JP2019086497A - Inspection device - Google Patents

Abstract

To provide an inspection device that has an exciting coil, a detection coil and a compensation coil coaxially disposed, conducts an inspection of a measured object by a difference between a detection result of the compensation coil and a detection result of the detection coil, and thereby miniaturizes the device, and stabilizes detection accuracy.SOLUTION: The inspection device comprises: an exciting coil 4 that is connected to an AC voltage source 2; a detection coil 5 that detects an amount of magnetic flux of an eddy current generating in a measured object 9 due to a magnetic field of the exciting coil 4; a compensation coil 6 that has almost the same magnetic characteristic as the detection coil 5; and a measurement unit that measures the amount of magnetic flux of the measured object 9 detected by the detection coil 5 on the basis of outputs of the detection coil 5 and the compensation coil 6. The exciting coil 4, the detection coil 5 and the compensation coil 6 are parallely arranged with a magnetic flux direction of the exciting coil 4 as the same axis, and respective coils are an air core coil, and with respect to the measured object 9, the detection coil 5 and the compensation coil 6 are disposed at a position different in a distance in the magnetic flux direction.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an inspection apparatus for inspecting an object to be measured using an eddy current flaw detection method.

  Patent Documents 1 and 2 disclose techniques related to inspection using an eddy current flaw detection method. According to the technique disclosed in Patent Document 1, the nondestructive inspection device includes an excitation coil 101 facing the subject 2, a reference coil 105 electromagnetically coupled to the excitation coil 101, and a detection coil 104 facing the subject 2. Excitation coil 101 is excited with an alternating voltage, and the amplitude and phase of the signal under test, which is the difference between the voltage generated in reference coil 105 and the voltage generated in detection coil 104, are referenced to the voltage of excitation coil 101 or reference coil 105. And a measurement processing unit 4 that detects a signal.

In the technique shown in Patent Document 2, two probes S ₁ and S ₂ each including an excitation coil M and a detection coil D are disposed at regular intervals in the circumferential direction in proximity to the inspection material 1, and the two probes An alternating current is supplied from the oscillator 3 to the excitation coils M ₁ and M ₂ of S ₁ and S ₂ to generate eddy currents in the test material 1, and the detection coils D _{1 of the two} probes S ₁ and S ₂ , D ₂ to find the difference value e ₀ of the induced voltages e ₁ , e ₂ and detect a defect by this e ₀ .

JP, 2015-087168, A JP, 2000-227420, A

  However, the techniques disclosed in Patent Documents 1 and 2 have a structure in which each has a detection coil and a reference coil respectively corresponding to a plurality of excitation coils, so there is a limit to the miniaturization of the device, and Because the measurement is performed at different points for each of the exciting coils in the above, the detection accuracy may not be stable.

  In the present invention, the excitation coil, the detection coil, and the compensation coil are coaxially disposed, and the device is miniaturized by inspecting the object to be measured based on the difference between the detection result of the compensation coil and the detection result of the detection coil. And provide an inspection device that stabilizes detection accuracy.

  The inspection apparatus according to the present invention includes an excitation coil connected to an AC power supply, a detection coil for detecting an amount of magnetic flux of an eddy current generated on an object to be measured by a magnetic field of the excitation coil, and magnetic characteristics substantially the same as the detection coil. And a measurement unit for measuring the amount of magnetic flux of the object detected by the detection coil based on the output of the detection coil and the compensation coil, the excitation coil, the detection coil and the compensation coil being an excitation coil Are arranged in parallel with each other in the same direction of magnetic flux, and each coil is an air core coil, and the detection coil and the compensation coil are disposed at different positions in the magnetic flux direction with respect to the object to be measured It is.

  As described above, in the inspection apparatus according to the present invention, the excitation coil connected to the AC power supply, the detection coil for detecting the amount of magnetic flux of the eddy current generated on the object by the magnetic field of the excitation coil, and the detection coil An excitation coil, a detection coil, and a compensation, comprising: a compensation coil having the same magnetic characteristic; and a measurement unit that measures an amount of magnetic flux of an object detected by the detection coil based on outputs of the detection coil and the compensation coil. The coils are arranged in parallel, with the magnetic flux direction of the exciting coil as the coaxial, and each coil is an air core coil, and arranged at a position where the detection coil and the compensation coil are different distances in the magnetic flux direction Since the detection coil and the compensation coil are coaxially provided for one excitation coil, the apparatus can be miniaturized.

