Optical inline inspection detecting 3D defects on complex free-form surfaces in harsh production environments

Abstract

Abstract In today’s industrial manufacturing, quality monitoring is one of the key elements to ensure continuous improvement and to trigger innovations. Especially inline inspection presents considerable challenges to the underlying measurement system. Besides the – in most cases – already not easily satisfiable demands concerning resolution, measurement uncertainty, and cycle time, the robustness of the chosen solution within the overall production environment is an essential requirement for a successful operation in the field. Furthermore, a sufficient ease of operation for the production staff plays a crucial role for the acceptance of the measurement technology. In sheet metal forming, for example, vibrations, ambient light variations and short cycle times as well as oiling and surface variations of the specimen under test challenge and simultaneously limit currently available measurement systems in their application. Especially in automotive manufacturing, visionary design concepts increase the complexity of components with respect to shape and forming capability, leading to critical areas on the object prone to constrictions and cracks. Varying geometry and surface defect characteristics call for an automated 3D inspection to maintain constant product quality for each single part. In this paper, an applied method for the inline inspection of free-form metal surfaces is presented, which allows for quality monitoring even in a harsh environment. So-called inverse fringe projection in combination with single-sideband demodulation facilitates the inspection of free-form metal surfaces in 3D, based on a single camera image (‘single-shot’), which allows for measurement even in a vibrating environment. Only one single camera and a projector are needed, simplifying the integration into series production considerably. Calibration-free, fast setup possibilities enable flexible handling and maintenance in a short-cycled production. First inline tests in a press shop at BMW proof the capability of this method. As an example, constrictions with a depth of about 80 microns could be detected automatically during series production within the press.