JPS6259776B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to an eddy current flaw detection device.

A known device of this type has a probe 0, as shown in the partial longitudinal sectional side view of FIG. 1 and the block diagram of FIG.
Circular coils 02, 03, coaxially wound around 1
04, and the circular coils 02 and 03 are flaw detectors 01
A high frequency voltage is applied by an oscillation circuit 034a built into the circular coil 03, and the outputs of the circular coils 02 and 03 are taken out via variable impedances 031a and 032a with the same characteristics that constitute a bridge with the same coil, and are amplified by an amplification circuit 035a. After that, the oscillation circuit 0
The phase detection circuit 036 refers to the voltage waveform of 34a.
It is converted into a DC voltage by a and recorded by the recorder 012, and the circular coil 04 is converted to a DC voltage by the flaw detector 0.
A high frequency voltage is applied by the oscillation circuit 034b built in the circular coil 04, and the output of the circular coil 04 is transmitted through the variable impedance 03 which forms a bridge with the same coil.
1b, 032b and the amplifying circuit 035
After being amplified in step b, the voltage waveform of the oscillation circuit 034b is referred to, and the phase detection circuit 036b converts the voltage into a DC voltage, which is then recorded by the recorder 012.

When probe 01 is placed in the air, the third
As shown in Figure A, the voltage of the oscillation circuit 034a is set to E
If i, the voltage applied to the circular coils 02 and 03 becomes E〓 ₂ ≒E〓 ₃ ≒E〓i/2, and the amplifier circuit 035
The input voltage E〓 ₀ of a is E〓 ₀ =E〓 ₃₂ −E〓 ₃ , but
When the probe 01 is inserted into the test tube 06, as shown in FIG.
3 induces a voltage in the circular coils 02, 03 by interlinking with the circular coils 02, 03.

Now, the induced voltage of circular coils 02 and 03 is V〓 ₂ ,
If V〓 is ₃ , then the voltage across the circular coils 2 and 3 is E〓
ca and E〓bc are respectively E〓ca=E〓 ₂ −V〓 ₂ and E〓bc
=
E〓 ₃ −V〓 ₃ , and since the circular coils 02 and 03 are evenly wound, V〓 ₂ ≒ V〓 ₃ , so E〓
₂ -V〓 ₂ ≒E〓 ₃ -V〓 ₃ ≒E〓ba/2 (Here, E〓ba is the voltage between point b and point a.) The amplification circuit 035a has a considerably high magnification. Therefore, if the input voltage is not very small, the output voltage will be saturated and it will not work properly.

In such a device, impedance 03
After adjusting the knobs 1a and 032a to set the bridge circuit's output E〓 ₀ = 0, when flaw detection is started, the circular coils 02 and 03 move as shown in Fig. 3C.
When it comes near the defect 07, the flow direction of the eddy current 0102 changes due to the defect 07, which causes the magnetic field 01
03 also changes, the changes ΔV〓 ₂ and ΔV〓 3 in the induced voltage of the circular coils 02 and _{03 are the changes in the induced voltage of the circular coils 02 and} 03.
Since it is closer to the defect 07 and is more affected by the defect, ΔV〓 ₂ > ΔV〓 ₃ , and the amplifier circuit 035a
The input voltage E〓 ₀ is E〓 ₀ =E〓bd−E〓bc=E〓 ₃ −V〓 ₃ +(ΔV〓 ₂ +ΔV〓 ₃ )/2−(E〓 ₃ −V〓 ₃ +ΔV〓 ₃ ) =
(Δ
V〓 ₂ −ΔV〓 ₃ ) 1/2, and the input voltage E ₀ of the amplifier circuit 035a is always proportional to the difference in the induced voltage between the circular coils 02 and 03, so the induced voltage of the two coils is The flaw detection method that utilizes the difference in is called the differential method.

Since defect 07 is a short-axis defect that is short in the axial direction,
As shown in FIG. 4A, the induced voltages ΔV ₂ and ΔV ₃ of the circular coils 02 and 03 are such that the center of each coil has no defect.
The output of the phase detection circuit 036a becomes the maximum as shown in A of the figure.However, since the defect 08 is a long-axis defect that is long in the axial direction, the induced voltage of the circular coils 02 and 03 ΔV ₂ , Δ
The outputs of V ₃ and the phase detection circuit 036a are as shown in Figure B. Although the differential method has the disadvantage that only a small signal can be obtained for the long-axis defect 08, The induced voltages in each circular coil cancel each other out due to the horizontal vibration of the moving probe, so the S/N ratio becomes high, and small defects are advantageous for detecting flaws.

