JPH0266447A - Eddy current test equipment - Google Patents

Abstract

PURPOSE:To shorten the length of a flaw-undetected area due to the response delay of a balance circuit, a filter circuit or the like and to improve quality control by holding a difference signal from a penetration coil when a detecting means for detecting the passage of an object to be inspected through the penetration coil by a previously determined length detects the passage. CONSTITUTION:An object (pipe) P to be inspected is inserted into the penetration coil 1 and moved in the axial direction. Eddy current is formed on the surface of the pipe P by current supply to coils 1a, 1b, and if a surface defect or the like exists, a difference is generated between the impedance values of both the coils 1a, 1b, a bridge circuit 4 is unbalanced and a flaw signal can be detected. When both photosensors PH1, PH2 detect the pipe P, a memory circuit 10 in the balance circuit 7 stores the flaw signal through a gate circuit 11 and outputs the signal to a subtractor 9. The subtractor 9 subtracts a signal inputted from the memory circuit 10 from the flaw signal and outputs the subtracted value through a switch circuit 12 to be turned on by the gate circuit 11.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an eddy current flaw detection device using a penetrating coil for detecting flaws on the surface of a cylindrical or rod-shaped object such as a steel pipe or steel bar.

[Prior art]

Eddy current flaw detection equipment using a through-coil usually consists of two coils arranged concentrically at a required distance in the axial direction, and the two coils are connected to form opposite sides of a bridge circuit. It is set so that it is always in an equilibrium state, and the through coil is fitted onto the object to be inspected and moved relative to it in the axial direction.The balance state of the bridge circuit is disrupted due to impedance changes in both coils that occur in the flawed area. Flaw detection is performed by detecting the voltage signal generated when the

By the way, when applying such a conventional device to an automatic flaw detection line, in order to eliminate the unbalanced output of the bridge circuit caused by impedance changes due to coil temperature fluctuations, the output is constantly controlled by a feedback loop with a time constant. A balance control circuit is provided to make it zero,
Further, a frequency filter is provided to detect flaw signals with good sensitivity (S/N).

FIG. 2 is a schematic diagram of a balance control circuit provided in a conventional device, in which a high frequency signal emitted from an oscillator 22 is amplified by an amplifier 23, passed through a bridge circuit 24, and outputted to a through coil 21. The synchronous detection circuit 25 detects and outputs an unbalanced signal of the bridge circuit 24 due to the change in impedance that occurs in the through coil 21, but this signal is fed back through the balance circuit 27 and added to the output from the bridge circuit 24. , control is performed such that the output of the synchronous detection circuit 25 is always zero.

[Problem to be solved by the invention]

By the way, in general, in an eddy current flaw detection device using a through coil, when the end of a test object such as a pipe or bar passes through the through coil,
Since an excessive shape signal is generated, it is impossible to perform flaw detection on this part, and it is inevitable in principle that an undetected area will occur.

Figure 3 (a) is a graph showing an example of the relationship between the coil diameter and the magnetic field, with x/d (d: inner diameter of the coil, X: distance from the end of the coil in the axial direction) plotted on the horizontal axis. Also, HX on the vertical axis
/Ho (Ha: Magnetic field at the end of the coil, HX
: Magnetic field at a point separated by dimension X in the axial direction from the end of the coil), and the coil width <b> is 2IIm.
This is a diagram for the case (see Figure 3 (b)).

As is clear from this graph, the magnetic field becomes smaller as it moves away from the coil, and the distribution of the magnetic field due to the coil is 0.6
It is within the range of d. Therefore, when the test object is inserted through the coils 21a and 21b constituting the two through-coils in the direction shown by the white arrow in FIG. The magnetic field of the coil 21a begins to act on the test object, and the front end of the coil 21a starts to act on the test object at a distance of 0.
When the position 6d is reached, all the magnetic fields of both coils 21a and 21b act on the test object, and the bridge circuit 24 becomes in an equilibrium state. During this period, an extremely large fluctuation signal is output, resulting in an area where flaw detection cannot be practically performed. becomes.

It should be noted that even when the rear end of the object to be inspected leaves the coils 21a and 21b, a large fluctuation signal is output in the range of 0.6 d just before that, making it impossible to carry out substantial flaw detection.

Note that the distribution range of the magnetic field varies depending on the width of the coil (C1l"), but in the case of a coil used for flaw detection on front bars, it is 0.
．． 5d to 0.7d, and this range is the undetected range in which flaws occur in principle.

Moreover, in the conventional device as described above, the balance circuit 27
, or a filter circuit is provided, which causes a response delay, and the length of the undetected area as a whole becomes longer than the undetected area that arises from the principle.

Incidentally, the length of the undetected area due to the response delay of the balance circuit and filter circuit is as follows.

■ Length of undetected area due to delay in balance circuit, Figure 4 shows the signal in principle when the through coil 21 passes, for example, the end of a pipe material (Figure 4 (a)) and the balance circuit 2
7 (Fig. 4 (b)), the output of the balance circuit 27 goes beyond the theoretical signal and outputs a similar excessive signal in response to the filtered large shape signal, and then further outputs an oversized signal. It converges by repeating shoots and undershoots,
This region B becomes an undetected region.

Therefore, when we performed step input as shown in Figure 5 (a) and measured the time constant, we found that the time constant was <50 as shown in Figure 5 (b).
There is a delay of about ~60 msec, and if the vessel speed is 90 m/min, the length of the undetected area B will be about 75 to 901 m.

