JPH0743349A - Method and device for eddy current flaw detection - Google Patents

Abstract

PURPOSE:To improve the detection sensitivity and S/N ratio for a very small defect by extracting a voltage difference induced in a secondary coil due to AC current supplied from an oscillator and then detecting the defect of a material whose flaw is to be detected according to the value. CONSTITUTION:Each adjacent head is driven in a reversed polarity manner to a magnetic field polarity produced by primary coils P of detection heads H1-H4, thus generating a magnetic flux distribution phi from the center pole of an E-type core C and making possible produced eddy current of a material 1 whose flaw is to be detected to indicate characteristics EC. When the produced magnetic fluxes cross in a beam-like manner in this configuration, the produced eddy current also becomes local. Therefore, when the head H is located at the mother material sound part of the material 1, magnetic fluxes produced from the center pole are divided and cross at a pair of secondary coils S1 and S2 which are laid out around the core C. Then, the material 1 moves in the X-direction and a defect I passes the coil S2, eddy current changes for the side of the coil S1 and magnetic flux density crossing the coil S2 changes due to reaction. By extracting voltage differences induced in the coils S1 and S2 and then measuring the values, the defect of the material 1 is detected.

Description

Detailed Description of the Invention

[0001]

[Application field of industry] It is possible to detect minute defects existing in conductive pipes and plate-shaped materials to be inspected, and it can be used for quality control and shipping inspection in the production line.

[0002]

[Prior art] Conventionally known techniques for detecting defects existing in conductive pipes and plate materials include (1) ultrasonic flaw detection method (2) leakage flux flaw detection method (3) eddy current flaw detection method, etc. The method corresponding to the purpose is widely used in the industry.

In particular, the eddy current flaw detection method is suitable for online inspection because it does not require a contact medium at the time of flaw detection and can perform flaw detection on a material to be inspected under non-contact conditions.

FIG. 7 is a block diagram showing the basic structure of a known technique of the eddy current flaw detection method. Reference numeral 1 is a flaw detection material, 2 is a rotary probe, and 3 is a slip ring.

The prior art will be briefly described below with reference to FIG. A resistance R from an oscillator (not shown) is attached to a probe coil installed outside the flaw detection material via a slip ring.
An alternating current is supplied to generate a local eddy current in the flaw detection material.

Next, when the probe coil composed of a slip ring and one body rotates at a constant speed on the outside of the material to be inspected, and the probe coil 2 crosses the defective portion I, it is generated from the probe coil 2. The eddy current due to the alternating magnetic field changes, and as a reaction to this, the impedance of the probe coil changes.

This change in impedance is extracted via the slip ring 3, and a defect affecting the flaw detection target material 1 is indirectly detected from the change value of impedance.

[0008]

Problems of the Prior Art In the prior art, there are the following problems.

Since the signal is transmitted through the slip ring, a noise signal due to poor contact of the slip ring is mixed.

The moving speed of the flaw detection target material is defined by the rotational speed of the probe coil.

In order to improve the inspection efficiency, there is no choice but to increase the number of probe coils or increase the rotation speed. However, in order to increase the number of probe coils, the number of CHs of the slip ring increases, and the size of the device increases, which deteriorates maintainability. In addition, there is a limit of the mechanical system regarding the increase of the rotation speed of the probe coil.

When the flaw detection material 1 and the probe coil 2 are misaligned with each other, the flaw detection material comes into contact with the probe coil 2 and the probe coil is damaged.

The user's needs further require high precision and high efficiency inspection of minute defects, and it is necessary to improve S / N.

[0014]

In order to improve the detection sensitivity and S / N for minute defects, it is required to generate an eddy current as locally as possible in the flaw detection material.

For this purpose, it is preferable that the distribution of the alternating magnetic field generated from the detection head installed facing the material to be inspected be made into a beam shape and intersect the material to be inspected.

