Abstract: One new kind of robot system which adopts a new wheeled-pedrailed mobile machine is introduced and the constitution of the system and operating principle are developed thoroughly. The robot system is composed of mobile machine, welding system, visual sensor system and control system, which is a kind of automatic welding system neither use track nor navigation. The mobile machine has strongly load capacity and move agility, which is satisfactory to the request of the all position on the large workpiece. Through abundance experimentation, the moving ability of mobile machine is analysis and it’s kinematics model is built up. A kind of switch controller is designed aim at the complexity of the robot system. Then the cooperative control between mobile machine and crosshead is obtained to achieve seam tracking accurately. The trackability experiment and the all position multilayer and mul- tipass welding experiment are proceeded, the satisfactory result is obtained. The automation welding process of the large workpiece is realized.

Key words: Crawling robot, Seam tracking technique, Welding on large workpiece