The circular coil 04 is provided to compensate for the drawbacks of such a differential method, and the flaw detector 011 has a fixed impedance 033b.If the circular coil 03 and the fixed impedance 033b are replaced, it is substantially the same as the differential method. Since the circuit configuration is the same and no voltage is induced in the fixed impedance 033b due to external factors, a voltage proportional to the voltage induced in the circular coil 04 due to the defect is input to the amplifier circuit 035b, so the output of the phase detection circuit 036b is circular coil 0
Since the absolute value of the induced voltage of the circular coil 04 is extracted as a signal, the circuit of the flaw detector 011 is said to be an absolute method as opposed to a differential method. In the absolute method, the detection sensitivity for long-axis defects is not as low as in the case of the differential method, but on the other hand, noise due to horizontal vibration of the probe is large, and the detectability for minute defects is poor.

The apparatus shown in FIGS. 1 and 2 uses both the differential method and the absolute method to increase the detection sensitivity for both short-axis defects and long-axis defects.

By the way, when testing thin tubes such as various heat exchangers, the sources of signals are not only defects, but also external structures such as support plates and tube plates outside the tube, changes in the diameter of the tube, etc. Since signals are generated, in order to estimate the cause of the occurrence and extract only those caused by defects from among the many signals generated, it is necessary to observe one type of signal, especially when signals from various factors are combined. This alone is not sufficient; it is necessary to compare at least two different types of signals.

Therefore, it is desirable to use the differential method flaw detection results and the absolute method flaw detection results shown in Figures 1 and 2 as two different types of signals for signal factor estimation. , circular coil 02,0
3 and the circular coil 04 are separated in the axial direction, the differential method and absolute method signals due to the same factor are not generated at the same time, and the generation interval changes depending on the moving speed of the probe. It becomes difficult to clarify the correspondence between the signals of the system.

016 in FIG. 1 is the differential method flaw detection result recorded on the recording paper 015, 018 and 020 are the signal waveforms due to short axis defect 07 and long axis defect 08, respectively, and 01
7 is the flaw detection result of the absolute method, and 019 and 021 indicate the signal waveforms due to short-axis defect 07 and long-axis defect 08, respectively.The signal waveform of the differential method and the signal waveform of the absolute method due to the same factor occur at the same time. I know it won't.

As shown in Figure 1, the signal generation factors (defects 0
7, 08) is simple, it is relatively easy to clarify the correspondence between the signal waveforms of both methods even if they do not occur at the same time. exist continuously, and in such a case, it becomes difficult to clarify the correspondence between the signal waveforms of the differential method and the absolute method, and the estimation accuracy of signal factors decreases, especially in actual flaw detection. Due to wide variations in the manual speed of the probe, the time intervals between the signals generated by both types also vary widely, making it almost impossible to determine the correspondence between the signals of both types.

The present invention was proposed in view of the above circumstances, and an object of the present invention is to provide an eddy current flaw detection device that improves flaw detection accuracy by providing complete simultaneity to the signals generated by the differential method and the absolute method. A dummy coil attached to the probe and shielded by a metal ring, an absolute detection coil connected in series with the dummy coil at a sufficient distance and energized by a high frequency oscillator, and an eddy current generated in the subject. an absolute method flaw detection circuit comprising a bridge circuit for detecting a change in the induced voltage induced in the absolute detection coil itself by a magnetic field due to a defect in the test object, and detecting an output signal of the bridge circuit; and the absolute detection coil. 1 placed close to each other with the two in the center.
It is equipped with a pair of differential detection coils having the same characteristics and a bridge circuit for detecting a change in the induced voltage caused by the differential detection coil due to a defect in the test object. A differential method flaw detection circuit that detects the difference in induced voltage between a pair of detection coils,
The present invention is characterized by comprising a phase detector that detects the outputs of the bridge circuits of the absolute method flaw detection circuit and the differential method flaw detection circuit, respectively, using the output of the high frequency oscillation circuit as a reference voltage.

An embodiment of the present invention will be explained with reference to the drawings.
Fig. 5 is a partial longitudinal sectional side view of the circuit, Fig. 6 is a block diagram of the circuit shown in Fig. 5, and Fig. 7 A, B, and C are voltages and circular Figure A shows the magnetic field distribution of the coil. Figure A shows the circular coil placed in the air, Figure B shows the circular coil inserted into the test tube, and Figure C shows the circular coil facing the defect. The following shows the cases in which the FIG. 8 is an explanatory diagram showing the voltage of the bridge circuit including the circular coils 4 and 25 of FIG. 6 and the magnetic field distribution of the circular coils.