■ The length of the undetected area due to the delay of the filter circuit.The filter circuit uses capacitors and resistors, and when targeting seamless pipes, the delay is about 20 msec, and the pipe speed is 90 m/min. Then, the length of undetected area C is 30n
+ degree.

Therefore, assuming the pipe speed is 90m/min, the inner diameter is 50n+ and the width is 2.
When n through-coils are used, the total length of the undetected area is the sum of these values, and is about 150 cm*.

The present invention has been made in view of the above circumstances, and its purpose is to avoid undetected areas due to response delays of the balance circuit 1 filter circuit, etc., even though the occurrence of undetected areas is unavoidable in principle. An object of the present invention is to provide an eddy current flaw detection device whose length is shortened and quality control can be improved.

[Means to solve the problem]

The eddy current flaw detection device according to the present invention includes means for detecting that the end of the object to be inspected has passed through the through-hole coil for a predetermined length in relation to the inner diameter and width of the through-hole coil; It is equipped with a circuit that holds a difference signal from the through-hole coil when the passing of the object to be inspected is detected, and a means for subtracting the hold circuit and a difference signal subsequently output from the through-hole coil and outputting the difference. do.

[Effect]

According to the present invention, it is thereby possible to bring the length of the undetected area closer to a length that cannot be avoided in principle.

〔Example〕

The present invention will be specifically described below based on drawings showing embodiments thereof.

Fig. 1 is a block diagram of an eddy current flaw detection device according to the present invention (hereinafter referred to as the device of the present invention), and in the figure 1 is 2 with a coil width of 2.
Solenoid coils (hereinafter simply referred to as coils) la
, lb concentrically arranged and assembled, P
indicates a tube as the object to be inspected.

A signal from an oscillator 2 connected to a constant current circuit is amplified by a constant current type amplifier 3 and energized through a bridge circuit 4 to each coil Ia, lb of the through coil 1. In this state, the tube P is inserted through the through coil 1 and moved relatively in the axial direction to perform flaw detection. As mentioned above, flaw detection cannot be theoretically performed during the period corresponding to 0.6 d before and after the through coil 1, respectively, when the pipe P is inserted into the through coil 1. and 0.6d behind the solenoid coil 1b.
When reaching the distance of 1. , P.H.
, is designed to detect this.

Eddy currents are formed on the surface of the tube P by energizing the coils la and Ib, and if there are surface defects, both coils la and Ib
, lb impedance difference occurs, and the bridge circuit 4
The balance is disrupted, and the signal is detected by the synchronous detection circuit 5 as a flaw signal.

This flaw signal is converted into a digital signal by an analog/'digital converter 8 in a balance circuit 7, and is input to a subtracter 9.

The memory circuit 10 has coils la and lb and predetermined dimensions (0,6d d: coil inner diameter) before and after the through-hole coil 1, respectively.
A pair of photosensors PH+, P) arranged apart from each other
I2 is connected via a gate circuit 11, and when both photosensors P)I, PH2 detect the tube P, they are activated by a command signal issued from the gate circuit 11, and synchronous detection is performed at that time. The detection signal input through the circuit 5 and the A/D converter 8 is stored and output to the subtracter 9. In addition, the gate circuit 11 is connected to both photosensors P.
1(,,) A short time (several microseconds) after both PH2 detect the tube P, an on command is issued to the switch circuit 12.The subtracter 9 then converts the signal input directly from the A/D converter 8 into the memory. The switch circuit 1 which subtracts the signal inputted from the circuit 10 and uses the value as turned on by the gate circuit 11
2 through a digital/analog (D/A) converter 13
Convert it to an analog quantity and output it.

The reason why a constant current circuit is used as a power supply is that although temperature rises and fluctuations in the coil are due to heat generation, the effect of constant current is less than the effect of constant voltage on the amount of heat generated, and that excessive impedance changes that occur at the ends of the tube are On the other hand, this is because there is no delay in response to changes in current value.

〔effect〕

As described above, in the device of the present invention, the balance operation is fast, and the response delay that occurs when detecting flaws at the edge of the pinched object is significantly reduced. Since no flaws are output, there is no delay in subsequent filter processing, the undetected area is significantly shortened, and the quality control of the test object can be improved.The present invention has excellent effects. .

[Brief explanation of the drawing]

Figure 1 is a block diagram of the device of the present invention, Figure 2 is a block diagram of a conventional balance circuit, Figure 3 (a) is a graph showing the theoretical flaw detection limit when using a through coil, and Figure 3 ( (B) is an explanatory diagram of the coil, Figures 4 (A) and (B) are four explanations showing the delay in the balance circuit, and Figures 5 (A) and (B) are the relationship between step input and its response output. FIG. P... Steel pipe PH+, pHz... Photo sensor 1... Penetration coil la, lb... Coil 2
...Oscillator 3...Amplifier 4...Bridge circuit 5...Synchronous detection circuit 7...Balance circuit 8...
・A/D converter 9...Subtractor 10...Memory circuit 11...Gate circuit 12...Switch circuit 1
3...D/^ converter

Claims (1)

[Claims] 1. A through coil consisting of two concentrically arranged coils is moved relative to the object to be inspected in its axial length direction, and a flaw signal is captured as a difference signal between the impedances of the through coils. The eddy current flaw detection device has a means for detecting that the end of the object to be inspected has passed through the through-hole coil for a predetermined length in relation to the inner diameter and width of the through-hole coil; A circuit for holding a difference signal from a through-hole coil when the passing of an object is detected, and means for subtracting the hold circuit from a difference signal subsequently output from the through-hole coil and outputting the difference. An eddy current flaw detection device characterized by: