On orientation control of functional redundant robots

Abstract

There are tasks that do not require a controlled motion in all spatial directions. These unused degrees-of-freedom (DOF) make the robot functionally redundant. Traditional methods for redundancy resolution developed for intrinsic redundant robots cannot be used directly for resolving functional redundancy. The functional redundancy is in general not reflected as rows in the Jacobian matrix and therefore, the unused DOFs cannot be mapped into the null-space. In the paper we present a novel approach for the orientation related functional redundancy resolution based on the task space rotation. We define a time-variant task space frame so that one or more frame axes are aligned with the unused directions. This allows that the rows of Jacobian matrix corresponding to functional redundant DOFs are excluded. As the standard quaternion control is not appropriate to control orientation of such functional redundant robots, we propose two control algorithms to control one or two orientation directions. Two examples then show how the proposed control algorithms can be implemented and finally, we present the application of the algorithm on a two-arm robot system.