First, in Figs. 5 and 6, 1 is a probe;
2, 3, 4, and 25 are circular coils wound coaxially around the probe 1, and the circular coil 4 is a differential detection coil 1 disposed close to each other.
The circular coil 25 is installed as an absolute detection coil in the center of a pair of circular coils 2 and 3 having the same characteristics, and the circular coil 25 is installed as a dummy coil shielded by a metal ring 26, which will be described later, at a position sufficiently distant from the circular coils 2, 3, and 4. It is provided. 23 is a flaw detector that outputs the difference in induced voltage between the circular coils 2 and 3; 31c and 32c are variable impedances that constitute a bridge circuit with the circular coils 2 and 3; and 35c is a flaw detector that outputs the difference in induced voltage between the circular coils 2 and 3 and the variable impedances 31c and 32c. 36c is a phase detection circuit 31c, 32c, 35c, 36c.
cooperate to form the flaw detector 23.

31d and 32d are variable impedances that constitute a bridge circuit with the circular coils 4 and 25, and 34d are the circular coils 4 and 25 and variable impedance 3.
An oscillation circuit that applies a high frequency voltage to the bridge circuit consisting of 1d and 32d, 35d an amplification circuit that amplifies the output of the bridge circuit, and 36d an oscillation circuit 3.
31 with a phase detection circuit that converts the output of the amplifier circuit 35d into a DC voltage by referring to the output voltage waveform of the amplifier circuit 35d.
d, 32d, 34d, 35d, and 36d cooperate to constitute the flaw detector 10.

5 is the cap of the probe 1, 6 is the test tube having the short axis defect 7 and the long axis defect 8, and 9 is the probe 1.
12 is a recorder that inputs the outputs of the flaw detectors 23 and 10 through cables 13 and 14, respectively. 16 is the flaw detection result of the flaw detector 23 drawn on a recording paper. 1
8 and 20 are the signal waveforms of the short axis defect 7 and the long axis defect 8, respectively, 17 is the flaw detection result of the flaw detector 10, and 1
9 and 21 are signal waveforms caused by the short-axis defect 7 and the long-axis defect 8, respectively; 24 is the oscillation circuit 34d of the flaw detector 10;
A cable 26 is a metal ring that shields the circular coil 25 for inputting the output as a reference voltage to the phase detection circuit 36c of the flaw detector 23.

In such a device, the flaw detector 23 to which the circular coils 2 and 3 are connected does not have an oscillation circuit, so no alternating current from the internal power supply flows through the circular coils 2 and 3, and no alternating magnetic field is caused by this. Does not occur. On the other hand, an alternating current is passed through the circular coils 4 and 25 to generate an alternating magnetic field, but since the magnetic field generated by the circular coil 25 is shielded by the metal ring 26 surrounding its outer periphery, not reached,
Conversely, no matter what state the circular coil 25 is placed in, its induced voltage does not change, and furthermore, the generated magnetic field does not affect the circular coils 2 and 3.

When the probe 1 is inserted into the test tube, the magnetic field of the circular coil 4 generates an eddy current in the thick part of the tube wall, and the circular coil 4 itself is influenced by the magnetic field due to this eddy current. Since the coil 3 is close to the circular coil 4, it is influenced by both the magnetic field generated by the circular coil 4 and the magnetic field generated by the eddy current in the thick part of the tube wall, thereby inducing a voltage.

That is, first of all, FIG. 7A shows the circular coils 2, 3,
4 is in the air, and in this case there is no influence from the test tube, so the induced voltage in the circular coils 2 and 3 is only due to the magnetic field generated by the circular coil 4, and the circular coil 4 is in contact with the circular coil 2. , 3, the circular coils 2 and 3 are almost uniformly affected by the magnetic field 111 generated by the circular coil 4, and the circular coils 2 and 3 are each wound uniformly. , the respective induced voltages e ₂ and e ₃ are approximately equal (e ₂ ≒ e ₃ ).

Next, Figure B shows a case where the circular coils 2, 3, and 4 are inserted into the test tube. Since they are affected by both the magnetic field 111 caused by the coil 4, the induced voltage in the circular coils 2 and 3 due to the magnetic field 113 caused by the eddy current 112 is expressed as v〓
₂ , v〓 ₃ , the voltages across the circular coils 2 and 3 are e〓ca=e〓 ₂ −v〓 ₂ , e〓bc=e〓 ₃ −
v〓
₃ , and since the circular coils 2 and 3 are arranged symmetrically with respect to the eddy current 112 and are uniformly wound, v〓 ₂ ≒v〓 ₃ , so e〓 ₂ −v〓 ₂ ≒e〓 ₃
−v〓
It is ₃ .

