JPH0142376B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electromagnetic acoustic transducer used for ultrasonic flaw detection of thin tubes, etc.

As an electromagnetic acoustic transducer (hereinafter abbreviated as EMAT) that is inserted into a thin tube to perform ultrasonic flaw detection, one having the structure shown in FIG. 1 is conventionally known. That is, 1 in the figure are permanent magnets arranged so that the upper and lower polarities are opposite to each other, and coils 2 are wound around these permanent magnets 1 so that, for example, five permanent magnets form one unit. This constitutes EMAT3. In addition, 4 in the figure
is the tubule into which EMAT3 is inserted. It takes
The operation of EMAT will be explained with reference to the second example.
When a high frequency is applied to the coil 2 of the EMAT 3, an eddy current I is generated in the thin tube 4 adjacent to the coil 2. On the other hand, a perpendicular and periodic magnetic flux B is applied from the permanent magnet 1 to the wall surface of the thin tube 4, and the eddy current I
Lorentz force F is generated by the interaction with
This Lorentz force F changes at the same period as the magnetic flux period, and this force F generates ultrasonic waves (plate waves) called SH waves in the thin tube 4. Note that the ultrasonic waves are detected by converting them into electrical signals in a reverse process to that described above.

However, since the ultrasonic waves generated by the EMAT 3 with the above structure propagate in the axial direction of the thin tube 4, they are strongly reflected against defects in the circumferential direction, but the reflectance is low against defects in the axial direction, and the defect detection becomes difficult. Therefore, it is sufficient to generate a wave that propagates in the circumferential direction, but if the diameter of the capillary is small and the duration of the ultrasonic wave that circulates around the circumference of the capillary and returns is long, the defect signal will be buried in it and detected. Can not.

The present invention has been made in order to eliminate the above-mentioned drawbacks, and aims to provide an electromagnetic acoustic transducer that is capable of extracting only axial defect signals of small-diameter thin tubes and that can reliably perform defect detection. It is something.

Hereinafter, the present invention will be explained in detail based on an embodiment shown in the third diagram.

In the figure, reference numeral 11 denotes a permanent magnet forming a magnetic circuit, and both end faces of this magnet indicating polarity are curved along the inner wall surface of the thin tube. Moreover, the magnet 1
A pair of support parts 12a and 12b whose distal ends are curved along the inner wall surface of the thin tube are attached to both side surfaces of the support part 1, respectively. A transmitting coil 13 is disposed on a curved surface at the tip of one of the support portions 12a in a meandering manner in a direction perpendicular to the direction of curvature.
Further, on the curved surface of the tip of the other support portion 12b, a receiving coil 14 is arranged in a meandering manner in a direction perpendicular to the direction of curvature. In other words, the above transmission,
The receiving coils 13 and 14 are arranged symmetrically with the permanent magnet 11 at the center. These coils 1
3 and 14 meander with a period T ₀ equal to the wavelength λ of the ultrasonic waves generated, and the length of the transmitting coil 13
L _T and the length L _R of the receiving coil 14 are expressed as follows.

L _R = T _p (n + 1/2) + L _T (however, n = 0, ±1, ±2...) Next, the action of the EMAT of the present invention is shown in Figures 4 to 6.
This will be explained with reference to the figures.

First, when EMAT is inserted into the thin tube 4 and a high frequency current is passed through the transmitting coil 13, the transmitting coil 13
are arranged in a meandering manner in the axial direction of the thin tube 4, so that an eddy current I is generated between the walls of the thin tube 4 adjacent to the transmitting coil 13. On the other hand, a magnetic flux B in the circumferential direction is applied from the permanent magnet 11 to the wall surface of the thin tube 4, and a Lorentz force F is generated by interaction with the eddy current I. This Lorentz force F is T _p
Since the direction changes with a period of , a plate wave called a Lamb wave with a wavelength λ equal to T _p is generated in the circumferential direction of the thin tube 4 . These plate waves are divided into those that propagate in a clockwise direction and those that propagate in a counterclockwise direction with the portion of the thin tube 4 where the transmitting coil 13 is located as a base point.

Therefore, if there is no defect in the thin tube 4, the plate wave propagates, and in the transmitting coil 13 and the receiving coil 14, which are arranged symmetrically with respect to the center of the thin tube 4,
Based on the positional relationship of the receiving coil 14, a signal with a phase shown in FIG. 5a is detected for a clockwise plate wave, and a signal with a phase shown in FIG. 5b is detected for a counterclockwise plate wave. A signal with a phase shift of 1/2 period is detected. As a result, the composite signal of the two plate waves is
As shown in Figure c, they cancel each other out and are not detected as echoes.

