GB2249169A

GB2249169A - Curved surface inspection method and apparatus using diffuse light

Info

Publication number: GB2249169A
Application number: GB9119372A
Authority: GB
Inventors: Shigetoshi Hyodo; Koji Okamura
Original assignee: Hajime Industries Ltd
Current assignee: Hajime Industries Ltd
Priority date: 1990-09-14
Filing date: 1991-09-11
Publication date: 1992-04-29
Also published as: FR2666884A1; JPH04122839A; CA2051032A1; DE4130217A1; AU8380591A; GB9119372D0

Description

1 22491 tS'

TITLE: SURFACE INSPECTION METHOD AND APPARATUS THEREOF BACKGROUND OF THE INVENTION Field of the Inventio

The present invention relates generally to a surface inspection method and apparatus thereof and more particularly, is directed to a surface inspection method and apparatus thereof for inspecting defect on the ground surfaces of steel tubes, steel bars or the like that are ground on the outer round surface thereof in the circumferential direction thereof. Description of the Prior Art

There are proposed optical inspection methods so as to automatically inspect flaws on the outer surface of a steel material. For example, in order to inspect the surface of a steel plate, high precision automatic inspection is conducted by scanning a laser beam on the surface of the steel material. However, although many proposals for the optical automatic inspection methods that apply to the outer round surface of steel tubes, steel bars or the like that have a circular cross section were made, hardly any method is placed on actual practice. The reason is that when the surface inspection is conducted by optical means for the outer round surfaces of steel tubes, bars or the like, its automatic inspection property Is disturbed by the ground traces of the steel tube, steel bar and so on or vibration thereof during the conveyance thereof.

In other words, the outer round surfaces of steel tubes, bars or the like that necessitate high accuracy optical surface inspections are normally subjected to grinding finish in the circumferential direction thereof. In attempting to detect subtle defects on the outer round surface by optical methods, it generally is the case that the very subtle ground traces on the 1 -2 outer round surface of the steel tube or the like in the circumferential direction thereof be detected together with the surface flaws or defects. Such ground trace will become a large external disturbance that will be clearly detected as higher the inspection device precision is. For such reason, it was impossible to conduct a high precisioned surface inspection on the outer round surfaces of steel tubes, bars with optical methods.

Further, in the case of inspecting the entire outer round surfaces of steel tubes, bars or the like, it is general that the materials such as steel tubes and so on are transferred in a spiral fashion. However, since the eccentricity and/or bend generally exist in steel tubes, bars or the like, the steel tube and so on will swing upon spiral conveyance which cause errouneous detection or overlooks of the flaws which disturb the automatic optical surface inspection for steel tubes, bar or the like.

Also, as for surface inspection methods of metal outer round surfaces by utilizing diffused lighting, there is a method as on Japanese Patent Publication Gazette No. 58-53861, which is a method where-the reflection light from an irradiated portion is input to a photosensing means via a frosted glass like projection plate and a slit. However, since in this method the reflected light from the irradiated portion is once projected on a projection plate of frosted glass and then picked up by the photosensing means, it cannot detect the extremely subtle flaws with high precision.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a surface inspection method and apparatus that avail the t --3- high precision optical surface inspection for the steel tube, bar or the like, which outer round surfaces are ground in the circumferential direction thereof, without the influence or disturbance of ground traces or vibration caused by eccentricity of the steel tube and so on.

According to an aspect of the present invention, there is provided a surface inspection method, comprising the steps of:

a) obliquely irradiating a portion of an outer round surface of an object to be inspected with a diffused light in a direction perpendicular to the center axis of said object, said outer round surface being ground in its circumferential direction; b) picking up an irradiated portion of said outer round surface of said object by optical photosensing means located along a path of light reflected on said irradiated portion; and c) processing a video signal from said optical photosensing means to thereby detect a flaw on said outer round surface of said object.

According to another aspect of the present invention, there is provided a surface inspection apparatus comprising:

a) a light source for emitting a light so as to obliquely irradiate an outer round surface of an object to be inspected in a direction perpendicular to the center axis of said object, said outer round surface being ground in its circumferential direction; b) a light diffuser means located in front of said light source and for diffusing the light from said light source and irradiating a portion of said outer round surface of said object uniformly; c) an optical photosensing means located along a path of light reflected on said irradiated portion and for picking up the L 4-- irradiate portion of said outer-round surface of said object to thereby produce a video signal; and d) a signal processing means for processing said video signal to detect a flaw on said outer round surface of said object.

