Abstract. In order to meet the demands of the welding of large scaled work- pieces, a welding robot is designed with 9 motion joints and 6 degrees of free- dom. Two groups of macro and micro motion mechanisms are adopted for large travel and high positioning precision. The welding trajectory is taught grossly in Cartesian space, and the motion of joints is planned in joint space with the kinematics model of the welding robot. A method of motion control of macro and micro joints is presented for stability and precision in seam tracking. Ex- perimental results verified the effectiveness of the mechanisms of welding robot and the control system.67749

Keywords: Welding robot, Motion control, Visual servo, Welding automation.

1 Introduction

The devices and intelligent technology for weld of large scale workpieces are widely applied in mechanical manufacturing industry, especially in pressure vessel and pipe construction. Nowadays the devices commonly used in weld industry include welding manipulators, rollers, positioning machines, tumbling machine, special welding ma- chine and welding robot station, etc. For large workpieces welding, the devices are commonly in gantry structure or based on walking mechanisms, combining with other auxiliary mechanisms. As to welding technology, auto submerged arc welding widely adopted for large workpieces weld for its high productivity and quality. With the development of equipment manufacturing and control technology, welding devices feature in mechanism maximization and precision[1]. There are some significant trends that more and more application of welding robot with high precision and reli- ability, and the application of digital intelligent control technology for advanced con- trol system for extreme manufacturing.

Although more and more welding robots and advanced control technology have been introduced to weld industry, there are still some challenges for the welding of large scale workpieces in mechanism and control, which include the large workspace of the robot with high positioning precision, the open robot controller, coordinated motion control of multi-axis, and macro- micro motion control.

Firstly, the mechanisms of welding manipulators commonly structured gantry[3] and walking mechanisms[4] for large scaled workpieces welding. A specific mecha- nisms was presented for boiler pipes welding[5]. In the process of the workpieces welding, the mechanisms with long travel and high precision are needed for the weld seam. That improves the cost of the welding equipments, so the applications of auto- matic welding are limited, especially for the small enterprises of weld manufacturing. Secondly, the controller of welding robot used to be designed for specific equipments, thus it is difficult to meet the need of generality and openness of controller. Xu de- signed a real time open robot controller for a standard industrial welding robot system SIASUN-GRC[6], and the general robot controller for welding manipulators  should be studied further. Compared to precisely teaching and offline programming[7], it is convenient to online tracking control base on few grossly teaching points. Then the third issue is the coordinated motion control of macro and micro mechanisms for the stablity and precision in process of seam tracking.

The remainder of this paper is organized as follows. Firstly, the structure of mechanisms of welding robot is introduced in Section 2, and the kinematics model of the robot is calcuted. Then, the system functions and control architecture are discribed in Section 3. In Section 4, the method of teaching and planning of the motion of the robot is developed, and the motion control for the coordination of macro and micro mechanisms is given. Experiments of seam tracking are conducted to verify the effec- tiveness of the mechanisms of welding robot and the control system in section 5. Finally, the paper is concluded in Section 6.

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