JPH05196608A - Separated eddy current flaw detecting method - Google Patents

Abstract

PURPOSE:To obtain a phase difference signal corresponding to the size of a flaw without any influence of the permeability of a material to be detected. CONSTITUTION:An exciting coil 2 and a receiving coil 4 is arranged with a space between each other in a steel pipe 3, being a material to be detected. A power source 1 excites the exciting coil 2, and magnetic flux generated from the exciting coil 2 is detected by the receiving coil 4. Phase difference phi1 between an exciting signal and a detecting signal is detected by a phase difference detecting circuit 5. The average amplitude A1 of the detecting signal is corrected according to a constant A0, (a) set in a constant setting circuit 8 by an amplitude correcting circuit 7. The detected phase difference is corrected by a phase difference correcting circuit 9. Thereby it is possible to obtain a phase difference signal allowing hard influence of the permeability mu of the steel pipe 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a separated eddy current flaw detection method and apparatus used for inspecting corrosion thinning of a material to be detected such as an underground steel pipe.

[0002]

2. Description of the Related Art Conventionally, a remote eddy current flaw detection method is also called a remote field eddy current flaw detection (abbreviated as "RFEC") method, and has been used for inspecting corrosion thinning of metal pipes such as underground steel pipes. There is. The remote eddy current flaw detection method is performed by arranging an exciting coil and a receiving coil in the tube with a space in the axial direction of the tube. This distance is required to be 2 to 4 times the normal diameter (that is, the outer diameter of the tube) so that the influence of the direct magnetic field from the exciting coil does not affect the receiving coil.

When an alternating current is passed through the exciting coil, the magnetic flux from the exciting coil penetrates the tube, passes through the external space, and is transmitted along the outer surface of the tube. The magnetic flux penetrating the tube is again received by the receiving coil. The electromagnetic wave of this magnetic flux has a significantly smaller speed when passing through the thick portion of the tube than when passing through the air. Therefore, this propagation time, that is, the phase difference between the excitation signal of the exciting coil and the detection signal of the receiving coil changes in proportion to the wall thickness of the tube. From this, it is possible to detect the corrosion thinning of the pipe corresponding to the phase difference.

[0004]

In the separated eddy current flaw detection method, the detected phase difference is proportional to the square root of the magnetic permeability μ of the material to be detected. The value of magnetic permeability varies greatly depending on the material.
Therefore, although it is possible to detect the occurrence of the corrosion thinning state by the partial change in the phase difference of the magnetic flux penetrating the material to be detected, it is difficult to quantitatively evaluate.
Therefore, the output of the phase difference signal by the separated eddy current flaw detection method can be output to the recorder, and the presence or absence of a defect can be known from the output. The size of the defect can only be estimated.

An object of the present invention is to provide a separated eddy current flaw detection method and apparatus capable of generating the same phase difference signal for the same defect regardless of the material of the material to be detected.

[0006]

According to the present invention, an exciting coil is excited by an exciting signal to generate a magnetic flux, the receiving coil detects the magnetic flux penetrating a material to be detected, and the amplitude average of the detection signal from the receiving coil is detected. With respect to a value obtained by normalizing A1 to a certain value A0, a power value with respect to a predetermined index a is obtained, and the obtained power value is multiplied by the phase difference φ1 between the excitation signal and the detection signal φ1 · (A1 / A0 ) A is ^a phase difference, and the separated eddy current flaw detection method is characterized by detecting a portion where the thickness of the material to be detected is reduced corresponding to the calculated phase difference.

According to the present invention, the exciting coil, the receiving coil arranged along the material to be detected and spaced from the exciting coil, the phase of the exciting signal for exciting the exciting coil, and the detected coil generated from the exciting coil. The phase difference φ is compared with the phase of the detection signal generated in the receiving coil by the magnetic flux penetrating the material.
In response to the detection signal from the receiving coil and the phase difference comparison means for deriving a signal representing 1, the amplitude average A1 of the detection signal is normalized by a certain value A0, and a signal representing a power value with respect to a predetermined index a is derived. The phase difference φ corresponding to the thickness of the material to be detected is calculated in response to the signals from the amplitude correction means and the phase comparison means and the amplitude correction means.

[0008]

## EQU1 ## A separated eddy current flaw detector characterized by including phase difference correction means according to φ = φ1 (A1 / A0) ^a .

