JPH06201654A - Method and apparatus for magneto-optical flaw detection - Google Patents

Abstract

PURPOSE:To provide a magneto-optical flaw-detection method wherein flaws on the whole face can be detected at high speed and to provide an apparatus used to execute it. CONSTITUTION:A magneto-optical element row 3 is arranged near a round steel rod K in such a way that its lengthwise direction corresponds to the axial direction of the round steel rod K, the beam shape of a beam radiated from a laser 4 is formed to be a slit shape in an element short-axis direction by a cylindrical lens 5, the beam is expanded by a beam expander 6 so that the magneto-optical element row 3 is irradiated uniformly. Its reflected beam is image-sensed by a CCD camera 9 after the rotation amount of its polarizing face has been detected by an analyzer 7 and has been amplified by an image intensifier 8, and its signal is given to a high-speed image processing apparatus 10. In addition, the title apparatus is provided with a phase shifter 13 which synchronizes the optical-intensity modulation period of the laser 4 with the frequency of an AC power supply 2 magnetizing the round steel rod K and with a phase shifter 14 which synchronizes the output of the CCD camera 9 with the frequency of the AC power supply 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for magneto-optical flaw detection of steel surface flaws and an apparatus used for the method.

[0002]

2. Description of the Related Art A magnetic flux leakage flaw detection method for a ferromagnetic material is a method for detecting a leakage magnetic flux generated by a defect when a material is magnetized. Leakage magnetic flux flaw detectors for surface flaws in steel materials such as pipes and rods generally employ AC magnetization as a magnetizing method, and utilize the AC magnetic field (skin effect) that gathers near the surface of the material to leak magnetic flux from defects. Is detected by the magnetic sensor.

FIG. 3 is a schematic diagram showing this conventional device.
It is disclosed in Japanese Patent Laid-Open No. 50-77081. In the figure, P is a steel pipe which is an object to be inspected, and a magnetizer 31 used for high-frequency AC magnetization is installed near the steel pipe P. The magnetizer 31 is provided with a flaw detection probe 32 for detecting a leakage magnetic flux generated due to a defect of the steel pipe P.
32 has a built-in search coil wound around a ferrite core, for example. The magnetizer 31 is arranged inside a cylindrical frame (not shown), and the cylindrical frame is rotatably installed around the axis of the steel pipe P so that the steel pipe P can be conveyed in the axial direction. Yes.

In this apparatus, when the magnetizer 31 is energized, a magnetic flux is generated on the surface of the steel pipe P. Then, the steel pipe P is conveyed in the axial direction while rotating the cylindrical frame. Here, if there is a defect on the surface of the steel pipe P, a leakage magnetic flux is generated, and the leakage magnetic flux is detected by the flaw detection probe 32 to perform flaw detection.

However, since the search coil used in this apparatus has a very narrow flaw detection range, the information obtained is only a one-dimensional flaw detection signal. Therefore, when detecting the entire surface of the steel pipe P, it is necessary to install a large number of these sensors and provide an independent signal processing system for each sensor, or to reduce the rotational speed of the cylindrical frame and the transport speed of the steel pipe P. Providing a large number of sensors increases equipment costs and maintenance costs. If the transport speed is reduced, high speed flaw detection is impossible. Further, even if the number of sensors is increased, a dead zone always exists between the sensors, so that the flaw detection signal is a series of one-dimensional information, and two-dimensional full-face flaw detection is difficult.

In order to solve these problems, a Faraday element (magneto-optical element) of an optical fiber magnetometer is used instead of the search coil to perform flaw detection magneto-optically. It is disclosed in the official gazette. FIG. 4 shows a schematic diagram of the flaw detection probe in this apparatus. In this device, the light beam emitted from the light source 41
After passing through a, it is collimated by rod lens 43a,
The polarized light having a single polarization direction is formed by 44a. Then, this polarized light beam is incident on the split mirror 46 in parallel with the steel material K as the material to be inspected, and the steel material K is split by the split mirror 46.
The direction is changed to a direction perpendicular to the surface of the (downward in FIG. 4).

