Method and device for measuring the thickness of a conductive object

Abstract

To provide a thickness measuring method and a thickness measuring device, utilizing an eddy current, that can easily measure a thickness of an electrically conductive analyte by reducing an influence of variation in magnetic characteristics.SOLUTION: A method of measuring a thickness of an electrically conductive analyte includes: a cumulative connection measuring step of cumulatively connecting a first coil 1 with a second coil 2 and applying an alternating magnetic field to the analyte to measure cumulative impedance Zcum; a differential connection measuring step of differentially connecting the first coil 1 with second coil 2 and applying the alternating magnetic field to the analyte to measure differential impedance Zdif; a component extracting step of extracting a component of differential impedance dZ which is a difference between the cumulative impedance Zcum and the differential impedance Zdif; and a thickness calculating step of calculating thickness of the analyte from a correlation between the component of the differential impedance Zdif and the thickness of the analyte.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a method for non-destructively measuring the thickness (wall thickness) of a test object, and more particularly to a method and apparatus for measuring the thickness of a conductive test object using a change in impedance due to the eddy current effect. [Background technology]

[0002] Devices for non-destructively measuring the thickness of an object are used, for example, to measure the finished thickness (wall thickness) of products such as plates and pipes, and to monitor changes in the thickness of an object over time, etc. One well-known method for measuring the thickness of an object is a measurement method that uses eddy currents. In this measurement method, an alternating magnetic field is applied to the specimen using a coil placed close to the specimen, changes in the coil's electrical properties are measured, and the thickness of the object is calculated from the changes in electrical properties. To achieve this, multiple standard samples with different thicknesses are prepared in advance, and a master curve (calibration curve) showing the relationship between the output of the measurement device and the thickness is created. The thickness of the specimen is then calculated from the output of the measurement device for the specimen and the previously prepared standard curve. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2021-152486 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-127888 Summary of the Invention [Problem to be solved by the invention]

[0004] As mentioned above, thickness measurement using eddy current requires a standard curve corresponding to the electromagnetic characteristics of the test object, and creating such a standard curve can require advanced technology. Furthermore, the magnetic properties of the object being measured can change depending on the manufacturing process and usage history. Conventional thickness measurement methods using eddy currents are easily affected by magnetic properties, and the object being measured is often limited to non-magnetic metals.

[0005] A main technical object of the present invention is to provide a thickness measurement method and a thickness measurement device that utilize eddy currents and that enable easy thickness measurement of a conductive object. Another object of the present invention is to make it possible to reduce the influence of magnetic properties. [Means for solving the problem]

[0006] The thickness measurement method according to the present invention comprises: 1. A method for measuring a thickness of a conductive specimen, comprising: The first coil 1 and the second coil 2 are summed and connected together, and an alternating magnetic field of a first frequency is applied to the subject, resulting in a summed impedance Z cum A summing connection measurement process for measuring the summing connection; The first coil 1 and the second coil 2 are differentially connected, and the alternating magnetic field is applied to the subject to generate a differential impedance Z dif a differential connection measurement step of measuring The summed impedance Z cum and the differential impedance Z dif a component extraction step of extracting a component of differential impedance dZ, which is the difference between and a thickness calculation step of calculating the thickness of the object from the correlation between the component of the differential impedance dZ and the thickness of the object.

[0007] Sum impedance Z cum and differential impedance Z dif By calculating the differential impedance dZ based on the difference between these, the influence of eddy currents can be effectively extracted, facilitating measurement of the thickness of the object to be inspected. Furthermore, it becomes possible to easily calculate the correlation between the components of the differential impedance dZ and the thickness of the object, which further facilitates measurement of the thickness of the object.

[0008] Further, in the above-mentioned configuration, the thickness measurement method according to the present invention comprises: The first frequency may be a low frequency.

[0009] Further, in the above-mentioned configuration, the thickness measurement method according to the present invention comprises: The first frequency may satisfy the Thickkin condition.

[0010] By using a low frequency for the alternating magnetic field applied to the object and satisfying the Thick Skin condition, it is possible to alleviate the limitations on the measurement of the object's thickness due to the skin effect.

[0011] Further, in the above-mentioned configuration, the thickness measurement method according to the present invention comprises: In the component extraction step, a differential resistance dR is extracted as a component of the differential impedance dZ, The differential resistance dR is divided by the square of the first frequency to obtain the specific resistance <dr>Further includes a resistivity calculation step of calculating In the thickness calculation step, the specific resistance is calculated in place of the component of the differential impedance dZ. <dr>The thickness of the object may be calculated from the correlation between the amplitude of the light and the thickness of the object.

[0012] Resistivity <dr>By measuring the thickness of the object using the above method, the magnetic influence of the object can be reduced.

[0013] Further, in the above-mentioned configuration, the thickness measurement method according to the present invention comprises: In the component extraction step, a differential inductance dL is extracted as a component of the differential impedance dZ, The first coil 1 and the second coil 2 are summatively connected, and an extremely low frequency alternating magnetic field of a second frequency lower than the first frequency is applied to the subject, thereby generating an extremely low frequency summation impedance Z VLcum a very low frequency sum connection measurement process for measuring the The first coil 1 and the second coil 2 are differentially connected, and the extremely low frequency alternating magnetic field is applied to the subject to generate an extremely low frequency differential impedance Z VLdif an extremely low frequency differential connection measurement process for measuring The extremely low frequency sum impedance Z VLcum and the extremely low frequency differential impedance Z VLdif The difference between the base differential inductance dL(f vl ) and calculate the differential inductance dL and the base differential inductance dL(f vl ) and a corrected differential inductance d(dL) is calculated from the difference between the first frequency and the corrected differential inductance d(dL), and the corrected differential inductance d(dL) is divided by the square of the first frequency to obtain an intrinsic inductance and a specific inductance calculation step of calculating the specific inductance. In the thickness calculation step, the component of the differential impedance dZ is replaced by the intrinsic inductance The thickness of the object may be calculated from the correlation between the amplitude of the light and the thickness of the object.

[0014] Intrinsic Inductance By measuring the thickness of the object using the above method, the magnetic influence of the object can be reduced.

[0015] Further, in the above-mentioned configuration, the thickness measurement method according to the present invention comprises: The object may be a pipe with a constant outer diameter.

[0016] By measuring the pipe thickness in this way, the inside diameter of the pipe can be inspected non-destructively.

[0017] The thickness measuring device according to the present invention comprises: A thickness measurement device for measuring the thickness of a conductive object, The device comprises a first coil 1, a second coil 2, a switching device 3, a measuring device 4, and a control device 5, the switching device 3 is capable of selectively switching between a summation connection state in which the first coil 1 and the second coil 2 are summation connected and a differential connection state in which the first coil 1 and the second coil 2 are differentially connected, the control device 5 controls the switching device 3 to set the connection between the first coil 1 and the second coil 2 to the additive connection state or the differential connection state, the control device 5 controls the measuring device 4 to measure a sum impedance in the sum connection state between the first coil 1 and the second coil 2, and a differential impedance in the differential connection state between the first coil 1 and the second coil 2, The control device 5 receives the sum impedance and the differential impedance from the measurement device 4 and calculates the thickness of the object.

[0018] According to such a thickness measurement device, the thickness of the object to be measured is measured from the summation impedance and differential impedance of the first coil 1 and the second coil 2, thereby effectively extracting the influence of eddy currents and easily measuring the thickness of the object to be measured. [Effects of the Invention]

