JP2001318080A - Detection coil and inspecting device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a detection coil and an inspecting device capable of detecting not only a surface of a sample but also a deep part or an internal part thereof without giving pretreatment to the sample. SOLUTION: In this detection coil comprising an exciting coil coaxially wound and integrated with an induction coil, the frequency f of a current impressed on the exciting coil satisfies 60 kHz>=f>=0.1 kHz, and the number N1 of turns of the exciting coil is related with the number N2 of turns of the induction coil so as to satisfy 1000>=N2>=N1>=50.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a detection coil for inspecting a sample using electromagnetic induction and an inspection apparatus using the detection coil.

[0002]

2. Description of the Related Art As a detection method using magnetism, a method called an eddy current method has been conventionally known. The eddy current method is a method that utilizes an eddy current generated in a sample due to an induced electromotive force when a coil to which an alternating current is applied approaches a conductive sample. Since the magnitude and distribution of the eddy current generated in the sample change depending on various conditions such as the defect and the shape and size of the sample, the change in the magnetic flux due to the state of occurrence of the eddy current is taken out, and the presence or absence and the shape of the sample are determined. It can inspect changes and the like and is widely used. The relationship represented by the following equation is known between the frequency of the alternating current applied to the coil and the induced electromotive force generated in the conductive sample.

[Formula 1] Here, E: electromotive force, f: frequency, and A: constant. Therefore, generally, in order to improve the detection sensitivity in the eddy current method, a method of increasing the frequency of the alternating current of the coil to increase the induced electromotive force (eddy current) generated in the test sample is adopted.

[0003]

However, the eddy current method has a problem that it is not suitable for detecting a deep portion of a sample. This is due to what is called the skin effect and is expressed by the following equation.

[Formula 2] Here, δ: penetration depth, f: frequency, μ: magnetic permeability, σ: conductivity. This indicates that the electromagnetic field generated in the sample is concentrated near the surface, and attenuates rapidly at a deeper portion, indicating that the higher the frequency, the greater the attenuation. For this reason, the eddy current method applying a high frequency has a high detection sensitivity near the surface, but is not suitable for detecting a deep part of the sample.

In the case of the eddy current method, the surface condition of the sample greatly affects the detection sensitivity. Therefore, when it is desired to detect a defect such as a welding sample or a casting sample having fine irregularities on the surface, for example, a pretreatment such as smoothing the surface or removing an oxide film or a coating film is required.

Further, the eddy current method has a problem that the distance (lift-off) between the coil and the surface of the sample greatly affects the detection sensitivity, so that the shape of the sample that can be measured is limited.

The present invention has been made in view of the above problems, and provides a detection coil and an inspection apparatus capable of detecting not only the surface of a sample but also a deep portion or the inside thereof without performing pretreatment on the sample. The task is to

[0007]

In order to solve the above-mentioned problems, a detection coil according to the present invention employs the following means.

That is, the present invention comprises an exciting coil which is excited by application of an alternating current to form a magnetic field, and an induction coil which electromagnetically induces an electromotive force by exciting the exciting coil to generate an electromotive force. Is coaxially wound with the above-mentioned exciting coil to form a single unit, and the frequency f of the current applied to the exciting coil is 60 k
Hz ≧ f ≧ 0.1 kHz, and the number of turns N1 of the exciting coil and the number of turns N2 of the induction coil are set to 1000 ≧ N2 ≧
It is characterized by a relationship of N1 ≧ 50.

In this means, the magnetic flux reaches the deep part of the test sample to be inspected by applying a low frequency current to the exciting coil. However, the electromotive force (hereinafter referred to as the electromotive force E) generated in the test sample because of the low frequency. ) Is reduced. To compensate for this, the number of turns N1 of the exciting coil is increased, the magnetic flux density generated by the exciting coil is increased, and the electromotive force E generated in the test sample is increased. As a result, the magnetic flux reaches the deep portion of the test sample and generates a large electromotive force E also in the deep portion. Further, by increasing the number of turns N2 of the induction coil, the mutual inductance is increased to increase the electromotive force generated in the induction coil (hereinafter, referred to as an electromotive force V to distinguish it from the electromotive force E). Even a small change in E is detected efficiently.

According to a second aspect of the present invention, in the detection coil according to the first aspect, the frequency f of the current applied to the exciting coil is set to a preferable range of 40 kHz ≧ f ≧ 1 kHz.

According to this means, it is possible to obtain a detection coil having a penetration depth suitable for allowing the magnetic flux to reach a deep portion of the test sample.