On the other hand, if there is a defect in the thin tube 4 through which the plate wave propagates, the receiving coil 14 receives the defect signal shown by the dashed line and is superimposed on the clockwise plate wave shown by the broken line as shown in FIG. The signal indicated by is detected,
As shown in FIG. 6B, the defect signal shown by the dashed line is superimposed on the counterclockwise plate wave shown by the broken line, and the signal shown by the solid line is detected. As a result, only a defective signal is detected in the composite wave detected by the receiving coil 14, as shown in FIG. 6c.

Therefore, even if a defective echo is difficult to detect because it is superimposed on an echo generated during transmission, it is possible to separate only the defective echo, and the defect can be detected reliably. Furthermore, if the defect echo is large and the level of the clockwise and counterclockwise echoes transmitted through the defect is unbalanced, the existence of the defect can be confirmed based on the difference.

It should be noted that the EMAT according to the present invention is not limited to the structure shown in the above embodiment, but can also exhibit the same effect even if it has the structure shown in FIG. 7, for example. That is, 15a and 15b in the figure are magnet coils 16a and 16.
b are horsehair-shaped electromagnets wrapped around each other and placed facing each other. However, a horse-shaped permanent magnet may be used instead of the electromagnet. A meandering transmitting coil 13 is attached to one electromagnet 15a.
It is arranged so as to be interposed between the poles of 5a.
Further, a meandering receiving coil 14 is disposed on the other electromagnet 15b so as to be interposed between the poles of the electromagnet 15b. The meandering period and length of these coils 13 and 14 are the same as in the embodiment described above. The electromagnet 15a of EMAT shown in FIG.
When the thin tube 4 is placed between the tubes 15b and 15b as shown in FIG. 8, and a high frequency current is passed through the transmitting coil 13, the transmitting coil 13 will be arranged in a meandering manner in the axial direction of the outer peripheral surface of the thin tube 4, so that the transmitting Eddy currents are generated on the wall surface of the thin tube 4 close to the coil 13. On the other hand, electromagnet 1
A circumferential magnetic flux is applied from 5a to the wall surface of the thin tube 4, and Lorentz force F is generated by interaction with the eddy current, and as shown in FIG. As a result, a plate wave propagating clockwise and a plate wave propagating counterclockwise are generated. The receiving coil 1 of the electromagnet 15b is arranged symmetrically with the transmitting coil 13 with respect to the center of the thin tube 4, such as clockwise plate waves and counterclockwise plate waves.
By performing signal detection in step 4, the
Like EMAT, it can only detect defect signals.

As detailed above, according to the present invention, by devising the shape and arrangement of the transmitter coil and the receiver coil, out of the ultrasonic waves generated in the small diameter tube,
It is possible to provide an electromagnetic acoustic transducer that can extract only the defect signal by canceling out the signal that has returned after going around the circumference, and can thereby perform defect detection reliably.

[Brief explanation of drawings]

Figure 1 is a perspective view showing the conventional EMAT, Figure 2 is a principle explanatory diagram showing the operation of the conventional EMAT, and Figure 3 is a perspective view showing the conventional EMAT.
The figure is a perspective view of EMAT showing one embodiment of the present invention.
Fig. 4 is a principle explanatory diagram showing the operation of the EMAT shown in Fig. 3, and Figs. 5 a to c are detection signals of a clockwise plate wave when there is no defect in the thin tube in which the plate wave propagates, respectively.
Waveform diagrams showing the detection signal and composite signal of a counterclockwise plate wave. Figures 6 a to c are the detection signals of a clockwise plate wave when there is a defect in the thin tube through which the plate wave propagates, respectively, and counterclockwise. FIG. 7 is a perspective view of EMAT showing another embodiment of the present invention, and FIG. 8 is a waveform diagram showing the plate wave detection signal and the combined defect signal.
FIG. 2 is a principle explanatory diagram showing the action of EMAT. 4...Thin tube, 11...Permanent magnet, 12a, 12
b... Support part, 13... Transmission coil, 14... Receiving coil, 15a, 15b... Electromagnet.

Claims (1)

[Claims] 1. A meandering transmitting coil disposed along the wall surface of a capillary tube as an object, and a meandering transmitting coil disposed along the wall surface of the capillary tube at a position symmetrical to the transmitting coil with respect to the center of the capillary tube, and having a length equal to that of the transmitting coil. n±1/ compared to coil
2 (n=0, ±1, ±2...) serpentine receiving coils with long or short periods, and a magnetic circuit that applies a static magnetic field to a thin tube portion in which each of the coils is adjacent. An electromagnetic acoustic transducer.