[0019] The present invention can provide a thickness measurement method and a thickness measurement device that utilize eddy currents and that can easily measure the thickness of a conductive object. Furthermore, it is possible to reduce the influence of magnetic properties. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is an equivalent circuit diagram showing the change in impedance due to eddy currents. [Figure 2] Figure 2(A) is a perspective view showing an example of a disk-shaped subject TP with a thickness D and a radius r0 placed in an alternating magnetic field H, and Figure 2(B) is a perspective view showing a schematic diagram of an alternating magnetic field being applied to the subject TP by a coil C. [Figure 3] FIG. 3 is a graph showing the effect of the penetration depth δ on the eddy current loss Peddy. [Figure 4] FIG. 4(A) is a graph showing the relationship between Rac and thickness calculated for material A with a relative magnetic permeability μr of 500 and material B with a relative magnetic permeability μr of 250. FIG. 4(B) is a graph showing the relationship between Rac and thickness calculated for material A and material B. <rac>10 is a graph showing the relationship between the thickness and the thickness. [Figure 5] FIG. 5(A) shows a circuit in which two identical coils are connected additively, and FIG. 5(B) shows a circuit in which two coils are connected differentially. [Figure 6] FIG. 6(A) shows the thickness measurement device 100 in a state where two coils are connected additively, and FIG. 6(B) shows the thickness measurement device 100 in a state where two coils are connected differentially. [Figure 7] FIG. 7 is a cross-sectional view showing the arrangement of a pipe-shaped subject TP, the first coil 1 and the second coil. [Figure 8] Figure 8(A) is a graph showing the cross-sectional area dependence of dR, and Figure 8(B) is <dr>8(C) is a graph showing the cross-sectional area dependency of dX, and FIG. 8(D) is a graph showing the cross-sectional area dependency of dL. [Figure 9] FIG. 9 shows a photograph showing the arrangement of the conductive subject TP, the first coil 1 and the second coil 2, and the measurement conditions. [Figure 10] Figure 10(A) shows the measured data showing the frequency dependence of dR, and Figure 10(B) shows <dr>Fig. 10(C) shows the actual measurement data showing the frequency dependence of <dr>10 shows actual measurement data showing the cross-sectional area dependence of [Figure 11] FIG. 11(A) is a graph showing the frequency dependence of dL, and FIG. 11(B) is a graph showing the cross-sectional area (ro 2 - ri 2) dependence of dL. [Figure 12] Figure 12(A) shows the frequency dependence of d(dL), and Figure 12(B) shows The frequency dependence of Fig. 12(C) is 2 is a graph showing the cross-sectional area (ro 2 - ri 2) dependence of [Figure 13] FIG. 13 is a perspective view showing an example of measuring the thickness of a flat plate-shaped specimen TP using the thickness measurement device 100. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the following embodiments are not intended to limit the scope of the present invention. Furthermore, the same or similar components will be designated by the same reference numerals, and their description may be omitted.

[0022] Furthermore, terms used in this specification that specify shapes, geometric conditions, and their degrees, such as "parallel," "orthogonal," and "identical," as well as values ​​of lengths and angles, are not to be construed as being bound by strict meanings, but rather as including a range within which similar functions can be expected.

[0023] (Measurement principle) The principle of non-destructive thickness (wall thickness) measurement of the test object TP using eddy current will be described below. The test object TP to be measured is made of a homogeneous material and is manufactured to have the same thickness. By measuring the thickness of the test object TP, it is possible to non-destructively inspect whether it is the designed (targeted) thickness. FIG. 1 is an equivalent circuit diagram showing the impedance change due to eddy currents, and shows the impedance change of one coil C. The impedance Z is measured by a measuring device T. In FIG. 1, Z0 represents the impedance of the coil itself, and ΔZ represents the change in impedance due to eddy currents.

[0024] As shown in Figure 1, if the DC resistance (ohmic resistance) of coil C is R0 [Ω] and the self-inductance of coil C is L0 [H], the impedance Z0 of the coil itself is: Z0=R0+jωL0[Ω] (Equation 1) This becomes: Here, ω is the angular frequency of the AC current applied to the coil C, and is defined as ω=2πf, where f is the frequency. The AC current may be, for example, a sinusoidal current. In the following description, for simplicity, the angular frequency ω may be abbreviated to "frequency".

[0025] The AC current is applied to coil C by measuring device T, and the impedance of coil C is measured by measuring device T. Measuring device T refers to the AC current applied to coil C, measures the impedance of the coil from the amplitude and phase difference of the detected AC current, and can output the real and imaginary parts of the measured impedance. The measuring device T can measure the impedance at each frequency while changing the frequency of the applied alternating current. For example, a commercially available general-purpose LCR meter (frequency sweep type LCR measuring instrument) can be used as the measuring device T. By using a general-purpose LCR meter, the measuring system shown in Fig. 1 can be constructed easily and inexpensively without using a specially designed device. Instead of using an LCR meter as the measuring device T, a source of alternating current with a variable frequency may be prepared, and an measuring instrument may be provided to apply the alternating current to the coil C and measure the electrical characteristics of the coil C. The applied alternating current may be compared with the measured electrical characteristics of the coil C using a personal computer PC or the like, and the impedance Z may be calculated from the amplitude and phase difference using a known calculation method.

[0026] When the alternating magnetic field generated by coil C is applied to the subject TP, the impedance Z of coil C changes by ΔZ. Z = Z0 + ΔZ (Equation 2) Generally, the magnetic field used in thickness measurement using eddy currents is weak, and ΔZ is the impedance change caused by eddy current loss if hysteresis loss is ignored. The change in resistance of coil C due to eddy current is R ac [Ω], the change in inductance of coil C due to eddy current is L ac [H], the impedance change ΔZ due to the generation of eddy current is ΔZ=R ac +jωL ac [Ω]...(Formula 3) Since ΔZ is the impedance change caused by the eddy current, the influence of the eddy current due to the test object TP can be detected by analyzing ΔZ. Eddy current loss (P eddy ) is proportional to the square of the frequency, so if the applied AC current is I0exp(j(ωt+θ)), the eddy current loss (P eddy ) and R ac The relationship is as follows: R ac =Re[ΔZ]≒P eddy / I0 2 ∝σω 2 ...(Formula 4) Therefore, approximately R ac ω 2 It will be proportional to. Here, I0 is the amplitude of the AC current, Re[ ] is the real part, and σ is the conductivity of the test object TP.

[0027] R ac ω 2 Since it is proportional to <R ac > is defined as follows: <R ac >=R ac / ω 2 [Ω / (rad / s) 2 ] ...(Formula 5) Resistivity <R ac > removes the dependence on ω. The approximate formula for impedance change ΔZ is the specific resistance <R ac Using > it becomes: ΔZ= <R ac >ω 2 +jωL ac [Ω] (Formula 6)

[0028] As mentioned above, R ac ω 2 Therefore, if the frequency of the alternating magnetic field is low, a sufficiently large impedance change ΔZ relative to Z0 cannot be obtained, and the influence of drift may be large, making it difficult to measure the thickness of the object TP. Therefore, when measuring the thickness of the test object TP using eddy current loss, in order to fully obtain the effect of eddy current loss, an alternating magnetic field with a high frequency, for example, several kHz to several MHz, is generally applied to the test object TP, and the impedance change ΔZ is analyzed.

[0029] In general, it is known that the penetration depth δ of a magnetic field into a material depends on frequency f, conductivity σ, and magnetic permeability μ, and can be calculated using the following formula: δ=1 / (πfμσ) 1 / 2 ...(Formula 7) That is, the penetration depth δ is inversely proportional to the square root of the frequency f of the alternating magnetic field. Therefore, the measurable range of the thickness of the specimen TP is limited by the penetration depth 6. In order to alleviate the limitation on the measurable range due to the skin effect, it is effective to lower the frequency of the alternating magnetic field. Therefore, the conditions for the frequency of the alternating magnetic field suitable for detecting the impedance change ΔZ due to eddy current and the conditions for the frequency of the alternating magnetic field suitable for reducing the restrictions on the measurable range are inversely related to each other.

[0030] (Thickness measurement of flat plates) An example of measuring the thickness of a flat plate specimen TP will be described below. Fig. 2(A) is a perspective view showing an example of a disk-shaped subject TP with a thickness D and a radius r0 placed in an alternating magnetic field H, and Fig. 2(B) is a perspective view showing a schematic diagram of an alternating magnetic field being applied to the subject TP by a coil C. In Fig. 2, the depth direction of the subject TP is the Y direction, and the surface of the subject TP is Y=0. In the following description, units of physical quantities may be omitted for simplicity.

[0031] As shown in Figure 2(A), an alternating magnetic field H is applied to the test object TP, and an eddy current Ie is generated as shown by the dotted arrow. If the magnetic flux density on the surface of the test object TP is B0, then the eddy current loss P eddy is as follows. P eddy =(r0 2 / 8)ω 2 σ(B0 2 / 2)δ(1-exp(-2D / δ)) ...(Formula 8) Under the condition where the penetration depth δ is sufficiently larger than the thickness D (δ>>D) (called the thick skin condition), P eddy =βω 2 σB0 2 D...(Formula 9) Here, β=r0 2 / 8. Therefore, R ac ∝P eddy ∝ω 2 σD (Equation 10) <R ac >=R ac / ω 2 ∝σD (Equation 11) This becomes: Since the conductivity σ is constant, <R ac > is proportional to D.

[0032] Below, we will examine the sick skin conditions in detail. Fig. 3 shows the eddy current loss P eddy 10 is a graph showing the effect of The horizontal axis represents the ratio of D to δ (D / δ), and the vertical axis represents the value of the function "1-exp(-2D / δ)." The function "1-exp(-2D / δ)" is P eddy The D / δ dependence of

[0033] In FIG. 3, it can be seen that the thick skin condition holds true when, for example, D / δ<0.3, that is, when the penetration depth δ is three times or more the thickness D of the specimen TP. If the penetration depth δ of the specimen TP is known, the thickness D of the specimen TP in the range of thickness D<0.3δ can be calculated by the above <R ac It can be measured using >.

