An electrode misalignment inspection system for resistance spot welding based on image processing technology

Weld quality in resistance spot welding (RSW) is greatly affected by any misalignment of the upper and lower electrodes. This work presents a non-contacting system for measuring electrode misalignment. It uses two industrial CCD cameras, oriented perpendicular to one another in the same plane. The electrode cap images taken by the CCD cameras are sent to a computer through an Ethernet switch. The original red, green and blue (RGB) images are converted to greyscale images. The left and right edge points of the cap images are determined by computing the first derivative of the grey level gradient of the images; the edge points are the locations of local maxima in the horizontal direction. Using the edge points, linear boundaries are fitted via the least squares method, and the noise points are eliminated. The axes of the electrode caps are calculated from the left and right linear boundaries of the images, and then the spatial and angular offsets of the two electrode axes can be determined by the two axis functions. The results of comparison experiments indicate that the abovementioned system can rapidly and accurately measure the misalignment of electrodes during the process of RSW. The measurement results will provide critical input for equipment adjustment and maintenance.