Therefore, in the present technology, two or more detection heads are installed close to each other at a fixed interval with respect to the material to be inspected, and the polarities of the alternating magnetic fields generated from the adjacent detection heads are driven to have opposite polarities. .

FIG. 1 is a configuration diagram for explaining the basic principle of the technique of the present application.

In FIG. 1, reference numeral 1 is a material to be detected, and H1 to H4.
Is a detection head composed of an E-shaped core, C is an E-shaped core, P is a primary coil arranged on the central pole of the E-shaped core,
S1 and S2 are a pair of secondary coils arranged on the left and right poles, and Φ is a distribution of magnetic flux generated from the primary coils.

In FIG. 1, when the polarities of the magnetic fields generated from the primary coils of the detection heads H1 to H4 are driven in the opposite polarity for each adjacent head, the magnetic flux distribution generated from the central pole of the E-shaped core C is as shown in FIG. Since it is indicated by Φ of 1, the eddy current generated in the flaw-detected material by each detection head has a characteristic indicated by EC.

[0020]

When the magnetic flux generated from one detection head intersects the material to be detected with the detection head having the above-described structure in a beam shape, eddy current is locally generated in the material to be detected. .

Therefore, when the detection head is in the sound part of the base material of the material to be detected, the magnetic flux generated from the central pole of the E-shaped core is divided into two parts by the pair of secondary coils arranged in the E-shaped core. Cross.

In FIG. 1, when the flaw I exists in the material to be detected and moves in the direction of the arrow, when the flaw I passes through the upper secondary coil S2, the eddy current due to the magnetic flux generated from the primary coil is , S1 to the coil side, and as a reaction to this, the magnetic flux density intersecting the coil S2 changes.

Therefore, by extracting the difference between the voltages induced in the pair of secondary coils S1 and S2 and measuring this value, it is possible to indirectly detect the defect existing in the flaw detection material.

If the magnetic flux generated from the primary coil of the detection head is in the form of a beam, the reason why the detection sensitivity to minute defects is improved is that, for example, N magnetic fluxes are generated from the poles of the primary coil, and every 50 magnetic fluxes are generated. Since each secondary coil intersects with each other and there is a defect on one secondary coil side, and all N / 2 intersect with the defective part, the electrical resistance R is ∞ at the defective part. The generation of eddy currents is greatly reduced.

For this reason, the differential voltage of the voltage induced in the pair of secondary coils fluctuates greatly, and it becomes possible to detect more minute defects.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS.

In FIG. 2, reference numeral 1 is a material to be detected, and H1 to Hn.
Is an E-shaped core, each detection head, C is an E-shaped core, P is a primary coil, S1 and S2 are a pair of secondary coils,
W is the distance between the poles and M is the width of the E-shaped core. Further, FIG. 3 shows an electronic circuit of the detection head part, where OSC is an oscillator and S11
-S1n, S21-S2n show each secondary coil of each head.

Hereinafter, referring to FIG. 2 and FIG.
Examples will be described in detail.

Two or more detection heads H1 to Hn composed of E-shaped cores C are arranged at regular intervals on the outer circumference of the material to be inspected 1, and the adjacent primary coils are connected as shown in FIG. They are reversely connected to each other and an alternating current is supplied from the oscillator OSC.

When the magnetic flux generated from each primary coil P crosses the flaw detection material 1, a local eddy current is generated in the circumferential direction of the flaw detection material as shown in FIG. 2 (C).

In the soundness of the base material of the flaw detection material, the magnetic flux generated from the primary coil P of each of the detection heads H1 to Hn is about 1 /
It is divided into two and intersects with each pair of secondary coils of each head H1 to Hn.

Therefore, when the pair of secondary coils of each detection head are reversely connected to each other as shown in FIG. 3, the voltages induced in the pair of secondary coils are equal to each other, and the difference between them is zero volts. .

