Purpose -The aim of this paper is to develop a redundancy-resolution (RR) algorithm to optimize the joint space trajectory of the six-rotation-axis industrial robot as performing arc-welding tasks. Design/methodology/approach -The rotation of the tool around its symmetry axis is clearly irrelevant to the view of the task to be accomplished besides some exceptional situations. When performed with a general 6-degrees-of-freedom (DOF) manipulator, there exists one DOF of redundancy that remains. By taking advantage of the symmetry axis of the welding electrode, the authors decompose the required instantaneous twist of the electrode into two orthogonal components, one lying into the relevant task subspace and one into the redundant task subspace, respectively. Joint-limits and singularity avoidance are considered as the optimization objectives. Findings -The twist-decomposition algorithm is able to optimize effectively the joint space trajectory. It has been tested and demonstrated in simulation. Originality/value -A new RR algorithm is introduced for the six-rotation-axis industrial robot performing welding tasks. A new kinetostatic performance index is proposed on evaluating the kinematic quality of robotic postures. It can also be used in other applications like milling, deburing and many other tasks requiring less than 6-DOF in tool frame.

This paper introduces the concept of functional redundancy of serial manipulators, and presents a new resolution scheme to solve such redundant robotic tasks requiring less than six degrees-of-freedom. Instead of projecting the secondary task onto the null space of the Jacobian matrix in order to take advantage of the redundancy, the twist of end-effector is directly decomposes into two orthogonal subspaces where the main and secondary tasks lie, respectively. The algorithm has shown to be computationally efficient and well suited to solve functionally-redundant robotic tasks, such as arc-welding.

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