This light flux passes through the Faraday element 45 in the steel material K.
Of the Faraday element 45 and is reflected upward by a reflection film (not shown) formed on the bottom surface of the Faraday element 45. That is, it passes through one Faraday element 45 twice. The light flux enters a corner prism 47 installed above the split mirror 46, the traveling direction of the light is converted by 180 degrees by the corner prism 47, and enters the Faraday element 45 in the next stage. Similarly, for example, the light flux that has passed twice through the six Faraday elements 45, 45, which are arranged in parallel in the width direction of the steel material K, passes through the polarizer 44b, and the rod lenses 43b arranged to face each other. , 43c is incident. Then, the polarization components of the light flux are taken out by these rod lenses 43b, 43c, and the respective polarization components are received through the optical fibers 42b, 42c to the light receiving element 48a,
It is incident on 48b. Further, this output is given to the divider 49, and its ratio is calculated.

In this device, when there is a leakage magnetic flux, the light flux passing through the Faraday element takes advantage of the fact that its polarization plane rotates in proportion to the amount of the leakage magnetic flux based on the Faraday effect. Then, the leak magnetic flux is detected from the total rotation angle of the polarization planes of the light flux that has passed through all of the plurality of Faraday elements 45, 45.

However, this device only provides a signal regarding the presence or absence of defects. Therefore, the defect position in the longitudinal direction of the steel material K can be obtained by detecting the conveying speed, but the position in the width direction of the steel material K is limited, that is, which of the six Faraday elements 45, 45 ... It is impossible to limit whether or not the plane of polarization of the light flux has rotated, and the detection sensitivity for minute defects is low. Further, similar to the above-mentioned device, since there is an element gap, a dead zone is generated and it is difficult to detect the flaws on the entire surface.

Further, Japanese Patent Application Laid-Open No. 3-245052 discloses a device in which a thin steel plate is used as a test material, and the thin steel plate is magnetized with direct current to detect flaws. FIG. 5 is a schematic view showing this conventional device. The thin steel plate H is DC magnetized by the magnetizer 51 and kept in a magnetically saturated state. In the vicinity of the thin steel plate H, a magneto-optical element array 52 equipped with an air floating follow-up device is installed with its longitudinal direction being the width direction of the thin steel plate H. While transporting the thin steel plate H in its longitudinal direction, the laser beam emitted from the laser is scanned by the polygon scanner 54 in the longitudinal direction of the magneto-optical element array 52, and the analyzer, the cylindrical lens, the optical rod and the PMT ( The reflected light is detected by a sensor unit 55 equipped with a photomultiplier tube, and flaw detection of the surface of the thin steel plate H is performed.

[0011]

The above-mentioned device has high sensitivity because it is magnetically saturated and amplified by PMT. However, since steel materials such as pipes and rods have a large cross-sectional area, a large output is required, it is difficult to magnetically saturate the entire portion to be inspected by direct current magnetization, and the detection sensitivity for minute defects is low. Therefore, FIG. 6 shows a magnetization state in the case where magnetization is performed using AC magnetization in order to efficiently magnetically saturate by utilizing the skin effect. The alternating current repeats the extreme value I _a and the extreme value I _b as shown in FIG. The saturation magnetization required for flaw detection is performed only near these extreme values I _a and I _b ,
The positions where the extreme values I _a and I _b appear are in an oblique direction as the steel material is conveyed. FIG. 6B shows a magnetized state when the thin steel sheet H is magnetized with an alternating current, and only the stripe region a indicated by hatching is the flaw detection range. As described above, in AC magnetization, the magnetization state varies depending on the flaw detection position, and it is difficult to keep the flaw detection conditions constant.

When the PMT is used, it is difficult to limit the leakage magnetic flux generation position within the irradiation range because only the change in the light amount is detected. Therefore, it is conceivable to limit the generation position by detecting the light intensity distribution in the irradiation range using a CCD camera, but this CCD camera is easily affected by ambient light.

The present invention has been made in view of the above circumstances, in which a steel material is subjected to AC magnetization, and an AC magnetizing current is applied during an irradiation period of light for irradiating a magneto-optical element and / or a detection period of reflected light. It is an object of the present invention to provide a magneto-optical flaw detection method and a device used for implementing the flaw detection method, which enable high-speed full-face flaw detection by efficiently flaw-finding only a magnetically saturated flaw-detectable range by synchronizing with a frequency.

[0014]

A magneto-optical flaw detection method according to the present invention magnetizes a steel material and irradiates a magneto-optical element array installed in the vicinity of the surface of the steel material with a light beam to form a flaw existing on the surface. In the method for detecting flaws by detecting the change in the reflected light from the magneto-optical element array due to the leakage magnetic field formed by, the steel material is AC-magnetized, the light beam irradiation period, and / or the reflected light is detected. It is characterized in that the period and the frequency of the alternating magnetizing current are synchronized.

