Abstract: A dichroitic mirror is designed to extract video signal from axis of laser beam with high power, which resolves the problem of difficultly extracting coaxial video signal from laser welding pool. The mirror is set in laser path whose inclination angle is 45°with axis of laser beam. The coaxial visual sensing system to monitor high power Nd：YAG laser welding process is established based on this light path of extracting video signal. The median filter and homomorphic filter are designed to remove low frequent noise between rows and to enhance images based on analyzing the characteristics of imaging signal. Edges of welding pool and keyhole are extracted based on image segmentation with threshold. In order to get precise shape parameters of welding pool and keyhole, edges are fitted by Hough transform and the sizes of welding pool and keyhole are obtained from the fitted results.

Key words: Coaxial, Curve fitting, Image processing, Nd：YAG laser welding with high power, Visual sensing