A machine vision system for laser welding of polymers

The development of innovative laser based manufacturing techniques for polymer welding represents a significant advance in industrial manufacturing, in particular compared to other classical joining methods. Among the most relevant benefits can be mentioned: the increase of process repetitiveness, the facility to implement on-line process control systems, the reduction of the heat affected zone and the implementation of non-contact joining technologies in industrial manufacturing. The introduction of the laser scanner into manufacturing processes could facilitate the increase of productivity, as well as to combine design flexibility and high speed. However, a precise automatic positioning of the laser beam relative to the workpiece is still a challenge. In this work, we present a machine vision system for scanner-based laser welding with automatic calibration that allows precise welding of polymers. With this system we intend to reduce the complexity of the clamping devices, allowing some uncertainty in the plastic parts position that is corrected by machine vision techniques. This system was implemented for two types of camera configuration: external camera and coaxial camera. In the external configuration, the camera is positioned next to the scanner and takes an image of the whole workspace by a single camera shot. In the coaxial camera configuration, the camera view is related to the laser beam driving system and thus the camera image shows a neighbourhood of the scanner position. The positioning accuracy of the system was experimentally determined for white lighting and monochromatic red lighting. The obtained results show that the coaxial configuration allows an accuracy of some hundredths of millimetre while the accuracy for the external camera is in the order of tenths of millimetre.