When the circular coils 2 and 3 are inserted into the test tube and before flaw detection starts, the input voltage of the amplifying circuit 35c of the flaw detector 23 must be approximately zero in order to prevent the amplifying circuit 35c from being saturated. The values Z〓 ₃₁ and Z〓 ₃₂ of 31c and 32c are approximately equal (Z〓 ₃₁ ≒Z〓 ₃₂ ).

Furthermore, C in the same figure shows a case where the circular coils 2 and 3 approach the defect 7, the flow direction of the eddy current 112 flowing through the thick part of the tube to be inspected changes, and the magnetic field 113 caused by the eddy current 112 also changes. Circular coils 2, 3 to
The induced voltage of will also change. In the figure, circular coil 2
is closer to the defect 7 than the circular coil 3, so the way the eddy current 112 flows changes greatly in the part close to the circular coil 2, so the amount of change in the induced voltage in the circular coils 2 and 3 is not the same. The change in the induced voltage in the circular coil 2 becomes larger.

If the changes in the induced voltages of the circular coils 2 and 3 at that time are Δv〓 ₂ and Δv〓, the terminal voltages of the circular coils 2 and 3 are e〓 ₂ −v〓 ₂ +Δv〓 ₂ , e〓 ₃ − respectively.
v〓
₃ +Δv〓 ₃ , and the voltage e〓ba between point b and point a is e〓 _ba = e〓 _bc +e〓 _ca = (e〓 ₃ −v〓 ₃ +Δv〓 ₃
) +(e〓 ₂ −v〓 ₂ +Δv〓 ₂ ) ≒2(e〓 ₃ −v〓 ₃ )+Δv〓 ₂ +Δv〓 ₃ .

Since Z ₃₁ ≒ Z ₃₂ , variable impedance 32
The voltage e〓 _bd applied to both ends of c is e〓 _bd ≒e〓 _ba /2=e〓 ₃ −v〓 ₃ +(Δv〓 ₂ +Δ
v〓
₃ )/2, and the input voltage _e〓0 of the amplifier circuit 35c is: _e〓0 = e〓 _bd −e〓 _bc ≒e〓 ₃ −v〓 ₃ + (Δv〓 ₂
+Δv〓
₃ )/2 −(e〓 ₃ −v〓 ₃ +Δv〓 ₃ ) = Δv〓 ₂ −Δv〓 ₃ )/2, and the circular coils 2 and 3 due to the presence of defects
A voltage proportional to the difference in induced voltage change is applied to the input terminal of the amplifier circuit 35c, and the circular coils 2 and 3
will operate as the detection end of the differential method.

As shown in FIG. 8, the circular coils 4 and 25 constitute a circuit similar to that shown in FIG.
0 to the amplification circuit 35d, but the circular coil 2
5 is shielded by the metal ring 26, its induced voltage does not change, and a voltage proportional to the amount of change in the induced voltage of the circular coil 4 is input to the amplifier circuit 35d.

In other words, the circular coil 4 operates as the detection end of the absolute method, and since the circular coils 4 and 25 are in almost the same atmosphere, changes in characteristics due to coil temperature and changes in cable capacitance are compensated for, resulting in stability. becomes higher.

The induced voltage in the circular coils 2 and 3, which are the detection terminals of the differential method shown in FIG. 7, uses the magnetic field generated by the circular coil 4, which is the detection terminal of the absolute method. When the circular coils 2 and 3 of the differential method are affected by changes in the flow of eddy current due to defects, etc., the differential method circular coils 2 and 3 are also affected by the defects, etc. Since the source of the signal and that of the absolute method are the same, there is a complete difference between the signals 18 and 19, 20 and 21 of both methods, as shown in the flaw detection results 16 and 17 of the recorder 12 in FIG. This provides excellent simultaneity, and the correspondence between the signals of both methods becomes clear.

According to this device, the two differential detection coils use the magnetic field generated by the absolute detection coil to perform differential detection, so the absolute detection and differential detection signals are the same as those of the probe. By installing a dummy coil on the probe that occurs at exactly the same time and is shielded by a metal ring, regardless of the moving speed, the effects of temperature-induced changes in characteristics of the absolute detection coil and changes in cable capacitance are eliminated. Since it is possible to carry out stable absolute method flaw detection, it becomes easy to estimate the cause of the generated signal, and as a result, the flaw detection accuracy is improved.

