JPS6255098B2 - - 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 circular coils 02, 03, and 04 coaxially wound around a probe 01, as shown in a partial longitudinal sectional side view in FIG. 1 and a block diagram in FIG. , 03 are applied with a high frequency voltage by an oscillation circuit 034a built in the flaw detector 010, and the outputs of the circular coils 02, 03 are taken out via variable impedances 031a, 032a with the same characteristics that constitute a bridge with the same coils, and are outputted to an amplification circuit. 0
35a, the voltage waveform of the oscillation circuit 034a is referred to, and the voltage waveform of the oscillation circuit 034a is converted into a DC voltage by the phase detection circuit 036a and recorded by the recorder 012. A high frequency generation voltage is applied by the circuit 034b, and the output of the circular coil 04 is transmitted through the variable impedance 031b, which forms a bridge with the circular coil 04.
032b, amplified by an amplification circuit 035b, converted into a DC voltage by a phase detection circuit 036b with reference to the voltage waveform of an oscillation circuit 034b, and recorded by a 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
If _i , the voltage applied to the circular coils 02 and 03 becomes ₂ ≒ ₃ ≒ _i/2 , and the width of the amplifier circuit 035a
The input voltage ₀ is ₀ = ₃₂ - ₃ , but when probe 01 is inserted into test tube 06,
As shown in the figure, an eddy current 0102 is generated in the test tube due to the magnetic field 0101 generated by the circular coils 02 and 03, and a high-frequency magnetic field 0103 is generated by the eddy current 0102.
interlinks with the circular coils 02, 03, thereby inducing voltage in the circular coils 02, 03.

Now, the induced voltage of circular coils 02 and 03 _{is 2} ,
₃ , the voltage _c across circular coils 2 and 3 is
_a and _bc are _ca = ₂ − ₂ and _bc =
₃ − ₃ , and since the circular coils 02 and 03 are evenly wound, ₂ ≒ ₃ , so ₂
− ₂ ≒ ₃ − ₃ ≒ _ba/2 (here _ba
is the voltage between point b and point a. ) Width increase circuit 035
Since a is a considerably high magnification, when the input voltage is not very small, the output voltage will be saturated and it will not work well.

In such a device, impedance 03
After adjusting the knobs 1a and 032a to set the bridge circuit output ₀ = 0, when flaw detection is started, the circular coils 02 and 03 will move as shown in Figure 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 Δ ₂ and Δ ₃ in the induced voltage of the circular coils 02 and 03 are
Since it is closer to defect 07, it is strongly influenced by the result, so Δ ₂ > Δ ₃ , and the amplification circuit 035
The input voltage E ₀ of a is ₀ = _bd − _bc = ₃ − ₃ + (Δ ₂ + Δ
₃ ) / ₂ - ( ₃ - ₃ + Δ ₃ ) = ( Δ ₂ - Δ ₃ ) 1/2, and the input voltage E ₀ of the amplifier circuit 035a is always proportional to the difference between the induced voltages of the circular coils 02 and 03. Therefore, a flaw detection method that utilizes the difference in induced voltage between two coils is called a 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 is at the defect 07.
The output of the phase detection circuit 036a becomes maximum as shown in FIG.
Since the defect 08 is a long-axis defect that is long in the axial direction, the induced voltages ΔV ₂ and ΔV ₃ of the circular coils 02 and 03 and the output of the phase detection circuit 036a are as shown in FIG. S /N ratio becomes high, which is advantageous for detecting minute defects.

The circular coil 04 is provided to compensate for such drawbacks of the differential method, and the flaw detector 011 has a fixed impedance 033b.If the circular coil 03 and the identified 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 do not occur at the same time. I understand that.

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. 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 the signal factors decreases, especially when using actual equipment for flaw detection. Due to the wide variations in the manual speed of the probe, the time intervals between the signals generated by both types also vary significantly, 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 its purpose 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. , extracts changes in the induced voltage induced in the detection coil itself due to defects in the object by an absolute detection coil attached to the probe and energized by a high-frequency oscillator and a magnetic field caused by eddy currents generated in the object. an absolute method flaw detection circuit comprising a bridge circuit and detecting the output signal of the bridge circuit;
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 oscillator as a reference voltage.

