Optical surface inspection

So-called chatter marks on rolled surfaces are an unwanted but frequently occurring phenomenon. This shading occurs perpendicular to the direction of manufacture but has an amplitude of just a few tenths of a micrometre. Nevertheless, due to their regularity, these marks are visually disturbing. Chatter marks have to be measured very precisely in order to pinpoint process disruption, which is quite difficult to do. If the occurring wavelengths are known, the source and therefore rectifying the cause of the chatter marks becomes much easier. However, there was no suitable method for precise measurements  that could provide the required results quickly and reliably. The mobile RC Sensor measuring system enables the measurement of chatter marks.

Only one sample, at least DIN A5 in size from the desired process section, is required for  measurements. This is placed under the RC Sensor system and the measurement is started:

A sine-shaped light/dark pattern alternating in its position is produced on a TFT display. A camera records the image reflected from the surface of the measuring object and passes the data to a PC for evaluation. The recorded mirror images are further processed in the computer in several CPU-intensive operations. Finally, distortions are evaluated which are caused by curvature changes of the surface. Therefore, even chatter marks that have amplitude of less than 1µm can be made visible on the display.

A significant benefit of this process is the two-dimensional measurement of the surface. By averaging the raw information for each column of the recorded image, local faults such as craters and possible inclusions in the film can be filtered or smoothed out before further analysis is undertaken.