GB2218199A - Method and apparatus for inspecting a smooth surface - Google Patents

Abstract

A smooth surface e.g. a motor body panel (12) is inspected for surface contour imperfections by traversing an illuminating beam (15) directed at an angle to the perpendicular to the surface and sensing light from the illuminated surface part along an optical path inclined oppositely to the beam and noting changes in the sensed light, during the traversing, due to changing contours of an imperfection. A line-scan camera (17) may be used for the sensing and the image may be analysed automatically. <IMAGE>

Description

METHOD AND APPARATUS FOR INSPECTING A SMOOTH SURFACE This invention relates to inspecting smooth surfaces.

So-called machine vision techniques are well known for fault detection and are documented inter alia in GB-A-2,107,858, EP-A20,058,028, US-4,221,297 and GB-A-1,298,953. Such techniques have been applied in the detection of faults such as black spots in potatoes and fruit blemishes, dyeing, weaving and knitting faults and imperfections in the emulsion of photographic film.

Essentially, such faults have been "visible" in the sense that they involve a colour or reflectivity imperfection. In many instances, the subject of the inspection is a travelling web - a textile or paper or a film - and the problem has been to image and analyse the image at speeds commensurate with the production or processing speed of the travelling web. In cases where objects e.g. potatoes have been conveyed past the imaging means, they have been effectively "stopped" by strobing techniques.

None of the prior art methods has been suitable for the detection of contour imperfections in smooth surfaces such as surfaces of pressed metal sheet parts for e.g. automobile panels. Such defects tend to be small and show no colour or refectivity change in most illumination conditions.

The present invention solves the problem of detecting these contour imperfections.

The invention comprises a method for inspecting a smooth surface for surface contour imperfections comprising relatively traversing the surface and an illuminating beam directed at an angle to the perpendicular to the surface and sensing light from the part of the surface illuminated by the beam along an optical path inclined oppositely to the beam with respect to the said perpendicular, and noting changes in the light sensed, as the surface and beam are relatively traversed due to the changing contour at an imperfection.

The light may be sensed by a line scan camera.

The surface and the illuminating beam may be relatively indexed while the line scan camera images a strip of the surfaces perpendicular to the relative traversing direction. The strip images thus formed may then be built up into an area image. This area image may be electronically modified and analysed automatically for fault detection.

The optical path and the beam may be inclined at substantially equal but opposite angles to the perpendicular to the surface.

The method may be adapted to inspecting manufactured articles and be connected to the manufacturing operation to raise an alarm or stop the operation on fault detection. In particular, the method may be adapted to the inspection of pressed metal articles, such as automobile panels, for contour imperfections.

The invention also comprises apparatus for inspecting a smooth surface for surface contour imperfections comprising traversing means relatively traversing the surface and illuminating means therefore directing a beam at an angle to the perpendicular to the surface and light sensing means sensing light from the part of the surface illuminated by the beam, said sensing means having an optical axis inclined oppositely to the beam with respect to the said perpendicular and being adapted to note changes in the light sensed as the surface and beam are relatively traversed.

The illuminating means and the sensing means (e.g. line scan camera) may be fixed while indexing means traverse the surface. The apparatus may comprise a bed for an article whose surface is to be inspected, and indexing means, which may include a stepper motor, traversing the bed by discrete steps.

The apparatus may comprise area image forming means building up an area image from a succession of line images, such for example as a frame store device in which a digitalised image is stored in random access memory.

The problem of inspecting smooth surfaces for contour defects is exacerbated by the fact that such surfaces as those of pressed metal for example for automobile panels may not be, and in general are not, plane surfaces, but convexly or concavely curved in two dimensions. This causes focussing problems for imaging devices especially when the apparatus must be adapted to inspecting articles of different shapes. This problem is alleviated by the use of a line scan arrangement building up a two dimensional image from a succession of strip images inasmuch as the focussing need only be considered along the strip, the traversing being adapted to bring successive strips into adequate focus.

One embodiment of apparatus and methods for inspecting smooth surfaces according to the invention will now be described with reference to the accompanying drawings, in which : Figure 1 is a diagrammatic side view of a surface with a contour defect and the arrangement of illuminating means and imaging means; Figure 2 is a plan view of an inspection apparatus according to the invention; Figure 3 is a side elevation of the apparatus illustrated in Figure 2; Figure 4 is a typical area image showing a contour defect; and Figure 5 is a diagrammatic illustration of a controlling arrangement for the apparatus and its connection to a manufacturing operation.

The drawings illustrate the inspection of a smooth surface 11 of a pressed metal article 12 e.g. an aluminium or steel sheet body panel for an automobile.

The object of the inspection is to detect surface contour imperfections such as bumps 13 and dimples 14 as shown (exaggerated) in cross-section in Figure 1. Such contour imperfections are difficult to see, yet can mar the appearance of, say, an automobile, by being apparent under some conditions of illumination and viewing.

Such imperfections, moreover, can indicate a press defect which, left uncorrected, can worsen and so ruin many body panels.

The method illustrated comprises relatively traversing the surface 11 and an illuminating beam 15 from a lamp 16 directed at an angle to the perpendicular P to the surface 11. As seen in Figure 3, the surface 11 may be curved and in speaking of the "perpendicular to the surface" regard has to be had to this.

Light is sensed from that part of the surface illuminated by the beam 15 by a line scan camera 17 directed along an optical path towards the part of the surface 11 illuminated by the beam 15. The optical path is equally and oppositely inclined to the beam 15 with respect to the perpendicular P.

The surface 11, being smooth, will exhibit specular reflection. A freshly pressed metal surface will be quite highly specularly reflective, while such surface painted with a gloss finish will also exhibit a degree of specular reflection. This specular reflection is utilised in the method according to the invention.

From a perfectly specular surface 11 the light from the lamp 16 is reflected specularly to the sensor 17. As the surface 11 moves in the direction of the arrow A relatively to the beam 15, a surface defect 13 or 14 crossing the beam 15 will alter the amount of light reaching the sensor 17. Even an imperfectly specularly reflecting surface will show this effect, albeit to a lesser extent than with the perfect such surface. The changes in the amount of light reflected to the sensor 17 can be detected and show up in the image as it is built up by the relative traversing of the surface 11 and the optical system.

A practical arrangement for the method is shown in Figures 2 and 3. The part 21, such as an automobile pressed panel straight from the press, is located on a traversing bed 22 which runs on tracks 23 pulled by chains or cords 24 driven on wheels or pulleys 25 from a geared stepping motor 26. The lamp 16 and the line scan camera 17 are set up above the bed 22 on adjustable mounts (not shown) and arranged in such a way that a required surface area is scanned by suitably traversing the bed 22 with the camera 17 in suitable focus and the lamp 16 directing its beam appropriately for the required specular reflection effect.

Of course, the surface 11 will not normally be flat, and indeed the panel 21 may have quite a complex shape, and not all of it may be scanned at once. It may be, for example, that not all areas of the panel 21 would be of interest, so that scanning three areas such as depicted at 31, 32 and 33 on Figure 3 might be sufficient. Auxillary optical systems of lamp and camera combinations might however be arranged to scan different parts of the panel 21, or the panel 21 might be reorientated on the bed 22 so as to bring different areas into play.

Such reorientation, the necessary traversing motion and the lamp and camera operation can of course be controlled by a robotics system such as are now becoming commonplace in industry. The arrangement can be set up to inspect a succession of like parts produced by a press, so that once set up to follow a predetermined sequence of operation, the arrangement can be left unattended to raise an alarm or stop the press whenever unacceptable contour defects are detected.

A typical contour defect produces an image (which may be displayed in the usual way on a CRT or other type of electronic display) as illustrated in Figure 4 in which the ring of reduced brightness against the level background is produced as a result of the light being specularly reflected away from the optical axis of the camera 17 by the angled sides of a bump or dimple. There might or might not be a bright central region depending on the nature of the defect.

The system will also, of course, show up scratches, wrinkles and surface roughness.

Some defects, if they do not indicate unacceptable press tool deterioration, might be repairable, as by local induction heating. The arrangement can be adapted to indicate the position of such defects and direct a repairing machine to them automatically, followed by a re-inspection.

A comprehensive arrangement is illustrated diagramatically in Figure 5.

An article detector 51 is activated by a signal from the press or by the presence of an article delivered therefrom and in turn actuates a positioning arrangement 52 which positions the article on the bed 22 of Figures 2 and 3 for the start of the inspecting operation. Once positioned, the stepper motor controller 53 is energised to index the article through the field of view of the camera 17. The lamp 16 can be switched on at the start of the operation and there may be built in a lamp failure detector that can terminate the operation or give a warning signal.

A signal from the controller 53 actuates the camera 17 to effect a new line scan. The output of the camera is passed to a frame store 54 in which data from successive line scans is stored, preferably in digital form.

Once then complete frame is built up, the image is electronically modified in a processor 55 as by predetermining a grey-scale level and changing lighter greys to white and darker greys to black, to maximise the contrast between defect and background. If desired, the camera can have several "takes" at each line before the bed 22 is indexed to the next position of the traverse, so that the average signal can be taken, in order to eliminate noise.

After processing, the area image can be displayed on a CRT or other display device 56. The digitised image can be analysed in an analyser 57 which can be programmed in accordance with known image analysis techniques (such as masking techniques, in which anticipated faults or defects are represented as masks which can be notionally fitted over the image to search for correspondence so that fault types can be identified and their positions noted.

Signals from the analyser 56 can be used to actuate an alarm 58, a press top 59 or a repairing machine 61, which can position an electrode over a bump or a dimple and reduce it by the application of induction heating.

Claims (18)

1. A method for inspecting a smooth surface for surface contour imperfections comprising relatively traversing the surface and an illuminating beam directedat an angle to the perpendicular to the surface and sensing light from the part of the surface illuminated by the beam along an optical path inclined oppositely to the beam with respect to the said perpendicular, and noting changes in the light sensed, as the surface and beam are relatively traversed due to the changing contour at an imperfection.

2. A method according to claim 1, in which the light is sensed by a line-scan camera.

3. A method according to claim 2, in which the surface and the illuminating beam are relatively indexed while the line-scan camera images a strip of the surface perpendicular to the relative traversing direction.

4. A method according to claim 3, in which the strip images are built up into an area image.

5. A method according to claim 4, in which the area image is electronically modified.

6. A method according to claim 4 or claim 5, in which the area image is analysed automatically for fault detection.

7. A method according to any one of claims 1 to 6, in which the said optical path and the beam are inclined at substantially equal but opposite angles to the perpendicular to the surface.

8. A method according to any one of claims 1 to 7, adapted to inspecting malufactured articles and being connected to the manufacturing operation to raise an alarm or stop the operation on fault detection.

9. A method according to any one of claims 1 to 8, adapted to inspecting pressed metal articles for contour imperfections.

10. Apparatus for inspecting a smooth surface for surface contour imperfections, comprising traversing means relatively traversing the surface and illuminating means therefore directing a beam at an angle to the perpendicular to the surface and light sensing means sensing light from the part of the surface illuminated by the beam, said sensing means having an optical axis inclined oppositely to the beam with respect to the said perpendicular and being adapted to note changes in the light sensed as the surface and beam are relatively traversed.

11. Apparatus according to claim 10, in which said sensing apparatus comprises a line scan camera.

12. Apparatus according to claim 10 or claim 11, in which the illuminating means and the sensing means are fixed and the surface is traversed.

13. Apparatus according to claim 12, comprising a bed for an article whose surface is to be inspected and indexing means traversing the bed by discrete steps.

14. Apparatus according to claim 13, said indexing means including a stepper motor.

15. Apparatus according to any one of claims 10 to 14, comprising area image forming means building up an area image from a succession of line images.

16. Apparatus according to claim 15, comprising image processing means adapted to process said area image to adapt it for automatic fault detection.

17. Apparatus according to claim 15 or claim 16, comprising image analysis means adapted to detect faults from said area image.

18. Apparatus according to any one of claims 10 to 17, adapted to inspecting pressed metal articles for contour imperfections by being arranged adjacent a press and to inspect articles produced by the press and to be interconnected with the press to halt press operations in the event of imperfections being detected.