capacitor C, and the voltage at both ends thereof increases. When the input side voltage Vin of the Schmitt inverter INV reaches _VH , the output of the Schmitt inverter INV is switched to the L level (0 V). When the output side voltage Vout of the Schmitt inverter INV becomes 0V, the capacitor C is discharged, and the input side voltage Vin of the Schmitt inverter INV gradually decreases. When the input voltage Vin of the Schmitt inverter INV drops to _VL , the output of the Schmitt inverter INV is switched to the H level.

By repeating the above operation, a rectangular wave having a predetermined frequency is obtained from the output side of the Schmitt inverter INV. The Schmidt oscillation circuit of the oscillation frequency f (= 1 / T) is determined by the energy storage time period _{T H} discharge period _{T L,} the electric storage period _{T H} and the discharging period _{T L} is determined by the constants of the capacitor C and a resistor element. Therefore, if the detection coils L1 and L2 are arranged in the feedback circuit as resistance elements, it is possible to cause a phase shift in the oscillation wave of the Schmitt oscillation circuit according to the inductance change of the detection coils L1 and L2.

  Of course, the oscillation circuit of the present invention is not limited to the Schmitt oscillation circuit. If the oscillation circuit generates a phase shift in the oscillation wave according to the inductance change of the detection coils L1, L2, the CR oscillation circuit, LC An oscillation circuit, a crystal oscillation circuit, or the like may be used.

  The detection circuit 4 is configured using, for example, a microcomputer (one-chip microcomputer) including a CPU, a ROM, a RAM, an I / O, and the like, and performs a strain detection process described later according to a program written in the ROM. Note that the detection circuit 4 may be configured by a plurality of microcomputers, or may be configured by one or a plurality of ICs.

  The detection circuit 4 detects the phase shift of the oscillation wave output from the first oscillation circuit 2 and the phase shift of the oscillation wave output from the second oscillation circuit 3, and obtains a difference between the two. In this way, strain on the subject surface can be detected with high accuracy. That is, in the oscillation wave output from the oscillation circuits 2 and 3 as described above, the permeability change according to the strain on the subject surface clearly appears as a phase shift (including frequency change and period change) In addition, since the phase shift in the oscillation wave is hardly affected by noise, the distortion of the subject H can be detected with high accuracy based on the phase shift in the oscillation wave.

  The detection circuit 4 counts a plurality of oscillation waves output from the first oscillation circuit 2 and performs an oscillation wave count process for determining whether or not the count number has reached a predetermined number N. Based on the time required for the counting process, a first direction permeability detecting means for detecting a change in permeability in the first direction and a plurality of oscillating waves output from the second oscillation circuit 3 are counted. Second direction permeability detection means for performing an oscillation wave count process for determining whether or not the predetermined number N has been reached, and detecting a change in permeability in the second direction based on a time required for the oscillation wave count process; Preferably, the apparatus includes a difference detection unit that detects strain of the subject H based on the difference between the permeability in the first direction and the permeability in the second direction.

  In this way, the detection accuracy of the magnetostrictive strain sensor 1 can be further improved. That is, the phase shift in the oscillating wave is accumulated by the number of oscillating waves, and the accumulated phase shift is measured as time, so that highly accurate detection can be performed using an inexpensive digital circuit. Moreover, since the resolution is determined by the counter speed for time measurement and does not depend on the reference frequency of the oscillation circuits 2 and 3, high-resolution distortion is achieved while optimizing the reference frequency of the oscillation circuits 2 and 3 according to the detection target. Detection can be performed.

  Further, the first detection coil L1 and the second detection coil L2 are wound around a core that forms a closed magnetic circuit with the subject surface in order to limit the detection region and the detection direction on the subject surface. In the detection region where the coils overlap each other, they are arranged in an intersecting manner in a plan view so as to detect a change in permeability in a direction intersecting each other. For example, the cores of the first detection coil L1 and the second detection coil L2 are configured by U-shaped cores 2a and 3a having different heights, and these U-shaped cores 2a and 3a are arranged so as to intersect in plan view. . By doing so, the occurrence of errors (temperature error, material, etc.) due to the deviation between the detection region of the first detection coil L1 and the detection region of the second detection coil L2 is suppressed, and the strain detection accuracy is further improved. be able to.

