Real-time thermal error compensation method for robotic visual inspection system

The thermally induced error is a critical element in the total errors for the robotic visual inspection system during its long-term operation. This paper investigates the thermal behavior of the robotic visual inspection system and proposes a real-time thermal error compensation method. Based on the fixed-point constraint, an error calibration model is derived by associating the thermally induced parameter errors with the deviations in the measured coordinates of the fixed reference point during the robot warm up and cool down. A joint-by-joint test is performed to investigate the link parameters that change significantly with respect to temperature variations. Standard spheres are adopted as the calibration targets, and optimum projected angles for sphere center measurement by visual sensor are obtained based on theoretical analysis and experimental data. Contrary to most other works, the method needs no exact knowledge of the temperature sensors and is well suited for online dynamic thermal error compensation for the robotic visual inspection system. Verification experiment is carried out on a car-body assembly line, and results show that the max/mean residual error for the tested points has been reduced from 0.441/0.195 to 0.136/0.078 mm with the significant parameters calibrated.