The magneto-optical flaw detector according to the present invention magnetizes a steel material, irradiates a magneto-optical element array installed near the surface of the steel material with a light beam, and forms a leakage magnetic field formed by a flaw existing on the surface. In a device for detecting flaws by detecting a change in reflected light from the magneto-optical element array by means of, a phase shifter for synchronizing the means for magnetizing the steel material with the irradiation period of the light beam and the frequency of the alternating magnetizing current. And / or a phase shifter for synchronizing the detection period of the reflected light and the frequency of the alternating magnetizing current.

[0016]

In the present invention, the steel material is AC-magnetized, and the irradiation period of the light for irradiating the magneto-optical element array is synchronized with the frequency of the AC magnetizing current, so that the magnetization peak period and the light irradiation intensity modulation period are set. Can be matched, and the flaw detection range formed only in the above-mentioned stripe shape is selectively irradiated, so that flaw detection can be performed in a constant magnetization state. Further, by synchronizing the detection period of the reflected light with the frequency of the alternating magnetizing current, it is possible to limit the flaw detection position within the flaw detection range and reduce the influence of aperiodic disturbance light. As described above, it is possible to efficiently perform flaw detection only in the flaw-detectable range, and it is possible to perform high-speed whole-face flaw detection.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below with reference to the drawings showing the embodiments. FIG. 1 is a schematic view showing an apparatus used for carrying out the magneto-optical flaw detection method according to the present invention. K in the figure
Is a round steel bar, and a magnetizer 1 is installed around the round steel bar K, and the round steel bar K can rotate about an axis and can be conveyed in the axial direction. This magnetizer 1
The coil is wound around a yoke having a rectangular tube shape with the central portion of its upper side being lacking. The upper side of the magnetizer 1 is installed close to the round steel bar K, and when the coil of the magnetizer 1 is energized by the AC power supply 2, both ends of the upper side become magnetic poles so that the round steel bar K is AC-magnetized. It has become. Further, at a position near the round bar K and substantially in the center of both magnetic poles of the magnetizer 1, a magneto-optical element array 3 having a plurality of elements each having a size of 5 mm × 50 mm is provided. Laser 4 which is arranged in the axial direction and emits linearly polarized light
Laser beam is radiated from the above, the beam shape is made into a slit shape in the element short axis direction by the cylindrical lens 5, and expanded in the longitudinal direction of the magneto-optical element array 3 by the beam expander 6.

On the light receiving side, a detection light having a length corresponding to the width of the reflected light in the optical path of the reflected light reflected by the magneto-optical element array 3 and detecting the rotation amount of the polarization plane of the reflected light. Photon 7
Of the image intensifier 8 which has a length corresponding to the width of the reflected light and which amplifies the light quantity. It is installed parallel to the analyzer 7. Further, a CCD camera 9 for picking up the amplified light and a high-speed image processing device 10 for processing this image are provided.

A phase shifter 13 for synchronizing the light intensity modulation period of the laser 4 with the frequency of the AC power source 2 and a phase shifter 14 for synchronizing the output of the CCD camera 9 with the frequency of the AC power source 2 are provided. . Further, a pulse generator 11 for detecting the rotation angle of the round bar K is brought into contact with the round bar K, and this signal is given to the high-speed image processing apparatus 10.

Next, a flaw detection method using the above-described device of the present invention will be described. FIG. 2 is an explanatory diagram showing each signal in the device of the present invention shown in FIG. The phase shifter 13 receives a timing signal shown in FIG. 2 (a) at which the magnetizing current is zero-crossed from the AC power supply 2 so that the peak period of the magnetizing current and the light intensity modulation period shown in FIG. 2 (b) coincide with each other. , The phase of this timing signal is shifted. Similarly, the phase shifter 14 phase-shifts the timing signal so that the peak period of the magnetizing current and the output of the CCD camera 9 shown in FIG.

In such a state, the magnetizer 1 is energized to AC magnetize the round steel bar K, rotate the round steel bar K around this axis, and further convey it in the axial direction. Then, the laser light emitted from the laser 4 is synchronized with the magnetizing current as described above,
The cylindrical lens 5 forms a slit shape in the lateral direction of the magneto-optical element array 3, and the beam expander 6 expands in the longitudinal direction of the magneto-optical element array 3 for scanning. In the analyzer 7, the linearly polarized light reflected by the magneto-optical element array 3 is converted into a light amount proportional to the Faraday rotation due to the leakage magnetic field generated by the presence of the defect. Then, after the amount of light is amplified by the image intensifier 8, it is picked up by the CCD camera 9 and the image signal is given to the high-speed image processing device 10.
Image processing is performed according to the rotation angle of.