  The first oscillation circuit 2 and the second oscillation circuit 3 are driven alternately to avoid mutual interference. For example, after the oscillation driving of the first oscillation circuit 2 is stopped after the oscillation drive of the first oscillation circuit 2 is stopped after the oscillation wave count processing related to the first oscillation circuit 2 is executed in a state where the oscillation drive of the second oscillation circuit 3 is stopped, 3 is executed, and thereafter the difference in measurement time required for each oscillation wave count process is obtained. In this way, even if the detection regions of the detection coils L1 and L2 are overlapped, mutual interference of the magnetic fields generated by the detection coils L1 and L2 is avoided, so that a reduction in detection accuracy due to the mutual interference is prevented. be able to.

  When the first detection coil L1 and the second detection coil L2 are arranged in a non-contact state or in a contact state with respect to the subject surface, it is preferable that the first detection coil L1 and the second detection coil L2 be fixed integrally to the subject H directly or indirectly. For example, the magnetostrictive strain sensor 1 including the first detection coil L1 and the second detection coil L2 is directly bonded to the subject surface or fixed to the subject H via a fixture such as a clamp. In this way, not only can the detection accuracy be improved by minimizing the gap between the core of each of the detection coils L1 and L2 and the subject surface, but the relative displacement between each of the detection coils L1 and L2 and the subject H is accompanied. Generation of errors can be avoided. That is, when the subject H is relatively displaced with respect to the first detection coil L1 and the second detection coil L2, an error factor existing on the subject surface (for example, variation in material and temperature on the subject surface, subject surface and core) The strain detection value may fluctuate regardless of the actual strain due to the influence of the gap fluctuation of the first detection coil L1 and the like, but the first detection coil L1 and the second detection coil L2 are fixed to the subject H integrally. Therefore, it is possible to prevent fluctuation of the strain detection value based on the error factor.

  The subject surface of the subject H whose strain is detected by the magnetostrictive strain sensor 1 is preferably a magnetostrictive film 5 formed by plating. For example, the magnetostrictive film 5 made of a nickel alloy is plated on the strain detection region of the subject H. In this way, not only can the strain be detected with high accuracy based on the inverse effect of magnetostriction in the magnetostrictive film 5 corresponding to the strain, but also hysteresis in strain detection can be suppressed. Moreover, in the magnetostrictive strain sensor 1 of the present invention, sufficient detection accuracy can be obtained even with the magnetostrictive film 5 formed by plating. Compared with the case where a magnetostrictive film is formed, not only can the cost be significantly reduced, but also high-accuracy strain detection can be performed for existing members (including resin) plated with nickel.

  Next, the phase shift accumulation action of the oscillation wave in the present invention will be described with reference to FIGS.

  FIG. 2 is an explanatory diagram showing the phase shift accumulation action of the oscillation wave (enlarged detection waveform start end), and FIG. 3 is an explanatory view showing the phase shift accumulation action of the oscillation wave (enlargement of the detection waveform end section). The waveforms shown in these figures are the output waveforms of the oscillation circuits 2 and 3 in one detection process. The number of oscillation waves output from the oscillation circuits 2 and 3 is counted, and the count number is set to a predetermined number N. Oscillation wave count processing is performed to determine whether or not the oscillation wave has been reached, and when measuring the phase deviation of the accumulated oscillation wave based on the time required for the oscillation wave count processing, the oscillation wave count in the oscillation wave count processing is counted. The waveform when the number N is 100 is shown. The upper waveform indicates a case where no strain is applied to the subject H, and the lower waveform indicates a case where the strain is applied to the subject H. As is clear from these figures, since the phase shift is not accumulated so much at the beginning of the detected waveform, that is, at the stage where the number N of oscillation waves in the oscillation wave count processing is small, the difference is not clear ( It can be seen that when the count number N increases, the phase shift of the oscillation wave is accumulated and the difference becomes clear, so that the phase shift can be easily measured (see FIG. 3). Since the phase shift of the oscillation wave increases in proportion to the distortion of the subject H, the distortion of the subject H can be measured with high accuracy based on the phase shift of the oscillation wave. The phase shift of the oscillation wave output from the first oscillation circuit 2 increases or decreases according to the distortion in the predetermined direction (detection target direction) on the subject surface, while the oscillation wave output from the second oscillation circuit 3 The phase shift does not change according to the strain in a predetermined direction on the subject surface, but changes according to the temperature and the distance to the subject surface. Therefore, the strain amount and strain polarity of the subject H are determined based on the difference. In addition to detection, a detection value in which temperature error and displacement error are canceled can be obtained.

  Next, a specific detection processing procedure of the detection circuit 4 will be described with reference to FIGS.