An embodiment of the present invention will be explained with reference to the drawings.
FIG. 5 is a partial vertical side view of the circuit, FIG. 6 is a block diagram of the circuit in FIG. 5, and FIG. Figure A shows the magnetic field distribution of a circular coil. Figure A shows when the circular coil is placed in the air, Figure B shows when the circular coil is inserted into the test tube, and Figure C shows when the circular coil is defective. The case where they are placed facing each other is shown.

First, in Figs. 5 and 6, 1 is a probe;
2, 3, and 4 are circular coils wound coaxially around the probe 1, and the circular coil 4 is a differential detection coil located in the center of a pair of circular coils 2 and 3 having the same characteristics and arranged close to each other. It is provided as an absolute detection coil. 23 is a variable impedance device 31c, 32c which outputs the difference in induced voltage between the circular coils 2, 3 and constitutes a bridge circuit with the circular coils 2, 3, and 35c is a variable impedance device 31c. An amplifier circuit 32c amplifies the output of the bridge circuit, and phase detection circuits 31c, 32c, 35c, and 36c cooperate to form the flaw detector 23.

31d and 32d are a circular coil 4, an identified impedance 33d and a variable impedance forming a bridge circuit; 34d is a circular coil 4, a fixed impedance 33d and a variable impedance 31
d, an oscillation circuit that applies a high frequency voltage to the bridge circuit 32d, 35d an amplification circuit that amplifies the output of the bridge circuit, and 36d an oscillation circuit 34.
A phase detection circuit converts the output of the amplifier circuit 36d into a DC voltage by referring to the output voltage waveform of the amplifier circuit 31d,
32d, 33d, 34d, 35d, and 36d cooperate to constitute the flaw detector 10. 5 is a cap of the probe 1, 6 is a test tube having a short axis defect 7 and a long axis defect 8, 9 is a cable for inputting the output of the probe 1 to the flaw detectors 23 and 10, and 12 is the flaw detector 2.
A recorder inputs the outputs of 3 and 10 via cables 13 and 14, respectively, 16 is the detection result of the flaw detector 13 drawn on the recording paper, and 18 and 20 are signal waveforms due to the short axis defect 7 and the long axis defect 8, respectively. , 17 are the flaw detection results of the flaw detector 10, 19 and 21 are signal waveforms due to the short axis defect 7 and the long axis defect 8, respectively, and 24 is the phase detection circuit of the flaw detector 23 using the output of the oscillation circuit 34d of the flaw detector 10 as a reference voltage. 36c.

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 applied to the circular coil 4 and the fixed impedance 33d, and the circular coil 4 generates an alternating magnetic field.

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 flaw detection power of the circular coils 2 and 3 is only affected by the magnetic field generated by the circular coil 4, and the circular coil 4 is , 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, B shows the case where circular coils 2, 3, and 4 are inserted into the test tube;
3 is a magnetic field 11 caused by an eddy current 112 in the thick part of the tube to be tested.
3 and the magnetic field 111 caused by the circular coil 4, the magnetic field 11 caused by the eddy current 112
The induced voltage in circular coils 2 and 3 due to 3 _{is 2} ,
₃ , the voltages across the circular coils 2 and 3 are _ca = ₂ - ₂ and _bc = ₃ - ₃ , respectively, and the circular coils 2 and 3 are arranged symmetrically with respect to the eddy current 112 and are uniformly wound. Due to the fact that
Since ₂ ≒ ₃ , ₂ − ₂ ≒ ₃ − ₃ .