According to a third aspect of the present invention, in the detection coil according to the first aspect, the number of turns N1 of the exciting coil is set to a preferable range of 600 ≧ N1 ≧ 80.

According to this means, it is possible to obtain a detection coil having a magnetic flux density suitable for generating a large electromotive force E to a deep part of the test sample.

Further, in order to solve the above-mentioned problem, an inspection apparatus using a detection coil according to the present invention employs the following means.

That is, in claim 4, claims 1 to 3
A detection coil unit having a detection coil according to any of the above, an AC power supply for applying an AC current to the detection coil unit, and a detection circuit unit for detecting a change in a magnetic field due to the test sample from an electromotive force output from the induction coil. Consists of

In this means, an alternating current output from the AC power supply is applied to the detecting coil section, thereby exciting the exciting coil of the detecting coil and generating an electromotive force V in the induction coil by electromagnetic induction. Is detected by a detection circuit section which has been input as an electric signal.

According to a fifth aspect of the present invention, the detection circuit section detects a change in the electromotive force output from the induction coil separately for a phase change amount and an amplitude change amount.

According to this means, the electromotive force V of the induction coil input as an electric signal to the detection circuit section is obtained by dividing the amount of change into a phase and an amplitude and obtaining the respective amounts of change.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A basic form of a detection coil according to the present invention will be described below with reference to the drawings. FIG.
Is a perspective view illustrating a configuration of a detection coil unit according to the embodiment (1), and FIG. 2 is a cross-sectional view illustrating a detection example using the embodiment (1).

The detection coil unit 10 includes an excitation coil 11 that is excited by the application of an alternating current to form a magnetic field 13, and an induction coil 12 that generates an electromotive force V by electromagnetic induction by excitation of the excitation coil 11. The excitation coil 11 and the induction coil 12 are coaxially wound and integrally formed.

The frequency f of the alternating current applied to the excitation coil 11 affects the depth δ of the magnetic flux penetrating into the test sample 30, as is clear from the above-mentioned equations (1) and (2). This also affects the electromotive force E generated. Therefore, when the magnetic flux reaches the inside of the test sample 30 by lowering the frequency f, the change in the test sample 30 cannot be sufficiently detected unless the electromotive force E is increased at the same time.

On the other hand, the following relationship is generally known between the number of turns N1 of the exciting coil 11 and the number of turns N2 of the induction coil 12. The relationship between the magnetic flux density B generated by electromagnetic induction of the exciting coil 11 and the number of turns N1 of the exciting coil 11 is as follows.

[Equation 3] It is represented by Where B: magnetic flux density, μ ₀ : vacuum permeability, l: length of exciting coil, N1: number of turns of exciting coil,
I ₁ : Indicates the current of the exciting coil. Therefore, when the number of turns N1 of the exciting coil 11 is increased, a larger magnetic flux density B can be generated. The relationship between the mutual inductance of the induction coil 12 with respect to the excitation coil 11 and the number of turns N2 of the induction coil 12 is as follows.

(Equation 4) It is represented by However, M _21: mutual inductance, N2:
The number of turns of the induction coil, φ ₂₁ : magnetic flux generated by the excitation coil and passing through the induction coil, I ₁ : current of the excitation coil. Therefore, as the number of turns N2 of the induction coil 12 increases, the mutual inductance increases, and the electromotive force V generated in the induction coil 12 increases. That is, the induction coil 12
Turns N2 greatly affects the detection sensitivity to changes in magnetic flux.

In view of the above-described relationship, in the detection coil section 10 of the present invention, the frequency f of the current of the exciting coil 11 is lowered in order to suppress the rapid attenuation of the magnetic flux in the test sample 30. An adverse effect that the electromotive force E of the sample 30 becomes small occurs. To compensate for this, the excitation coil 1
A measure was taken to increase the magnetic flux density B by increasing the number of turns N1 of 1 and thereby increase the electromotive force E generated in the test sample 30. However, this does not mean that the frequency f should be low or that the number of turns of the exciting coil 11 be large N1. These are based on an appropriate balance, and are optimal in a relative relationship depending on the purpose of detection. The detection coil unit 10 is formed.

In other words, the detection coil unit 10 can select the frequency f of the alternating current of the exciting coil 11, the number of turns N1 of the exciting coil 11, and the number of turns N2 of the induction coil 12 from the optimum range of the present invention to obtain the penetration depth. The balance between the δ and the electromotive force E allows the magnetic flux to reach not only the vicinity of the surface of the test sample 30 but also the inside thereof, and a feature that a large electromotive force E can be generated in the test sample 30 can be produced.

