GB2126716A

GB2126716A - Automatic checking of surfaces

Info

Publication number: GB2126716A
Application number: GB08321411A
Authority: GB
Inventors: Dr Gerhard Benz; Eberhard Lohner; Gunter Schneider
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1982-09-04
Filing date: 1983-08-09
Publication date: 1984-03-28
Also published as: GB2126716B; GB8321411D0; JPS5960344A; DE3232885A1

Abstract

In the automatic checking of a workpiece surface 16, especially a precision treated surface, by means of visible laser light 11 with which the surface 16 is scanned linearly or by areas, the surface condition is evaluated from the reflected light 18 which strikes a light sensitive detector element Do in the bright field and from the scattered light which strikes a plurality of light sensitive detector elements D1-Dn which are arranged around the first detector element and in the same plane to detect the entire angular distribution of the light 18 in the dark field. Damage to the work piece surface 16 produces variations in the dark field or the light field. <IMAGE>

Description

SPECIFICATION A method for the automatic checking of surfaces State of the art The invention originates from a method for automatically checking surfaces according to the preamble to the main claim. The majority of the known methods and apparatuses of this kind operate in an optical manner in that the different reflection characteristic of the error location is detected predominantly with suitable detector systems such as light conducting arrangements with attached individual receivers, linear sensors or cameras by illuminating with incoherent white light in the bright field. The signal variation due to scratches and similar surface irregularities when measuring with such apparatuses generally amounts to only a small percentage so that an expensive signal processing is required wherein particularly longer period natural reflection deviations of the surface must be compensated.

Furthermore, it is known to use laser scanning devices for surface inspection particularly of strip material (for example sheet metal, paper, textiles) which either only permit surface checking in the bright field or in a limited region of the dark field.

Advantages of the invention As opposed to this, the method in accordance with the invention and the associated apparatus for the automatic checking of surfaces have the advantage that, with simple means, precise information relating to the location, the angular orientation and the extent of a surface error on a work piece can be derived. This is possible due to the simultaneous measurement in the bright field and in the dark field wherein the dark field measurement is carried out according to angular resolution. The light scattered by a surface error has a pronounced directional structure in particular, which is moreover aligned perpendicularly with respect to the angular position of the surface damage-when the scanning of the surface is carried out with coherent laser light of high light intensity and high local resolution.It is of particular advantage when the detector elements detect the diffraction image generated by the scanned surface through a lens produced by fourier transformation and which is displacement invariant with respect to the different positions of the laser beam on the surface. That means it is known that, independent of the insidence point of the laser beam on the work piece surface, similar surface damage (for example scratches of the same depth and angular orientation) generate the same diffraction pattern at the same location in the focal plane of the lens which is used.

When, the light scattered in the dark field is analysed for signal processing, surface recesses, such as bores and grooves, are automatically discriminated during the evaluation without expensive calculating operations. Possible edge truncations not regarded as errors can be detected by the symmetrical property of the light scattered by the work piece surface. Flaws, scratches and the like produce a dispersion lobe in the dark field which is symmetrical with respect to the bright field reflection. With edge truncations, the dispersion lobe is very strongly assymmetrical.

In addition to the dark field signal, the bright field signal can also be analysed either on its own or in combination with the dark field measurement. In that way, particular surface errors, such as cavities or cracks, can also be recognised or the general optical contrast can be still further increased.

With the method in accordance with the invention, it is possible to check surfaces with a high resolution factor without contact. The method can be undertaken with very little technical expenditure and in particular permits ari automation in a simple manner which provides considerable saving in costs particularly when checking mass produced products.

Advantageous further developments and improvements of the method set forth in the main claim and of the associated apparatus are made possible by the measures set forth in the sub claims. The method is preferable and easily carried out by using a semi-transparent mirror which facilitates on the one hand the passage of an oscillating beam over the surface of a stationary work piece and which on the other hand transmits the light reflected by the work piece surface to an evaluating device. In that way, the arrangement can be constructed particularly simply.

Moreover, it has proved to be preferable for the laser beam to strike the work piece surface to be checked sharply focused which is achieved by a focusing lens in the path of the rays of the insident laser light. In so doing, a laser beam which originally had a beam diameter of about 1 mm, is preferably focused in such a manner that its beam diameter then amounts to about 0.1 mm when arriving at the surface to be checked.

Further details and advantageous developments of the method in accordance with the invention and of the associated apparatus will become apparent from the sub claims in association with the following description.

Drawing The invention is illustrated diagrammatically in the drawing and is explained in more detail in the following description.

Figure 1 shows an embodiment with the aid of an arrangement for checking a stationary level work piece surface.