  In the strain detection process shown in FIG. 4, first, after initial setting (S11: including initial value setting of the oscillation wave count number N), the count number change process (S12), the first direction permeability detection process (S13). : First direction permeability detection means) and second direction permeability detection processing (S14: second direction permeability detection means) are executed in order. Then, the difference between the first direction permeability detection value and the second direction permeability detection value obtained in the permeability detection process (S13, S14) is calculated (S15: difference detection means), and the calculated difference (strain detection). (Value) is converted into a predetermined detection signal format and output (S16), and one strain detection process is completed.

  In the count number changing process shown in FIG. 5, first, the input of the count number change signal is determined (S21). If the determination result is YES, the oscillation wave count number N included in the count number change signal is read (S22). In accordance with this, the oscillation wave count number N is changed (S23).

  In the first direction magnetic permeability detection process shown in FIG. 6, after the drive of the first oscillation circuit 2 is started (S31), the counter clear process (S32), the oscillation wave count process (S33, S34), and the time measurement process (S35) is executed, and then the driving of the first oscillation circuit 2 is stopped (S36). The counter clear process is a process for clearing the oscillation wave counter and the time measurement counter (S32). The oscillation wave counting process is a process for counting the number of oscillation waves output from the first oscillation circuit 2 (S33) and determining whether the count number has reached a predetermined number N (S34). . The time measurement process is a process of reading a time measurement counter value (first-direction magnetic permeability detection value) when the count number of oscillation waves reaches N (S35).

  In the second direction magnetic permeability detection process shown in FIG. 7, after the driving of the second oscillation circuit 3 is started (S41), the counter clear process (S42), the oscillation wave count process (S43, S44), and the time measurement process (S45) is executed, and then the driving of the second oscillation circuit 3 is stopped (S46). The counter clear process is a process for clearing the oscillation wave counter and the time measurement counter (S42). The oscillation wave counting process is a process for counting the number of oscillation waves output from the second oscillation circuit 3 (S43) and determining whether or not the count number has reached a predetermined number N (S44). . The time measurement process is a process of reading a time measurement counter value (second-direction magnetic permeability detection value) when the count number of oscillation waves reaches N (S45).

  According to the present embodiment configured as described above, the magnetostrictive strain sensor 1 detects the strain of the subject H, and is arranged to detect the permeability change in the first direction on the subject surface. A first detection coil L1 that detects a change in magnetic permeability as a change in inductance, and a second detection coil L2 that is arranged to detect a change in permeability in the second direction on the surface of the subject and detects the change in permeability as a change in inductance. And the first oscillation circuit 2 that oscillates autonomously at a predetermined reference frequency and generates a phase shift in the oscillation wave according to the inductance change of the first detection coil L1, and oscillates autonomously at the predetermined reference frequency. In addition, the second oscillation circuit 3 that causes a phase shift in the oscillation wave according to the inductance change of the second detection coil L2, and the level of the oscillation wave output from the first oscillation circuit 2 And a detection circuit 4 for detecting a phase shift of the oscillation wave output from the second oscillation circuit 3 and obtaining a difference between the two, and the first detection coil L1 and the second detection coil L2 are provided on the surface of the subject. In order to limit the detection region and the detection direction, a detection region that is configured to include a core that forms a closed magnetic path between the surface of the subject and a coil that is wound around the core, and that overlaps with each other, In order to detect the change in permeability in the direction crossing each other, they are arranged in a cross shape in plan view, and furthermore, the first oscillation circuit 2 and the second oscillation circuit 3 are driven alternately to avoid mutual interference. The strain on the subject surface can be detected with high accuracy.

  That is, in the oscillation wave output from the oscillation circuits 2 and 3 as described above, the distortion (permeability change) on the surface of the subject clearly appears as a phase shift (including frequency change and period change). Since the phase shift in the oscillation wave is not easily affected by noise, the distortion of the subject H can be detected with high accuracy based on the phase shift in the oscillation wave. In addition, the first detection coil L1 and the second detection coil L2 limit the detection region and the detection direction on the subject surface, and detect the permeability change in the direction intersecting each other in the detection region overlapping with each other. It is possible to suppress the occurrence of an error due to a deviation between the detection region of the one detection coil L1 and the detection region of the second detection coil L2, and further improve the detection accuracy. Furthermore, since the first oscillation circuit 2 and the second oscillation circuit 3 are driven alternately, even if the detection regions are overlapped, the mutual interference of the magnetic fields generated by the detection coils L1 and L2 can be prevented and the mutual interference can be prevented. It is possible to avoid a decrease in detection accuracy due to.

