Method and an apparatus for the inspection of the surface of a tire

An apparatus for the inspection of the surface (1) of a tire (2) or the like comprises a projector (3) for the projection of a pattern onto the tire (2) and a camera (4) for the taking of an image of the pattern projected onto the tire. A simplified apparatus of this type comprises an apparatus for the rotation of the tire (2) about its axis (5).

The invention relates to a method for the inspection of the surface of a tire or the like in which a pattern is projected onto the tire and an image is taken of the pattern projected onto the tire. The invention furthermore relates to an apparatus for the inspection of the surface of a tire or the like, in particular to an apparatus to carry out the method for the inspection of the surface of a tire or the like comprising a projector for the projection of a pattern onto the tire and a camera for the taking of an image of the pattern projected onto the tire. The invention is suitable for the inspection of the surface of tires of all types, in particular of passenger car tires, truck tires or tires of other vehicles or aircraft. It is furthermore suitable for the inspection of the surface of other rotationally symmetrical objects.

Defective positions in the substructure of tires represent a substantial safety risk. Since defective positions of this type are not visible from the outside or are only visible from the outside with difficulty, tire manufacturers and tire retreaders are dependent on reliable inspection methods for the recognition of defects of this type.

A known practice consists of carrying out a pressure inspection of the tires at a very elevated tire pressure and to make a tactile examination of the sidewalls of the tires manually. The local strength of the tire changes at defective positions of the sub-structure, whereby the corresponding positions of the side wall stretch more under pressure strain than regions with an intact substructure. Trained personnel can feel the light bulges on the sidewall of the tire which arise in this manner by tactile examination.

This manual determination of the defective positions involves a very high risk of accident for the inspecting personnel, however, since the tires standing under high pressure or very high pressure can burst during the inspection. A further disadvantage is the poor defect detection in regions in which the sidewall is not smooth, for example due to stamped lettering.

Further inspection methods are known in the prior art in which the shape of the tire is measured at different tire pressures. DE 100 19 387 C2 discloses a method for the examination of tires in which the inner pressure of the tire is changed and the shape change of the tire caused by the change in the inner pressure is determined. Those shape deviations caused by the change in pressure can be determined from at least two shape data sets which indicate defective positions in the substructure of the tire. It is, however, necessary to digitize the tire sidewalls with shape detection systems such as stripe projection systems a multiple of times and subsequently to extract the defects from the data sets.

The method known from EP 0 823 623 A1 is based on interferometric shearography. In this method, images, namely shearograms, are taken at different tire pressures. The gradient difference is determined from the different images. However, due to the very high measurement sensitivity of shearography, this method can only be used with low pressure differences. Furthermore, it is also necessary with this method to measure the shape of the tire at different tire pressures.

It is the object of the invention to provide a simplified method and a simplified apparatus of the initially recited type.

In a method of the initially recited type, this object is solved by the features of claim1. The tire is rotated about its axis, a further image is taken of the pattern projected onto the tire and the images are compared with one another. It is possible in this way to obtain a clear representation of the defects.

A stripe pattern is preferably projected onto the tire.

It is furthermore advantageous to form a difference image from the images.

In an apparatus of the initially recited type, the object underlying the invention is solved by the features of claim4. The apparatus includes an apparatus for the rotation of the tire about its axis.

Advantageous further developments are described in the dependent claims.

It is advantageous for the projector to be suitable for the projection of a stripe pattern onto the tire.

A further advantageous further development is characterized by an image processing device, which can be formed, for example, by a PC or other computer, to compare two images taken at different position of rotation of the tire. The image processing device preferably forms a difference image from two images taken at different positions of rotation of the tire.

In accordance with a further advantageous further development, the projector comprises a flashlight source. The flashlight source is preferably synchronized with the apparatus for the rotation of the tire and with the camera.

In accordance with a further advantageous further development, the projector projects the pattern onto a partial surface of the tire. It is, however, also possible for the projector to project the pattern onto the total tire area. It is possible that the camera takes images of a partial surface of the tire. This can be the same partial surface onto which the projector projects the pattern. It is, however, also possible for the camera to take images of the total tire surface. In this case, it is advantageous for the projector to project the pattern onto the total tire surface.

In accordance with a further advantageous further development, the apparatus comprises a plurality of projectors and/or a plurality of cameras. It is of particular advantage for the apparatus to respectively comprise at least one projector and at least one camera on two sides of the tire. In this case, both sides of the tire can be inspected simultaneously.

A further advantageous further development is characterized by one or more adjustment devices for the adjustment of the projector or of the projectors and/or of the camera or cameras.

The method in accordance with the invention and the apparatus in accordance with the invention are based on a combination of the obligatory pressure inspection and on the measurement of the sidewall surface or sidewall surfaces at a pressure kept constant. In this process, the global shape of the tire is utilized to obtain a clear representation of the defects. The surface deformations occurring due to pressure strain at defective positions can be determined very fast and simply with the method in accordance with the invention and with the apparatus in accordance with the invention, without having to expose the tire to an additional strain by a change in pressure. Since the tire does not have to be exposed to any pressure change, it is furthermore generally possible to carry out the method faster.

The method in accordance with the invention and the apparatus in accordance with the invention can be used with all rotationally symmetrical bodies on whose surfaces defective positions have to be determined. It can in particular be used with all rotationally symmetrical elastic hollow bodies which are charged with pressure or which can be charged with pressure for the purpose of the inspection. The invention can, however, also be used with any other geometrical bodies in order to identify bulges or indentations or similar defects.

