JPS612065A - Flaw detector using eddy current - Google Patents

Abstract

PURPOSE:To improve the accuracy of phase analysis by arranging concentrically a testing coil on the outside of an exciting coil to constitute a probe. CONSTITUTION:A flaw detecting probe using eddy current is constituted by arranging the exciting coil 2 having a small diameter concentrically with the testing coil 3 having a diameter larger than that of the coil 2. A high frequency current is supplied from an oscillator 31 to the coil 2, so that an eddy current is generated on a testing surface and a mutually induced voltage due to the eddy current is generated on the coil 3. The change of the induced voltage is amplified 32 and inputted to the 1st and 2nd synchronization detectors 5, 6. The detector 5 outputs a component directed to a control signal for an output signal V0 outputted from the coil 3 on the basis of the setting of a 90 deg. phase shifter 50. Consequently, a distance signal due to the variation of lift-off is erased and only a flaw signal due to the existence of a defect is obtained. On the other hand, the detector 6 outputs a component, i.e. a distance signal, directed to a control signal for the signal V0 of the coil 3 on the basis of a 360 deg. phase shifter 60. The flaw signal and the distance signal are inputted to a CRT54 respectively through filters 51, 61, amplifiers 52, 62 and signal processors 53, 63.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an improvement in an eddy current flaw detection device for detecting defects such as cracks and corrosion existing on or near the surface of metals such as steel pipes and steel strips.

[Prior art]

Conventionally, defects in metal surfaces have been inspected using an eddy current flaw detector using an electromagnetic induction coil. This involves passing a high-frequency current through a coil, and when the coil is brought close to the inspection surface, the high-frequency alternating magnetic field generated by the coil generates an eddy current on the metal wall, which bridges the induced back electromotive force generated in the coil due to the influence of this eddy current. It is detected by a circuit or the like. At the defect location on the inspection surface, the eddy current is disturbed and this appears as a change in the induced voltage, making it possible to detect the defect on the inspection surface. There are two types of coil configurations for eddy current flaw detectors: a mutual induction type and a self-induction type.The self-induction type coil configuration generates a high-frequency alternating magnetic field and at the same time detects the eddy current generated on the inspection surface as a self-induced back electromotive force. In contrast, a mutual induction type coil configuration consists of a large-diameter excitation coil (2) that generates a high-frequency alternating magnetic field and a small-diameter inspection coil (2) that detects eddy currents generated on the inspection surface, as shown in Figure 6, for example. The coils (3) are arranged concentrically with respect to the test object (8).

In the flaw detection method using an eddy current flaw detection probe, it is necessary to keep the distance (liftoff) between the inspection surface and the coil constant, and if the liftoff changes, the induced voltage in the coil will be simultaneously affected by the defect on the inspection surface and the variation in liftoff. As a result, it becomes impossible to distinguish between the two. However, in actual flaw detection inspection, the coil is not moved during flaw detection, so it is extremely difficult to keep the lift-off strictly constant. In order to eliminate the influence of lift-off changes on the output signal of the coil and separate only the flaw signal, conventionally the signal obtained from the coil is subjected to phase analysis using a synchronous detector to extract only the flaw signal. method is used.

The principle of this method will be explained with reference to FIG. When the probe is scanning a defect-free inspection surface on a voltage plane where the excitation high-frequency signal of the excitation coil is taken as the phase reference signal x on the X-axis, and the signal vy in the direction perpendicular to this is taken on the y-axis. Coil output vI due only to variations in lift-off! occurs in the standard phase direction shifted by phase θ from the X axis.

The direction of this standard phase θ is determined by the vibration of the probe or inspection surface,
It does not change due to external factors such as non-uniformity of the material of the metal wall, and the coil output Vl only increases or decreases in the direction of the standard phase θ. However, when there is a defect on the inspection surface, the coil output vO
The phase is shifted by a small angle dθ. Therefore, defects can be inspected by detecting the component plane of the output VO projected onto the a-a line in a direction phase-shifted by 90 degrees from the standard phase direction.

However, regardless of whether a self-induction type or mutual induction type coil configuration is used as an eddy current flaw detection probe, the deviation angle dθ of the output vO at the defect location is extremely small, so the output VO
projection component of. -9 was minute, and there was a problem that the precision of phase analysis was poor and reliability was lacking.

〔the purpose〕

SUMMARY OF THE INVENTION An object of the present invention is to provide an eddy current flaw detection apparatus that solves the above-mentioned problems with a flaw detection probe having a mutual induction type coil configuration and that can provide highly reliable test results.

[Principle] In the present invention, the coil configuration of the eddy current flaw detection probe is a mutual induction type, and after repeated experiments with various dimensions and placement positions of the excitation coil and detection coil, it was found that the excitation coil If the probe is constructed by placing an inspection coil concentrically outside the probe, the phase deviation dθ at the defective location on the metal surface will be smaller than a probe consisting of a combination of a large-diameter excitation coil and a small-diameter detection coil. I found out that it grows.

By utilizing this phenomenon, they succeeded in separating signals caused by defects and signals caused by external factors with high precision.

〔composition〕

The configuration of the present invention is that an eddy current flaw detection probe is formed by an excitation coil that generates a high-frequency alternating magnetic field and an inspection coil that is placed concentrically outside the excitation coil, and a high-frequency power source is connected to the excitation coil for inspection. A synchronous detector is connected to the test coil, and a phase shifter is connected to the synchronous detector to input a control signal in which the phase of the high frequency signal is shifted to the standard output phase of the test coil. The detector is characterized in that it outputs a component (flaw signal) whose phase is shifted by 900 with respect to the standard output phase in the output signal from the test coil.

[For production]

The excitation coil generates a high-frequency alternating magnetic field by being energized by a high-frequency power source, and is held at a predetermined height above the test surface while generating eddy currents on the test surface and scans the test surface. When the probe passes over a defective spot on the inspection surface, disturbances and drops occur in the eddy current, causing fluctuations in the mutually induced voltage in the inspection coil. The synchronous detector performs phase analysis on the output signal of the test coil by setting the phase shifter, and extracts and outputs only the flaw signal.

[Special effects]

The probe of the eddy current flaw detection device consists of a small-diameter excitation coil and a larger-diameter inspection coil than the coil, and is made into a mutual induction type.The probe consists of a large-diameter excitation coil and a small-diameter inspection coil. Compared to the above, the output signal of the test coil was generated in the direction of a large deviation angle with respect to the standard output phase due only to lift-off fluctuations. Therefore, when a flaw signal is extracted from the output signal of the test coil using a phase analysis circuit consisting of a phase shifter and a synchronous detector, the error is small (and the accuracy of phase analysis is improved).

〔Example〕

Examples illustrating the effects of the present invention will be described in detail below with reference to the drawings.

Figure 1 shows a situation in which the eddy current flaw detection device of the present invention was installed on a self-propelled vehicle (11) in order to inspect the inner surface of a large-diameter steel pipe. The self-propelled vehicle (11) is loaded with a motor, transmission, etc. and drives the traveling mechanism (12) installed on both sides. ) An arm (10) is attached to the front part of the apparatus so as to be able to swing back and forth, and an eddy current flaw detection probe (1) is equipped at the tip of the arm.

The probe (1) is placed at a constant height of approximately 3 r from the inspection surface (7).
approximately 200 to 4 per minute in the tube circumferential direction while being held at
At the same time, the self-propelled vehicle (11) scans the inspection surface (7) by moving at a speed of 4 m/min. The probe (1) is electrically connected to a signal processing device (141) placed in the monitoring room via a cable (13).The signal processing device supplies high frequency current to the probe (1) and It is equipped with a signal processing circuit that extracts a flaw signal from the output signal of the probe.

As shown in Fig. 2, the eddy current flaw detection probe (1) has a small-diameter excitation coil (2) and a larger-diameter inspection coil (3) arranged concentrically, each with a holder CI! 0) (
30) and attached to the lower end of the arm (10).

FIG. 3 shows a block diagram of a signal processing circuit included in the signal processing device (14). A high frequency current is supplied to the excitation coil (2) from an oscillator (31). As a result, an eddy current is generated on the inspection surface (7), and a mutually induced voltage due to this eddy current is generated in the inspection coil (3). This change in induced voltage is amplified by the differential amplifier (32) and input to the first synchronous detector (5) and the second synchronous detector (6). The high frequency signal from the oscillator (31) is passed through a 360 degree phase shifter (
ω) and a 90 degree phase shifter (50). The 360 degree phase shifter tint uses the high frequency signal from the oscillator (31) as the reference signal Vx shown in FIG. ) It is set in advance so that a signal in the direction matching Vl is output as a control signal. Also, 90
The 360 degree phase shifter (50) is designed in advance to output a signal in a direction further 90 degrees phase shifted from the output of the 360 degree phase shifter (60) (in the direction of line a-a in FIG. 5) as a control signal. It is set. The first synchronous detector (5) outputs the component of the output signal Vo from the test coil (3) in the control signal direction, that is, the component plane of vo shown in FIG. 5, by setting the 90-degree phase shifter (50). do. As a result, distance signals due to lift-off fluctuations are eliminated, and only flaw signals due to the presence of defects are obtained.