FIG. 5 shows an example in which the outer diameters of the circular coils 2 and 3, which are the detection ends of the differential method, are larger than the outer diameter of the circular coil 4, which is the detection end of the absolute method.
The coil diameter is not limited to the above embodiment; all the circular coils may have the same diameter, or the circular coil 4 may be larger than the circular coils 2 and 3. Further, although two flaw detectors are used in FIGS. 5 and 6 for ease of explanation, they may be combined into one.

In short, according to the present invention, a dummy coil attached to a probe and shielded by a metal ring, an absolute detection coil connected in series with the dummy coil at a sufficient distance and energized by a high frequency oscillator, and a an absolute method flaw detection circuit comprising a bridge circuit for detecting a change in the induced voltage induced in the absolute detection coil itself due to a defect in the object to be inspected by a magnetic field caused by an eddy current, and detecting an output signal of the bridge circuit; and a pair of differential detection coils having the same characteristics that are disposed in close proximity to each other with the absolute detection coil in the center, and a bridge circuit that detects changes in the induced voltage due to the defect in the test object due to the differential detection coil. a differential method flaw detection circuit that detects the output signal of the bridge circuit to detect the difference in induced voltage of the pair of differential detection coils; and the absolute method using the output of the high frequency oscillation circuit as a reference voltage. The present invention is industrially extremely useful because it provides an eddy current flaw detection device that improves flaw detection accuracy by including a flaw detection circuit and a phase detector that detects the output of a bridge circuit of a differential method flaw detection circuit. .

[Brief explanation of the drawing]

Fig. 1 is a partial vertical side view showing a known eddy current flaw detection device, Fig. 2 is a block diagram showing the circuit of Fig. 1, and Figs. This is an explanatory diagram showing the magnetic field distribution of a circular coil. Figure A is when the probe is placed outside the test tube, Figure B is when the probe is inserted into the test tube, Figure C is
4A and 4B show the positions of the circular coils, respectively, where the circular coils are placed facing the defect.
These are explanatory diagrams showing the induced voltage of a circular coil and the output signal of a phase detection circuit, in which Figure A shows the influence of a short-axis defect, and Figure B shows the influence of a long-axis defect. 5 is a partial longitudinal sectional side view showing one embodiment of the present invention, FIG. 6 is a block diagram showing the circuit configuration of FIG. 5, and FIGS. 7A, B, and C are the circular coil 2, 3 is an explanatory diagram showing the voltage of the bridge circuit and the magnetic field distribution of the circular coil. Figure A is the case where the probe is outside the test tube, and Figure B is the same diagram when the probe is inserted into the test tube. Figure C shows the case where a circular coil is placed opposite the defect. FIG. 8 is an explanatory diagram showing the voltage of the bridge circuit including the circular coils 4 and 25 of FIG. 6 and the magnetic field distribution of the circular coils. DESCRIPTION OF SYMBOLS 1... Probe body, 2, 3, 4... Circular coil, 5... Cap, 6... Test tube, 7... Short axis defect, 8... Long axis defect, 9... Cable, 10...
...Flaw detector, 12... Recorder, 13, 14... Cable, 15... Recording paper, 16, 17... Flaw detection result, 18, 19, 20, 21... Defect signal, 22
...Probe moving direction, 23...Flaw detector, 24
... Cable, 25 ... Circular coil, 26 ... Metal ring, 31c, 32c, 31d, 32d ...
Variable impedance, 34d...Oscillation circuit, 35
c, 35d... amplification circuit, 36c, 36d... phase detection circuit.

Claims (1)

[Claims] 1. A dummy coil attached to the probe and shielded by a metal ring, an absolute detection coil connected in series with the dummy coil at a sufficient distance and energized by a high-frequency oscillator, and a magnetic field caused by eddy currents generated in the subject. an absolute method flaw detection circuit comprising a bridge circuit for detecting a change in the induced voltage induced in the absolute detection coil itself due to a defect in the test object, and detecting an output signal of the bridge circuit; The bridge comprises a pair of differential detection coils having the same characteristics and arranged in close proximity to each other with the center sandwiched therebetween, and a bridge circuit for detecting a change in the induced voltage caused by the differential detection coil due to a defect in the test object. A differential method flaw detection circuit detects the difference in induced voltage between the pair of differential detection coils by detecting the output signal of the circuit, and the absolute method flaw detection circuit and the differential method flaw detection circuit use the output of the high frequency oscillation circuit as a reference voltage, respectively. An eddy current flaw detection device comprising: a phase detector that detects the output of a bridge circuit of a circuit.