Description of the embodiment In Figure 1, a HeNe-Laser is referenced 10 the beam 11 of which strikes the surface 1 6 of a work piece 1 7 to be checked through a first lens 12, a mirror arrangement 13, a second lens system 14 and a semi-transparent mirror 1 5. The laser beam is reflected and scattered at the work piece surface 1 6. The reflected light represented diagrammatically at 18 passes through the semitransparent mirror 1 5 and strikes a third lens 1 9 and then a detector element arrangement 21 which is spaced from the third lens 1 9 by the amount of its focal length f3. If necessary, a crosssectional converter 20 can be arranged in front of the detector element 21 for providing segmentlike receiving surfaces.In that case, the lens 1 9 has the focal length f'3 so that the entry surface of the cross-sectional converter is likewise located in the focal plane.

The lenses 12, 14 and 19 can be individual lenses or lens systems. The mirror arrangement 13 consists either of a single mirror or the linear scanning of a work piece surface 1 6 or of a system comprising two or more mirrors by which the laser beam 11 is deflected in two different directions for scanning a level work piece surface 16 over an area. In the case where the mirror arrangement 13 consists of 2 movable mirrors, the lens combination 1 4 is preferably constructed from a spherical and a cylindrical lens 1 4a, b so that the combination 14 includes two different focal lengths f2 and f'2 for two successive perpendicularly arranged scannings of the lens aperture. The mirror which deflects the light beam parallel to the axis of the cylindrical lens 1 4b is then located at the focal point of the longer focal length f2.The mirror which deflects the light beam perpendicular to the axis of the cylindrical lens 1 4b is arranged at the distance of the shorter focal length f'2. The reflection-transmission ratio of the lens 1 5 preferably amounts to 50/50%. For reasons of clarity, the absorption of the nonevaluated light components is not discussed in detail with respect to the respect to the representation of the invention.

The detector element arrangement 21 is illustrated on the one hand in section and on the other hand in plan view in the embodiment of Figure 1. The plan view shows that around a central fixed detector element D,, a plurality of further detector elements D1 to Dn are arranged in a circle for detecting the surface scattering in accordance with degree and direction. Figure 2 shows a further embodiment in plan view wherein an outer circle is introduced around an inner circle consisting of an even or odd number of detector elements. Each of the detector elements in the outer circle are arranged in the spaces between the inner elements.

Figure 3 shows an arrangement in plan view with annularly segmented receiving surfaces D, to Dn. As an alternative to that, a cross-sectional converter 20 consisting of light conducting fibres can be inserted in front of the detector element proper in a more elaborate and expensive detector arrangement. The light entry area of the crosssectional converter corresponds to the plan view of the arrangement in Figure 3. The light exit area can be formed according to Figure 1 or Figure 2 or according to a suitable arrangement of photo detectors. With the proposed method, the surface to be checked with the laser beam 11 is scanned linearly or over areas wherein the mirror arrangement 13 serves for the controlled deflection of the beam.In so doing, it is preferably a question of two electrically controlled deflection units which produce a line deflection and a column deflection of the laser beam 11. In so doing, the laser beam is first of ail guided linearly over the surface 16 of the work piece 1 7 and at the end of the lines is displaced by a line distance so that an area scanning of the surface 1 6 is provided. The first lens 1 2 prevents, on the one hand, the previous divergence of the laser beam 11, moreover it focuses the laser beam in cooperation with the second lens system 1 4 so that the beam strikes the surface 1 6 of the work piece 1 7 focused substantially to one tenth of its original beam diameter.The lens system 14 produces a collimation of the light deflected by the mirror arrangement 1 3 so that the light strikes the work piece surface 1 6 directed parallel by the semi-transparent mirror 1 5. Thus, the laser beam 11 is collimated on the one hand by the lens system 1 2 and 1 4 and is focused on the other hand through the semi-transparent mirror 1 5 on the work piece surface 1 6 with a greater depth of focus. With the surface 1 6 illustrated, it is a question of a level surface of a stationary work piece. Curved surfaces require special measures.

With cylindrical external surfaces, the surface is scanned linearly along a surface line whilst the work piece is rotated about the cylindrical axis.

With through bores where the ratio of bore diameter to bore length is not too small, an analogue checking of the internal surface area is possible with the assistance of a deflecting mirror in front of the measuring device.

The light 1 8 reflected by the work piece surface 1 6 is received by a detector element arrangement 21 arranged at the rear focal plane of the lens 1 9. Since coherent laser light is used for scanning the work piece surface 16, a diffraction pattern, additionai to a low intensity variation of the directly reflected light, occurs due to surface damage, for example due to a scratch, perpendicular to the direction of the scratch. The Fourier transformation of this diffraction pattern appears in the focal plane at the distance f3 behind the lens 1 9 and the position of which is not altered on the work piece surface 1 6 with respect to a different position of a scratch having the same angular orientation. Thus, the reflection characteristics of the work piece surface 1 6 can be recorded with one fixed detector element D,, for example with a photo diode, at the focal point of the lens 1 9 and the surface scatter according to degree and direction can be recorded with further fixed detector elements D, to Dn arranged in one or more circles or in the form of a segmented annulus, independently of the respective scanning point on the surface 1 6.

