For the hardening process, screws are transported through the hardening oven on a metal conveyor belt. The high temperatures make the belt surface corrode over time. Due to the continuous wear, the meshes become enlarged, and screws could get caught in the enlarged meshes, pass through the oven multiple times, and be hardened excessively. Excessively hardened, faulty screws cannot be sorted out anymore at a later point in time. This could lead to critical subsequent faults e.g. in body construction (potential recalls). It is the task of the image processing system to monitor the mesh sizes and to warn the user before the mesh size exceeds the critical limit. Strong heat and high dust levels pose a real challenge for the image-processing system.
The conveyor belt control makes sure that the bulk goods are not exposed to excessive hardening.
Fig. 1: Screws passing through the hardening oven on a belt
On one side of the conveyor belt, a line-scan camera is installed; on the opposite side, a line lighting is installed. With this inspection concept, the meshes can be viewed by transmitted light. The camera displays holes as white objects. An encoder that is connected to the conveyor belt via a wheel generates a position-independent line trigger for the camera, independently from the speed of the belt. Images with a length of 12 cm are captured. The correct installation position of the lighting and a sufficient distance between camera and conveyor belt are crucial for the inspection. The dust-tight housing protects the camera from dust and dirt, while the arising trapped heat may be reduced by installing a fan. The inspection concept of SAC GmbH makes it possible to monitor the mesh sizes continuously. Thus, the user is always informed about the current quality grade of the conveyor belt and is warned in time as soon as the mesh size reaches a critical value. The position and the image of the last error are displayed so that the belt can also be repaired at this position.
The conveyor belt is assigned one of four quality grades:
If holes of a settable critical hole size are detected, the belt is immediately rated “bad”.