Specifically, the frequency f of the alternating current applied to the exciting coil 11 is 60 kHz ≧ f ≧ 0.1 kHz
And more desirably, 40 kHz ≧ f ≧ 1 kHz. By selecting from this range, a penetration depth δ sufficient to obtain information not only from the surface of the test sample 30 but also from a deep portion is obtained. Since the actually required depth of arrival of the magnetic flux varies depending on the physical properties and configuration of the test sample, the purpose of the test, and the like, the frequency f is appropriately adjusted within the above range.

On the other hand, the number of turns N1 of the exciting coil 11 and the number of turns N2 of the induction coil 12 formed by the detection coil section 10 are as follows.
Specifically, the relationship of 1000 ≧ N2 ≧ N1 ≧ 50 is satisfied, and desirably, the number of turns N1
Is preferably selected from the range of 600 ≧ N1 ≧ 80. Thereby, even in the deep part of the test sample 30, a large electromotive force E can be generated, and the detection coil unit 10 having high detection sensitivity can be formed.

As for the shape of the exciting coil 11, if the number of turns is the same, it is effective to form the coil so that the cross-sectional area of the coil is large so that the magnetic flux reaches far.
Specifically, the wire diameter of the conductive wire of the coil is increased,
It is wound in multiple stages, or the inner hollow hole 21 is wound at the time of winding.
It is good to make the diameter of the part to become large.

The part to be inspected 20 on which the inspection sample 30 is placed is
Either the internal hollow hole 20a of the detection coil unit 10 or one of the opposing positions 20b and 20c at one end can be selected. This is a feature obtained by the fact that the magnetic flux density generated by the detection coil unit 10 is large. Conventional eddy current method coils or the like had to allow the test sample 30 to pass through the coil, but this is not necessary in the present invention, so there is no need to provide large-capacity holes inside. A solid structure without an internal hollow hole may be used. Therefore, the size and shape of the detection coil unit 10 can be freely changed, and downsizing becomes easy. In addition, the facing position 20
When the test sample 30 is placed at the positions a and 20b, the position may be set at any position as long as the magnetic flux of the magnetic field 13 can reach, and it is not necessary to strictly control the lift-off unlike the eddy current method.

The exciting coil 11 and the induction coil 12 form a detection coil unit 10 by coaxially winding an insulated conductive wire. The induction coil 12 is linked with the magnetic flux generated by the excitation coil 11 with high efficiency by being wound coaxially with the excitation coil 11, but by further increasing the adhesion of the windings of these two coils, The leakage magnetic flux of the induction coil 11 decreases, and the mutual inductance of the induction coil 12 increases. Thereby, the induction coil 12
The generated electromotive force V increases, and the detection sensitivity can be improved.

As described above, when the excitation coil 11 and the induction coil 12 are wound closely, the detection sensitivity is improved. However, if necessary, an electromagnetic shield layer or the like may be provided between the excitation coil 11 and the induction coil 12. An insulating sheet layer may be interposed. Further, the excitation coil 11 may be wound around a bobbin.

In the embodiment (1), the structure in which the induction coil 12 is formed on the outer peripheral surface of the exciting coil 11 is shown. Alternatively, the structure may be changed. Also, the coaxial means that the center axes of the coils coincide with each other.
2 may be configured to be juxtaposed, or may be configured to be wound by overlapping only a part thereof, or other configurations.

Further, the exciting coil 11 and the induction coil 12
Is not limited to the shape shown in the drawing, and the length and cross-sectional area of each coil or the wire diameter of the conductive wire of each coil are appropriately determined according to the design specifications.

As described above, the detection coil unit 10 of the present invention
In, the magnetic flux generated by the excitation coil 11 can reach not only near the surface of the test sample 30 but also deep inside, and a large electromotive force E is generated in the test sample 30. Furthermore, since the induction coil 12 has a very high efficiency mutual inductance, a subtle change in the test sample 30 can be detected. Therefore,
The use of the detection coil section 10 of the present invention makes it possible to detect even a small change in the magnetic flux of the test sample 30, so that the material of the test sample 30 may be a conductor, a magnetic material, a non-magnetic material. It can be detected regardless of the body, and the test sample 30
Has an excellent feature that it can be detected even if it has irregularities on its surface or an oxide film, a coating film, a metal film, or the like.

