The overall welding robot system is composed of the robot body, welding system, a host computer, a console, motion control and driving device, and vision sensors mounted on the welding torch for monitoring, etc.

2.1 Mechanism Design

A robot is designed with 9 joints and 6 degree of freedom for large scaled workpieces welding. The prototype structure of welding robot is shown in figure 1. The robot is structured in gantry. In order to enlarge workplace of the robot, there are two groups of orthogonal translational mechanisms with 6 translational joints for positioning. Thus the macro and micro motion mechanisms are composed of redundant joints in same directions. One group of macro joints can move with large travel, and the other group micro joints can position with high precision. In the process of multi-axis mo- tion of the welding robot, the micro joints can compensate positioning precision by intelligent sensing and control. Furthermore, three rotational joints are installed on the gantry structure of translational joints to adjust the pose of the weld torch. Welding equipments are integrated with the gantry and joints of the robot.

The travel and resolve of the joints of welding robot are shown in Table 1. The 3 joints of macro mechanisms provide large travel with low resolution of 1mm. Corre- spondingly, the 3 joints of micro mechanisms provide small travel with higher re- solves of 0.01mm.

Fig. 1. The prototype structure of welding robot

Table 1. The travel and resolution of the joints of welding robot

Travel Resolution

X 3000mm 1mm

Y 1000mm 1mm

Z 1200mm 1mm

x 200mm 0.01mm

y 200mm 0.01mm

z 100mm 0.01mm

α 120 0.05

β 120 0.05

γ 180 0.05

2.2 Kinematics Model

The definition of the coordinates frame of welding robot is shown in Figure 1. The origin point of the coordinates of world frame is defined as the intersections of the two axes of roll and yaw rotational joints when every joint is at zero position. Then the transformation matrix for the robot is as (1):

where ci is the abbreviation for cosθi, and si for sinθi; x, y and z are the variables of translational joints, and θ1, θ2, and θ3 are the angles of 3 rotational joints; a1, a2, a3 are the parameters of the 3 rotational joints.

The inverse kinematics model of the welding robot can be deduced by formula (1). The variables of translational and rotational joints θ1, θ2, θ3, x, y and z can be calcu- lated as follows.

Note that the variables of micro translational joint are redundant to macro ones, so they need not be calculated in the inverse kinematics of the robot.

3 System Functions and Control Architecture

The welding robot provides positioning and motion of weld torch to meet the de- mands in process of welding, which are implemented by corresponding function modules and control system.

3.1 System Functions

There are some system functions for the welding process, which include teaching, planning, multi-axis linkage, motion control, monitoring, human-compute interface, and manual manipulation function.