Kinematic Control of A Robot-Positioner System for Arc Welding Application

Welding is one of the most successful applications for industrial robots, which encourages intensive research and development in the area of the CAD-based and off-line programming (Pires et al., 2003, Yagi, 2004). At present, relevant software tools allow to optimise the process parameters, which directly influence the product quality and the system working cycle. Besides, they enable users to complete most of the process preparation actions in advance, without access to the workcell, and therefore, to make the robotic systems competitive for both large and small series, or even for unique products (Ranky, 2004). However, resent advances in the arc welding technology motivate rethinking of some postulates and conventions incorporated in the existing off-line programming methods. One of related problems, the kinematic control of a robot-positioner system, is addressed in this chapter. The welding position (or, more precisely, the weld joint orientation relative to gravity) is an essential issue in both manual and robotic welding, associated with the problem of the weld puddle control. As is known from long-term experience, the highest quality and productivity are achieved for the downhand (or flat) welding position, where the workpiece is oriented so that the weld tangent line is horizontal, and the weld normal vector is the opposite of the gravity direction (Cary 1995). This orientation is preferable because gravity draws the molten metal downward into the joint allowing it to flow appropriately along the weld contour, which makes the welding faster and easier. For this reason, the downhand welding has been adopted in robotics as a de facto standard (Bolmsjo 1987, Fernandez and Cook 1988). To ensure the desired weld orientation, a typical robotic arc welding station (Fig. 1) includes two separate moving mechanisms: (i) a fiveor six-axis industrial robot (welding tool manipulator) aimed at moving the weld touch with the required speed and orientation relative to the weld joint; and (ii) a twoor three-axis positioning table (workpiece manipulator), which ensures the downhand (or close to it) orientation of the weld joint with respect to gravity. In contrast to the robot, a skilled human-welder is capable to perform such operations in other positions, such as the horizontal, vertical, or overhead ones. To make such type of welding easier, several companies recently proposed their technological innovations, the flux-cored wires, that create a protective coating, supporting the metal against gravity. This