CALIBRATION AND VALIDATION OF A STEREO CAMERA SYSTEM AUGMENTED WITH A LONG-WAVE INFRARED MODULE TO MONITOR ULTRASONIC WELDING OF THERMOPLASTICS

Abstract. Ultrasonic welding of thermoplastics has become an important industrial manufacturing process in the fields of aerospace and transportation. High quality standards are demanded and a reliable quality assessment routine is fundamental. Most on-site and off-site approaches are time consuming or require additional active illumination. Especially temperature models have proven to be good indicators for welding quality. We present a methodology to measure the surface temperature in real-time and visualize it simultaneously as a 3D model. Through augmenting a stereo camera system with an additional passive thermal infrared camera, we are able to map the heat data of multiple successive welds of large, free-form structures into a common 3D data representation. A challenging calibration approach is used to derive the inner and exterior orientation for the trifocal camera system. Geometric and radiometric improvements for an aluminium chessboard allow the usage of wide-angle optics for the thermal infrared camera. Consequently, we verify the quality of each camera by means of their resolving power. Therefore, a Siemens star test pattern is used for the thermal camera as well. We demonstrate the effectiveness of our methodology on a robot-guided ultrasonic welding tool.