Next, the detection operation using the embodiment (1) will be described. FIG. 2 shows that the test sample 30 is
In this example, the presence or absence of the foreign matter 31 inside is detected. First, since it is necessary for the test sample 30 to make the magnetic flux reach the inside of the envelope 32, the number of turns of the exciting coil 11 and the induction coil 12 is adjusted according to various conditions such as the material and shape of the test sample 30. 1000 ≧ N2 ≧
It is determined within the range of N1 ≧ 50, and the detection coil unit 10 is manufactured. Alternatively, from among several types of detection coil units 10 prepared by changing the number of turns in advance, a detection coil unit suitable for various conditions of the test sample 30 is selected.

Next, the inspection target portion 20 of the detection coil portion 10
The sample in the normal state without the foreign matter 31 (hereinafter referred to as reference sample 3)
0a) and the electromotive force V generated in the induction coil 12 is measured as a reference value Vref. At this time, the frequency f at which an appropriate Vref is obtained in accordance with the test sample 30
Is adjusted in the range of 60 kHz ≧ f ≧ 0.1 kHz.

Next, the test sample 30 to be checked is arranged at the same position as the position where the reference sample 30a is arranged, and the electromotive force V generated in the induction coil 12 is measured as a measured value Vout. At this time, when the test sample 30 is in the steady state, which is the same as the reference sample 30a, that is, when the test sample 30 has no abnormality such as contamination of foreign matter, the measured value Vout becomes the reference value Vref
Matches. On the other hand, when the measured value Vout is different from the reference value Vref, the presence of a foreign substance can be detected.

In the above description, as an example of the abnormal state of the test sample 30, the mixing of the foreign matter 31 in the envelope 32 has been described. Can be detected. For example, generation of cracks and pinholes, change in composition, corrosion, change in shape, and the like can be similarly detected. Further, the position of the abnormality can be detected not only at the deep part but also at the surface or near the surface.

Alternatively, the frequency f at which the abnormality is most prominently detected may be checked in advance, and then the steady state and the abnormal state may be compared. Further, not only the measured value for a specific frequency f may be compared, but also the frequency characteristic of the measured value Vout may be compared with the frequency characteristic of the reference value Vref by changing the frequency f.

Next, as a second embodiment, an inspection apparatus using the detection coil unit 10 of the first embodiment will be described. FIG. 3 is a configuration diagram of the inspection apparatus, and FIG. 4 is a model diagram of an electric signal. Note that the same description as in the embodiment (1) will not be repeated.

The inspection apparatus 1 of the embodiment (2) includes the detection coil unit 10 of the embodiment (1), an AC power supply 40 for applying an AC current to the exciting coil 11 of the detection coil unit 10,
The detection circuit unit 50 detects a change in the magnetic field due to the test sample 30 from the electromotive force V output from the induction coil 12.

The inspected portion 20b includes a reference sample 30a,
The test samples 30 are sequentially arranged, and each time the electromotive force V of the induction coil is detected as a measured value. Each test sample does not need to be close to the detection coil unit 10, but the test sample 30 and the reference sample 30 a must be placed at the same position with respect to the detection coil unit 10.

The detection circuit section 50 receives the electromotive force V output from the induction coil 12 as an electric signal and compares and detects a change in the magnetic field. The comparison circuit 51 outputs the state of the test sample 30 based on the output of the comparison circuit 51. Determination circuit 52 for determining
It is composed of

In the comparison circuit 51, the reference sample 30a
Of the induction coil 12, ie, the reference value Vref
And the output measurement value V of the induction coil 12 by the test sample 30
out is compared. Then, the change amount of the measured value Vout with respect to the reference value Vref is, as shown in FIG.
The phase change amount Δθ and the amplitude change amount ΔH are detected separately.
The phase change amount Δθ is an amount that depends on the magnetic characteristics of the test sample 30, that is, the magnetic permeability, and the amplitude change amount ΔH is an amount that depends on the electric characteristics of the test sample 30, that is, the electric conductivity. Therefore, by distinguishing and detecting the magnetic characteristics and the electrical characteristics of the test sample 30, a subtle change in the state of the test sample 30 can be detected. That is, it is possible to detect a heterogeneous portion 33 of the test sample 30, for example, corrosion or deterioration of the inside, a subtle difference in the structure, and the like.