Under this condition, when the material to be inspected 1 moves in the direction of the arrow and the defect I crosses under the specific (for example, H3) detection head, the generation of the eddy current in the defect portion is compared with the sound portion of the base material. Decrease. This reaction causes a difference in the values of the voltages (V3, V3 ′) induced in the pair of secondary coils of the detection head H3, and the difference voltage ΔV3 is obtained.

In order to extract this differential voltage ΔV3, when a pair of secondary coils are connected so as to have opposite polarities,
A defect signal corresponding to the defect I can be extracted.

FIG. 4 shows an example of the flaw detection result for the artificial defect processed in the flaw detection material 1 according to the technique of the present application.

As shown in FIG. 4, the hole diameters are 0, 1, 0, 2,
Sufficient detection sensitivity and high S / N can be obtained for 0,4,0,8 mm through holes.

FIG. 5 shows the results of testing the detection sensitivity characteristics with respect to the lift-off change to the surface of the material to be inspected, the pole intervals of the E-type core, the pole lower end of the E-type core, and the present technique.

The lift-off L is set to 2 as the flaw detection condition.
~ 5mm, and the pole spacing W of the E-shaped core is 0.5 to 10m
It was set to m and tested for S / N characteristics against artificial defects.

As shown in FIG. 5, the S / N changes in accordance with the pole spacing of the E-type core and the lift-off, and the W / L for -3 dB or less is 0.7 <W / L <2,1.

Therefore, the flaw detection performance can be improved by selecting the relative ratio between the pole spacing W of the E-shaped core and the lift-off L.

FIG. 6 shows the relative S with respect to the relative ratio between the width M of the E-shaped core and the artificial defect (through hole) in the technique of the present application.
It is a result of testing / N.

As the test condition of FIG. 6, the width M of the E-shaped core is
, The artificial defect is 0,
The target was a 2 mm through hole.

As shown in FIG. 5, when the value of M / d increases, the relative S / N decreases. The reason for this is E
This is because the distribution of the magnetic flux generated from the central pole of the E-shaped core becomes broad when the width M of the mold core becomes large.

Assuming that the relative S / N allowable value is -3 dB, the desired S / N can be obtained when the value of M / d is M / d <40.

In the present application, the smaller M / d value is 20, but when M / d = 20, the width M of the E-shaped core is 2 mm. Due to the difficulty in manufacturing, the limit is actually set to around 1 mm.

Therefore, by setting the relative ratio between the width M of the E-shaped core and the artificial defect d such that 10 <Md <40, a minute defect can be detected with a high S / N.

In the embodiment of the present application, the principle of operation is explained by using the pipe as the flaw detection material 1. However, as shown in FIG. 2C, the pipe of the flaw detection material 1 is apparently disassembled in the axial direction of the pipe. Then, as shown by the distribution of the eddy current generated by the magnetic flux from the primary coil of each detection head, even if the material to be inspected is a plate material, it is possible to perform flaw detection in the same manner.

When the material to be inspected is a plate material, detection heads are installed in the width direction of the plate material so as to face the plate material at regular intervals, and under this condition, the material to be inspected is moved to be present in the plate material. It is possible to detect the defects.

If the material to be detected is a ferromagnetic material and magnetic noise is generated, the material to be detected is magnetically saturated with an electromagnet or the like in the same manner as in the conventional eddy current flaw detection method.
It is possible to improve / N.

[0050]

As described above, the following effects can be obtained by using the eddy current flaw detection method and the eddy current flaw detection device of the present invention.

Even if the diameter of the material to be detected (in the case of a pipe) or the width of the plate material increases, the detection sensitivity for minute defects is deteriorated by increasing the number of detection heads set corresponding to the diameter and width. This enables high-precision flaw detection.

The AC magnetic flux generated from the primary coil of the detection head can be converted into a beam to locally generate an eddy current in the material to be inspected, and high sensitivity to minute defects (twice or more compared with the prior art). Can be improved.