[0034] Figure 4 is a graph showing the results of simulating the resistance component of the impedance change due to eddy currents for materials with different magnetic properties but the same conductivity σ. Figure 4(A) shows the R calculated for material A with a relative permeability μr of 500 and material B with a relative permeability μr of 250. ac The graph in Figure 4(B) shows the relationship between the thickness and the thickness. <R ac 1 is a graph showing the relationship between the temperature and thickness.

[0035] Figure 4(A) shows the R ac The results of calculating R and R for material B with different thicknesses using alternating magnetic fields with frequencies of 20 Hz (▲ in the figure) and 8 Hz (△ in the figure) ac The results of the calculation are shown below. An alternating magnetic field can be generated by applying a sinusoidal alternating current to coil C, which is placed close to materials A and B.

[0036] From Figure 4(A), as the thickness increases, R ac It can be seen that increases. In addition, when the frequency of the alternating magnetic field is lowered, R ac It can be seen that the decrease

[0037] From Figure 4(B), for materials A and B that have the same conductivity but different relative permeabilities, <R ac > thickness dependence is the same. <R ac >It can be understood that the dependence on relative permeability, which is a magnetic property, is eliminated. Resistivity <R ac By eliminating the dependency on the relative permeability using >, it is possible to alleviate the constraints on measurement due to variations in magnetic properties in thickness measurement of the specimen TP.

[0038] In addition, under the condition that an alternating magnetic field with a frequency of 10 Hz was applied to material A (● in the figure) and under the condition that an alternating magnetic field with a frequency of 20 Hz was applied to material B (▲ in the figure), up to a thickness of 5 mm, <R ac > is proportional to the thickness, which means that the thick skin condition holds up to a thickness of 5 mm. On the other hand, when an alternating magnetic field with a frequency of 4 Hz was applied to material A (circle in the figure) and an alternating magnetic field with a frequency of 8 Hz was applied to material B (triangle in the figure), up to a thickness of 8 mm, <R ac > is proportional to the thickness, which means that the thick skin condition holds up to a thickness of 8 mm.

[0039] When the penetration depth δ of the test object TP is unknown, whether the measurement of the test object TP satisfies the thick skin condition can be determined by the following: <R ac >It can also be determined from the frequency dependence of If the frequency of the alternating magnetic field does not satisfy the Thick Skin condition, changing (increasing) the frequency <R ac For example, if the thickness of the material A, which is the specimen TP, becomes larger than 5 mm, the value of <R ac From the value of >, the frequency of the alternating magnetic field is 10 Hz. <R ac In the example shown in Figure 4, there is a boundary between the frequency of 4 Hz and the frequency of 10 Hz where the thickness D exceeds the penetration depth δ. Since the penetration depth δ decreases with increasing frequency, for an alternating magnetic field with a frequency (f+α) slightly different from the measurement frequency f, for example, f+2Hz or f+4Hz, <R ac In other words, increase the frequency by a few Hz (e.g., α = 1 to 5 Hz) and compare the results of the measurements in this range. <R ac If the frequency does not fluctuate or can be kept to a predetermined amount, for example, 5% or less, this fluctuation range can be considered as the allowable range and the Thick Skin condition can be satisfied. The allowable range can be set taking into consideration the accuracy (stable performance) of the frequency control of the measuring device T, etc. If it is determined that the thick skin condition is met, the thickness of the specimen TP is <R ac >The proportional relationship between thickness and thickness indicates that it can be easily and correctly measured. Similarly, in the other thickness measurements described below, the thick skin condition can be determined by a frequency variation test. Satisfaction of the Thick Skin condition can be determined by satisfying D / δ<0.3 when the skin depth δ is known, or by the resistivity satisfying a frequency variation test when the skin depth δ is unknown. In the case of substance A shown in Figure 4 above, <R ac >Due to the thickness dependency, a frequency of 4 Hz is deemed to satisfy the thick skin condition, so the allowable frequency range is less than 6 Hz.

[0040] If the resistance of coil C is low enough, ideally with a superconducting coil, it will reflect the effects of eddy currents. <R ac > can be calculated by actual measurement, <R ac The proportional relationship between the thickness of the specimen TP and the thickness of the specimen TP makes it possible to measure the thickness of the specimen TP. The proportionality coefficient is σ for the frequency f of the alternating magnetic field used for measurement. f Then, <R ac >=σ f D...(Formula 12) This becomes: Proportional coefficient σ f is measured with respect to a standard sample having a known thickness D0 and made of the same material as the specimen TP to be measured. <R ac > and calculate the resistivity of the standard sample obtained. <R ac > <r0>Then, <r0>=σ f D0...(Formula 13) From this, σ f = <r0> / D0...(Formula 14) In this way, the standard sample <r0>The proportionality coefficient is σ f It can be calculated. Proportional coefficient σ f Using the above, the value obtained by measuring the subject TP <R ac > and the proportionality coefficient, the thickness of the object TP to be measured can be measured.

[0041] In addition, as can be seen from Figures 4(A) and (B), the resistivity decreases as the frequency of the applied alternating magnetic field decreases. <R ac The thickness measurement range using the proportional relationship between impedance and thickness increases, but the real part of the impedance change ΔZ, R ac It can be seen that it decreases. In other words, ΔZ decreases relatively to the total impedance Z, and it can be understood that in reality it becomes difficult to detect ΔZ. Therefore, it is generally difficult to simultaneously detect eddy current loss and alleviate the restrictions on the measurable range.

[0042] Below, we will explain a method for extracting and analyzing impedance change ΔZ from impedance Z using a low-frequency (10 Hz to 100 Hz) alternating magnetic field in order to expand the measurable range of thickness while also achieving accurate thickness measurement using eddy currents.

[0043] Figure 5(A) shows a circuit with two identical coils connected in a summation manner, and Figure 5(B) shows a circuit with two coils connected differentially. The magnetic flux generated by two summation-connected coils is in the same direction, while the magnetic flux generated by two differentially-connected coils is in opposite directions.

[0044] Also, FIG. 5(A) shows the impedance change of two identical coils connected in a summation manner, and FIG. 5(B) shows the impedance change of two identical coils connected in a differential manner. The impedance of the two connected coils is measured by a measuring device T. In Figure 5, Z0 represents the impedance of the coil itself, ΔZ represents the change in impedance due to eddy currents, and ΔZ cum is the impedance change due to the eddy current in each coil connected in a summation, ΔZ dif is the impedance change due to the eddy current in each differentially connected coil. It should be noted that "the same coil" means "substantially the same coil", meaning that the coils are identical within the range of manufacturing variations.

[0045] In FIG. 5(A), the impedance of the first coil C1 is Z 1cum , the impedance of the second coil C2 is Z 2cum Then, since the first coil C1 and the second coil C2 are the same coil, Z 2cum =Z 1cum ...(Equation 15) Z 0_1cum , R 0_1 , L 0_1 are the impedance, resistance, and inductance of the first coil, and ΔZ _1cum , R ac_1cum , L ac_1cum are the impedance change, resistance change, and inductance change of the first coil C1 due to the eddy current, respectively, Z 1cum =Z 0_1cum +ΔZ _1cum ...(Equation 16) =(R 0_1 +jωL 0_1 )+(R ac_1cum +jωL ac_1cum ) =(R 0_1 +R ac_1cum )+(jωL 0_1 +jωL ac_1cum ) ...(Formula 17) This becomes: Impedance Z due to the summatively connected first coil C1 and second coil C2 cum If the mutual inductance is M, then Z cum =2Z 1cum +2jωM (Equation 18) Z cum =2(R 0_1 +R ac_1cum )+2(jωL 0_1 +jωL ac_1cum +jωM) ...(Formula 19) This becomes: In addition, impedance Z cum The sum of the impedance Z cum It is sometimes referred to as.

[0046] In FIG. 5(B), the impedance of the first coil C1 is Z 1dif , the impedance of the second coil C2 is Z 2dif Let's say. Since the first coil C1 and the second coil C2 are the same coil, Z 1dif =Z 2dif ...(Formula 20) Z 0_1dif , R 0_1 , L 0_1 are the impedance, resistance, and inductance of the first coil C1, and ΔZ _1dif , R ac_1dif , L ac_1dif are the impedance change, resistance change, and inductance change of the first coil C1 due to the eddy current, respectively, Z 1dif =Z 0_1dif +ΔZ _1dif ...(Formula 21) =(R 0_1 +jωL 0_1 )+(R ac_1dif +jωL ac_1dif ) =(R 0_1 +R ac_1dif )+(jωL 0_1 +jωL ac_1dif ) ...(Formula 22) The impedance Z due to the differentially connected first coil C1 and second coil C2 is dif If the mutual inductance is M, then Z dif =2Z 1dif -2jωM (Equation 23) Z dif =2(R 0_1 +R ac_1dif )+2(jωL 0_1 +jωL ac_1dif -jωM) ...(Formula 24) This becomes: In addition, impedance Z dif is the differential impedance Z dif It is sometimes referred to as.

