JP4674416B2 - Self-comparing eddy current flaw detector - Google Patents

Description

【００１９】
上記実施例は、被検査体が溶接鋼管の場合であるが、本発明は、それに限るものではなく、コイルの形状やサイズを変更し、棒鋼、線材、平鋼板等の欠陥検査にも利用できることは、言うに及ばない。
【００２０】
【発明の効果】
以上述べたように、本発明によれば、
ノイズを欠陥とする誤判定が従来に比べ確実に少なくなる。その結果、被検査体の欠陥検査に要する時間や作業者の省力が達成され、該被検査体の生産性が向上したばかりでなく、製造コストの削減も達成できる。
【図面の簡単な説明】
【図１】本発明に係る自己比較方式の渦流探傷装置の欠陥判定回路を示す図である。
【図２】従来の自己比較方式渦流探傷装置のフロー図である。
【図３】ブラウン管上に出現させた検波器を経た信号の波形を示す図である。
【図４】コイル形状による信号の極性変化の様子を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a self-comparing eddy current flaw detector, and in particular, is a technique for preventing erroneous detection of a surface defect of a steel pipe.
[0002]
[Prior art]
When a coil carrying an alternating current is brought close to a conductor such as metal, a defect (for example, a surface flaw) existing in the metal or the like is detected as a change in current and voltage induced in the coil. Moreover, if necessary, it is also possible to measure metal material discrimination, film thickness measurement, shape / dimension, and the like. Defect detection devices that use this principle are called eddy current flaw detectors, which can detect at high speed and the detection results can be extracted with electrical signals, so that they are widely used for inspection of surface defects such as steel bars, wire rods, and steel pipes in steel. Has been. The coil includes a penetration method that surrounds an object to be inspected, such as a steel pipe, a wire rod, a probe method that simply approaches the object to be inspected, and an insertion type that is inserted inside the object to be inspected. Further, a flaw detection apparatus in which coils are connected at a predetermined distance as shown in FIG. 2 so that the same defect is detected twice is called a self-comparison type.
[0003]
The basic configuration of this self-comparing eddy current flaw detector is shown in FIG. 2 as a flow chart, in which an alternating current generated by an oscillator is applied to a coil and an alternating magnetic field is applied to an object to be inspected (for example, a steel pipe). It has become. And a coil detects the eddy current which arose in the to-be-inspected object, and sends the output to a balanced circuit. Further, in this flaw detection apparatus, since a very small change in current must be detected, the balance circuit is adjusted in advance so that the output when there is no defect is zero. The output signal from this balanced circuit is amplified by an amplifier and sent to a detector (in the apparatus shown in FIG. 2, two detectors are provided so that the X and Y axis phases of the signal waveform can be processed separately). ing). These detectors analyze the phase of the input signal using a control signal applied from a phase shifter, remove noise other than a defective signal by a filter, and display information from the object to be inspected on a CRT (CRT) or the like. Further, the signal that has passed through the filter is finally input to the defect determination circuit, and is compared with a predetermined determination condition to determine the presence or absence of a defect, and is output from the comparator. The detected signal is represented by a waveform having a phase angle θ and an amplitude A as shown in FIG. In the self-comparison method, the polarity (+ −) of the signal when the same defect is detected as an eddy current change by the coil A and the coil B is inverted as shown in FIG. 4 due to the coil structure. In addition, a signal generated due to a normal defect always has the same waveform on the cathode ray tube (always swings to the + side and then swings to the-side) when the phase angle θ is set to a certain angle.
[0004]
By the way, in the conventional defect determination circuit, if the input signal is larger than a preset value (hereinafter referred to as a threshold), it is determined as a defect. In addition, since the signal size is the same on the + and − sides, a threshold is provided only on one side.
[0005]
However, even when such a self-comparison eddy current flaw detector is used, a defect may still be erroneously determined in a defect inspection of a steel pipe, and an improvement is desired.
[0006]
[Problems to be solved by the invention]
In view of such circumstances, an object of the present invention is to provide a self-comparing eddy current flaw detector with fewer erroneous determinations of defects than in the past.
[0007]
[Means for Solving the Problems]
In order to achieve the above-mentioned object, the inventor considered that it is necessary not to determine noise (such as noise generated around the object to be inspected and its surroundings) received by the coil during inspection as a defect. And earnestly researched about the means to embody the idea, and completed the present invention.
[0008]
That is, the present invention is connected to an AC oscillator, separated by a certain distance, two coils having different polarities for detecting an eddy current generated by applying an AC magnetic field to the object to be inspected, and when the object to be inspected is not defective. A balanced circuit that adjusts the output of the balanced circuit to 0, an amplifier that amplifies the output from the balanced circuit, a detector that analyzes the phase of the amplified signal using the control signal from the phase shifter, In the self-comparison type eddy current flaw detection apparatus comprising a filter for suppressing noise and a defect determination circuit that receives a signal that has passed through the filter and determines the presence or absence of a defect in comparison with a preset threshold value, the defect determination As a circuit, according to the polarity of the signal from the detector, the + side polarity determination circuit and the − side polarity determination circuit that compare each with a threshold, and the time when the + side polarity determination circuit detects a + polar signal exceeding the threshold -Side polarity Circuit exceeds a threshold value - an eddy-current flaw detection device of the self-comparison method, characterized by comprising a latency measurement circuit for measuring the difference between the time of detecting the polarity signal. In this case, the number of the detectors is set to 2, and the signal waveform is preferably divided into an X-axis component and a Y-axis component for phase analysis. Further, it is preferable that a CRT for externally displaying the output of the filter is provided between the filter and the defect determination circuit, or the object to be inspected is a steel pipe.
[0009]
According to the present invention, erroneous determination with noise as a defect is surely reduced as compared with the prior art. As a result, the time required for defect inspection of the inspection object and labor saving of the operator are achieved, and not only the productivity of the inspection object is improved but also the manufacturing cost can be reduced.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
In the following, embodiments of the present invention will be described in accordance with the circumstances leading to the invention.
[0011]
First, the inventor investigated the actual state of flaw detection inspection. As a result, it has been found that the cause of the erroneous determination is excessive detection of noise caused by extraneous electricity or transport play (the object to be inspected rattles while traveling on the inspection line). These noises are not regular in the waveform characteristics. For example, they are swung to the minus side first and then to the plus side (the phase is reversed 180 °) or only one side (the plus side or the minus side). I also learned that it was often a signal. In the conventional defect determination circuit, these noise signals are also determined as defects.
[0012]
Therefore, as described above, the inventor, in the self-comparison type eddy current flaw detection apparatus, sequentially passes the inspected object having the true same defect to the coil A and the coil B, and detects the eddy current change in each coil. If the polarity (+-) of the obtained signal is reversed in coil A and coil B, and the phase is set to a certain angle, the same waveform (always swings to the + side and then swings to the-side) is always obtained. In particular, the present invention was completed.
[0013]
That is, in the defect determination circuit, when the signal first exceeds the + side threshold value, “+ side defect signal exists” is held, and after a certain time (for example, 20 msec), the − side threshold value is exceeded. If there is no "defect signal on the side" information, it is not determined that there is a defect. If there is "-defect signal present" information that exceeds the -side threshold within a certain time, it is determined that there is a defect. I did it.
[0014]
As a concrete means for doing this, as shown in FIG. 1, a defect judgment circuit of a conventional eddy current flaw detector is remodeled, and each is compared with a threshold according to the polarity of the signal from the detector. A determination circuit, a negative side polarity determination circuit, and a waiting time measurement circuit that measures the time from the determination of the positive side polarity signal to the detection of the negative side polarity signal corresponding to the signal may be incorporated. .
[0015]
Note that the input signal to the defect determination circuit is one of the signals from the detector used for the phase analysis of the X-axis side and the Y-axis side of the signal waveform as in the past. This is because the same determination result is obtained regardless of which is used for the inspection. In addition, although the time required to detect the -side polarity signal corresponding to the + side polarity signal varies depending on the traveling speed of the object to be inspected and the distance between the coils A and B, it is usually about 1 to 30 msec in actual inspection. . Furthermore, the threshold value may be determined by using a reference inspection object having an artificial defect.
[0016]
【Example】
In the production of a welded steel pipe having an outer diameter of 48.6 mmφ, a wall thickness of 3.2 mm, and a length of 5500 mm, the self-comparing eddy current flaw detector according to the present invention was applied to inspect the surface flaw. At that time, a coil having a distance between A and B of 2 mm and a length of A and B of 5 mm was used, and the traveling speed of the welded steel pipe of the test object was set to 60 m / min. Therefore, the difference between the appearance times of the positive and negative signals based on the same defect is about 7 msec.
[0017]
The inspection results are shown in Table 1 together with the inspection results of the conventional apparatus. From Table 1, it is clear that the error detection rate of about 1.7% in the prior art is reduced to about 0.3% in the apparatus according to the present invention.
[0018]
[Table 1]

