JPH0441303B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention applies to large-diameter pipes and long metal plates, such as cargo oil pipes installed in oil tankers, city water and sewer pipes, and long metal plates, the surface area of which is cracks near the surface.
This invention relates to an inspection method for detecting defects such as corrosion.

Conventionally, defects in metal surfaces have been inspected by eddy current flaw detection. The method involves bringing a detector equipped with a coil close to a metal surface, and applying the coil's high-frequency varying magnetic field to the metal surface. Eddy currents generated on the metal surface affect the coil through self-induction or mutual induction, producing an induced electromotive force.
By detecting this, abnormalities in the metal surface can be known.

FIG. 3 shows the detection signal A and its phase due to the electromotive force induced in the coil when the detector is moved close to the metal surface. The detection signal A is output in the x-axis direction with its phase shifted by an angle θ with respect to the reference voltage input to the coil due to the impedance between the detector and the metal surface.

The vector value of the detection signal A fluctuates due to movement of the detector or metal surface, vibrations received from the surrounding environment, non-uniformity of the material of the metal surface, etc. Depending on these factors, the phase θ of the output signal may change. (assumed to be standard phase) remains unchanged.

However, when there is a defect on the metal surface, the eddy current generated on the metal surface is disturbed at the defect location, and the vector direction of the detection signal A is affected.
A pulse-like waveform is output in a direction in which the phase of is shifted by a small angle Δθ. Since Δθ is a very small amount, signal A is normally used as the tube inspection signal. Therefore, when a waveform Q having the same shape as the waveform P indicating a defect occurs in the inspection signal A for some reason during the relative movement between the detector and the metal surface as shown in FIG. There was a problem in which it was difficult to distinguish whether the problem was due to a defect in the system or to the influence of the outside world.

In addition, during inspection, the distance (lift-off) between the detector and metal surface should be kept constant and the two should be moved relative to each other, but lift-off may occur during inspection due to waviness of the metal surface or imperfection of the moving device. It often fluctuates. When the detector gets closer to the metal surface and the lift-off decreases, the output signal due to the defect on the metal surface becomes larger than the actual one. Conversely, when the detector moves away from the metal surface and the lift-off increases, the defect detection signal becomes larger than the actual defect signal. The output is smaller than the actual size, and there is a risk of misjudgment when evaluating the metal surface based on the inspection data.

The present invention removes the waveform Q caused by external factors other than defects on the metal surface from the detection signal A, and outputs only _the signal The purpose of the present invention is to clarify a test method that corrects test signals to eliminate the influence of lift-off and output test signals under the same conditions.

_The present invention passes _the inspection signal A of the detector through a high-pass filter and _outputs the flaw signal The flaw signal X ₁ due to a defect on the metal surface is detected by logically multiplying the component Y ₀ in the 90° phase shift direction and the flaw signal X ₀ .

The present invention also passes the test signal A through a low-pass filter, outputs a low-frequency lift-off signal B in the test signal A, and combines the above-mentioned 90° phase-shifted component Y ₀ and lift-off signal B in the test signal A. and outputs a lift-off correction signal C, corrects the flaw signal _X1 due to the metal surface defect with the lift-off correction signal C, and outputs a flaw signal X due to the metal surface defect in which the influence of lift-off fluctuation is corrected. It is characterized by:

In addition to the waveform P caused by defects on the metal surface, the inspection signal A includes a waveform Q caused by external factors. However, by ANDing the two signals X ₀ and Y ₀ ,
If the pulse-like waveform in the inspection signal A is due to a defect on the metal surface, the waveforms P in the signals X ₀ and Y ₀ are aligned, and a flaw signal X ₁ due to the metal surface defect is output.
Furthermore, if the pulse-like waveform in the test signal A is due to an external factor, even if the pulse-like waveform Q exists in the signal X ₀ , there is no corresponding waveform in the signal Y ₀ ; When the two signals X ₀ and Y ₀ are ANDed, pulses that do not produce an output and are caused by external factors are removed, and only the flaw signal X ₁ due to defects on the metal surface can be output.

Furthermore, by passing the inspection signal A through a low-pass filter, short-cycle pulse-like waveforms P and Q caused by metal surface defects and external factors are removed, and only the long-cycle waveform B caused by lift-off fluctuations is removed. Output.

Since the lift-off signal B above almost faithfully represents the lift-off at each moment during the inspection, it is multiplied by the lift-off correction signal C, which is the logical product of the _Y0 signal, and an appropriate correction coefficient a, which is inversely proportional to the increase or decrease in lift-off. Forming E=aC, flaw signal due to metal surface defect
Signal processing is performed on X ₁ by a correction circuit,
When the lift-off correction signal C is large, such as X ₁ −aC or X ₁ /aC, that is, when the detector is close to the metal surface and the sensitivity is increased, the value of the flaw signal X ₁ is lowered,
Also, when the lift-off signal B becomes small, that is, when the detector moves away from the metal surface and the sensitivity decreases, the flaw signal
By increasing the value of X ₁ and correcting the influence of lift-off fluctuations, it is possible to output a flaw signal X that is as close to the same lift-off condition as possible.

FIG. 1 shows a situation in which the method of the invention is carried out to inspect the metal surface of a tube. In this system, the upper end of a detection arm 3 is swingably supported on the front part of a trolley 2 equipped with a traveling mechanism 1 such as a caterpillar, an endless belt, and wheels, and a detector 4 is provided at the lower end of the detection arm 3. A swing mechanism 6 is connected to the rotating shaft 5 of the rotary shaft 5, and while the trolley 2 is running in the pipe at a low speed of 4 m/min, the detection arm 3 is moved perpendicularly to the running direction in the circumferential direction of the pipe at a speed of 250 to 250 m/min. The tube wall is inspected within the transition range of the detector 4 by reciprocating the tube at a speed of 400 times.
Note that the object to be inspected according to the present invention is not limited to pipe walls, but can also be applied to metal plates by swinging the detection arm back and forth with respect to a moving metal plate.

