Robot-based in-line measurement system for high-precision optical surface inspection

Abstract

Integration of metrology systems directly in the manufacturing process is seen as a key feature for future production lines to enhance product quality, manufacturing efficiency and to provide continuous information about manufactured goods. Fast, contactless inspection systems allow surface characterization, defect detection, dimensional inspection, and integration in industrial networks allows the adjustment of process parameters by utilizing measurement data. Industrial robots provide the necessary flexibility for the inspection of freeform surfaces and complex shapes. Optical inspection systems and sensors are a prefered choice due to their non-tactile character, submicrometre resolution and high measurement frequency but typically require a supplementary fine positioning system respectively actuation technique to perform a scanning motion. An additional positioning system also results as a consequence of limited accuracy and repeatability of industrial robots for both, pose and path. Within the course of this thesis, a fully-automated robot-based in-line metrology system for optical surface inspection is designed and developed. A system based on a dual-stage approach consisting of a six-degree-of-freedom industrial robot arm for coarse positioning and additional stepper-motor-based linear stages is implemented as a prototype, which utilizes a triangulation sensor. To provide necessary flexibility regarding sample size, optimally utilizing the optical sensor’s measurement range and compensating for position and orientation deviations within a production line, feature detection, appropriate scanning trajectory selection and adjustment of the robot pose is performed. Based on the system requirements an error budget is created, followed by a systematic analysation of individual system components to identify error sources and improve the overall performance. For a measurement volume of 50mm × 50mm × 25mm an out-of-plane resolution of 0.385 μm, an accuracy of 3.4 μm, a precision during a single scan of 22 μm and an overall repeatability for accumulative measurements of 7.2 μm is achieved.