In FIG. 2, for example, the magnetization frequency is 10 kHz.
(Cycle 100 μsec.), And the laser beam is irradiated only during the time t when the magnetizing current is near the maximum value (FIG. 2 (b)).
As shown in (c), the reading by the CCD camera 9 is performed a little longer than this time t, for example, about 10 μsec., and the output is made to coincide with this time t. Then, it can be considered that the magnetization state is constant at time t and a strong magnetic field is formed. Therefore, the information from the magneto-optical element array 3 is imaged by the CCD camera 9 every 100 μsec. In this way, high-speed flaw detection is possible. By shortening the reading time of the CCD camera 9, it is possible to reduce the influence of non-periodic ambient light, and to reduce the CC of 100MHz, 1024bit.
When using a D camera, a reading time of about 10 μsec. Is appropriate.

The laser irradiation is synchronized with the magnetizing current signal by the phase shifter 13, and the intensity is modulated for a certain period of time including the time when the magnetizing current has a positive maximum. Here, since the polarity of the flaw detection signal is reversed at the time when the magnetizing current has the negative maximum, only the time when the positive maximum is detected for the sake of convenience. As described above, the irradiation time is preferably about ⅕ or 1/10 of the magnetization period in consideration of the influence of ambient light.

Also, by synchronizing the reading period of the CCD camera 9 with the magnetizing current signal by the phase shifter 14, it is possible to selectively limit the flaw detection range in which the magnetization state is constant and to perform flaw detection. Furthermore, if the reading time of the CCD camera 9 is matched with the laser irradiation time, the influence of ambient light can be reduced.

In this embodiment, the irradiation period and the reading period are synchronized with the magnetizing current signal. However, if the light irradiation range is very narrow and there is no limitation on the position where the magnetic flux leaks, the irradiation period is simply synchronized. Good. If the light intensity is sufficiently high and the influence of ambient light can be ignored, it is sufficient to synchronize the reading period.

[0026]

As described above, in the magneto-optical flaw detection method and the apparatus thereof according to the present invention, the irradiation period of the light for irradiating the magneto-optical element array with the alternating current magnetization of the steel material and / or the detection period of the reflected light is set. By synchronizing with the frequency of the alternating magnetizing current, only the magnetically saturated flaw-detectable range can be efficiently flaw-tested to enable high-speed full-face flaw detection. The present invention has excellent effects.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a magneto-optical flaw detector according to the present invention and an implementation state thereof.

FIG. 2 is an explanatory diagram showing a waveform of each observation signal in the device of the present invention.

FIG. 3 is a schematic diagram showing a conventional device.

FIG. 4 is a schematic view showing a conventional device and its implementation state.

FIG. 5 is a schematic view showing a conventional device and its implementation state.

FIG. 6 is a schematic diagram showing a magnetized state in a conventional device.

[Explanation of symbols]

 1 Magnetizer 2 AC power supply 3 Magneto-optical element array 4 Laser 5 Cylindrical lens 6 Beam expander 7 Analyzer 8 Image intensifier 9 CCD camera 10 High-speed image processor 11 Pulse generator 13, 14 Phase shifter K Round bar steel

Claims (2)

[Claims] 1. A steel material is magnetized, a magneto-optical element array installed near the surface of the steel material is irradiated with a light beam, and a magnetic field leaked from the magneto-optical element array is formed by a leakage magnetic field formed by a flaw existing on the surface. In the method of detecting a change in reflected light and performing flaw detection, the steel material is AC-magnetized, and the irradiation period of the light beam and / or the detection period of the reflected light and the frequency of the AC magnetizing current are synchronized. Magneto-optical flaw detection method. 2. A steel material is magnetized, a magneto-optical element array installed near the surface of the steel material is irradiated with a light beam, and the magneto-optical element array from the magneto-optical element array is formed by a leakage magnetic field formed by a flaw existing on the surface. In a device for detecting a change in reflected light and performing flaw detection, means for alternating-current magnetizing the steel material, a phase shifter for synchronizing the irradiation period of the light beam and the frequency of alternating-current magnetizing current, and / or the reflected light A magneto-optical flaw detector comprising a phase shifter that synchronizes a detection period with the frequency of the alternating magnetizing current.