The apparatus shown schematically inFIG. 1in a view from above for the inspection of the surface1, namely the sidewall, of a tire2mounted on a rim and standing under pressure includes a projector3for the projection of a stripe pattern onto the surface1of the tire2and a camera4which includes an optical taking device and a CCD sensor for the taking of an image of the stripe pattern projected onto the surface1of the tire2. The tire2can be rotated about its axis5, namely its running axis, by a further apparatus (not shown in the drawing). The axis5extends in the horizontal direction.

The plane constructed by the projection beam6and the taking beam7likewise extends horizontally. The axis5is a component of this plane. The projector3and the camera4are mounted at the ends of a rail8which is fixed to the housing and which extends in a horizontal plane perpendicular to the axis5. The taking beam7of the camera4is incident onto the surface1of the tire2in a perpendicular manner. The projection beam6is incident onto the surface1of the tire2at an angle of approximately 30 to 45° (other angles are also possible). As can be seen fromFIG. 1, the projector3only projects a stripe pattern onto a partial surface of the sidewall of the tire2. The camera4takes images of this partial surface.

The apparatus furthermore comprises an image processing device (not shown in the drawing) which is in particular realized by a corresponding program on a PC or other computer. The image processing device compares two images of the camera4taken at different positions of rotation of the tire2, in particular in that it forms a difference image from two images taken at different positions of rotation of the tire2. This procedure is then repeated for a plurality of positions of rotation of the tire2preferably sequential to one another.

The projector3is furthermore equipped with a flashlight source (not shown in the drawing) which is synchronized with the camera4and the angle of rotation of the tire2about the axis5. In this manner, images can be taken in a very brief time, for example in 1/1000th of a second or even shorter times. The images can be taken when the tire2is at a standstill. It is, however, also possible to have the tire2rotate continuously about the axis5. The tire2should rotates “slowly” in relation to the shutter time of the camera4and/or to the Illumination period of a flash of the flashlight source. The rotational speed of the tire2about the axis5should therefore be sufficiently low in relation to the exposure time of the camera4and/or to the illumination period of a flash of the flashlight source in order to obtain images which can be evaluated.

The apparatus for the inspection of the surface1of the tire2consists, as shown inFIG. 1, of a stripe projection system which is mounted in a pressure inspection bench and consists of a stripe projector3and a camera4with connected image processing, with the triangulation plane of the stripe projection system expediently being arranged, as shown, radially to the tire2. In the embodiment, the arrangement is aligned to a sector of the tire sidewall. It can, however, also be aligned onto the total tire sidewall.

The defect recognition takes place in a manner such that two images of the surface1of the tire2are taken, with the fire2being rotated by a small angle in its axis5between the image taking. The relative shape of the surface1of the tire2is measured with each of the two images in the observed image region (with a suitable calibration of the camera4it is also possible to obtain absolute shape data). When the two images taken with a slightly rotated tire are compared, it is found that the global shape is similar in both images since the tire2is rotationally symmetrical, but that clear shape deviations result at defect positions.

The pictorial difference of the two images taken thus represents the difference of the relative (or absolute) shape of the surface1of the tire2and approximately corresponds to the representation of the local shape change known from shearography.

An example is shown inFIG. 2. The upper curve9shows a section through the first image. A first amplitude10is generated by a first defect position; a second amplitude11is generated by a second defect position.

The middle line12shows a section through the second image on whose taking the tire2was rotated slightly about its axis5. Accordingly, the amplitudes10′,11′ are displaced slightly to the right with respect to the associated amplitudes10,11.

The lower line13shows the difference result, with the values of the middle line12having been subtracted from the values of the upper line9. The deflections of the difference amplitudes10″ and11″ are greater than the deflections of the amplitudes10,11and10′,11′. The defective positions on the surface1of the tire2can hereby be reliably recognized.

FIG. 3shows a comparison image taken in this manner, namely a difference image, of the sidewall of a tire. The defect position14is clearly visible and undoubtedly recognizable. It corresponds to a bulge or to an indentation.

The difference image representation in accordance withFIG. 3can still be evaluated quantitatively by a suitable image processing and/or be stored as an inspection record. The sensitivity of the measurement process can be set in a simple manner by a change of the angle of rotation of the tire between the taking of the images.

For the simultaneous measurement of both sidewalls of the tire2, a plurality of projectors and cameras can also be used, in particular two projectors and cameras disposed opposite one another. An embodiment is shown inFIG. 4. The arrangement is symmetrical to the tire2. In addition to the components already present in the embodiment in accordance withFIG. 1, namely the projector3and the camera4, a further projector3′ and a further camera4′ are present on the other side of the tire2by which the oppositely disposed surface1′, that is the other side wall, of the tire2is inspected. Since the setup is symmetrical, it does not have to be described again in detail.

In the embodiment in accordance withFIG. 4, the projectors3,3′ and the cameras4,4′ are each adjustable. The adjustability of the projector3and of the camera4is described in the following. The projector3′ and the camera4′ are adjustable in a corresponding manner.

The rail8is fastened approximately at its center to a feed axle15. The rail can be displaced in the direction of the double arrow16, that is in the direction of the axis5, by an actuation of the feed axle15. An adjustment in this direction serves the adaptation of the projector3and of the camera4to different tire widths. The projector3and the camera4are moved so far along the feed axle15in the direction of the double arrow16until the surface1of the tire is focused.

The rail8can furthermore be fed in the direction of the double arrow17. As the rail8moves, the projector3fixedly mounted thereon and the camera4fixedly mounted thereon move. By a feed in the direction of the double arrow17, that is in a horizontal plane in a direction perpendicular to the axis5, the position of the projector3and the camera4can be adapted to different tire diameters.

Instead of the feed possibility in the direction of the double arrow17, a feed possibility in a direction perpendicular to the image plane can also be realized.