[0009]

According to the present invention, the magnetic flux generated from the exciting coil penetrates the material to be detected and is detected by the receiving coil. The amplitude average A1 of the detection signal is corrected by a certain value A0 and a predetermined index a. The phase difference φ1 between the excitation signal for exciting the exciting coil and the detection signal is multiplied by the correction value for the amplitude to obtain φ1 · (A1 / A0) ^a as the phase difference. By correcting the phase difference in this way, it is possible to reduce the influence of changes in the magnetic permeability of the material to be detected. Since the location where the thickness of the material to be detected is reduced is detected in accordance with the phase difference, the size of the defect such as the thickness reduction can be easily detected.

According to the invention, the isolated eddy current flaw detector has an exciting coil, a receiving coil, a phase difference comparing means,
It includes an amplitude correction means and a phase difference correction means. The exciting coil and the receiving coil are arranged at intervals along the material to be detected, and are generated in the receiving coil by the phase of the excitation signal for exciting the exciting coil and the magnetic flux generated from the exciting coil and penetrating the material to be detected. The phase of the detection signal to be compared is compared by the phase difference comparison means. The amplitude average A1 of the detection signal is corrected by the interval correction means. The phase difference correction means corrects the phase difference φ1 from the phase comparison means with a signal from the amplitude correction means to obtain a phase difference φ,

[0011]

Mathematical Expression 3 φ = φ1 · (A1 / A0) ^a is obtained. By this correction, the phase difference φ is not affected by the change in the magnetic permeability μ of the material to be detected, so that it is possible to detect the defect in which the thickness of the material to be detected is reduced due to corrosion or the like, and to know its size.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the overall electrical construction of an embodiment of the present invention. In order to carry out the remote eddy current flaw detection method, the power source 1
An exciting coil 2 that is excited by the receiving coil 4 and a receiving coil 4 are provided at intervals along the tube axis of a steel pipe 3 that is a material to be detected.

FIG. 2 is a schematic perspective view for explaining the principle of the embodiment shown in FIG. In order to inspect the corrosion thinning state of pipes such as steel pipes 3 buried in the ground,
The isolated eddy current flaw detection method according to the present invention is used. As the receiving coil 4, four coils 4a, 4b, 4c and 4d are arranged along the inner peripheral surface of the steel pipe 3 while being shifted by 90 °. By moving the exciting coil 2 and the receiving coil 4 in the pipe axial direction while keeping the distance L in the pipe axial direction of the steel pipe 3 constant, the corrosion thinning state of the steel pipe 3 can be inspected. The distance L between the exciting coil 2 and the receiving coil 4 is determined so that the direct magnetic flux from the exciting coil 2 is not detected by the receiving coil 4. Generally, it is selected to be, for example, three times or more the outer diameter D of the steel pipe 3.

The magnetic flux from the exciting coil 2 passes through the steel pipe 3, passes through the external space in the air, and again passes through the steel pipe 3, which is the material to be detected, and reaches the receiving coil 4. Since the velocity of the electromagnetic wave as the magnetic flux is significantly smaller in the steel pipe 3 than in the external space, the propagation time, that is, the phase difference between the excitation signal of the excitation coil 2 and the detection signal of the reception coil 4 is
It corresponds to the thickness of the steel pipe 3.

The phase delay between the excitation signal and the detection signal, that is, the phase difference φ1 is expressed by the following equation (4).

[0016]

[Equation 4]

Here, d is the thickness of the steel pipe 3, π is the circular constant, and f
Is the frequency of the excitation signal, μ is the permeability of the steel pipe 3, and σ is the steel pipe 3.
Shows the electrical conductivity of. That is, the phase difference φ1 is monotonically increasing with respect to the magnetic permeability μ of the steel pipe 3.

Amplitude average A1 of the detection signal of the receiving coil 4
Is expressed by the following equation (5).

[0019]

[Equation 5]

Here, d, π, f, μ and σ are the same as those in the above equation (4). That is, the amplitude average A1 of the detection signal
Is a monotonic decrease in magnetic permeability μ. Although the magnetic permeability μ varies greatly depending on the material of the steel pipe 3, the conductivity σ varies little.

In the embodiment shown in FIG. 1, the phase difference φ1 between the exciting coil 2 and the receiving coil 4 is the phase difference detecting circuit 5
Detected by. The amplitude of the detection signal from the receiving coil 4 is measured by the amplitude measuring circuit 6. This measured value is corrected by the amplitude correction circuit 7 using the values of the constants A0 and a set in the constant setting circuit 8. A
A value of 0 is a value for normalizing the amplitude average A1,
The value of a is an index for the normalized amplitude. The output from the amplitude correction circuit 7 represents (A1 / A0) ^a .