[0047] The sum impedance Z of two sum-connected coils cum and the differential impedance Z of the two differentially connected coils dif If the difference between these is dZ (called differential impedance), then dZ=Z cum -Z dif ...(Formula 25) =2[R ac_1cum -R ac_1dif ]+2jω[L ac_1cum -L ac_1dif ]+4jωM ...(Formula 26) This becomes: If the resistance component, reactance component, and inductance component of the differential impedance dZ are dR, dX, and dL, respectively, then dR=Re[dZ]=2[R ac_1cum -R ac_1dif ] ...(Formula 27) dX=Im[dZ]=2ω[L ac_1cum -L ac_1dif ]+4ωM ...(Formula 28) dL=2[L ac_1cum -L ac_1dif ]+4M...(Formula 29) Here, Im[] indicates the imaginary part. The impedance of the coil itself is excluded from dZ, dR, dX, and dL, so these values ​​can be used to effectively extract the electrical characteristics caused by eddy current loss at low frequencies, facilitating thickness measurement of the object under test. As will be described later, it is possible to measure the thickness of the object TP by using dR and dL in particular. Here, dR is called differential resistance, dL is called differential inductance, and dX is called differential reactance.

[0048] In addition, the specific resistance is <dr>is defined as follows: dR=ω 2 <dr>...(Formula 30) inherent resistance <dr>is the quantity with frequency components removed, and the specific resistance <dr>The thickness of the specimen TP can be measured using the following equation. Even if dR contains frequency-independent components, <dr>ω is dR 2 For example, at a low frequency of 10 Hz, ω is 20π, and the contribution of frequency-independent components is less than 0.03%.

[0049] (Measuring equipment) To extract dZ, two identical coils are connected in a summation state and the impedance Z is measured with the measuring device T. cum The frequency dependence of the impedance Z dif The frequency dependence of Z cum and Z dif The difference needs to be calculated. The connection of the two coils is manually switched, and the impedance Z is measured by the measuring device T. cum , and impedance Z dif The measurement of impedance Z cum , and impedance Z dif It is also possible to measure and calculate dZ.

[0050] Fig. 6 is a circuit diagram showing a schematic example of the main configuration of a thickness measurement device 100 that can measure impedance by switching between summation and differential connection of two coils. Fig. 6(A) shows the thickness measurement device 100 in a state where the two coils are summation connected, and Fig. 6(B) shows the thickness measurement device 100 in a state where the two coils are differentially connected. In order to minimize the resistance of the coil itself, Litz wire is preferably used for winding the coil. As will be described later, the thickness of the object TP can be measured using dR and dL, and the thickness measuring device 100 can also be configured to automatically calculate the thickness of the object TP from dZ.

[0051] The thickness measuring device 100 includes a first coil 1, a second coil 2, a switching device 3, a measuring device 4, and a control device 5. The first coil 1 and the second coil 2 are substantially identical and have substantially the same electrical and magnetic properties. The switching device 3 can be configured with, for example, a C-contact relay or a multiplexer. The switching device 3 can selectively switch between a summation connection of the first coil 1 and the second coil 2 and a differential connection of the first coil 1 and the second coil 2.

[0052] The control device 5 has an input / output unit, a storage unit, and a central processing unit (CPU), and can be configured, for example, by a personal computer (PC), but can also be configured by a microcomputer and a storage device (for example, a semiconductor memory). The control device 5 stores necessary data in the storage unit. The control device 5 controls the switching device 3, and can control switching between the summation connection of the first coil 1 and the second coil 2 and the differential connection of the first coil 1 and the second coil 2 by the switching device 3. The control device 5 controls the measuring device 4 and can instruct the measuring device 4 to measure the impedance.

[0053] The first coil 1 corresponds to the coil C1 in FIG. 5, the second coil 2 corresponds to the coil C2 in FIG. 5, and the measuring device 4 corresponds to the measuring device T in FIG. 5 or FIG. The first coil 1 and the second coil 2 are the same coil and have the same electromagnetic properties. The measuring device 4 applies an AC current, for example a sinusoidal AC current, to the first coil 1 and the second coil 2, and measures the impedance of the connected first coil 1 and second coil 2 while applying the AC current to the first coil 1 and the second coil 2. The impedance can be calculated from the applied AC current and the amplitude and phase difference of the current in the connected first coil 1 and second coil 2.

[0054] The measuring device 4 has the function of an LCR meter, and a commercially available LCR meter can be used, for example. Specifically, the measuring device 4 has a source of an AC signal (AC current or AC voltage). The source can output the AC signal, i.e., AC current or AC voltage, to the first coil 1 and the second coil 2. The measuring device 4 also has a detector that measures the output signal, output current, or output voltage across the first coil 1 and the second coil 2 in synchronization with the output AC signal. The impedance, i.e., the real and imaginary parts of the impedance, is detected by analyzing the amplitude and phase of the supplied AC signal and the output signals from the first coil 1 and the second coil 2.

[0055] 6, the first coil 1 and the second coil 2 constitute a detection unit 6, and the switching device 3, the measuring device 4, and the control device 5 constitute an analysis unit 7. The analysis unit 7 analyzes the electrical signal from the detection unit 6 and can also output an AC current to the detection unit 6.

[0056] As shown in FIG. 6(A), when the contact P1 and the contact P3 in the switching device 3 are connected and the contact P2 and the contact P4 are connected, the first coil 1 and the second coil 2 are summarily connected. The measuring device 4 measures the sum impedance Z of the sum-connected first coil 1 and second coil 2 in the frequency range of low frequencies (10 Hz to 100 Hz). cum Measure. In addition, the sum impedance Z cum is frequency dependent.

[0057] The measuring device 4 records the value of the measurement frequency and the measured sum impedance Z cum (Sum impedance Z cum The control unit 5 outputs the frequency value and sum impedance Z cum are associated with each other and stored in the storage unit. The control device 5 may specify the value of the measurement frequency to the measurement device 4 via the input / output unit.

[0058] The control device 5 controls the switching device 3 via the input / output unit, and switches the connection between the first coil 1 and the second coil 2. As shown in FIG. 6(B), when the contact P1 and the contact P4 in the switching device 3 are connected and the contact P2 and the contact P5 are connected, the first coil 1 and the second coil 2 are differentially connected.

[0059] The measuring device 4 measures the differential impedance Z of the differentially connected first coil 1 and second coil 2 in the frequency range of low frequencies (10 Hz to 100 Hz). dif Measure. In addition, the differential impedance Z dif is frequency dependent. The measuring device 4 records the value of the measurement frequency and the measured differential impedance Z dif (Differential impedance Z dif The control device 5 outputs the real and imaginary parts of the frequency input via the input / output unit and the differential impedance Z dif are associated with each other and stored in the storage unit. The control device 5 may specify the value of the measurement frequency to the measurement device 4 via the input / output unit.

[0060] The control device 5 calculates the sum impedance Z cum and differential impedance Z dif Calculate the differential impedance dZ, which is the difference between the two. dZ is a function of frequency. The control device 5 calculates and stores the differential resistance dR and the differential inductance dL. The control device 5 can calculate the thickness of the object from the values ​​of dR and / or dL and store it in the storage device. The above measurement may be performed at multiple frequencies, and dZ, dR, and dL may be calculated for each frequency. 2 The validity of the measurement can be assessed by evaluating the dependency.

[0061] In addition, the sum impedance Z cum and differential impedance Z dif The order of measurements may be reversed.

[0062] When measuring at multiple frequencies, the sum impedance Z cum After measuring the differential impedance Z at multiple frequencies, the switching device 3 switches between the sum junction and the differential junction. dif Alternatively, the summation impedance Z may be measured by switching between the summation junction and the differential junction for each frequency using the switching device 3. cum and differential impedance Z dif and may be measured. However, at one frequency, the summed impedance Z cum and differential impedance Z dif and then change the frequency to measure the sum impedance Z cum and differential impedance Z dif It is preferable to measure the temperature and humidity because it is less susceptible to drift.

[0063] If the penetration depth δ of the specimen TP is known, the thickness D of the specimen TP in the range of, for example, thickness D<0.3δ can be measured using the above dR. Furthermore, when the penetration depth δ of the specimen TP is unknown, it is possible to determine whether or not the measurement of the specimen TP satisfies the Thick Skin condition from the frequency dependency of the above dR. As described above, if the frequency of the alternating magnetic field is slightly varied, for example, by 5 Hz or less relative to the frequency f, and dR does not vary within this variation range or can be suppressed to a predetermined amount, for example, 5% or less, it can be determined that the Thick Skin condition is met. If it is determined that the thick skin condition is satisfied, it is determined that the thickness of the subject TP can be measured normally.

