JP2008025990A - Method and apparatus for detecting surface flaw of steel strip - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical detection method for precisely detecting the surface flaw of a steel strip in a steel strip continuous transferring line, and to provide an optical detector therefor. <P>SOLUTION: A light source (1) for irradiating the surface of the steel strip, and a line camera (2) for photographing the optical image of the steel strip are arranged so as to be inclined with respect to a steel strip transfer direction, and the line camera (2) is scanned along a scanning line Z (inclined at an angle θ with respect to the width direction of the steel strip) to take in an optical image. As the effect of this oblique scanning, an angle of inclination is formed with respect to a camera scanning direction and a flaw component in surface flaw detection operation setting the fine linear flaw or the like of the same component as that in the width direction of the steel strip, and the data of a normal part and the data of a flaw part are present in image data obtained by one scanning. Under the flaw detection operation of the actual operation accompanied by the variation of base noise (the signal caused by the surface roughness, gloss and other factors of the steel strip), the presence and size of the surface flaw of the steel strip are precisely judged. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method and an apparatus for optically detecting a surface defect of a steel strip while transporting a steel strip which is a continuous strip-like body in a predetermined direction, and in particular, a direction orthogonal to the transport direction of the steel strip (of the steel strip). The surface defects such as fine linear wrinkles generated in the width direction) can be detected with high accuracy.

  As a technology to detect the surface defects of the steel strip in the transfer line while feeding the steel strip in a predetermined direction, the optical image is taken with a line camera while illuminating the steel strip surface, and the obtained image signal is processed to a certain degree An optical method for determining the presence or absence of surface defects and the shape thereof is known. The general detection mode is shown in FIG. 6 (front view) and FIG. 7 (overhead view). The steel strip (S) is continuously transferred in a predetermined direction (arrow Y), and a light source (1) and a line camera (2) are arranged above it. Line X (the width direction axis of the steel strip perpendicular to the steel strip transfer direction Y) indicates the scanning direction of the line camera (2) that images the steel strip surface. The scanning line X is irradiated over the entire width of the steel strip by the light source (1), and the line camera (2) for capturing the reflected light image is installed at a position where the entire width of the scanning line X is within the field of view. The image signal captured by the line camera (2) is subjected to predetermined processing by a signal processing unit (not shown), and based on the difference in strength of the image signal between the surface defect portion and the normal portion, The grade is determined.

As a device to improve the detection accuracy of the surface defects,
An optical filter is installed on the path of illumination light from the light source (or on the path of the optical image between the object to be inspected and the camera), and the optical image is captured by selectively transmitting light of a predetermined wavelength band. Enhances the brightness difference between the normal part and the defective part to facilitate detection (Patent Document 1).
Multiple light sources are placed at different heights to irradiate multiple surfaces with different angles of incidence, and each reflected light image is individually captured by multiple line cameras, resulting in color tone changes, unevenness changes, etc. Making it easy to detect different types of surface defects (Patent Document 2),
Or in order to eliminate base noise (random signals due to differences in surface roughness and gloss of steel strips) from the image signal captured by the line camera and improve detection accuracy, the captured image signal is converted to digital data and smoothed. To obtain a threshold value, binarize the positive value of the difference between the threshold value and the original image signal, and determine the presence or absence of a defect based on the area ratio of the binarized image signal (Patent Document 3),
Such defect detection techniques have been proposed.
JP-A-8-136474 Japanese Patent Laid-Open No. 7-218451 Japanese Patent Laid-Open No. 5-180781

The surface defects of the steel strip include relatively coarse to fine ones, as well as those extending in the longitudinal direction of the steel strip (steel strip transport direction) and the width direction of the steel strip (direction substantially perpendicular to the transport direction). ) Extends to a wide variety of forms. Among them, fine linear wrinkles extending in the width direction of the steel strip (surface wrinkles called “folding wrinkles”) have the same components as the scanning direction of a line camera that images the steel strip surface. Therefore, the difference in the intensity level of the image signal necessary for distinguishing the surface defect from the normal part is not clear, and therefore it is difficult to set an appropriate threshold value for determining the presence or absence of the surface defect. It is not easy to increase accuracy.
The present invention copes with the above, and various surface defects on the surface of the steel strip, including fine linear wrinkles extending in the plate width direction of the steel strip, can be accurately detected without requiring complicated detection scanning and apparatus configuration. It is an object of the present invention to provide a steel strip surface defect detection method and detection apparatus which can be well detected.