  The detection circuit 4 counts a plurality of oscillation waves output from the first oscillation circuit 2 and performs an oscillation wave count process for determining whether or not the count number has reached a predetermined number N. Based on the time required for the counting process, a first direction permeability detecting means for detecting a change in permeability in the first direction and a plurality of oscillating waves output from the second oscillation circuit 3 are counted. Second direction permeability detection means for performing an oscillation wave count process for determining whether or not the predetermined number N has been reached, and detecting a change in permeability in the second direction based on a time required for the oscillation wave count process; Since the difference detecting means for obtaining the difference between the magnetic permeability in the first direction and the magnetic permeability in the second direction is provided, the strain detection accuracy can be further improved. That is, the phase shift in the oscillating wave is accumulated by the number of oscillating waves, and the accumulated phase shift is measured as time, so that highly accurate detection can be performed using an inexpensive digital circuit. Moreover, since the resolution is determined by the counter speed for time measurement and does not depend on the reference frequency of the oscillation circuits 2 and 3, high-resolution distortion is achieved while optimizing the reference frequency of the oscillation circuits 2 and 3 according to the detection target. Detection can be performed.

  Note that the present invention is not limited to the above-described embodiment, and arbitrary modifications can be made without departing from the scope of the claims. For example, the present invention is not limited to a magnetostrictive strain sensor, and it goes without saying that the present invention can also be implemented in metal state detection devices such as torque sensors, stress sensors, material inspection devices, metal fatigue inspection devices, damage inspection devices, and defect inspection devices.

It is a block diagram which shows the structure of the magnetostriction type strain sensor which concerns on embodiment of this invention. It is explanatory drawing which shows the phase shift accumulation effect | action (enlarged detection waveform start end part) of an oscillation wave. It is explanatory drawing which shows the phase shift accumulation effect | action (an enlarged detection waveform termination | terminus part) of an oscillation wave. It is a flowchart which shows the process sequence of the distortion | strain detection process in a detection circuit. It is a flowchart which shows the process sequence of the setting number change process in a detection circuit. It is a flowchart which shows the process sequence of the 1st direction magnetic permeability detection process in a detection circuit. It is a flowchart which shows the process sequence of the 2nd direction magnetic permeability detection process in a detection circuit.

Explanation of symbols

1 magnetostrictive strain sensor 2 first oscillation circuit 3 second oscillation circuit 4 detection circuit 5 magnetostrictive film L1 first detection coil L2 second detection coil H subject

Claims (2)

A metal state detection device for detecting a metal state,
A first detection coil arranged to detect a change in permeability in a first direction on the surface of the metal, and detecting the change in permeability as a change in inductance;
A second detection coil arranged to detect a change in permeability in the second direction on the surface of the metal and detecting the change in permeability as a change in inductance;
A first oscillation circuit that oscillates autonomously at a predetermined reference frequency and causes a phase shift in an oscillation wave in accordance with an inductance change of the first detection coil;
A second oscillation circuit that oscillates autonomously at a predetermined reference frequency and causes a phase shift in the oscillation wave in accordance with an inductance change of the second detection coil;
A detection circuit that detects a phase shift of the oscillation wave output from the first oscillation circuit and a phase shift of the oscillation wave output from the second oscillation circuit and obtains a difference between the two,
The first detection coil and the second detection coil include a core that forms a closed magnetic path between the metal surface and a coil wound around the core in order to limit a detection region and a detection direction on the metal surface. Are arranged in a crossed manner in a plan view so as to detect a change in permeability in a direction crossing each other in detection regions that overlap each other.
Further, the first oscillation circuit and the second oscillation circuit are alternately driven in order to avoid mutual interference. The detection circuit includes:
A plurality of oscillation waves output from the first oscillation circuit are counted, an oscillation wave count process is performed to determine whether or not the count number has reached a predetermined number N, and based on the time required for the oscillation wave count process A first direction permeability detecting means for detecting a change in permeability in the first direction;
A plurality of oscillation waves output from the second oscillation circuit are counted, an oscillation wave count process is performed to determine whether or not the count number has reached a predetermined number N, and based on the time required for the oscillation wave count process Second direction permeability detection means for detecting a change in permeability in the second direction;
The metal state detection device according to claim 1, further comprising difference detection means for obtaining a difference between the magnetic permeability in the first direction and the magnetic permeability in the second direction.

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