  In addition, since each coil is an air core coil, the distance between the detection coil and the object to be measured and the distance between the compensation coil and the object to be measured are different from each other. The magnitude of the magnetic flux detected by the current can be made different, and the difference can be used to accurately inspect the object to be measured. That is, for example, in the case of having a core having a high permeability such as an iron core, even if the distance between the detection coil and the compensation coil with respect to the object to be measured is different, the respective coils are disposed coaxially Then, the detected magnetic flux due to the eddy current of the object to be measured is detected in the same manner, and the difference can not be accurately detected. However, in the present invention, such a problem is solved by using an air core coil. can do.

  Furthermore, by setting each coil as an air-cored coil, the effect of the loss due to the magnetic hysteresis can be ignored to realize a highly accurate inspection.

  Furthermore, by setting each coil as an air-cored coil, the attraction force due to the magnetic force generated between the coil and the object to be measured can be suppressed, and the coil can be freely moved on the object to be measured.

  Furthermore, since the inspection apparatus can be configured only with three coils (for example, one obtained by winding three copper coils on a coil support), a power supply, and a measuring unit, it can be manufactured extremely easily and inexpensively. It plays an effect.

  In the inspection apparatus according to the present invention, the detection coil and the compensation coil are disposed at positions where the distance between the excitation coil and the detection coil in the magnetic flux direction is equal to the distance between the excitation coil and the compensation coil in the magnetic flux direction. is there.

  Thus, in the inspection apparatus according to the present invention, the detection coil and the compensation coil are disposed at positions where the distance between the excitation coil and the detection coil in the magnetic flux direction is equal to the distance between the excitation coil and the compensation coil in the magnetic flux direction. Therefore, the influence of the excitation magnetic field by the excitation coil received by the detection coil and the compensation coil can be made almost the same, and the excitation magnetic field and the disturbance unrelated to the measurement result can be cancelled, and only the detected magnetic flux can be measured. The effect of being able to

  In the inspection apparatus according to the present invention, the detection coil and the compensation coil are connected in reverse polarity so as to cancel each other's magnetic fields.

  As described above, in the inspection apparatus according to the present invention, since the detection coil and the compensation coil are connected in reverse polarity so as to cancel each other's magnetic field, the effect of the excitation magnetic field on the detection coil or the compensation coil or disturbance is With the effect of eliminating the influence, only the magnetic flux from the object to be measured can be detected with high sensitivity.

  In the inspection apparatus according to the present invention, the exciting coil is wound in a spiral shape whose diameter gradually increases from the center.

  As described above, in the inspection apparatus according to the present invention, since the exciting coil is wound in a spiral shape whose diameter gradually increases from the center, the exciting magnetic field can be generated from the central portion of the core. On the other hand, not only the peripheral portion of the coil but also the surface can be inspected from the central portion to the peripheral portion.

  The inspection apparatus according to the present invention includes arrangement changing means for changing the distance of the magnetic flux direction of the detection coil to the excitation coil and / or the distance of the magnetic flux direction of the compensation coil to the excitation coil.

  As described above, in the inspection apparatus according to the present invention, since the arrangement changing means for changing the distance of the magnetic flux direction of the detection coil to the excitation coil and / or the distance of the magnetic flux direction of the compensation coil to the excitation coil is provided Even if there is an error in the magnetic field characteristics of the motor and the compensation coil, the influence of the excitation magnetic field on the detection coil and the compensation coil is adjusted by changing the arrangement of the detection coil and the compensation coil to minimize the error. The effect of being able to be reduced to

  The inspection apparatus according to the present invention uses the excitation coil, the detection coil and / or the compensation coil as a superconducting coil.

  As described above, in the inspection apparatus according to the present invention, since the excitation coil, the detection coil, and / or the compensation coil is a superconducting coil, a large magnetic field can be produced with a large current flowing through the superconducting coil. The effect is that it can be improved dramatically.

  The inspection apparatus according to the present invention is provided with adjusting means for adjusting the voltage applied to the exciting coil in accordance with the distance between the object to be measured and the detection coil.

  As described above, the inspection apparatus according to the present invention includes the adjustment means for adjusting the voltage applied to the excitation coil in accordance with the distance between the object to be measured and the detection coil. The effect of being able to apply a magnetic field is exerted.

  The inspection apparatus according to the present invention comprises calibration means for calibrating the output of the detection coil and the output of the compensation coil in a state where the object to be measured is arranged.

  As described above, in the inspection apparatus according to the present invention, since the calibration means for calibrating the output of the detection coil and the output of the compensation coil in a state where the object to be measured is arranged, the object to be measured is, for example, a magnetic body In such a case, the excitation magnetic field is not evenly distributed due to the permeability of the object to be measured, and the influence of the excitation magnetic field on the detection coil and the influence of the excitation magnetic field on the compensation coil become unbalanced. The balance is adjusted, and the effects of the excitation magnetic field on the detection coil and the effects of the excitation magnetic field on the compensation coil can be made identical.

It is a circuit block diagram which shows the structure of the test | inspection apparatus which concerns on 1st Embodiment. It is a 1st figure which shows the arrangement configuration of the coil in the test | inspection apparatus which concerns on 1st Embodiment. It is a 2nd figure showing the arrangement composition of the coil in the inspection device concerning a 1st embodiment. It is a figure which shows the connection structure of the coil in the test | inspection apparatus which concerns on 1st Embodiment. It is a 1st figure which shows the structure of the coil in the inspection apparatus which concerns on other embodiment. It is a 2nd figure showing the composition of the coil in the inspection device concerning other embodiments. It is a 3rd figure which shows the structure of the coil in the test | inspection apparatus which concerns on other embodiment. It is a figure which shows the other circuit structure of the test | inspection apparatus which concerns on other embodiment. It is a figure which shows the system outline | summary of the test | inspection apparatus used in the Example. It is a figure which shows the coil dimension of the inspection apparatus used in the Example. It is a graph which shows the measurement result of the impedance change with respect to the plate thickness d in case a test body is SM490A steel plate. It is a graph which shows the measurement result of the impedance change with respect to the plate thickness d in case a test body is an aluminum plate. It is a graph which shows the measurement result of the impedance change with respect to the plate | board thickness d of SM490A steel plate in a hand-held type inspection apparatus.

  Hereinafter, embodiments of the present invention will be described. In addition, the same reference numerals are given to the same elements throughout the present embodiment.

First Embodiment of the Present Invention
An inspection apparatus according to the present embodiment will be described using FIGS. 1 to 4. The inspection apparatus according to the present embodiment measures the thickness, corrosion, magnetic properties, etc. of the object to be measured using the eddy current flaw detection method, and it is possible to perform measurement with low magnetic field and low frequency without magnetic saturation. It is

  FIG. 1 is a circuit configuration diagram showing a configuration of an inspection apparatus according to the present embodiment. In FIG. 1, the inspection apparatus 1 is disposed in parallel to the exciting coil 4 coaxially with the exciting coil 4 disposed in the vicinity of the DUT 9 and generating a magnetic field in the direction perpendicular to the DUT 9. Detection coil 5, a compensation coil 6 for canceling and compensating the magnetic field of the excitation coil 4 for the detection coil 5, an AC voltage source 2 for applying an AC voltage to the excitation coil 4, and a voltage from the AC voltage source 2 A current limiting resistor 3 for controlling the excitation current, an amplifier 7 for amplifying a signal detected by the detection coil 5, and a lock-in amplifier 8 for measuring the voltage of the amplified signal.

  The detection coil 5 and the compensation coil 6 are connected in reverse polarity in a configuration having the same magnetic characteristic in order to cancel the excitation magnetic field that affects them. Moreover, in order to make the influence of the excitation magnetic field on the detection coil 5 and the influence of the excitation magnetic field on the compensation coil 6 identical, the distance from the excitation coil 4 to the detection coil 5 and the distance from the excitation coil 4 to the compensation coil 6 It is disposed at the same position as the distance. Furthermore, in order to reliably detect the magnetic flux induced by the eddy current of the object to be measured 9, the side on which the object to be measured 9 is disposed with the detection coil 5 interposed between the excitation coil 4 and the object to be measured 9. Is placed on the side where is not placed. Furthermore, each coil is an air-cored coil, and the coil winding on which the coil is wound is nonmagnetic and nonconductive. By measuring the voltage by the lock-in amplifier 8 with such a circuit configuration, it is possible to estimate the thickness, thickness reduction and the like of the object to be measured.

  As shown in FIG. 1, in the present embodiment, the resistance change of the exciting coil is detected indirectly by the mutual induction method in which the detecting coil is provided separately from the exciting coil. In the mutual induction method, the compensation coil 6 is excluded, and a circuit consisting only of the exciting coil 4 and the detecting coil 5 can be considered. However, with regard to the magnetic flux linked to the detecting coil 5, the direction of the magnetic flux generated by the exciting coil 4 However, since it is larger than the magnetic flux induced by the eddy current flowing to the object to be measured 9, it is difficult to measure the magnetic flux of the latter. Therefore, the configuration provided with the compensation coil 6 as shown in FIG. 1 is extremely effective. By providing the compensation coil 6, the influence of the magnetic flux generated by the excitation coil 4 can be canceled out and ignored, and the detection coil 5 detects only the magnetic flux induced by the eddy current flowing to the object 9 by the excitation magnetic field. It is possible to

  Further, if using the self-induction method in which the exciting coil and the detecting coil are the same coil, the measured voltage is expressed by the following equation.