BRIEF DESCRIPTION OF THE DRAWING

A better understanding of the objects, features and advantages of the present invention can be gained from a consideration of the following detailed description of the preferred embodiments thereof, in conjunction with the figure of the accompanying drawing, wherein;

Fig. 1 is a schematic diagram showing an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before describing embodiments of the present invention, a general aspect of the present invention will be explained.

For the surface inspection of an inspected object, it is generally the practice to irradiate the surface of the material with light, make photoelectric conversion of the reflected light with such optical photosensing means like ITV camera or the like and process the signal therefrom to detect flaws. When there is a flaw or minor defect as caused by a foreign particle on the inspected material surface, the light will be scattered when it hits the same. Accordingly, by eliminating generation of strong direct reflection light at the other portion surrounding the flaw or the like and detecting only scattered light, subtle flaws, defects or the like by foreign particles can be detected with high SIN ratio. However, in the case that the inspected material is metal material, a strong direct reflection light will generate when light directly hits the inspected material surface.

-15 Therefore, by applying a light with highly diffusing property and nondirectionality on the surface of the metal, the direct reflection light is suppressed so that the scattered light component can be easily extracted.

In the present invention, the light emitted from a light source is irradiated upon a light diffusion plate such as a milky white light diffusion board in order to diffuse the light and the light passed through the light diffusion board is irradiated upon the inspected object surface. This milky white light diffusion board will also diffuse light inside the board so that the entire surface of the milky white light diffusion board will become bright to function like a flat board light source. As such, a highly diffused light irradiation is made available and by lighting the inspected object surface therewith, it is possible to lowly suppress the surface refraction light from the inspected object surface as well as the direct reflection light from the ground trace.

As for the ground trace on the surface of the inspected object, since the light is irradiated thereon along the ground trace direction, there is no fear that the optical photosensing means will pickup the ground trace as shadows.

Further, the present invention is applied to the case that the inspected object has a round column shape, even though some minor vibration occurs in the inspected object at the tine of inspection, since it is being irradiated in a broad range by diffused light from the entire milky white light diffuser plate, the disturbances by the center axis vibration of the object can be prevented.

Further, as the inspected material surface is irradiated by the white light, the optical photosensing means will pick up the material surface that is irradiated in white, as a white bright surface. Generally, white coloured light is formed of a continuous spectrum without any enforcement nor deterioration of specific wave length light components so that it becomes the appropriate light source for irradiation in defect inspection on metal surface. And also when there is defective portion on the surface of the metal material, a light scattering occurs at such defective portion that reduces the white light components at that portion to become a shadow so that the optical photosensing means will pick up the same portion clearly as a black defective portion.

As for the milky white light diffusion board that is used at the surface inspection method and apparatus of the present invention, the milky white plastic plate that simulates opalglass or the like used as construction materials will suffice. While the light diffusion effects of milky white glass will be the same to the plastic plate, in the case of the present invention, from the apparatus construction stand point of breakproof, the plastic plate is better preferred to glass and is easier to handle. This type of milky white plate contains extremely minor white grains therein as a medium so that when the light passes the same, the light progressing direction is refracted into multi spacial directions and possess the feature to provide a very high degree of light diffusion. Further, when the irradiation light is a white light of continuous spectrum, it becomes an adequate lighting against the metal surfaces as described above.

An embodiment of the present invention shall be explained in reference with the drawing. Fig. 1 is a schematic diagram showing an embodiment of the present invention.

In this embodiment a steel tube 1 that is an inspected material, is spirally conveyed in the horizontal direction or in its center axis 0 direction.

Light source 2 is a stroboscope that uses, for example, a xenon lamp which irradiates an upper outer round surface of steel tube 1 from the above of the steel tube 1 in an inclined direction. An optical axis 2A of light source 2 is placed on a perpendicular plane in right angle to the center axis 0 of steel tube 1, while an angle between the optical axis 2A and a normal N at an incident point I at which the light from the light source 2 is incident on the outer round surface 1A of steel tube 1 (incident angle a) will be selected, for example, about 45 degrees. In front of light source 2, a light diffusion plate such as a milky white light diffusion plate 5 mentioned above is placed at a right angle to the optical axis 2A of light source 2.

A power source 3 for the light source 2 is switched on and off by a trigger signal delivered from a signal processor 4 in order to flash the light source 2 or the stroboscope.

An optical photosensing means 6 such as an ITV camera (CCD camera) is placed on a reflection light path OP in order to pick up an upper portion of the outer round surface 1A of steel tube 1 which is irradiated. A video signal from the optical photosensing means 6 is sent to the signal processor 4, and is memorized therein in sychronism. with the flash timing of light source 2. While the memorized video signal is supplied to and displayed on monitor 7, at signal processer 4, it is directly binary digitized or binary digitized after being analized by differentiating operation in order to emphasize the brightness difference, whereas the surface defect signal is delivered to an output terminal 8 based upon such brightness difference.