The outputs from the phase difference detection circuit 5 and the amplitude correction circuit 7 are given to the phase difference correction circuit 9. The phase difference correction circuit 9 obtains the corrected phase difference φ according to the following equation (6).

[0023]

Φ = φ1 · (A1 / A0) ^a This corrected phase difference φ is given to the comparison circuit 10,
It is compared with the reference value of the phase difference preset in the reference value setting circuit 11. When the phase difference φ is equal to or greater than the reference value, it is determined that the defect is caused by corrosion wall thinning, and the output circuit 12 outputs an alarm or the like.

Table 1 below shows an example of the experimental results according to the embodiment shown in FIG.

[0025]

[Table 1]

The graph shown in FIG. 3 is obtained from this experimental data. The value of the index a can be obtained from this graph.

In this experimental data, when the vertical axis represents the phase difference φ1 and the horizontal axis represents the amplitude V, the variation in the phase difference value becomes large as shown in FIG. This variation is due to a change in magnetic permeability μ of the material to be detected. When the phase difference is corrected, the variation in the corrected phase difference becomes small as shown in FIG. Figure 6
Shows the test results for more samples. Of these, the effect of correction for excitation of 5 V is shown in FIG. In this way, it is possible to obtain a phase difference corresponding to the wall thickness of the corrosion-reduced portion without depending on the magnetic permeability of the steel pipe 3.

[0028]

As described above, according to the present invention, it is possible to obtain a phase difference signal corresponding to the size of a defect such as corrosion wall thinning without being affected by the change in magnetic permeability depending on the material of the material to be detected. it can. This makes it possible to correctly detect the thick portion.

[Brief description of drawings]

FIG. 1 is a block diagram showing a schematic electrical configuration of an embodiment of the present invention.

FIG. 2 is a schematic perspective view showing the principle of the present invention.

3 is a graph showing the relationship between the amplitude of the detection signal and the phase difference with respect to the excitation signal according to the embodiment shown in FIG.

FIG. 4 is a graph showing the relationship between the amplitude of the detection signal and the phase difference with respect to the excitation signal according to the embodiment shown in FIG.

5 is a graph showing the relationship between the corrected phase difference and amplitude according to the embodiment shown in FIG.

6 is a graph showing the relationship between the amplitude of the detection signal and the phase difference with respect to the excitation signal according to the embodiment shown in FIG.

7 is a graph showing the effect of correction by the sample shown in FIG.

[Explanation of symbols]

 1 Power Supply 2 Excitation Coil 3 Steel Tube 4 Receiver Coil 5 Phase Difference Detection Circuit 7 Amplitude Correction Circuit 8 Constant Setting Circuit 9 Phase Difference Correction Circuit 10 Comparison Circuit

Claims (2)

[Claims] 1. An exciting coil is excited by an exciting signal to generate a magnetic flux, the receiving coil detects the magnetic flux penetrating a material to be detected, and the amplitude average A1 of the detection signal from the receiving coil is set to a certain value A0.
Then, a power value with respect to a predetermined exponent a is calculated for the value normalized to, and a value φ1 · (A1 / A0) ^a obtained by multiplying the calculated power value by the phase difference φ1 between the excitation signal and the detection signal is used as the phase difference. A separated eddy current flaw detection method, which is characterized by detecting a location where the wall thickness of the material to be detected is reduced corresponding to the obtained phase difference. 2. An exciting coil, a receiving coil arranged along the material to be detected and spaced from the exciting coil, a phase of an excitation signal for exciting the exciting coil, and a material to be detected generated from the exciting coil. Phase difference comparison means for comparing the phase of the detection signal generated in the receiving coil by the penetrating magnetic flux and deriving a signal representing the phase difference φ1 and the amplitude average A1 of the detection signal in response to the detection signal from the receiving coil. Amplitude correction means for deriving a signal representing a power value with respect to a predetermined index a, normalized by a certain value A0, and a position corresponding to the thickness of the material to be detected in response to the signals from the phase comparison means and the amplitude correction means. A separated eddy current flaw detector comprising ^a phase difference correction means for calculating the phase difference φ according to an arithmetic expression φ = φ1 · (A1 / A0) ^a .