  Where I is the excitation current of the excitation coil, V is the voltage of the excitation coil, ω is the angular frequency of the excitation current, R and L are the resistance and inductance of the excitation coil, and ΔR and ΔL are the resistances of the excitation coil produced by the object to be measured And the amount of change in inductance. When the excitation coil is a general copper coil and measurement is performed at a low frequency and a low magnetic field, since ΔR is much smaller than the resistance R of the excitation coil itself, measurement of ΔR becomes difficult. Further, since the voltage | ΔRI | generated by ΔR is smaller than the voltage | jω LI | generated by the inductance L of the excitation coil, measurement of ΔR becomes difficult. Furthermore, since R changes with temperature and the amount of change is larger than ΔR, there is a problem that the temperature of the exciting coil must be kept constant when measuring ΔR.

  On the other hand, when the mutual induction method is used as in the present embodiment, the effects of R and L can be ignored, and the resistance and inductance of the detection coil are input impedances of the amplifier connected thereto. Can be ignored if is large enough. This is because if the input impedance is sufficiently large, no current flows in the secondary side, that is, the detection coil and the compensation coil. This will be described below using equations.

The voltage V _d of the detection coil 5 is expressed by the following equation.

Here, M coil mutual inductance between the detection coil 5 and the exciting coil 4, [Delta] R _M resistance apparent caused by the object to be measured 9 (hereinafter, referred to as mutual equivalent resistance) variation, .DELTA.M is measured This is the mutual inductance change amount caused by the object 9. On the other hand, the voltage V of the compensation coil 6 is expressed by the following equation, ignoring the influence of the object 9 to be measured.

However, it was assumed that the mutual coil inductance between the exciting coil 4 and the compensating coil 6 was equal to the mutual coil inductance M between the exciting coil 4 and the detecting coil 5. Therefore, the differential voltage ΔV _M is expressed by the following equation.

  From the above equation, it is possible to avoid the deterioration of the measurement accuracy due to the resistance and inductance of the exciting coil 4, the detecting coil 5 and the compensating coil 6 itself.

  In order to supply a stable current to the exciting coil 4, the impedance of the exciting coil 4 is reduced by increasing the wire diameter of the coil and reducing the number of turns, and further, the current limiting resistor 3 large enough for the impedance of the exciting coil 4 By connecting in series, a stable constant current can be supplied. However, even if a large current is supplied, the heat dissipation is sufficiently enhanced so that the resistance change due to the temperature change of the current limiting resistor 3 does not occur.

  A more specific arrangement of coils will be described. FIG. 2 is a view showing an arrangement configuration of coils in the inspection apparatus according to the present embodiment. As shown in FIG. 2, the excitation coil 4, the detection coil 5, and the compensation coil 6 are arranged such that the axis of the coil is in the vertical direction with respect to the flat object to be measured.

  As described above, since the influence of the excitation magnetic field by the excitation coil 4 needs to be the same for the detection coil 5 and the compensation coil 6, the magnetic characteristics of the detection coil 5 and the compensation coil 6 are the same. There is. Further, the distances from the exciting coil 4 are also arranged at the same distance. The detection coil 5 and the compensation coil 6 are disposed at both ends with the exciting coil 4 interposed therebetween. The detection coil 5 is disposed closer to the object 9 to be measured, and the compensation coil 6 is disposed farther to the object 9 to be measured. 5 and the compensation coil 6 are connected so as to be opposite in polarity. That is, the influence of the excitation magnetic field can mutually cancel each other, and the influence of the magnetic flux induced to the eddy current of the object to be measured 9 can be made different between the detection coil 5 and the compensation coil 6. It becomes possible to detect only the magnetic flux from the object 9 to be measured.

  At this time, the core of each coil is empty. That is, since the core is nonmagnetic and nonconductive, the magnetic permeability is extremely low, and the magnetic flux induced by the eddy current of the DUT 9 does not affect the compensation coil 6. Therefore, the detection coil 5 detects the excitation magnetic field + the magnetic field from the object to be measured 9, and the compensation coil 6 detects only the excitation magnetic field. Taking the difference between them detects only the magnetic field from the object to be measured 9 with high sensitivity. It is possible to detect

  In addition, by using an air-cored coil, the magnetic force generated by the exciting coil 4 can suppress the force of attracting the object to be measured 9 and the coil can be freely moved on the object to be measured 9. It is possible to improve dramatically.