It is possible to easily determine the number of detection heads to be installed with respect to the defect detection object (hole diameter) existing in the flaw detection material.

If the material to be inspected is conductive, it can be inspected regardless of whether it is a non-magnetic material or a ferromagnetic material.

As in the prior art, since no signal is transmitted by a slip ring or the like, there is no deterioration of detection sensitivity or deterioration of S / N due to contact failure or the like.

As in the prior art, it is possible to provide a flaw-detection apparatus which does not have a drive unit for rotating the detection head, etc., has a low failure rate, and is maintenance-free.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the basic configuration of the present invention.

2A, 2B, and 2C are diagrams showing a principle of flaw detection according to an embodiment of the present invention. FIG. 2A is a relative positional relationship diagram between a material to be detected and a detection head. (C) The material to be inspected (pipe) is disassembled in the axial direction of the pipe to be flattened, and the eddy current locally generated in the detection head and the material to be inspected with respect to the material to be inspected. Schematic diagram of distribution

FIG. 3 is a signal system diagram of the present invention, showing a connection of cables supplied from an oscillator to each primary coil and a differential connection method of each primary and secondary coil.

FIG. 4 is an output voltage waveform of a flaw detection result for a minute defect according to an embodiment of the present invention.

FIG. 5 is a relative S / N characteristic diagram of artificial defects with respect to lift-off between the poles of the E-shaped core and the material to be inspected in the example of the present invention.

FIG. 6 shows relative S / N characteristics with respect to the ratio of the width of the E-shaped core used in the detection head and the hole diameter of the artificial defect in the example of the present invention.

FIG. 7 is a basic configuration diagram of a conventional technique (known technique).

[Explanation of symbols]

H ... Detection heads H1 to Hn ...
Two or more detection heads P ... Primary coil C ... E-shaped core S1, S2 ... A pair of secondary coils L ... Lift-off 1 ... Detected material T ... Top of detected material Ec ... Eddy current characteristic Φ ... Magnetic flux Distribution B ... Bottom of flaw-detected material I ... Artificial defect X ... Axial direction of flaw-detected material W ... Pole interval of E-shaped core M ... E-shaped core width OSC ... Oscillator 2 ... Rotating probe (probe coil) 3 ... Slip Ring R ... Resistance 0 ° and 360 ° ... Position where the flaw detection material is disassembled in the X-axis direction and displayed ΔV1 to ΔVn ... Differential voltage between a pair of secondary coils for each detection head

Claims (4)

[Claims] 1. Two or more detection heads, each having a primary coil in the central pole of an E-shaped coil and a pair of secondary coils in the other two poles, are installed facing the material to be inspected. In the eddy current flaw detection method in which an alternating current is supplied from the oscillator to the secondary coil to detect defects existing in the flaw detection material, the magnetic fluxes generated from the primary coils of the adjacent detection heads should have opposite polarities. It is characterized in that a secondary coil is connected to supply an alternating current from an oscillator, a difference between voltages induced in a pair of secondary coils of each detection head is extracted, and a defect of a flaw detection target material is detected from this value. Eddy current flaw detection method and its equipment 2. In the eddy current flaw detection method according to claim 1, the relative distance (hereinafter referred to as lift-off) L between the flaw detection material and the lower end of the pole of the E-shaped core of the detection head and each pole of the E-shaped core. Eddy current flaw detection method characterized by setting the relative ratio to the interval W to 0.7 <W / C <2.1 and its apparatus 3. In the eddy current flaw detection method according to claim 1, the relative ratio of the hole diameter d of the defect existing in the flaw-detected material to the width M of the E-shaped core of the detection head is 10 <M / Eddy current flaw detection method characterized by setting d <40 and its apparatus 4. The eddy current flaw detection method according to claim 1, wherein a pair of secondary coils for each detection head are connected in opposite polarities to induce a pair of secondary coils. An eddy current flaw detection method and an apparatus therefor, which are characterized by extracting a difference in applied voltage.