[0064] For verification, a simulation was performed on a pipe-shaped specimen TP with a constant outer diameter. 7 is a cross-sectional view showing the arrangement of a pipe-shaped object TP and the first and second coils 1. The parameters used in the simulation are shown in FIG. The object TP has a cylindrical shape and is inserted into the center of the first coil 1 and the second coil 2. The inner diameter of the object TP is Dci (=2r i ), outer diameter is Dco(=2r o ), the cross-sectional area of ​​the specimen TP is π(r o 2 -r i 2 ) and the magnetic flux Φ passing through the cross section of the test object TP is Φ=π(r o 2 -r i 2 )B Here, B is the magnetic flux density. The magnetic flux Φ applied to the test object TP by the connected first coil 1 and second coil 2 generates eddy current loss in the test object TP made of a conductor, resulting in an impedance change in the first coil 1 and the second coil 2. The product of the cross-sectional area and the conductivity σ of the test object TP reflects the resistance in that cross section. The cross-sectional area of ​​the specimen TP is π(r o 2 -r i 2 ) and dR, which reflects the above impedance change, <dr>The cross-sectional area and frequency dependence of the alternating magnetic field of dX, dL were simulated. The thickness of the specimen TP is r o -r i and the cross section of π(r o 2 -r i 2 ) and a known constant r o It can be calculated from

[0065] FIG. 8 is a graph showing the simulation results. Figure 8(A) shows the cross-sectional area dependence of dR, and Figure 8(B) shows <dr>8(C) shows the cross-sectional area dependence of dX, and FIG. 8(D) shows the cross-sectional area dependence of dL. The cross-sectional area of ​​the specimen TP is π(r o 2 -r i 2 ), but in Figure 8, instead of the cross-sectional area, "r o 2 -r i 2 " is used. "r o 2 -r i 2 " to the cross-sectional area π(r o 2 -r i 2 ) can be easily converted by multiplying it by the constant π, so in the following we will refer to it as "r o 2 -r i 2 " is explained as the cross-sectional area. In FIG. 8, square indicates the simulation results under the conditions of a frequency of 1 Hz, a diamond indicates a frequency of 10 Hz, a triangle indicates a frequency of 20 Hz, a diamond indicates a frequency of 40 Hz, and a black circle indicates a frequency of 80 Hz.

[0066] From FIG. 8(A), it can be seen that dR depends on the cross-sectional area and also on the frequency. From Figure 8(B), <dr>It can be seen that increases linearly with the cross section but does not depend on the frequency. However, when the frequency is 80 Hz, the cross-sectional area (r o 2 -r i 2 ) is 48mm 2 In the above, <dr>The cross-sectional area (r o 2 -r i 2 ) is 48mm 2 This is because the Thick Skin condition is no longer satisfied. From Figure 8(C), we can see that dX varies linearly with the cross-sectional area, but the cross-sectional area dependence is not clear. Also, dX depends on the frequency. On the other hand, from FIG. 8(D), it can be seen that dL clearly changes depending on the cross-sectional area, and that it does not depend on the frequency. Also, <dr>Similarly, the cross-sectional area (r o 2 -r i 2 ) is 48mm 2 From the above, it can be seen that dL at a frequency of 80 Hz differs from dL at other frequencies.

[0067] <dr>The cross-sectional area of ​​the specimen TP can be calculated from the linear relationship between the cross-sectional area and the thickness of the specimen TP. <dr>By first determining the linear relationship between the cross-sectional area and the mass, and then preparing that linear relationship as a master curve (calibration curve), <dr>From the measurement results, the cross section (r o 2 -r i 2 ) can be output as a measured value. Since the master curve is a linear function, <dr>From the measurement results, the cross section (r o 2 -r i 2 ) is easy to calculate. Furthermore, the obtained cross section (r o 2 -r i 2 ) and the known (or otherwise measured) outer diameter 2r o The thickness of the object TP can be output as a measurement value. Specifically, the outer diameter of the object TP is 2r o From the radius r of the outer wall of the specimen TP o Calculate r o squared and (r o 2 -r i 2 ) and calculate the square root of the difference, and the radius r of the inner wall of the object TP is i , and the thickness r of the specimen TP is calculated. o -r i can be calculated. In this way, the cross-sectional area (r o 2 -r i 2 ) to thickness r o -r i It is also easy to calculate

[0068] In addition, as shown in Figure 8(A), the frequency is fixed, and the cross-sectional area (r o 2 -r i 2 In this case, a master curve created by measuring at the same frequency as the measurement frequency is required. but, <dr>Since is independent of frequency, if a master curve for a representative frequency is prepared, a master curve corresponding to the measurement frequency is not necessarily required. This makes it easy to create a master curve. In addition, the calculation was performed at multiple frequencies. <dr>Calculate the average value of <dr>Using the cross-sectional area (r o 2 -r i 2 ) or thickness can be calculated to improve the reliability of thickness measurements.

[0069] As will be described later, it is also possible to detect the cross-sectional area and thickness of the specimen TP by utilizing the inductance component dL.

[0070] The results of measurements on a pipe-shaped test object TP will be described below. FIG. 9 shows a photograph showing the arrangement of the conductive test object TP, the first coil 1, and the second coil 2, and also shows the measurement conditions. Impedance measurements were performed on samples 1 to 7 as the test object TP, and the above analysis was carried out. Samples 1, 2, 3, 4, 5, and 6 each had a cross-sectional area (r o 2 -r i 2 ) are 28, 39, 48, 55, 57.75, 60mm 2 The specimen TP is a pipe-shaped specimen with a cross-sectional area (r o 2 -r i 2 ) is 64mm 2 The specimen TP is a solid rod-shaped specimen. The outer diameters of Samples 1 to 7 were all 16 mm, and the thicknesses (wall thicknesses) of Samples 1, 2, 3, 4, 5, and 6 were 2, 3, 4, 5, 5.5, and 6 mm, respectively. In actual thickness measurement, the outer diameter of the test object TP can be easily measured directly from the outside thereof, and therefore the outer diameter of the test object TP can be treated as a known value.

[0071] Figure 10(A) shows the measured data showing the frequency dependence of dR, and Figure 10(B) shows <dr>Fig. 10(C) shows the actual measurement data showing the frequency dependence of <dr>Figure 10(C) shows the cross-sectional area dependence of the alternating magnetic field frequency in the range of 10 Hz to 20 Hz. <dr>10 is a graph showing the cross-sectional area dependence of In Figure 10(A) and Figure 10(B), ▲, □, ■, ●, ◇, △, and ◆ represent the cross-sectional area (r o 2 -r i 2 ) are 28, 39, 48, 55, 57.75, 60, 64mm 2 The data shown is: In FIG. 10(C), ■, ○, ◆, □, and ● represent data at measurement frequencies of 12, 14, 16, 18, and 20 Hz.

[0072] It can be seen from FIG. 10(A) that dR increases depending on the frequency and also depending on the cross-sectional area. On the other hand, as shown in Figure 10(B), when the frequency is 10 Hz or higher, <dr>It can be seen that the frequency dependence of becomes very small. This result supports the above theoretical formula. It is also stable against frequency fluctuations. <dr>It can be seen that it is possible to measure In addition, at 10Hz or less <dr>The frequency dependence of is due to the influence of parasitic capacitance. In addition, a solid rod-shaped specimen TP (cross-sectional area 64 mm 2 ) compared to other pipe-shaped (hollow) specimens TP, <dr>The frequency dependence of

[0073] As shown in Figure 10(C), when the frequency is in the range of 10 to 20 Hz, the cross-sectional area (r o 2 -r i 2 ) is 50mm 2 In the following, <dr>No frequency dependence is observed (or the frequency dependence is very small). In addition, <dr>No frequency dependence is observed (or the frequency dependence is very small) means that even if the measurement frequency f is increased by a few Hz (1 to 5 Hz) (f + a few Hz), <dr>indicates that the Thick Skin condition is satisfied without fluctuation (or with small fluctuation). Also, the cross-sectional area (r o 2 -r i 2 ) is 50mm 2 In the following, the cross-sectional area (r o 2 -r i 2 )and <dr>It can be seen that there is a linear relationship between these two, which supports the above theoretical considerations. In addition, the cross-sectional area (r o 2 -r i 2 ) is 50mm 2 The cross-sectional area (r o 2 -r i 2 ) as a linear function of <dr>The area where it is possible to reproduce <dr>The cross-sectional area (r o 2 -r i 2 ) is called the linear approximation region.

