Subpixel drilling and riveting circular hole detection method based on an improved Zernike moment

Robotic drilling and riveting systems use visualization to precisely position the workpiece and for quality assurance. This paper describes a multi-step method to improve the visual detection accuracy of the reference and connection holes. A pixel-level edge point set based on the Canny operator is used first for coarse positioning of the circular hole. The edge parameters of the three-grey transition model are accurately derived using four Zernike orthogonal moments for the region of interest around the hole to be detected. Then, the relationships between the edge parameters and thresholds is used to calculate the subpixel coordinates of the edge points. A least squares analysis is then used to detect the center location and hole radius. Simulations show that the error in the center coordinates is approximately 0.01 pixels and that of the radius is 0.1 pixels, which is higher detection accuracy than the traditional algorithm. The system works for various size holes and is less sensitive to noise. Thus, this method effectively improves the detection precision of circular holes, which is important in robotic drilling and riveting visual measurement systems. © 2019, Tsinghua University Press. All right reserved.