JPH0215018B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electromagnetic acoustic transducer for a flaw detection device that detects flaws in conductive structural members such as piping.

For example, in order to detect defects in thin pipes such as water pipes using ultrasonic waves, a transducer as shown in FIG. 1 is used to generate ultrasonic waves. This transducer includes, for example, a plurality of permanent magnets 11
a, 11b, ... are set as one unit, and the magnets 11a, 11b, ... are arranged so that the upper and lower polarities thereof are opposite to each other, and a coil 12 for passing a high-frequency current is wound around the group of magnets forming one unit. It consists of

That is, as shown in FIG. 2, when a high frequency current Ih is passed through the coil 12, eddy currents I ₁ , I ₂ , . . . are generated in the thin tube 13 in contact with the coil 12.
These eddy currents I ₁ , I ₂ ,... are caused by the magnets 11a, 11b,...
As a result, magnetic fields B ₁ , B ₂ , .
Lorentz forces F ₁ , F ₂ , ... are generated by the interaction with These Lorentz forces F ₁ , F ₂ , ... change with the same period as the magnetic fields B _{1 ,} B ₂ , _... mentioned above, and the thin tube is 13, ultrasonic waves (plate waves) called SH waves are generated.

In other words, if there is a defect such as a scratch on the thin tube 13,
The ultrasonic waves are reflected at the location where the flaw exists, and this reflected wave is converted into an electrical signal by the transducer and detected.

However, in the transducer configured in this way, since the ultrasonic waves generated in the thin tube 13 mainly propagate in the tube axis direction X, defects such as scratches may occur in the thin tube 13 in the circumferential direction Y If there is a difference between the two, the ultrasonic wave will be propagated reliably and the correct reflected wave will be returned. Therefore, on the contrary, if a defect exists in the tube axis direction As a result, sufficient flaw detection sensitivity cannot be obtained.

This invention was made to solve the above-mentioned problems. For example, when performing ultrasonic flaw detection on thin pipes such as water pipes, even if the flaws to be detected exist in the direction of the pipe axis. The object of the present invention is to provide an electromagnetic acoustic transducer that can obtain sufficient flaw detection sensitivity.

That is, the electromagnetic acoustic transducer according to the present invention includes a plurality of magnets formed so as to be spirally wound at a constant pitch and having alternately different polarities in the circumferential direction about the same axis, and the plurality of magnets. It is equipped with high frequency current coils which are wound and set in correspondence with each other.

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 3 shows its configuration. For example, six elongated magnet bands 20a to 20f are connected to a cylindrical core 21.
With respect to the surrounding surface of the adjacent magnetic strips 20a-2
The magnetic pole of 0f is alternately different from the N pole and the S pole, and is wound so that it is arranged in a spiral shape with the same pitch Tm. A high frequency current coil 22 is wound between them to form an integrated structure as a cylindrical magnet body.

Each of the magnet bands 20a to 20f wound around the cylindrical core 21 may be a rubber magnet 23 having an elongated shape and having a magnetization direction perpendicular to the peripheral surface of the cylindrical core 21 as shown in FIG. 4a, or It is formed by arranging in series small magnets 24 having the same magnetization direction as shown in FIG. 4b. And these adjacent magnetic bands 20a~
The magnetization directions of 20f are set to be opposite directions. The coil 22 wound around the magnetic circuit made of such a rubber magnet 23 or small magnet 24 has its direction reversed at pitch T ₀ as shown in FIG. 5 and expanded in FIG. 22 is wound around the outer peripheral surface of the magnet body shown in FIG. The winding position of this coil 22 is approximately at the center of the magnetic pole face of the magnetic bands 20a to 20f and between each magnetic band. The direction of the high frequency current Ih flowing through the wound coil 22 is reversed at the pitch _T0 , and flows in a spiral shape along the magnet surfaces wound around the magnet bands 20a to 20f.

Here, the pitch Tm of each of the magnet bands 20a to 20f and the winding pitch _T0 of the coil 22 are set to intervals that satisfy the following equation.

Tm=λ/2 T ₀ =λ/4 where λ is the wavelength of the generated ultrasonic wave.

In other words, in a magnetic circuit constructed in this manner, as shown in FIG. 6, a part of its surface is expanded, there is a pitch T ₀ around the surface of the magnetic circuit due to the magnetic action of each of the magnet bands 20a to 20d. Magnetic fields B ₁ to _{B 7} whose directions change by 90° are generated.

FIG. 7 shows a diagonal cross section 25 of the thin tube 13 corresponding to plane A-A in FIG. 6 when this magnetic circuit is inserted into the thin tube 13, that is, the magnetic field B applied by the magnetic circuit is shown. Coil 2 for ₁ ~ B ₇
When a high frequency current Ih is passed through the coil 2, eddy currents _I1 to _I7 are generated in the oblique section 25 parallel to the coil 22.

These eddy currents I ₁ to I ₇ interact with the magnetic fields B ₁ to _{B 7} due to the Lorentz force whose direction changes by 90° at pitch T _0.
This Lorentz force generates F ₁ to F ₇ .
Lamb waves (plate waves) are generated in the oblique section 25 due to the directionality shown by the broken line that F ₁ to F ₇ have at a period of 2Tm, and as shown in FIG. will be propagated semi-infinitely.

As described above, according to the present invention, a plurality of magnets are formed so as to be wound in a spiral shape with a constant pitch and have alternately different polarities in the circumferential direction about the same axis, and each of the plurality of magnets is Since the high-frequency current coil is provided with a high-frequency current coil wound in a manner corresponding to the above-mentioned, it is possible to generate ultrasonic waves that propagate helically to the thin tube. In other words, by rotating and propagating ultrasonic waves along the tube axis direction of the thin tube, for example, not only when defects such as cracks exist in the circumferential direction of the thin tube, but also when defects exist in the tube axis direction, Ultrasonic waves are effectively propagated and reflected. Therefore, no matter where the defect as described above exists in the capillary, it is possible to obtain sufficient flaw detection sensitivity, and the reliability of ultrasonic flaw detection in the capillary is surely improved.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a conventional transducer, FIG. 2 is a diagram explaining ultrasonic waves generated by the above-mentioned transducer, and FIG. 3 is a diagram showing an electromagnetic acoustic transducer according to an embodiment of the present invention. Fig. 4a shows a rubber magnet that can be used as a magnet in the above embodiment, Fig. 4b shows a small magnet that can be used as a magnet in the above embodiment, and Fig. 5 shows a rubber magnet that can be used as a magnet in the above embodiment. FIG. 6 is an exploded view of a wound coil in the example, FIG. 6 is an expanded view of a part of the surface of the coil in the above example, and FIG. 7 is a diagram illustrating the ultrasonic waves generated in the example above. , FIG. 8 is a diagram showing the ultrasonic waves propagated to the thin tube in the above embodiment. 13...Thin tube, 20a to 20f...Magnetic band, 2
1...Cylindrical core, 22...High frequency current coil.

Claims (1)

[Claims] 1 A magnet body is created by winding a plurality of elongated magnet strips on the outer circumferential surface of a cylindrical core in a spiral shape so that the magnetic poles of adjacent magnet strips are different from each other. 1. An electromagnetic acoustic transducer comprising: coils having different current flow directions wound around the center of each magnet band and between each magnet body on the outer peripheral surface of the magnet body.