Inverse Kinematic Functions for Redundant Spherical Wrists

Abstract

A three-axis wrist manipulator is redundant when applied to the two-degree-of-freedom (DOF) task of pointing. Because of widespread commercial applications of pointing tasks, such as spray-painting, sealing, and arc-welding, manipulators with two-axis wrists are widely available. However, by upgrading to a manipulator with a three-axis wrist, one can expand the workspace and largely eliminate the effects of singularities. In this paper, we achieve these goals using the inverse kinematic function approach to redundancy. We show that an ideal wrist should be mounted with its first axis perpendicular to the major axis of the manipulator’s forearm. With regard to commercially available manipulators, we show that the pointing workspace of a PUMA-560 can be expanded by approximately 20 percent by using all three wrist axes as compared to using only the first two wrist axes. The results can also be applied to kinematic control of a four-axis spherical wrist applied to 3-DOF orientation tasks. © 1989 IEEE