Micro-Defect Inspection on Curved Surface Using a 6-DOF Robot Arm with One-Shot BRDF Imaging

The need for automated visual inspection in Japan is increasing due to labor shortages caused by the declining birthrate and aging population. We have developed an imaging system called One-shot BRDF (Bidirectional Reflectance Distribution Function), which can easily detect even micro-defects that are conventionally difficult to capture using parallel-beam illumination and an image sensor. By color-coding the direction of light, micro-defects can be instantly captured as a clear image. This paper proposes a novel micro-defect inspection system capable of automatically inspecting the surface of materials with complex shapes by mounting One-shot BRDF optics on the tip of a 6-degree-of-freedom (DOF) robot arm and scanning the material's surface. The 6-DOF robot arm can track the scanning trajectory and keep the optics squarely against the curved surface at a constant distance. We developed a simple algorithm to automatically determine the trajectory from the material's 3D CAD data so that the entire surface to be scanned can be inspected quickly and within the 6-DOF robot arm's range of movement. For high-speed and high-precision inspection, we built a nonlinear dynamic model of the 6-DOF robot arm and model-based control to track the trajectory while suppressing vibration. In scanning experiments using mirror-finished aluminum with a curved surface, the position and orientation control errors were sufficiently small in high-speed operation at the limit of the robot arm's performance. Several minute scratches on the mirror-finished aluminum could be detected and their locations were mapped as 3D CAD data.