Process and Device for Inspecting the Surface of a Material

The invention relates to a process for checking the surface of a material, in particular cast aluminum strip, by visual inspection of an image of the material surface provided by at least one camera monitoring the material and supplying the image to a screen via a video signal. The invention also relates to a device for this purpose.

Known so-called on-line quality control methods are carried out for example using magnetic scattering, ultra-sonics, induction currents, or also in a trivial manner by visual inspection of the surface of the material by an observer. Today considerable use is made of television so that the surface is imaged either as a whole or in part and displayed on one or more TV monitors. By freezing the image briefly stationary pictures are obtained of the moving material. By selecting TV cameras of specific spectral sensitivity (e.g. in the infra red range) and by filtering the incident light it is possible to optimize the contrast of the colored picture for ease of recognition of a fault by the observer. The linear scanning of a TV picture permits a computerized record of surface defects by comparison with standards; this can then be processed to register commands for sorting or control for treatment of the surface.

The advantages of the optical methods lie in their applicability to items at any elevated temperature. The optical method enables the avoidance of wear or damage to detectors by the moving item thanks to the large camera-object distance which can be selected. In additon, they have the ability to image the whole or the greater part of the surface of interest instead of the spot or linear inspection possible with all other methods.

On the other hand it is a disadvantage that this surface inspection is possible only at relatively slow strip speeds. At higher speeds only individual parts are displayed as a stationary picture, which then permits only partial inspection e.g. 20% of the total surface area.

The faster the movement of the strip, the poorer and less often are the flaws detected.

It is therefore an object of the invention to overcome these disadvantages and to develop a system of quality control which is simple and as economic as possible, and such that the cameras and TV monitor screens available up to now can be readily used.

This object is achieved by way of the invention whereby the connection between screen and camera is made only when a defect is detected on the material.

To this end the video signal is delimited within an upper and lower signal strength beyond which the video signal activates a trigger signal for a freely variable duration .tau.. Between two trigger signals the image of the defect is held on the monitor screen by means of an image storage facility such as a video-recorder. This storage facility also has such a large capacity that it can store one or more successive images. This method has the great advantage that only pictures of areas featuring flaws appear on the screen while defect-free lengths of strip do not generate any pictures. As a result no unnecessary demands are made of the concentration of the observer.

Furthermore, the material e.g. a cast strip can run at relatively high speed as a stationary picture is displayed only if and when a surface defect appears. As defects do not occur very frequently, all defects are, as a rule, correctly and accurately assessed. The picture remains displayed on the screen until the camera senses a new surface defect.

Further advantages, features and details of the invention are revealed in the following descrption of a preferred exemplified embodiment and with the help of drawings viz.,

FIG. 1: A schematic representaton of a facility for inspecting the surface of a material.

FIG. 2: A diagram of a video signal along with a diagram of a trigger signal corresponding to the video signal.

In accordance with the present invention, preferably the image storage facility has capacity for a plurality of successive pictures which can be called up on demand so that a new defect does not immediately cause the earlier defect image to be removed from the screen.

Furthermore, in accordance with the invention, a delay facility is provided between the trigger switch and the image storage device, by means of which an image displayed as a result of triggering of the system can be projected on a monitor at other downstream positions. This produces a displacement of the image on the monitor with respect to the point of origin of the defect.

Also, the trigger mechanism can be connected to a second camera by means of which an image displaced with respect to that taken by the first camera can be registered and fed to the image storage facility. This too considerably improves the recognition of defects and in particular the evaluaton of these.

A preferred device for performing the process according to the invention is such that a trigger mechanism and an image storage facility are provided between the screen and the camera.

With this arrangement it is possible to install, displaced with respect to the first camera, a second camera which takes a picture displaced with respect to that taken by the first camera.

Usefully a delay facility is provided between the trigger mechanism and the image storage facility.

The storage facility is usually of the FIFO (first in first out) type with a capacity for one or more successive images.

As a rule known inspection facilities already have a camera and a screen, therefore it is sufficient to install between these facilities a storage facility and a trigger mechanism along with a delay facility as an extra instrument.

Referring to the drawings, a video camera 1 televises the surface of a cast strip 2 and controls a trigger mechanism 3 which is connected to an image storage facility 5 via a switch 4. The storage facility 5 is preferably fitted with a selection device. The material stored by the storage facility 5 can be imaged on screen 6.

A delay facility 19 is provided between the trigger mechanism 3 and the storage facility 5.

Connected to the trigger mechanism is optionally a second video camera 7 to televise a part of the strip displaced with respect to that taken by the first camera 1.

As shown in FIG. 2 a video signal 10 occupies a range within an upper level 11 and a lower level 12. Defects on the cast strip 2 produce a deviation in the video signal 10 as a result of which the signal 10 enters the range beyond levels 11 or 12. In the present example the video signal 10 activates a trigger signal 14. At point 15 the video signal 10 exceeds the upper level 11. After a freely selectable time .tau. this produces the trigger impulse 16 which leads to the connection being made between camera 1 and image storage facility 5. This picture remains on display until, as a result of the lower level 12 being crossed at point 17, the command is given to record the next image via a second trigger impulse 18.