The determination circuit 52 determines whether or not the test sample 30 is in a steady state based on the output of the comparison circuit 51. If both the phase change amount Δθ and the amplitude change amount ΔH are zero, the test sample 30 is determined to be in a steady state.
Further, the above-described phase change Δ
If the θ and the amplitude change ΔH are converted into data in association with the state of the test sample, the state of the abnormality of the test sample 30, that is, the state of a composition change, a foreign matter, a crack, or the like is specifically identified. You can also understand.

As described above, the inspection apparatus 1 of the present invention can detect even a slight change in the test sample 30 by detecting the amount of change in the state of the test sample 30 while distinguishing between the magnetic characteristics and the electrical characteristics. It has an excellent feature that a difference due to a change factor can be identified. Thereby, regardless of whether the test sample 30 is a conductor, a magnetic material, or a non-magnetic material, it is possible to detect an abnormality such as a change in the material and dimensions of the test sample 30 or the presence or absence of a defect, and accurately inspect not only the surface but also the inside. it can.

The configuration of the detection circuit 50 is not limited to the comparison circuit 51 and the determination circuit 52 described above, and may be appropriately added with necessary circuits such as an amplification circuit, a conversion circuit, and a display circuit. Good.

In the embodiment (2), as an application example, as shown in FIG.
The same coil structure as that described above may be provided in advance as the reference coil unit 14, and the coil unit 15 may be configured to extract a differential output. At this time, the inspection sample 30 and the reference sample 30a are sequentially arranged only in the detection coil unit 10 without disposing any inspection sample 30 in the reference coil unit 14. If the differential output between the reference coil unit 14 and the detection coil unit 10 is taken out in this configuration, the background effect of the inspection environment can be removed, and the detection accuracy of the detection coil unit 10 can be further improved.

Alternatively, as another application example of the embodiment (2), as shown in FIG. 6, the same coil structure as that of the detection coil unit 10 is provided in advance in the inspection apparatus 1 as the reference coil unit 14, The coil unit 15 is configured to take out a differential output, and the reference coil unit 14 includes a reference sample 30.
a, the test sample 30 may be arranged in the detection coil unit 10. Thereby, if the reference sample 30a arranged in the reference coil unit 14 and the test sample 30 arranged in the detection coil unit 10 are the same, the differential output becomes zero, and if the test sample 30 is abnormal, The balance is lost, a differential output occurs, and an abnormality is recognized. Note that the test sample 30 and the reference sample 30a must be relatively located at the same position with respect to the detection coil unit 10 and the reference coil unit 14.

As described above, the inspection apparatus 1 of the present invention can extract information not only from the vicinity of the surface of the detection sample 30 but also from a deep part. Further, even if there are fine irregularities or oxide films on the surface state of the test sample 30, it can be detected without any pretreatment. Furthermore, there is no need to bring the test sample close to or through the test sample. Therefore, it is suitable for use in detecting cracks, pinholes, defects, and the like of metal workpieces, such as deterioration and composition change of metal parts, and foreign substances and the like mixed therein. Specific inspection targets include weld inspection, castings,
It can be used for forging inspection, quenching / tempering inspection, steel / alloy discrimination inspection, metal corrosion inspection, metal fatigue inspection, metal nitridation inspection, brazing inspection, sintered metal crack inspection, etc. Further, it may be configured to be installed on a factory line or the like as a permanent type, or may be configured to be taken out to a site where inspection is required and carried out as a portable type.

[0050]

Next, the present invention will be described in more detail with reference to examples. (Example 1) Using the inspection apparatus 1 according to the embodiment (2) of the present invention, an experiment was conducted to detect the presence or absence of cracks in an inspection sample shielded by a metal plate. FIG. 7 illustrates a configuration diagram of the first embodiment. The test sample 30 is an aluminum die-cast product having a stepped portion having a depth of 5 mm on the surface, and a crack 35 having a width of 100 μm and a depth of 3 mm near the bottom of the stepped portion.

The detection coil section 10 is coaxially adhered to the outer peripheral surface of the exciting coil 11 around which the conductive wire is wound 100 times (N1), and the conductive coil is used as the induction coil 12.
It was wound 0 times (N2) and integrally formed.

The experiment was performed under the following four conditions. Condition a was a normal test sample without cracks (reference sample), condition b was a test sample with cracks, and condition c was an iron plate interposed between the detection coil unit 10 and the sample under condition b. In the case, the condition d is a case where an aluminum plate is interposed between the detection coil unit 10 and the sample under the condition b. In the experiment a ~
The same detection coil unit 10 was used under any condition of d.
The distance (lift-off) ΔL from one end of the detection coil section 10 to the test sample 30 was 10 mm, and the thickness of the metal plate 36 was 0.3 mm for both aluminum and iron.