In the above, the magnetic characteristics of the detection coil 5 and the compensation coil 6 is the same, the same as the distance l ₁ from the exciting coil 4 to the detecting coil 5, and a distance l ₂ from the excitation coil 4 until the compensation coil 5 ( Although it is assumed that l ₁ = l ₂ ), for example, as shown in FIG. 3, the distance l ₁ from the excitation coil 4 to the detection coil 5 and the distance l ₂ from the excitation coil 4 to the compensation coil 5 are not the same ( l ₁ ≠ l ₂ ) may be used, and the magnetic properties of the detection coil 5 and the compensation coil 6 may not be the same. In this case, since the influence of the excitation magnetic field is detected as the difference between the detection coil 5 and the compensation coil 6, calibration means (not shown) for performing calibration in advance is provided. That is, in the state where the exciting coil 4 is excited, the difference between the detection coil 5 and the compensation coil 6 is previously adjusted so as to be 0, and the coil is brought close to the object 9 in this state. It becomes possible to detect only the magnetic flux. At this time, for adjustment of the detection coil 5 and the compensation coil 6, for example, numerical values may be corrected on a computer, or the number of turns of the coil, the thickness of the conducting wire, the coil diameter, the coil position, etc. You may adjust by changing to.

  Further, in the above description, the detection coil 5 and the compensation coil 6 are connected in reverse polarity, but without connecting the detection coil 5 and the compensation coil 6, the difference is obtained by calculation from the respective detection results. It is also good. Also in this case, by providing the calibration means (not shown) as described above, in the state where the exciting coil 4 is excited, the difference between the detection coil 5 and the compensation coil 6 is adjusted in advance so as to be zero. You may do so. In connection with this, for example, as shown in FIG. 4, the connection of the detection coil 5 and the compensation coil 6 may be switched. That is, the configuration is provided with a switch unit 40 as shown in FIG. 4 that switches between the state in which the detection coil 5 and the compensation coil 6 are connected in reverse polarity and the state in which the detection coil 5 and the compensation coil 6 are not connected. It may be

  In particular, when the object to be measured 9 is a magnetic body, the excitation magnetic field is not evenly distributed due to the permeability of the object to be measured 9, and the influence of the excitation magnetic field on the detection coil 5 and the influence of the excitation magnetic field on the compensation coil 6 Although the balance is unbalanced by the above calibration means, the influence of the excitation magnetic field on the detection coil 5 and the influence of the excitation magnetic field on the compensation coil 6 can be made the same.

(Other Embodiments of the Present Invention)
The inspection apparatus according to the present embodiment will be described with reference to FIGS. 5 to 8. In addition, the description which overlaps with the said 1st Embodiment in this embodiment is abbreviate | omitted.

  FIG. 5 is a first view showing a configuration of a coil in the inspection apparatus according to the present embodiment. In FIG. 5, the exciting coil 4 is formed by winding a conducting wire spirally so that the coil diameter gradually increases from the central portion of the coil. When the excitation coil 4 is wound in this manner, the excitation magnetic field is distributed in a plane from the central portion of the coil to the periphery of the coil as a whole. It becomes possible to expand the range which can detect it and to raise detection accuracy.

  FIG. 6 is a second view showing the configuration of the coil in the inspection apparatus according to the present embodiment. In FIG. 6, the arrangement positions of the excitation coil 4, the detection coil 5 and the compensation coil 6 can be mechanically varied. This function is mainly used when performing calibration by the calibration means (not shown) described above in the first embodiment. That is, the arrangement of each coil is adjusted so that the magnetic field excited by the exciting coil 4 interlinks with the detecting coil 5 and the compensating coil 6 by the same amount of magnetic flux.

  As shown in the first embodiment, the detection coil 5 and the compensation coil 6 have the same magnetic characteristics, and the distance from the excitation coil 4 is also the same position, thereby completely canceling out the excitation magnetic field. Although it is possible in the design, even if it is configured as such, there is a possibility that the excitation magnetic field can not be completely canceled due to a manufacturing error or the like. In such a case, it is possible to completely cancel out the exciting magnetic field by performing the calibration by the calibration means and finely adjusting the arrangement of the coils as shown in FIG.