The surface inspection method of the present invention is 1 -- R practiced in the following manner by utilizing the above mentioned apparatus.

While the steel tube 1 is spirally conveyed, the upper portion of its outer round surface 1A is irradiated by the light source 2 or stroboscope. The light that irradiates the upper portion of outer round surface 1A of the steel tube 1 has passed through the milky white light diffuser plate 5, while it is irradiated in the direction perpendicular to the center axis 0 of steel tube 1, so that at the irradiated portion of steel tube 1, the ground traces that are formed on the outer surface 1A of steel tube 1 in the circumferential direction, which is perpendicular to the center axis 0 of steel tube 1, will disappear to show the good portion in white colour but flaws only will appear as dark shadows. Therefore, by photosensing the irradiated portion in a magnified fashion by the optical photosensing means 6, watching the image thereof by monitor 7, and processing the video signal thereof at the signal processer 4, the flaws can be accurately detected without causing the ground traces as external disturbance. At the same time, by the aforementioned reason, the influence as caused by vibration owing to steel tube 1 warp, etc. is also eliminated. As such, by repeating the strobe photosensing of the spirally conveyed steel tube 1, the entire outer round surface of steel tube 1 is surface-inspected with high precision.

To a steel tube with an outer diameter 6.5mm and wall thickness 0.5mm that was outer-round-surface-finished by PVA grinding method and applying the above mentioned method, surface flaws of 50 micron diameter and 20 micron depth were detected. In the case that the milky white light diffuser plate 5 was removed, by the scattering of the irradiated light on the ground C trace, many signals equal to the above surface flaws were detected.

Further, although the milky white light diffuser plate 5 was used, in the case that the outer round surface of the steel tube 1 was irradiated from the direction tilted to the center axis 0 of the steel tube 1, many ground trace shadows were detected.

Also, the optical photosensing means 6 used at this occasion was a CCD camera with a fixed 50 times magnification rate, the light source 2 was a xenon stroboscope, and the milky white light diffuse plate 5 was a plastic plate similar to opalglass used as construction material.

As it is apparent from the above explanation, according to the surface inspection method and apparatus of the present invention, the disturbances as caused by ground trace and warp etc., of the inspected material are removed so that it is possible to conduct the ground outer round surface inspection with high precision. Therefore, the automation of this type inspection that was difficult in the prior art is accomplished with the merits of magnificient reduction in the processing.

it should be understood that the above description is presented by way of example on the preferred embodiments of the invention and it will be apparent that many modifications and variations thereof could be effected by one with ordinary skill in the art without departing from the spirit and scope of the novel concepts of the invention so that the scope of the invention should be determined only by the appended claims.

Claims

CLAIMS 1. A surface inspection method, comprising the steps of:

a) obliquely irradiating a portion of an outer round surface of an object to be inspected with a diffused light in a direction perpendicular to the center axis of said object, said outer round surface being ground in its circumferential direction; b) picking up an irradiated portion of said outer round surface of said object by optical photosensing means located along a path of light reflected on said irradiated portion; and c) processing a video signal from said optical photosensing means to thereby detect a flaw on said outer round surface of said object.
2. A surface inspection method as claimed in claim 1, wherein said diffused light is a white light.
3. A surface inspection method as claimed in claim 1, wherein an incident angle of said diffused light on the outer round surface is about 45'.
4. A surface inspection apparatus comprising:

a) a light source for emitting a light so as to obliquely irradiate a portion of an outer round surface of an object to be inspected in a direction perpendicular to the center axis of said object, said outer round surface being ground in its circumferential direction; b) a light diffuser means located in front of said light source and for diffusing the light from said light source and irradiating the portion of said outer round surface of said object uniformly; c) an optical photosensing means located along a path of light reflected on said irradiated portion and for picking up the irradiate portion of said outer round surface of said object to 1 A 1 thereby produce a video signal; and d) a signal processing means for processing said video signal to detect a flaw on said outer round surface of said object.
5. A surface inspection apparatus according to claim 4 further comprising a power source which is controlled by said signal processing means to drive said light source.
6. A surface inspection apparatus according to claim 5 further comprising a monitor which receives the video signal from said signal processing means to display an image of said object thereon.
7. A surface inspection apparatus as claimed in claim 4, wherein said light diffuser means is a milky white light diffuser plate.
8. A surface inspection apparatus as claimed in claim 1, wherein said milky white light diffuser plate is made of plastics similar to opalglass.