Table 1 summarizes the results of detection under each condition.

[Table 1]

From the experimental results, in the inspection apparatus 1, regardless of whether the inspection sample 30 is shielded by a magnetic (iron plate) or non-magnetic (aluminum) metal plate, the magnetic flux penetrates these metal plates. And it was found that cracks could be detected. In this case, by adjusting the frequency f according to the metal plate, the detection sensitivity was improved and the detection was remarkable. In addition, it was found that the detection coil section 10 can sufficiently detect the position of the crack 35 even when the crack 35 is not located on the surface of the test sample 30 but is located one step lower.

(Example 2) Using the inspection apparatus 1 according to the embodiment (2) of the present invention, an inspection of a crack of an iron casting was performed. The detection coil unit 10 is coaxially adhered to the outer peripheral surface of the exciting coil 11 around which the conductive wire is wound 350 times (N1), and is integrally formed by winding the conductive wire 350 times (N2) as the induction coil 12. did. The applied frequency f is 9
kHz.

As a result of the above experiment, the detection coil unit 10
Cracks having a width of 100 μm and a depth of 3 mm of the iron casting were detected separately from the surface irregularities. In this way, even if the surface of the iron casting is rough, the test sample 3
No. 0 could be inspected without any pretreatment.

[0057]

As described above, the detection coil of the present invention is
The frequency f of the current applied to the exciting coil is 60 kHz ≧
f ≧ 0.1 kHz, and the number of turns N1 of the exciting coil and the number of turns N2 of the induction coil are 1000 ≧ N2 ≧ N
By forming a coil coaxially so as to have a relationship of 1 ≥ 50, regardless of whether the test sample is a conductor, magnetic or non-magnetic material, anomalies existing not only on the surface but also inside are detected with high sensitivity. can do.

Further, an inspection apparatus is constituted by using the detection coil, and the change of the magnetic field due to the test sample is separated into the phase change and the amplitude change to be detected. Can be detected, and the abnormality can be identified and grasped according to the state.

[Brief description of the drawings]

FIG. 1 is a perspective view illustrating a configuration of a detection coil unit according to a first embodiment of the present invention.

FIG. 2 is a cross-sectional view showing a detection example using the embodiment (1) of the present invention.

FIG. 3 is a configuration diagram of an inspection device according to an embodiment (2) of the present invention.

FIG. 4 is a model diagram of an electric signal according to the embodiment (2) of the present invention.

FIG. 5 is a configuration diagram of an application example of the inspection device according to the embodiment (2) of the present invention.

FIG. 6 is a configuration diagram of an application example of the inspection device according to the embodiment (2) of the present invention.

FIG. 7 is a configuration diagram of the first embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Detection coil part 11 Excitation coil 12 Induction coil 13 Magnetic field 14 Reference coil part 15 Coil part 20 Inspection part 21 Internal hollow hole 22a, b Opposition position 30 Inspection sample 30a Reference sample 31 Foreign substance 32 Enclosure 33 Heterogeneous part 35 Crack 36 Metal plate 37 Unique part 40 AC power supply 50 Detection circuit part 51 Comparison circuit 52 Judgment circuit

Claims (5)

[Claims] 1. An excitation coil which is excited by application of an alternating current to form a magnetic field, and an induction coil which electromagnetically induces an electromotive force by excitation of the excitation coil to generate an electromotive force. In the detection coil formed integrally by coaxial winding, the frequency f of the current applied to the excitation coil is set to 60 kHz ≧ f ≧ 0.1 kHz, and the number of turns N1 of the excitation coil and the number of turns N2 of the induction coil are determined. A detection coil having a relationship of 1000 ≧ N2 ≧ N1 ≧ 50. 2. The detection coil according to claim 1, wherein a frequency f of a current applied to the excitation coil is set to a preferable range of 40 kHz ≧ f ≧ 1 kHz. 3. The detection coil according to claim 1, wherein the number of turns N1 of the exciting coil is set to a preferable range of 600 ≧ N.
A detection coil, wherein 1 ≧ 80. 4. A test sample comprising: a detection coil unit having the detection coil according to claim 1; an AC power supply for applying an AC current to the detection coil unit; and an electromotive force output from an induction coil. An inspection device comprising a detection circuit unit for detecting a change in a magnetic field. 5. The inspection apparatus according to claim 4, wherein the detection circuit detects a change in the electromotive force output from the induction coil separately for a phase change amount and an amplitude change amount.