  FIG. 7 is a third diagram showing the configuration of the coil in the inspection apparatus according to the present embodiment. In FIG. 7, the excitation coil 4, the detection coil 5, and the compensation coil 6 are superconductive coils, and are used in a state of being immersed in the refrigerant 71. By making each coil superconducting, it is possible to excite with a large current, and it is possible to dramatically improve the detection sensitivity. Further, since the resistance of the coil and the temperature change can be ignored, more accurate detection can be performed. Furthermore, by using a SQUID (superconducting quantum interferometer) as a detection circuit, it is possible to perform detection with higher accuracy and sensitivity.

  Here, although the exciting coil 4, the detecting coil 5 and the compensating coil 6 are all superconducting coils, for example, only the exciting coil 4 is a superconducting coil, the detecting coil 5 and the compensating coil 6 are normal conducting coils (for example, copper Alternatively, only the exciting coil 4 may be a normal conducting coil (for example, a copper coil), and the detecting coil 5 and the compensating coil 6 may be superconducting coils. In this case, only the superconducting coil may be immersed in the refrigerant 71, or all the coils may be immersed in the refrigerant 71.

  FIG. 8 is a view showing another circuit configuration of the inspection apparatus according to the present embodiment. Here, a hand-held type configuration is shown which is miniaturized so as to be convenient for carrying the inspection apparatus 1. The principle of measurement is the same as in the case of FIG. 1 in the first embodiment, but in the case of FIG. 8, a shunt resistor 81 is inserted to detect a current phase shift due to the impedance of the exciting coil 4; The current is obtained by measuring the voltage. At this time, the connection is switched by the relay circuit 82, and the voltage of the shunt resistor 81 and the voltage of the detection coil 5 are measured.

  A sine wave is generated using the D / A converter of the microcomputer 83, and a signal is amplified using the power amplifier 84. A resistor 85 is inserted between the exciting coil 4 and the power amplifier 84, and the output voltage value of the D / A converter is set so that a sine wave current of a predetermined amplitude flows. The excitation coil 4, the detection coil 5 and the compensation coil 6 used are the same as those described in the first embodiment. The voltage of the detection coil 5 is amplified by the amplifier 7, the signal is synchronously detected by the lock-in amplifier 8, and finally the voltage is measured by the Δ-Σ A / D converter built in the microcomputer 83.

When there is only one A-D converter of the .DELTA .-. SIGMA. System of the microcomputer 83 to be used, voltages of in-phase and quadrature components are measured while switching the connection with a multiplexer in the microcomputer 83. Measured voltage value is transferred to a personal computer or the like (not shown), and outputs the measurement result of [Delta] R _M and ΔM on your computer. With such a configuration, it is possible to realize a handheld inspection apparatus 1 that is portable.

  In each of the configurations in the first embodiment and the present embodiment, the voltage applied to the exciting coil 4 is controlled according to the distance (lift-off) between the detection coil 5 and the object 9 to be measured, or The liftoff may be controlled in accordance with the excitation current of For example, depending on the use environment of the inspection apparatus of the present embodiment, the lift-off can not be made zero (measurement in a state in which the detection coil is in contact with the object 9), and inspection in a state separated from the object 9 May be required. In such a case, if the lift-off is large, the detection sensitivity is lowered according to the lift-off, so the excitation sensitivity may be controlled so as to flow more and the detection sensitivity may be maintained.

  On the contrary, even if a large amount of excitation current is flowing, if the lift-off is made zero, the detection accuracy may decrease, so the coil is floated from the object 9 to control the lift-off. May be

  Further, when it is desired to stabilize in a state where the liftoff is greater than 0, for example, a dielectric having a certain thickness is provided on the coil surface of the detection coil 5 on the side of the object 9 to be measured. By measuring with the body and the object 9 in contact, it is possible to keep liftoff constant at all times.

  About the inspection apparatus which concerns on this invention, the inspection apparatus was actually produced and the following experiments were done. FIG. 9 is a diagram showing a system outline of the inspection apparatus used in the present embodiment. A sine wave was generated by a function generator (WF1974, NF Corp.) and a signal was amplified by a power amplifier (HSA 4014, NF Corp.) to excite the exciting coil. The voltage amplitude and frequency of the function generator were controlled from a computer via a USB interface. A resistor (10 Ω) was inserted between the power amplifier and the excitation coil, and the voltage value of the function generator was set so that a sine wave current with an amplitude of 1 A would flow.

  FIG. 10 is a diagram showing coil dimensions of the inspection apparatus used in the present embodiment. The compensation coil and the detection coil are reversely connected by a coil of the same shape, so as to cancel the magnetic field from the excitation coil. Each of the excitation coil, detection coil and compensation coil has an inner diameter of 20 mm and an outer diameter of 28 mm, the excitation coil is 50Turn, and the detection coil and compensation coil are 600Turn.

The voltage of the differentially connected detection coil is amplified by the preamplifier (SA-400F3, NF Corp.), and the lock-in amplifier (LI 5640, NF Corp.) acquires voltages in phase and in phase with the excitation current, and GPIB Data was collected on the computer through the interface. In the above equation (4), I can be calculated from the voltage command value to the function generator and the frequency, and since ΔV _M is measured by the lock-in amplifier, ΔR _M and ΔM can be obtained. SM490A steel plate and aluminum plate were used as the object to be measured.

FIG. 11 is a graph showing the measurement results of the change in impedance with respect to the plate thickness d when the test body is a SM 490 A steel plate. 11 (A) shows mutual equivalent resistance change, FIG. 11 (B) shows mutual equivalent resistance change normalized with the maximum value, FIG. 11 (C) shows mutual inductance change, and FIG. 11 (D) shows the maximum value. Indicates mutual inductance change. That is, FIG. 11 (A), (B) the results of [Delta] R _M for the plate thickness d, FIG. 11 (C), the the result of ΔM for the plate thickness d is (D). 11B and 11D are normalized with the maximum value when the plate thickness d is changed with the frequency f fixed. 11 (A) and 11 (C) show the results of f = 1 to 10 Hz, while FIGS. 11 (B) and 11 (D) show only the results of f = 1 to 4 Hz for simplification. it's shown.

As shown in FIGS. 11A and 11B, in the case of f <4 Hz, it can be seen that the thickness monotonically increases with respect to the plate thickness d. On the other hand, in the case of f ≧ 4 Hz, the skin effect does not increase monotonically at dd12 mm. That is, if the f <4 by measuring the [Delta] R _M can estimate the thickness d.

  As shown in FIGS. 11C and 11D, it can be seen that ΔM is almost constant even if the plate thickness d changes. This is because the change in coil linkage flux due to the ferromagnetic material is larger than the change in coil linkage flux due to the eddy current, so the magnitude of coil linkage flux hardly changes even if the plate thickness d changes. Conceivable. This indicates that .DELTA.M is determined by the lift-off regardless of the change of the plate thickness d, and the .DELTA.M can be measured to estimate the lift-off.

FIG. 12 is a graph showing the measurement results of the change in impedance with respect to the plate thickness d when the test body is an aluminum plate. Fig. 12 (A) shows mutual equivalent resistance change, Fig. 12 (B) shows mutual equivalent resistance change normalized by the maximum value, Fig. 12 (C) shows mutual inductance change, and Fig. 12 (D) is normalized by the maximum value. Indicates mutual inductance change. That is, FIG. 12 (A), (B) the results of [Delta] R _M for the plate thickness d, FIG. 12 (C), the the result of ΔM for the plate thickness d is (D). 12B and 12D are normalized with the maximum value when the thickness d is changed with the frequency f fixed. 12 (A) and 12 (C) show the results of f = 1 to 10 Hz, but FIGS. 12 (B) and 12 (D) show only the results of f = 1 to 4 Hz for simplification. it's shown.

  As shown in FIG. 12A, in the case of f ≦ 8 Hz, it can be seen that the thickness monotonically increases with the plate thickness d. On the other hand, in the case of f> 8 Hz, the skin effect does not increase monotonically at d で は 19 mm. As shown in FIG. 12B, it can be seen that in the case of f ≦ 4 Hz where the skin effect is small, there is almost no change even if f changes.

  It can be said that the skin effect of the aluminum plate is smaller than that of the SM490A steel plate when comparing the results of FIG. 11 (A) and FIG. 12 (A), but aluminum is also a good conductor and the skin effect is relatively strong. In the above case, it is suggested that it is difficult to measure the plate thickness at 10 Hz or more.

  As shown in FIGS. 12 (C) and 12 (D), ΔM tends to monotonically decrease except for the 1 Hz result where the noise at the time of measurement is large. This represents a change in coil linkage flux due to an eddy current, and is considered to be a change due to a change in plate thickness d. This is a feature different from the results (FIGS. 3C and 3D) when the magnetic body is used as a test body.

Next, a hand-held type inspection apparatus was produced and experimented. The system configuration of the inspection apparatus is as shown in FIG. The measurement principle is almost the same as above. A sine wave was generated using a D / A converter (resolution: 10 bits) of a microcomputer (PSoC 5LP CY8CKIT-059, Cypress Semiconductor), and a signal was amplified using a power amplifier (OPA 569, Texas Instruments). A resistor (5Ω) was inserted between the D / A converter and the power amplifier, and the output voltage value of the D / A converter was set so that a sine wave current with an amplitude of 800 mA flows. The excitation coil and detection coil used are the same as those shown in FIG. The voltage of the detection coil is amplified by an in-amp (AD 8429, Analog Devices, Inc.), the signal is synchronously detected by a lock-in amplifier, and finally, the Δ-Σ A / D converter (resolution: The voltage was measured at 20 bit). Since only one Δ‐ system A / D converter is used in the microcomputer used, the voltages of the in-phase and quadrature components were measured while switching the connection with the multiplexer in the microcomputer. Measured voltage value is transferred to a computer or the like, and displays the measurement result of [Delta] R _M and ΔM on your computer. In addition, SM490A steel plate was used as a test body.

FIG. 13 is a graph showing the measurement results of the change in impedance with respect to the thickness d of the SM 490A steel plate in a hand-held type inspection apparatus. FIG. 13A shows mutual equivalent resistance change, and FIG. 13B shows mutual inductance change. That is, FIG. 13 (A) is the result of [Delta] R _M for the plate thickness d, FIG. 13 (B) is the result of ΔM for the plate thickness d. Similar to the result of FIG. 11, in FIG. 13 (A), it is monotonically increasing with respect to the plate thickness d, and in FIG. 13 (B), it can be seen that it is almost constant even if the plate thickness d changes.

  As described above, it has been found that in the inspection apparatus according to the present invention, it is possible to measure the thickness of a steel plate whose thickness exceeds 10 mm. In addition, for practical use, a prototype of a hand-held type inspection device was made, but it was shown that the thickness of more than 10 mm can be measured. It is thought that more stable measurement can be expected if the amount of excitation current and frequency, the number of coil turns and dimensions are optimized by finite element analysis.

DESCRIPTION OF SYMBOLS 1 inspection apparatus 2 AC voltage source 3 current limiting resistance 4 excitation coil 5 detection coil 6 compensation coil 7 amplifier 8 lock-in amplifier 9 measured object 40 switch part 71 refrigerant 81 shunt resistance 82 relay circuit 83 microcomputer 84 power amplifier 85 resistance

Claims (8)

An exciting coil connected to an AC power supply,
A detection coil that detects the amount of magnetic flux of an eddy current generated on an object to be measured by the magnetic field of the excitation coil;
A compensation coil having substantially the same magnetic characteristics as the detection coil;
And a measurement unit configured to measure an amount of magnetic flux of the object detected by the detection coil based on outputs of the detection coil and the compensation coil.
The excitation coil, the detection coil and the compensation coil are arranged in parallel with the magnetic flux direction of the excitation coil as the same axis, and each coil is an air core coil, and the detection coil and the compensation coil are in the magnetic flux direction An inspection apparatus characterized in that the inspection apparatus is disposed at different distances. In the inspection apparatus according to claim 1,
An inspection apparatus in which a detection coil and a compensation coil are disposed at positions where the distance between the excitation coil and the detection coil in the magnetic flux direction is equal to the distance between the excitation coil and the compensation coil in the magnetic flux direction. In the inspection device according to claim 1 or 2,
An inspection device in which a detection coil and a compensation coil are connected in reverse polarity so as to cancel each other's magnetic fields. The inspection apparatus according to any one of claims 1 to 3.
An inspection device in which an exciting coil is wound in a spiral shape in which the diameter gradually increases from the center. The inspection apparatus according to any one of claims 1 to 4.
An inspection apparatus comprising arrangement changing means for changing a distance of a magnetic flux direction of a detection coil to an excitation coil and / or a distance of a magnetic flux direction of a compensation coil to the excitation coil. In the inspection apparatus according to any one of claims 1 to 5,
An inspection apparatus in which the excitation coil, the detection coil and / or the compensation coil are superconducting coils. The inspection apparatus according to any one of claims 1 to 6.
An inspection apparatus comprising adjustment means for adjusting a voltage applied to an excitation coil according to a distance between an object to be measured and a detection coil. The inspection apparatus according to any one of claims 1 to 7.
An inspection apparatus comprising calibration means for calibrating an output of a detection coil and an output of a compensation coil in a state in which an object to be measured is placed.