The image processing specialist Bi-Ber has developed a new optical inspection system for a chocolate manufacturer. In this application, packaging is removed from discarded chocolate bars and scraps so that the chocolate can be recycled by animal fodder manufacturers. The hygienically designed inspection system is specifically constructed for the detection of reflective surfaces and can identify both metal foil and plastic films.
The inspection system consists of one stainless-steel control cabinet with a touch panel PC and another stainless-steel housing for lighting and camera. The latter in particular is tailor-made for the production line where it is installed directly above a slightly sloped conveyor belt, with the camera oriented directly at the conveyor. The camera uses a Sony polarisation sensor with a grid of on-chip polarisation filters – a quarter each of the sensor pixels are sensitive only to light waves oscillating in one direction. To reduce interference caused by unpolarised light and show up reflective surfaces as clearly as possible, Bi-Ber also fitted the light sources with polarisation filters. The camera captures three images of a conveyor band position at a time, with alternating illumination from above, from right & left and from the front & back. The polarisation direction of each LED module pair corresponds to one polarisation direction of the Sony sensor. Polarised light is reflected onto the sensor from pieces of foil, while the matt surfaces of the chocolate and the conveyor belt reflect it unpolarised so that they are very poorly exposed in comparison. Due to alternating illumination from different angles, any pieces of foil are captured regardless of their orientation. The software combines the three individual images into one resulting image and marks bright pixels (binarisation). In addition, the system captures each position on the conveyor three times, so that no piece of film goes undetected even in unfavourable positions.
Made of stainless steel and suitable for steam-jet cleaning, the inspection system meets the high hygienic standards of food applications. Water can drain off easily. The PC cabinet doors are sealed, compliant with IP67. The LED modules also comply with protection class IP67 and are fully enclosed individually.
Conventional inspection methods have limited use for detecting foil mixed in with discarded chocolate. Human-eye inspection, which used to be employed in this factory, realistically only allowed random checks. Metal detectors identify metal foil very reliably, but not plastic film. Air blowers can be used to remove pieces of foil loosely mixed in with the scrap, but do not pick up packaging that is still wrapped around products. To overcome these challenges, Bi-Ber made use of the reflective properties of foil and has developed an inspection system based on a polarisation camera that reliably detects foil and ensures 100 % monitoring.