[0074] Therefore, the values ​​obtained by measuring multiple standard samples prepared in advance with the same outer diameter but different thicknesses were <dr>and cross-sectional area (r o 2 -r i 2 ) and a master curve is created from the linear relationship with the TP. <dr>The cross-sectional area (r o 2 -r i 2 ) can be measured. Since the outer diameter of the specimen TP is known, the obtained cross-sectional area (r o 2 -r i 2 ) the thickness of the specimen TP can be measured. Furthermore, thickness can be measured stably regardless of frequency, and by measuring at multiple frequencies, the reliability of the data can be verified.

[0075] It should be noted that the cross-sectional area or thickness of the test object TP can also be measured using the differential inductance dL. A method for measuring the thickness of the specimen TP using dL will be described below.

[0076] Figure 11 shows the frequency dependence of dL and the cross section (r o 2 -r i 2 11A is a graph showing the frequency dependence of dL, and FIG. 11B is a graph showing the cross-sectional area (r o 2 -r i 2 ) dependency. In Figure 11(A), ▲, □, ■, ●, ◇, △, and ◆ represent the cross-sectional area (r o 2 -r i 2 ) are 28, 39, 48, 55, 57.75, 60, 64mm 2 The data shown is: In FIG. 11(B), squares, diamonds, circles, and triangles represent data when the frequencies f used in the measurements were 4 Hz, 6 Hz, 8 Hz, and 10 Hz, respectively.

[0077] As shown in FIG. 11(A), it can be seen that dL is almost independent of frequency at frequencies below 10 Hz. Figure 11(B) shows the cross-sectional area of ​​dL (r o 2 -r i 2 ) indicates dependency. As shown in Figure 11(B), at frequencies below 10 Hz, the cross-sectional area (r o 2 -r i 2 ) is 48mm 2 Until then, dL is the cross-sectional area (r o 2 -r i 2 ) shows a linear increase. Therefore, the cross-sectional area (r o 2 -r i 2 ) is 48mm 2 The cross-sectional area (r o 2 -r i 2 ) can be reproduced as a linear function of the cross section (r o 2 -r i 2 ) is a linear approximation region for the thickness (r o -r i ) can be calculated.

[0078] However, 48mm 2 More than 60mm 2 In the range of , dL is the cross-sectional area (r o 2 -r i 2 ) decreases linearly to 48 mm 2 The cross section of dL (r o 2 -r i 2 ) It is understood that dependencies change. As mentioned above, dL=2[L ac_1cum -L ac_1dif ]+4M, and dL includes the term "2[L ac_1cum -L ac_1dif ]” and also the mutual inductance M. 48mm 2 The cross section of dL (r o 2 -r i 2 ) dependency is thought to be due to the influence of mutual inductance M, which is a magnetic property. By removing the mutual inductance M that depends on the magnetic properties, it becomes possible to further extract the influence of eddy currents. A method for removing the influence of mutual inductance M will be described below.

[0079] As mentioned above, the effect of eddy currents increases in proportion to the square of the frequency. Therefore, under conditions where the frequency is lower than the low frequency (extremely low frequency conditions), abs(2[L ac_1cum -L ac_1dif ] / 4M)<<1 ...(Formula 31) Note that abs() indicates the absolute value. Extremely low frequency f vl For example, select a value that is one tenth of the low frequency or less as the extremely low frequency f vl For example, 1 Hz can be selected as the frequency. If dL, which is a function of frequency f, is dL(f), then f vl At =1Hz, dL(f vl =1)=4M (Formula 32) is approximated as follows. In addition, the extremely low frequency f vl for dL(f vl ) is called the base differential inductance.

[0080] L as a function of frequency f ac_1cum , L ac_1dif L ac_1cum (f), L ac_1dif (f), then dL(f) and base differential inductance dL(f vl ), d(dL), which is a function of frequency f, is defined as follows: d(dL)=d(dL(f)) (Equation 33) =dL(f>10)-dL(f vl )...(Formula 34) Base differential inductance dL(f vl )=4M, so d(dL)=2[L ac_1cum (f>10)-L ac_1dif (f>10)] ...(Formula 35) The d(dL) defined in this way is called the corrected differential inductance. Note that dL(f>10) is dL(f vl Select a frequency f (>10) where the contribution of eddy currents is large compared to the frequency f (=1). As the frequency f, for example, the above-mentioned low frequency (10 to 100 Hz) can be suitably selected, but the frequency is not limited to this. As an example of extremely low frequency, f vl = 1Hz was selected, but it is not limited to this. For example, the extremely low frequency f vl An extremely low frequency such as 0.1 Hz, 2 Hz, or 3 Hz may be selected as the frequency.

[0081] Furthermore, for the differential resistance dR, dR=ω 2 <dr>The resistivity defined by <dr>Similarly, the corrected differential inductance d(dL) is replaced by the intrinsic inductance is defined as follows: d(dL)=ω 2 ...(Formula 36) For simplicity, d(dL(f)) is written as d(dL), where ω is the frequency of the alternating magnetic field used to measure dL(f>1).

[0082] Figure 12 shows the corrected differential inductance d(dL) and the intrinsic inductance The actual measured values ​​are shown. Figure 12(A) shows the frequency dependence of d(dL), and Figure 12(B) shows The frequency dependence of Fig. 12(C) is The cross-sectional area (r o 2 -r i 2 ) dependency. In Figures 12(A) and 12(B), ▲, □, ■, ●, ◇, △, and ◆ represent the cross-sectional area (r o 2 -r i 2 ) are 28, 39, 48, 55, 57.75, 60, 64mm 2 The data shown is: In FIG. 12(C), ■, ◯, ♦, ◇, and ▲ represent data at measurement frequencies of 22 Hz, 24 Hz, 26 Hz, 28 Hz, and 30 Hz, respectively.

[0083] From FIG. 12(A), it can be seen that the absolute value of d(dL) exhibits the same frequency dependency as the absolute value of dR. From Figure 12(B), when the frequency is 10 Hz or more, tends to change linearly with frequency, but the amount of change is small. Particularly at frequencies above 20 Hz, The increase in frequency is very small. Therefore, for a change in frequency, It can be seen that it is stable.

[0084] From Figure 12(C), at frequencies between 20Hz and 30Hz, is the cross-sectional area (r o 2 -r i 2 ) and it can be seen that it decreases linearly with Thus, the specific resistance <dr>Similarly, the intrinsic inductance has little frequency dependence, and the cross section (r o 2 -r i 2 ) and it was confirmed by actual measurements that it changes (decreases) linearly with

[0085] Intrinsic Inductance and cross-sectional area (r o 2 -r i 2 ) as a master curve, From the cross-sectional area (r o 2 -r i 2 ) and calculate the cross-sectional area (r o 2 -r i 2 ) the thickness (wall thickness) of the specimen TP can be calculated.

[0086] Also, from Figure 12(C), the intrinsic inductance is the cross-sectional area (r o 2 -r i 2 ) is the range that changes linearly with the cross-sectional area (r o 2 -r i 2 ) is 28mm 2 From 60mm 2 The specific resistance shown in Figure 10(C) is in the range of <dr>Compared to the cross-sectional area (r o 2 -r i 2 ) is expanding. Therefore, By using this, it becomes possible to easily measure the thickness of the specimen TP over a wider range. and the cross-sectional area of ​​the specimen TP (r o 2 -r i 2 ) and by using the master curve, it is possible to measure the thickness of the specimen TP.

[0087] <dr>as well as is independent of frequency (or has little frequency dependence), A master curve for may be created for a representative frequency.

[0088] In this way, by calculating the two types of impedance detected in the summation and differential junction states of the two through coils (first coil 1, second coil 2), the influence of the resistance of the coil itself can be eliminated, and the resistance change due to eddy current can be calculated as the specific resistance. <dr>It can be extracted as: In addition, by taking the difference with the inductance at extremely low frequency (1 Hz), the inductance change due to eddy current can be calculated as the intrinsic inductance. It can be extracted as: Compared to electrical conductivity, magnetic properties tend to change easily depending on the processing and usage history of the object. <dr>and intrinsic inductance is dependent on the electrical conductivity of the test object TP but is hardly affected by the magnetic properties. Therefore, the thickness of the test object TP can be measured stably. Furthermore, by applying a low-frequency alternating magnetic field to the specimen TP and measuring the thickness under thick skin conditions, the limitations on the measurable thickness of the specimen TP are alleviated. Resistivity <dr>and intrinsic inductance Since there is a linear relationship between the cross-sectional area and the master curve, the thickness of a pipe with a constant outer diameter can be easily evaluated.

[0089] Also, <dr>and By using both of these, the reliability of the measurement data can be ensured.