On the other hand, the second synchronous detector (6) is a 360 degree phase shifter (60)
By setting , a component of the output signal Vo of the inspection coil (3) in the control signal direction, that is, a distance signal is output. The flaw signal and the distance signal are transmitted through filters +51+ +61+, amplifiers (52) ((towards), and signal processors (531 (63)), respectively.
1, the signals are subjected to noise reduction, gain control, linear rice, etc., and then inputted to the cRT (54) as a Y-axis signal vy and an X-axis signal yx.

Figure 4 shows a CRT-(5) obtained by scanning above an artificial defect formed on the inspection surface using the above-mentioned eddy current flaw detection device.
111, and represents the locus of the output signal when the horizontal axis is Vx and the vertical axis is vy.

The excitation coil (2) used has an impedance of 66
．． 30Ω, resistance value is 34370, number of turns is 70,
On the other hand, the test coil (3) has an impedance of 81.2.
The resistance value is 7Ω, the resistance value is 265Ω, and the number of turns is 50. A high frequency current having a frequency of 32 KH2 is supplied to the excitation coil (2). The material of the inspection surface (7) is steel 5S41, and the size of the defect (71) to be detected is a diameter a of 10
mm, and the depth b is 7.3 mn+. In the figure, the solid line a shows the results obtained using the signal processing circuit shown in Fig. 3, while the broken line a shows the results obtained using the excitation coil (2) and the testing coil for the signal processing circuit shown in Fig. 3. This is the result of scanning with the connection to the coil (3) reversed.

That is, the broken line indicates the test results obtained using a probe consisting of a large-diameter excitation coil and a small-diameter test coil.

As a result, in the eddy current flaw detection device according to the present invention, compared to the eddy current flaw detection device in which the connections shown in FIG. 3 are reversed, the output signal obtained for defects appears as a sharper peak, and It has become clear that this occurs in the direction of the deviation angle that is larger by the angle ψ shown in the figure (approximately 6° in this example) with respect to the standard output phase. Even if the shapes and dimensions of the excitation coil (2) and the inspection coil (3) are varied, as long as the inspection coil (3) is placed outside the excitation coil,
It has been confirmed that there is an effect similar to that described above.

In addition, the excitation coil (2) of the probe (1) shown in Figure 2
It is also possible to provide an inspection coil with a smaller diameter inside the holder, and detect defects using the two inspection coils, large and small. In this case, small diameter inspection coils can detect minute defects such as cranks and pinholes.

[Brief explanation of the drawing]

Fig. 1 is a perspective view of an example in which the present invention is implemented for flaw detection on the inner surface of a steel pipe, Fig. 2 is an enlarged sectional view of an eddy current flaw detection probe, Fig. 3 is a block diagram of a signal processing circuit of a mutual induction flaw detection device, and Fig. 3 is a block diagram of a signal processing circuit of a mutual induction flaw detection device. FIG. 4 is a CRT screen of output signals showing the effects of the present invention, FIG. 5 is a vector diagram of output signals explaining the principle of phase analysis, and FIG. 6 is a partially cutaway view of a conventional eddy current flaw detection probe. (1)... Eddy current flaw detection probe (2)... Excitation coil (3)... Inspection coil (31)... Oscillator (51 [6)... Synchronous detector (50) (lliO
)...Phase shifter (7)...Inspection surface

Claims (1)

[Claims] (1) An eddy current flaw detection probe is formed by an excitation coil that generates a high-frequency alternating magnetic field and an inspection coil placed concentrically outside the excitation coil.A high-frequency power source is connected to the excitation coil, and the inspection coil is connected to a synchronous detector, and to the synchronous detector is connected a phase shifter that inputs a control signal in which the phase of the high frequency signal is shifted to the standard output phase of the test coil, and the synchronous detector is connected to the test coil. component (90° phase shifted from the standard output phase in the output signal from the coil)
An eddy current flaw detection device characterized by outputting a flaw signal.