[0019]
Although the said Example is a case where a to-be-inspected object is a welded steel pipe, this invention is not restricted to it, The shape and size of a coil can be changed and it can be utilized also for defect inspections, such as a bar steel, a wire, and a flat steel plate. Needless to say.
[0020]
【The invention's effect】
As described above, according to the present invention,
The erroneous determination that the noise is a defect is surely reduced as compared with the conventional case. As a result, the time required for defect inspection of the inspection object and labor saving of the operator are achieved, and not only the productivity of the inspection object is improved but also the manufacturing cost can be reduced.
[Brief description of the drawings]
FIG. 1 is a diagram showing a defect determination circuit of a self-comparing eddy current flaw detector according to the present invention.
FIG. 2 is a flow diagram of a conventional self-comparing eddy current flaw detector.
FIG. 3 is a diagram showing a waveform of a signal that has passed through a detector that appears on a cathode ray tube.
FIG. 4 is a diagram illustrating a change in polarity of a signal depending on a coil shape.

Claims (4)

Two coils having different polarities for detecting an eddy current generated by applying an AC magnetic field to the object to be inspected and connected to an AC oscillator at a certain distance, and the output of the object to be inspected without any defects are zero. A balanced circuit that adjusts the output, an amplifier that amplifies the output from the balanced circuit, a detector that analyzes the phase of the amplified signal using a control signal from the phase shifter, and a filter that suppresses noise other than the defective signal, In the self-comparison eddy current flaw detection apparatus provided with a defect determination circuit that receives a signal that has passed through a filter and determines the presence or absence of a defect in comparison with a preset threshold value,
As the defect determination circuit, a + side polarity determination circuit and a − side polarity determination circuit that compare each with a threshold according to the polarity of the signal from the detector, and a + pole signal in which the + side polarity determination circuit exceeds the threshold is detected. A self-comparison type eddy current flaw detector characterized in that a waiting time measuring circuit for measuring the difference between the measured time and the time when the -side polarity determination circuit exceeds a threshold value is detected .   2. The self-comparing eddy current flaw detector according to claim 1, wherein the number of the detectors is 2, and the signal waveform is divided into an X-axis component and a Y-axis component and phase analysis is performed.   3. The self-comparing eddy current flaw detector according to claim 1, wherein a CRT for externally displaying the output of the filter is provided between the filter and the defect determination circuit.   The self-comparison type eddy current flaw detector according to any one of claims 1 to 3, wherein the object to be inspected is a steel pipe.

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