[0090] In addition, the inherent inductance Although the measurable range of the thickness of the test object TP is limited compared to the above, it is possible to measure the cross-sectional area or thickness of the test object TP using the differential inductance dL.

[0091] It should be noted that the impedance measurement of the first coil 1 and the second coil 2 may be performed in the absence of the subject TP for periodic inspection of the thickness measurement device 100. However, it is not necessary to perform the impedance measurement of the first coil 1 and the second coil 2 in the absence of the subject TP for calibration every time the thickness of the subject TP is measured.

[0092] Furthermore, the detection unit 6 of the thickness measurement device 100 may be moved relatively with respect to the object TP to measure the thickness of different locations on the object TP. As a result, the thickness distribution of the object TP can be measured. For example, for a pipe-shaped object TP, the thickness of the object TP can be measured in the longitudinal direction to evaluate the uniformity of the thickness of the object TP and detect local thickness abnormalities.

[0093] (Application to plate thickness measurement) The thickness measuring device 100 including the two coils, the first coil 1 and the second coil 2, can also be used to measure the thickness of a flat plate-shaped object TP. FIG. 13 is a perspective view showing an example of measuring the thickness of a flat plate-shaped specimen TP using the thickness measurement device 100. A detection unit 6 is arranged on the surface of the flat-plate test object TP, and the sum impedance Z cum and differential impedance Z dif and measure. Then, the sum impedance Z cum and differential impedance Z dif Using <dr>, dL can be calculated. Furthermore, the summed impedance Z at low frequencies cum and differential impedance Z dif The measurement results of the summed impedance Z at extremely low frequencies VLcum (Very low frequency sum impedance Z VLcum ) and the differential impedance Z at extremely low frequencies VLdif (Very low frequency differential impedance Z VLdif ) and the measurement results, can be calculated. A master curve can be created using standard samples of the same material as the specimen TP but with different thicknesses, and the thickness of the specimen TP can be measured using the master curve.

[0094] The detection unit 6 of the thickness measurement device 100 can be moved relatively over the surface of the flat-plate-like specimen TP to measure the thickness of different points on the specimen TP. This makes it possible to evaluate the in-plane thickness uniformity of the specimen TP.

[0095] (Measurement procedure) The steps for measuring the conductive specimen TP using the thickness measurement device 100 will be described below. It is assumed that the specimen TP is made of a homogeneous material and has a uniform conductivity. (1) Create a master curve using a standard sample. (1-1) A low-frequency (for example, 10 Hz to 100 Hz) alternating magnetic field (first frequency) is applied to standard samples made of the same material, with the same outer diameter but different inner diameters by the first coil 1 and the second coil 2 of the thickness measurement device 100, and the summation impedance Z at the low frequency is measured by the summation-connected first coil 1 and the second coil 2. cum The differentially connected first coil 1 and second coil 2 produce a differential impedance Z dif and measure. As mentioned above, the first coil 1 and the second coil 2 are substantially identical. The frequency of the alternating magnetic field in this process may be referred to as the reference frequency. (1-2) Furthermore, an alternating magnetic field of extremely low frequency (a second frequency that is one-tenth or less of the first frequency) is applied to the standard sample by the first coil 1 and the second coil 2 of the thickness measuring device 100, and an extremely low frequency sum impedance Z VLcum The differentially connected first coil 1 and second coil 2 measure the extremely low frequency differential impedance Z VLdif Measure. In order to distinguish it from the alternating magnetic field of (1-1), the alternating magnetic field of extremely low frequency is called an extremely low frequency alternating magnetic field.

[0096] In addition, in (1-1) and (1-2), the sum impedance Z cum and differential impedance Z dif The order of measurements is arbitrary.

[0097] (1-3) Sum impedance Z at low frequencies cum and differential impedance Z dif From the differential impedance dZ, which is the difference between these, the differential resistance dR at low frequencies and the differential inductance dL at low frequencies are calculated. The specific resistance is calculated by dividing the differential resistance dR by the square of the frequency (or the square of the angular frequency) used in the measurement. <dr>Calculate. In addition, <dr>, dL is calculated for each different inner diameter.

[0098] (1-4) Specific Resistivity <dr>It is confirmed that the frequency used in the measurement satisfies the Thick Skin condition by using the above formula. The frequency of the alternating magnetic field used in the measurement is increased by a predetermined amount (for example, 2 Hz, 4 Hz) and the specific resistance is <dr>Evaluate the variation of <dr>If the variation is a predetermined amount (for example, 5% or 10% or less), it is determined that the thick skin condition is satisfied. If the Thick Skin condition is not satisfied, change the frequency used for measurement. <dr>, dL, and determine whether or not the Thick Skin condition is satisfied. Or, in advance, at multiple frequencies <dr>, dL are calculated, and the frequency that satisfies the Thick Skin condition is calculated. <dr>, dL is adopted. If a frequency that satisfies the thick skin condition cannot be obtained, it may be determined that the thickness of the object TP is not measurable, and the thickness may be stored in the control device 5.

[0099] (1-5) Obtained resistivity <dr>and cross-sectional area (r o 2 -r i 2 ) is created as a first master curve (master curve of the resistance component) (see Figure 10(C)). The first master curve is stored in the control device 5 of the thickness measurement device 100 . The first master curve is the resistivity <dr>and cross-sectional area (r o 2 -r i 2 ) but with a known r o and cross-sectional area (r o 2 -r i 2 ) to thickness (wall thickness) r o -r i can be calculated uniquely, so the first master curve is indirectly the specific resistance <dr>and the thickness of the specimen TP.

[0100] In addition, the specific resistance <dr>and thickness (r o -r i ) may be calculated as a first master curve and stored in the control device 5 of the thickness measurement device 100. Therefore, the first master curve is indirectly or directly the resistivity. <dr>and the thickness of the specimen TP (r o -r i ) and the correlation with

[0101] (1-6) Also, the obtained differential inductance dL and cross-sectional area (r o 2 -r i 2 ) is calculated as a second master curve (master curve of the inductance component) (see Figure 11(C)). The second master curve is stored in the control device 5 of the thickness measurement device 100. The second master curve indirectly defines the correlation between the differential inductance dL and the thickness of the specimen TP. In addition, the differential inductance dL and the thickness (r o -r i ) may be calculated as a second master curve and stored in the control device 5 of the thickness measurement device 100. Therefore, the second master curve is indirectly or directly related to the thickness of the specimen TP (r o -r i ) and the correlation with As a master curve for measuring the thickness of the test object TP using the inductance component, it is possible to create only the third master curve described below without creating the second master curve. As described above, when the influence of the magnetic properties is taken into consideration, This is because the thickness measurement performance is further improved with the use of the ion beam splitter.

[0102] (1-7) Furthermore, the extremely low frequency sum impedance Z VLcum and the extremely low frequency differential impedance Z VLdif and the base differential inductance dL(f vl ) and the differential inductance dL at low frequencies (corrected differential inductance d(dL)) to obtain the intrinsic inductance Calculate. Intrinsic Inductance d(dL) is the square of the frequency (reference frequency) ω of the alternating magnetic field in the (1-1) process (ω 2 ) is calculated by dividing by In addition, is calculated for each different inner diameter. The resulting specific inductance and cross-sectional area (r o 2 -r i 2 An approximate curve (especially a linear function) that reproduces the correlation with the inductance component is calculated as a third master curve (master curve of the corrected inductance component) (see Figure 12(C)). The third master curve is stored in the control device 5 of the thickness measurement device 100. The third master curve indirectly represents the intrinsic inductance and the thickness of the specimen TP.

[0103] In addition, the inherent inductance and thickness (r o -r i ) may be calculated as a third master curve and stored in the control device 5 of the thickness measurement device 100. Therefore, the third master curve is indirectly or directly and the thickness of the specimen TP (r o -r i ) and the correlation with

[0104] (1-8) After that, for different outer diameters, first, second, and third master curves are calculated using specimens TP whose inner diameters are similarly changed, and the calculated master curves are stored in the control device 5.

[0105] Similarly, first, second, and third master curves are calculated for different outer diameters and different inner diameters of specimens TP made of different materials, and are stored in the control device 5.

[0106] The control device 5 of the thickness measurement device 100 stores first, second, and third master curves that can indirectly or directly determine the thickness of the test object TP for each test object TP made of different materials and with different outer diameters.

[0107] (2) The thickness of the specimen TP is measured using the master curve. The operator specifies the material and outer diameter of the test object TP to the control device 5 of the thickness measurement device 100.