Before the circular coils 2 and 3 are inserted into the test tube and flaw detection starts, the input voltage of the amplifying circuit 35c of the flaw detector 23 must be almost zero to prevent the amplifying circuit 35c from being saturated, so the input voltage is variable. _The values ₃₁ and ₃₂ of the impedances 31c and 23c are approximately equal ( _31≈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 voltage of the circular coils 2 and 3 at that time are Δ ₂ and Δ ₃ , the terminal voltages of the circular coils 2 and 3 _{are 2} − ₂ + Δ ₃ and ₃ − , respectively.
₃ +Δ ₃ , and the voltage _ba between point b and point a is
_ba = _bc + _ca = ( ₃ − ₃ + Δ ₃ ) + ( ₂
− ₂ + Δ ₂ ) ≒ 2 ( ₃ − ₃ ) + Δ ₂
+ _Δ3 .

Since Z ₃₁ ≒ Z ₃₂ , variable impedance 32
The voltage _bd applied across c is _bd ≒ _ba/2 ≒ ₃ − ₃ + (Δ ₂ + Δ ₃ )
/ ₂ , and the input voltage ₀ of the amplifier circuit 35c is ₀ = _bd − _bc ≒ ₃ − ₃ + (Δ ₂ + Δ
₃ )/ ₂ − ( ₃ − ₃ + Δ ₃ ) = (Δ ₂ − Δ ₃ )
/ ₂ , and the circular coils 2 and 3 due to the presence of defects
A voltage proportional to the difference in induced voltage changes 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.

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 circular coils 2 and 3 of the differential method are also affected by the defect, 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 method detection coils utilize the magnetic field generated by the absolute method detection coil to perform differential method flaw detection, so the absolute method and differential method flaw detection signals are as follows: Regardless of the moving speed of the probe, the signals are generated at the same time, making it easy to estimate the cause of the generated signals, which in turn improves flaw detection accuracy.

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, and the circular coil 4 may be larger than the circular coils 2 and 3. Also, Figures 5 and 6
Although two flaw detectors are used in the figure for ease of explanation, they may be combined into one.

In short, according to the present invention, an absolute detection coil attached to a probe and energized by a high-frequency oscillator detects a defect in the object due to an induced voltage induced in the detection coil itself by a magnetic field caused by an eddy current generated in the object. an absolute method flaw detection circuit that extracts the change caused by the change through a bridge circuit and detects it using a phase detector (to be described later), and a pair of differential detectors with the same characteristics that are placed close to each other with the absolute detection coil in the center. A differential method in which a change in the induced voltage caused by the detection coil due to the defect in the test object is extracted through a bridge circuit and detected by a phase detector described later to detect the difference in the induced voltage between the differential detection coil pair. By including a flaw detection circuit and a phase detector that uses the output of the high frequency oscillator as a reference voltage and detects the output of the bridge circuit of the absolute method flaw detection circuit and the differential method flaw detection circuit, respectively, the eddy current improves flaw detection accuracy. Since a flaw detection device is obtained, the present invention is extremely useful industrially.

[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 shows the voltage when the probe is outside the test tube, and Figure B shows the same result when the probe is inserted into the test tube. Figure C shows the case where a circular coil is placed opposite the defect. 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, 31c, 3
2c, 31d, 32d...variable impedance,
33d... fixed impedance, 34d... oscillation circuit, 35c, 35d... amplification circuit, 36c, 3
6d...Phase detection circuit.

Claims (1)

[Claims] 1. Extracting changes in the induced voltage induced in the detection coil itself due to defects in the object to be inspected by an absolute detection coil attached to the probe and energized by a high-frequency oscillator and a magnetic field caused by eddy currents generated in the object to be inspected. an absolute method flaw detection circuit comprising a bridge circuit and detecting the output signal of the bridge circuit; a pair of differential detection coils having the same characteristics and disposed in close proximity to each other with the absolute detection coil in the center; a bridge circuit for detecting a change in the induced voltage caused by the motion detection coil due to the defect in the object to be inspected, and detecting an output signal of the bridge circuit to detect a difference in the induced voltage between the pair of differential detection coils. Eddy current flaw detection comprising: a dynamic flaw detection circuit; and a phase detector that detects the outputs of the bridge circuits of the absolute flaw detection circuit and the differential flaw detection circuit, respectively, using the output of the high frequency oscillator as a reference voltage. Device.