The method for detecting defects on the steel strip surface according to the present invention (Claim 1)
Illumination light is projected over the entire width of the surface of the steel strip traveling in a predetermined direction, an optical image of the entire width of the irradiated steel strip surface is captured with a line camera, and the captured optical image signal is processed and the steel is processed. In the method of detecting the surface defect of the strip, the steel strip surface is irradiated in the direction inclined with respect to the steel strip transfer direction, and the surface of the steel strip is scanned by a line camera installed in the direction orthogonal to the inclination direction. It is characterized by imaging.

The steel strip surface defect detection device according to the present invention (Claim 2)
A light source that illuminates the entire width of the surface of the steel strip traveling in a predetermined direction, a line camera that captures an optical image with the entire width of the irradiated steel strip surface within the field of view, and an image signal captured by the line camera In a defect detection apparatus including a signal processing unit that detects surface defects by processing and an output unit that outputs the signal processing result, the light source and the line camera are arranged to face each other in a direction inclined with respect to a steel strip transfer direction. In addition, the line camera is installed in a direction orthogonal to the tilt direction and scanned.

  The present invention is different from the conventional detection technique (scanning the line camera in the steel strip width direction X orthogonal to the steel strip transport direction Y), and the light source is inclined with respect to the steel strip transport direction (Y). The belt surface is irradiated obliquely (inclination angle θ), and a line camera for imaging the steel strip surface is installed in a direction orthogonal to the inclination direction (inclination angle θ with respect to the steel strip width direction axis X) for scanning. I am going to do that. As an effect of this oblique scanning, in the detection of surface defects such as fine line wrinkles with the same component as the width direction of the steel strip, there is an inclination angle between the scanning direction of the camera and the defect component, and image information obtained by one scanning The information on the normal part and the defective part is present in this, and as a result, the defect detection conditions of the actual operation accompanied by fluctuations in the base noise (signal due to the surface roughness, gloss, and other factors of the steel strip) Originally, it is possible to accurately determine the presence and size of surface defects.

1 and 2 schematically show an embodiment and apparatus configuration of surface defect detection according to the present invention (FIG. 1: front view, FIG. 2: overhead view).
The defect detection apparatus of the present invention includes a light source (1) that illuminates the surface of a steel strip (S), a line camera (2) that images the irradiated steel strip surface, and a steel strip surface imaged by the line camera (2). Image signal processing unit (3) for processing the optical image signal of the image and determining the presence and degree of surface defects, and an output unit (printer, monitor, etc.) (4) for outputting the processing result. ing.
In the figure, the straight line X indicates the steel strip width direction axis (direction perpendicular to the steel strip transport direction Y), and the straight line Z indicates the scanning zone and scanning direction of the steel strip surface by the line camera (2). The scanning line Z is inclined with respect to the steel strip width direction X by a preset angle θ.

  The light source (1) is installed at a position displaced with an angle θ (corresponding to the inclination angle θ of the scanning line Z) with respect to the steel strip transport direction Y so as to uniformly illuminate the entire width of the scanning line Z. Yes. As the light source (1), a halogen lamp that emits continuous light, a high-frequency fluorescent lamp that emits light repeatedly at high speed, a high-intensity fluorescent lamp, or the like is used. If the tubular light source (1) extending in the scanning line Z direction of the steel strip is used as the light source (1) as shown in FIG. 5, the illumination over the entire width of the scanning line Z of the steel strip is made more uniform. be able to.

  The line camera (2) is deviated from the steel strip transport direction Y by an angle θ corresponding to the inclination of the scanning line Z on the steel strip surface, and faces the light source (1) to capture the entire width of the scanning line Z. It is installed at a position where a corner can be obtained. As the line camera (2), an industrial television camera (CCD camera) having a charge coupled device as an imaging device is preferably used. The CCD camera is suitable for detecting color shading and shading on the surface of the object to be inspected, and is advantageous in terms of maintenance, etc. due to its small size and light weight.

3 and 4 show other examples of the arrangement of the light source (1) and the line camera (2) in the present invention. This is done by arranging multiple line cameras (2) (three cameras 2 ₁ , 2 ₂ , 2 _{3 in the} figure) and dividing the field of view of the scanning line Z on the steel strip surface into approximately three equal parts. Scanning by the cameras (2 ₁ ) (2 ₂ ) (2 ₃ ) is performed. The imaging field of view between adjacent line cameras is given an appropriate lapping margin (for example, 20-30 mm in the scanning line Z direction). The configuration of the image signal processing unit (3) and the output unit (4) is the same as that of FIG.
If the scanning line Z is divided into a plurality of areas and scanned by a plurality of line cameras (2 ₁ ) (2 ₂ ) (2 ₃ ) in this way, the distortion of the image signal taken into the line camera is reduced, and the camera scanning direction is reduced. Pixel resolution is improved and surface defect detection accuracy is increased.