The detector 4 is composed of a large-diameter excitation coil 7 that generates a high-frequency alternating magnetic field, and a small-diameter detection coil 9 that is coaxially arranged in the center of the coil 7 and has an iron core 8. fixed inside. The lift-off of detector 4 is set to 5 to 10 mm in consideration of unevenness caused by corrosion on the metal surface.
The excitation coil 7 applies a high frequency alternating magnetic field to a metal surface within the coil diameter to generate eddy currents. When the detector 4 and the metal surface are moved relative to each other while keeping the lift-off constant, the output of the detection coil 9 is constant if the metal surface has no defects and the eddy current is uniformly distributed. However, if a defect 11 occurs on the metal surface, the eddy current will be disturbed in this area, and the detection coil 9
Fluctuations occur in the phase and output value of the output.

FIG. 6 shows an example of a configuration for generating the test signal A. A high frequency voltage of about 50 kHz is generated by the oscillator 12, amplified, and applied to the excitation coil 7. In the small-diameter detection coil 9 disposed coaxially with the excitation coil 7, induced electromotive force is generated due to mutual induction from eddy currents on the metal surface, and this is amplified to obtain the inspection signal A. I can do it.

In addition, when implementing the present invention, the excitation coil 7
It is also possible to omit the detection coil 9 itself by applying a high frequency current to excite it, detect the induced electromotive force generated in the coil itself by self-induction using a bridge circuit, and amplify it. Test signal A can be obtained.

For the inspection signal A, the phase of the reference signal Dx from the phase shifter 13 is set to the standard phase (lift-off voltage phase) angle θ of the inspection signal with respect to the normal metal surface, and the gate 14 performs an AND operation. By passing it through the high-pass filter 15, it is possible to output the component X ₀ of the test signal A in the direction of the standard phase angle θ.

Further, the phase of the reference signal Dx from the phase shifter 13 is shifted by 90° by the phase shift circuit 16 to obtain another reference signal Dy.
is generated, ANDed by the gate 17, and passed through the high-pass filter 18 to output the component _Y0 in the direction of phase shift of 90° with respect to the direction of the standard phase θ in the test signal A. I can do it.

Since the X ₀ signal contains a mixture of a pulse-like waveform P caused by defects on the metal surface and a waveform Q caused by external factors, in the device of the first embodiment shown in FIG. By ANDing the signal Y _{0 with the signal Y 0} that is phase shifted by 90 degrees with respect to the standard phase θ, it is possible to output the flaw signal X ₁ in which the waveform Q caused by external factors is eliminated.

Furthermore, as in the inspection apparatus of the second embodiment shown in FIG.
is ANDed with the _Y0 signal to create a lift-off correction signal C, which is sent to the correction circuit 2 together with the flaw signal _X1 .
By signal processing in step 2, the flaw signal _X1 is corrected for the influence of lift-off variation, and a flaw signal X close to the actual size of the defect can be output.

The flaw signal , flaw signal X
The value of is sufficiently reliable and has the advantage of improving the accuracy of judgment.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the implementation status of the present invention, and FIG.
3 is a phase analysis diagram of the test signal, FIG. 4 is a block diagram of the first embodiment of the test signal, and FIG. 5 is a block diagram of the second embodiment of the test device. FIG. 6 is a block diagram showing the signal processing of the tester, and FIG. 7 is a waveform diagram at each step of the signal processing. A...Inspection signal, B...Lift-off signal, C...
...Lift-off correction signal, Dx, Dy...Reference signal,
P...defect waveform, Q...wrong waveform, _X0 ...component along the standard phase direction in the inspection signal A, that is, the flaw signal,
Y ₀ ...90° to the standard phase direction in test signal A
Component in the phase-shifted direction, X ₁ ...flaw signal due to metal surface defects,
Detection coil, 13... Phase shifter, 15, 18...
High pass filter, 20...Low pass filter, 22...Correction circuit.

Claims (1)

[Claims] 1. A high-frequency varying magnetic field is applied to the metal surface, and an inspection signal A due to the induced electromotive force generated in the coil is passed through a high-pass filter and a flaw signal X ₀ is output,
In the inspection signal A, a component Y ₀ in a direction shifted by 90° from the phase of the flaw signal X ₀ is detected, and the component Y ₀ in the 90° phase shifted direction and the flaw signal X ₀ are ANDed. , a flaw signal X ₁ due to a defect on the metal surface is output, and at the same time, the inspection signal A is passed through a low-pass filter and a lift-off signal B is output, and the 90° phase shift direction component Y ₀ and the lift-off signal B are ANDed and lifted off. It is characterized by outputting a correction signal C, correcting the flaw signal _X1 due to the metal surface defect by the lift-off correction signal C, and outputting a flaw signal X due to the metal surface defect in which the influence of lift-off fluctuation has been eliminated. Method of inspecting metal surfaces. 2. Claims in which the coil is placed on a self-propelled vehicle running on the back, and at the same time as it moves in the traveling direction of the self-propelled vehicle, it is swung back and forth in a direction perpendicular to the traveling direction to inspect the metal surface of the tube. Inspection method in Section 1. 3 The flaw signal _X1 due to _a metal surface defect is: An inspection method according to claim 1, which outputs a flaw signal X due to a defect.