[0108] (2-1) When measuring the thickness of a conductive object TP having an unknown thickness, a low-frequency (for example, 10 Hz to 100 Hz) alternating magnetic field is applied to the object TP by the first coil 1 and the second coil 2 of the thickness measurement device 100, and a summation impedance Z at the low frequency is obtained by the summation-connected first coil 1 and the second coil 2. cum The differentially connected first coil 1 and second coil 2 produce a differential impedance Z dif and measure. The frequency of the alternating magnetic field in this step is referred to as a reference frequency (first frequency). (2-2) Furthermore, an alternating magnetic field (extremely low frequency alternating magnetic field) of extremely low frequency (second frequency equal to or less than one-tenth of the first frequency) is applied to the subject TP by the first coil 1 and the second coil 2 of the thickness measurement device 100, and the summation impedance Z cum (Very low frequency sum impedance Z VLcum ) and the differentially connected first coil 1 and second coil 2 result in a differential impedance Z dif (Very low frequency differential impedance Z VLdif ) and measure. In (2-1) and (2-2), the summed impedance Z cum and differential impedance Z dif The order of measurements is arbitrary.

[0109] (2-3) The control device 5 controls the sum impedance Z at low frequencies. cum and differential impedance Z dif Therefore, the resistivity at low frequencies is <dr>, and the differential inductance dL at low frequencies.

[0110] (2-4) Resistivity <dr>is used to confirm that the frequency used in the measurement satisfies the Thickskin condition. If the Thick Skin condition is not satisfied, the frequency is changed to determine the frequency that satisfies the Thick Skin condition, and measurements are made at the frequency that satisfies the Thick Skin condition. <dr>, dL is adopted. If there is no frequency that satisfies the Thick Skin condition, it can be determined that the thickness measurement device 100 is unable to perform measurement.

[0111] (2-5) The control device 5 reads out from the storage unit the master curves (first, second, and third master curves) corresponding to the material and outer diameter of the test object TP specified by the operator.

[0112] (2-6) The control device 5 detects the temperature of the object TP obtained by measuring the temperature of the object TP. <dr>and the first master curve, a first thickness (thickness based on the resistance component) of the object TP is obtained. Furthermore, the control device 5 obtains a second thickness (thickness based on the inductance component) of the test object TP from dL obtained by measuring the test object TP and the second master curve.

[0113] (2-7) The control device 5 controls the extremely low frequency sum impedance Z VLcum and the extremely low frequency differential impedance Z VLdif The base differential inductance dL(f vl ) and the differential inductance dL at the low frequency, the corrected differential inductance d(dL) is calculated, and the corrected differential inductance d(dL) is calculated by multiplying the square of the frequency (reference frequency) ω of the alternating magnetic field in (2-1) (ω 2 ) to obtain the specific inductance Calculate. (2-8) The control device 5 and the third master curve, a third thickness of the object TP (thickness based on the corrected inductance component) is calculated.

[0114] (2-9) The control device 5 stores the first, second, and third thicknesses of the object TP in a storage unit, and outputs the first, second, and third thicknesses of the object TP to the outside via an input / output unit. The output destination may be, for example, a display, a storage device, an external computer, or the like.

[0115] In the above, an example has been shown in which the control device 5 calculates and stores the first, second, and third master curves, and calculates the first, second, and third thicknesses of the subject TP corresponding to each of them, but it may also be configured to calculate at least one of the first, second, and third master curves and output the thickness of the subject TP corresponding to that master curve, or, for example, it may create only the first master curve and output only the first thickness. In addition, it may be configured to calculate two of any combinations of the first, second, and third master curves and output two thicknesses of the specimen TP corresponding to each master curve; for example, a first master curve and a third master curve may be created and the first thickness and the third thickness may be output.

[0116] The operator can arbitrarily select one or more thickness measurement results from the first, second, and third thicknesses that are output, depending on the material and shape of the test object TP.

[0117] The thickness measuring device 100 can also detect variations in the relative thickness of the object TP. For example, it can be used to search for areas of the object TP where the thickness is abnormally larger or smaller than the surrounding area. In this case, the absolute value of the thickness is not necessarily required, and it is sufficient to be able to monitor the relative thickness. Resistivity <dr>, differential inductance dL, and intrinsic inductance Since is a value that reflects thickness, it may be output without converting it to thickness. The detection unit 6 of the thickness measurement device 100 is moved relative to the object TP, and the resistivity <dr>, differential inductance dL, or intrinsic inductance By monitoring at least one of the above, it is possible to search for an abnormality, for example, where the value suddenly increases or decreases. In this case, a master curve is not required. [Industrial Applicability]

[0118] The thickness measurement method using eddy currents of the present invention alleviates limitations on the measurable range and makes it possible to easily measure the thickness of an object to be measured. This enables non-destructive measurement of the wall thickness of a conductive pipe-like object to be measured, and can be used for various purposes, such as pre-shipment inspection of products such as pipes, diagnosis of deterioration over time of the object, and detection of abnormal wall thickness, and has great industrial applicability. [Explanation of symbols]

[0119] 100 Thickness measuring device 1 First coil 2 Second Coil 3 Switching Device 4. Measuring equipment 5. Control device 6. Detection unit 7 Analysis section C, C1, C2 coils T measuring device < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / dr> < / rac> < / dr> < / dr> < / dr>

Claims

1. 1. A method for measuring a thickness of a conductive specimen, comprising: A first coil (1) and a second coil (2) having the same configuration as the first coil (1) are summed and connected together, and an alternating magnetic field of a first frequency is applied to the subject to measure the summed impedance (Z ｃｕｍ a summation connection measurement step for measuring the The first coil (1) and the second coil (2) are differentially connected, and the alternating magnetic field is applied to the subject to generate a differential impedance (Z ｄｉｆ a differential connection measurement step of measuring The sum impedance (Z ｃｕｍ ) and the differential impedance (Z ｄｉｆ a component extraction step of extracting a resistance component (dR) and an inductance component (dL) as components of a differential impedance (dZ) which is a difference between the a thickness calculation step of calculating a thickness of the object from a correlation between the resistance component (dR) or the inductance component (dL) of the differential impedance (dZ) and the thickness of the object; A thickness measurement method comprising:

2. 2. The method of claim 1, wherein the first frequency is in the range of 10 Hz to 100 Hz.

3. 3. The thickness measuring method according to claim 1, wherein the first frequency satisfies the Thick Skin condition.

4. The method further includes a resistivity calculation step of calculating a resistivity (<dR>) by dividing the differential resistance (dR) by the square of the first frequency, 4. The thickness measurement method according to claim 1, wherein in the thickness calculation step, the thickness of the object is calculated from the correlation between the resistivity (<dR>) and the thickness of the object instead of the differential impedance (dZ) component.

5. The first coil (1) and the second coil (2) are connected in a summing manner, and an extremely low frequency alternating magnetic field of a second frequency lower than the first frequency is applied to the subject, thereby generating an extremely low frequency summing impedance (Z ＶＬｃｕｍ a very low frequency sum connection measurement step for measuring the The first coil (1) and the second coil (2) are differentially connected, and the extremely low frequency alternating magnetic field is applied to the subject to generate an extremely low frequency differential impedance (Z ＶＬｄｉｆ a very low frequency differential connection measurement step for measuring The extremely low frequency sum impedance (Z ＶＬｃｕｍ ) and the extremely low frequency differential impedance (Z ＶＬｄｉｆ ) and the base differential inductance (dL(f ｖｌ )) and the differential inductance (dL) and the base differential inductance (dL(f ｖｌ and calculating a corrected differential inductance (d(dL)) from the difference between the first frequency and the second frequency, and calculating an intrinsic inductance () by dividing the corrected differential inductance (d(dL)) by the square of the first frequency, 4. A thickness measurement method according to claim 1, wherein in the thickness calculation step, the thickness of the object is calculated from the correlation between the intrinsic inductance () and the thickness of the object instead of the differential impedance (dZ) component.

6. 6. The thickness measuring method according to claim 1, wherein the test object is a pipe having a constant outer diameter.

7. A thickness measurement device for measuring the thickness of a conductive object, The device comprises a first coil (1), a second coil (2) having the same configuration as the first coil (1), a switching device (3), a measuring device (4), and a control device (5), the switching device (3) is capable of selectively switching between a summation connection state in which the first coil (1) and the second coil (2) are summation connected and a differential connection state in which the first coil (1) and the second coil (2) are differentially connected; The control device (5) controls the switching device (3) to set the connection between the first coil (1) and the second coil (2) to the additive connection state or the differential connection state, The control device (5) controls the measuring device (4) to measure a sum impedance between the first coil (1) and the second coil (2) in the sum connection state, and a differential impedance between the first coil (1) and the second coil (2) in the differential connection state, The control device (5) receives the sum impedance and the differential impedance from the measurement device (4), A thickness measurement device characterized by extracting a resistance component and an inductance component as components of a differential impedance, which is the difference between the summed impedance and the differential impedance, and calculating the thickness of the object from the correlation between the resistance component or the inductance component and the thickness of the object.

Images