The bottom layer is for servo control. The velocities of the motors are controlled by the servo drivers with the feed back of the encoder signals of the joints.

4 Motion Planning and Control

4.1 Teaching and Planning

According to the teaching sample points of welding robot and the prior knowledge of seam groove， the trajectory of weld seam is generated with B-spline or high degree polynomial. The pose and position of welding torch are planned in Cartesian space. Then the joint motion is planned in joint space with inverse kinematics for solution of joint coordinates. As shown in figure 3.

Fig. 3. Teaching and motion planning of the welding robot

4.2 Motion Control of Macro and Micro Mechanisms

The motion control of macro and micro mechanisms aim at large workplace, high accuracy and stability for seam tracking. In the process of planning motion, the joints with macro motion provide main motion trajectory, and the motion of micro mecha- nisms compensates the accuracy of positioning. Concretely, the motion of macro mechanism is given by teaching path, and micro motion is automatically adjusted by intelligent algorithms with vision sensing information. Furthermore, since the  travel of micro mechanism is small, the two groups of translational joints are linked in syn- chronization to ensure the smoothing trajectory of the welding torch, when the micro mechanisms are limited. According to the position of the micro mechanisms, the expected offset of the macro mechanisms is calculated, which is implemented by the coordination module of motion control. The motion of macro mechanisms is intro- duced to the motion control as feed forward signal for the motion stability and the coordination of two groups of joints in the process of seam tracking. The control block diagram of welding robot is shown in figure 4.

Fig. 4. Control block diagram of welding robot

5 Experimental Results

According to the design of the welding robot, a prototype of the welding robot was manufactured with 9 joints and 6 degree of freedom. The mechanism of the robot is shown as in figure 5, composed of the joints, servo motors, the sensors and welding equipments.

The experiments of teaching and the motion control of seam tracking were con- ducted with the designed mechanisms of the welding robot and the motion control system. A workpiece with preprocessed S-shaped weld groove was employed for seam tracking. There are two experiments conducted for comparison. Firstly, the motion of weld torch was taught with 20 points, and the trajectory of welding torch was generated with B-spline. Then using the same workpiece, the motion of weld torch was taught grossly with 4 points, and the trajectory was generated with linear interpolation in Zigzag weld groove. The seam tracking control and the coordinated control of macro and micro mechanisms were investigated in two cases.

The Motion of macro and micro mechanisms in the first experiment is shown in figure 6, and the tracking error is shown in figure 7; the unit of the coordinates is mm.

Fig. 5. The mechanisms of welding robot

In the process of seam tracking control, the two groups of joints move in synchroniza- tion, and micro joints provide the motion compensation in high precision. As a result, the error of tracking is less than 1mm, and the average error is less 0.5mm.

In the second experiment, although the welding seam trajectory was taught grossly, the desirable results of seam tracking control were still obtained. As shown in figure 8 and figure 9, the weld groove was repeated precisely, and the average error is less 0.5mm. The twice experimental results verify the effectiveness of the mechanisms of welding robot and the motion control with synchronization and coordination.