Since the surface scatter is measured according to degree and direction, a surface texture caused by grinding tracks not considered as an error, can be eliminated in an electronic manner with the signal evaluation (not shown). With surface errors, such as lapping errors, scratches and the like, due to precision mechanical treatment or previously existing, are detected by the measuring method in accordance with the invention through the characteristic diffraction pattern of the error or through its Fourier transformation. Since bores or for example darkly coloured non-precision grooves reflect no or very little light in the dark field, such interruptions of the surface are automatically discriminated during the evaluation.

Edge bevels produce a different kind of light diffraction pattern than do scratches and can likewise be discriminated if necessary during the evaluation. Cavities and other dark locations on the surface 1 6 are recognised by the detector element Do due to the reflection received.

Claims

1. A method for the automatic checking of surfaces, especially of precision treated surfaces, with coherent laser light which is guided linearly or by areas over a work piece surface, wherein the light reflected by the surface is evaluated as a criterion of the surface condition, characterised in that, the reflected light (18) strikes a light sensitive detector element (Do) in the bright field on the one hand, and strikes a plurality of light sensitive detector elements (D, to Dn) arranged around the first detector element (Do) andin the same plane, on the other hand, which detect the entire angular distribution according to standard measuring techniques of the light reflected from the work piece surface (1 6) in the dark field.

2. A method according to claim 1, characterised in that, variations in the dark field of the reflected light (18) according to degree and direction are detected by the outer light sensitive detector elements (D, to Dn)

3. A method according to claim 1 or 2, characterised in that, the laser beam (11) and the reflected light (18) are conveyed to the work piece surface (16) and to the detector elements (D,, D, to D n) through a semi-transparent mirror (15).

4. A method according to one of the preceding claims, characterised in that; the light (1 8) reflected by a level surface (1 6) of a stationary work piece (17) is transmitted through a lens (19) onto light sensitive detector elements (D,, D, to Dn) arranged in the rearfocal plane (f3) of the lens.

5. A method according to one of the preceding claims, characterised in that, the laser beam (11) is focused and collimated by a lens system (12, 14) and strikes the work piece surface (1 6) sharply focused.

6. A method according to one of the preceding claims, characterised in that, the laser beam (11) is guided over the surface (16) of a stationary work piece (1 7) by at least one movable deflecting mirror (13).

7. Apparatus for carrying out the method according to one of the preceding claims, characterised in that, the detector element arrangement (21) for the light (1 8) reflected by the work piece surface (16) to be checked has a central detector element (Do) for detecting the bright field and a plurality of detector elements (D,Dn) arranged in a circle around the central detector element (Do) for detecting the dark field.

8. Apparatus according to claim 7, characterised in that, the detector elements (21 ) consists of a central detector element (Do) and a plurality of detector elements (D,Dn) arranged in two or more circles around the central detector element (D,).

9. Apparatus according to claim 7 or 8, characterised in that, the detector elements (D,Dn) are arranged around the central detector element (Do) in the form of segments of an annulus.

10. Apparatus according to claim 7, characterised in that, the detector elements (D,.

D,Dn) are formed by photo sensitive surfaces, particularly by photo diodes.

11. Apparatus for carrying out the method according to one of the preceding claims, characterised in that, a cross-sectional converter (20) of light conducting material is arranged in front of the detector element arrangement (21) and is provided with a light entry area in the form of a central element about which n annular segmented elements are arranged corresponding to the detector surfaces D,, D,--D,.

12. Apparatus according to claim 11, characterised in that, the round or rectangular light exit surfaces of the n annular segments of the cross-sectional converter (20) are arranged in one or more circles.

13. Apparatus according to one of claims 7 to 12, characterised by an arrangement consisting of two electrically controlled deflection mirrors (13) through which the laser beam (11) is guided line by line over the surface (16) of a level stationary work piece (17).

14. Apparatus according to one of claims 7 to 13, characterised in that, a mirror arrangement (13) and a lens combination (14) consisting of a spherical and a cylindrical lens (1 4a, b) are located in the path of the insident light beam (11) wherein each deflecting mirror (13) is arranged at one of the two focal points of the lens system.

1 5. Apparatus according to one of claims 7 to 14, characterised in that, a semi-transparent mirror (15) and a lens (19) are arranged in the path of the beam (1 8) between the work piece surface (16) and the detector element arrangement (21) the lens being spaced from the detector element arrangement (21) by the amount of its focal length (f3) or being spaced from the light entry surface of the cross-sectional converter (20) by the amount of its focal length (fit3).

16. A method for the automatic checking of surfaces substantially as herein described.

1 7. Apparatus for use in the automatic checking of surfaces, substantially as herein described with reference to Figure 1, Figure 2 or Figure 3 of the accompanying drawings.