  The inclination angle θ of the line camera (2) disposed obliquely is suitably in the range of about 5 to 20 °. Specifically, the scanning frequency of the line camera (2), the line speed (steel strip transfer speed), etc. It is set appropriately according to The light projection angle (incident angle) of the light source (1) with respect to the steel strip surface and the light reception angle (reflection angle) of the line camera (2) for capturing an optical image are, for example, 10 to 20 °. As the light reception of the reflected light, regular reflection light reception (light projection angle = light reception angle) or irregular reflection light reception (light projection angle ≠ light reception angle) is appropriately adopted.

  The optical image of the steel strip surface picked up by the line camera (2) is inputted as an electric signal to the image signal processing section (3). In the image signal processing unit (3), predetermined processing of image analysis, for example, smoothing (shading correction to remove the influence of base noise (a signal generated randomly due to the surface roughness, gloss, and other factors of the steel strip) ) Processing, processing for correcting the deviation of the signal strength in the width direction, binarization processing using a preset threshold value, and the like are performed, and the presence or absence and the type and grade (size, etc.) of the surface defect are determined. The signal processing may be performed by appropriately adopting various known methods, and the determination result is notified to the operator through a printout by a printer, display of a monitor screen, and the like in the output unit (4).

The surface defect detection apparatus of FIG.1 and FIG.2 was installed in the transfer line of the cold rolled steel strip, and the surface flaw was detected under the continuous transfer of the steel strip.
[1] Detection conditions
(1) Steel strip width size: 1400mm
(2) Steel strip transfer speed: 132m / min
(3) Inclination angle θ of scanning line Z: 15 °
(4) Light source: Halogen lamp (voltage max. 5V)
Emitting angle: 15 ° Distance: 300mm
(5) Line camera: CCD camera (pixel: 2048, scanning frequency: 20MHz)
Light reception angle: 15 ° Distance: 1000mm

[2] Detection result The detection signal from the surface wrinkle inspection is clear, and a wire wrinkle in the width direction of the steel strip called “Folding Wrinkle”, which is difficult with the conventional detection method, is about 0.3 mm in size. It was possible to detect even certain things reliably.
In addition, the detection accuracy of the surface defect differs depending on the size of the surface defect to be detected, the number of pixels of the line camera, and the line speed (steel strip transfer speed). In the above embodiment, as a result of the oblique scanning of the line camera, a fine surface flaw up to a size of about 0.3 mm is detected with high accuracy. However, for a finer surface flaw, higher speed (line speed) is targeted. In the case of performing detection with the above, good detection accuracy can be obtained by adopting, for example, a line camera having a high scanning frequency.

  According to the present invention, it is possible to accurately detect fine line wrinkles in the width direction that are difficult to detect on the surface of the steel strip, and contribute greatly to appropriate management of the steel strip manufacturing process, maintenance of product steel strip quality, and the like. Is. Moreover, the defect detection can be performed with a simple configuration in which the light source and the line camera are arranged at an angle without requiring any special equipment or cumbersome processing operation. It is what has.

It is a front view which shows the arrangement configuration of the detection apparatus of this invention. FIG. 2 is an overhead view of the arrangement configuration of FIG. 1. It is a front view which shows the other arrangement structure of the detection apparatus of this invention. FIG. 4 is an overhead view of the arrangement configuration of FIG. 3. It is a front view which shows the other arrangement structure of the detection apparatus of this invention. It is a front view which shows the arrangement configuration of the conventional common detection apparatus. FIG. 7 is an overhead view of the arrangement configuration of FIG. 6.

Explanation of symbols

1: Light source 2: Line camera 3: Image signal processing unit 4: Output unit S: Steel strip Y: Steel strip transport direction X: Axis Z indicating steel strip width direction Z: Scanning line

Claims (2)

  Illumination light is projected over the entire width of the surface of the steel strip traveling in a predetermined direction, an optical image of the entire width of the irradiated steel strip surface is captured with a line camera, and the captured optical image signal is processed and the steel is processed. In the method of detecting the surface defect of the strip, the steel strip surface is irradiated in the direction inclined with respect to the steel strip transfer direction, and the surface of the steel strip is scanned by a line camera installed in the direction orthogonal to the inclination direction. A method for detecting a surface defect of a steel strip, comprising imaging.   A light source that illuminates the entire width of the surface of the steel strip traveling in a predetermined direction, a line camera that captures an optical image with the entire width of the irradiated steel strip surface within the field of view, and an image signal captured by the line camera In a defect detection apparatus including a signal processing unit that detects surface defects by processing and an output unit that outputs the signal processing result, the light source and the line camera are arranged to face each other in a direction inclined with respect to a steel strip transfer direction. In addition, the steel strip surface defect detection apparatus is configured to scan by installing a line camera in a direction orthogonal to the inclination direction.

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