An implementation of inverse kinematic functions for control of a redundant wrist

A class of industrial tasks requires two-degree-of-freedom wrists to point an axisymmetric tool mounted at the end of a robot arm in a specified direction. Due to inherent deficiencies of wrists, such as joint limits and singular positions, the workspace of the tool on a pointing sphere is limited. A three-degree-of-freedom wrist is a redundant mechanism for point tasks. Using inverse functions, it is shown that with a redundant mechanism the workspace on the pointing sphere can be significantly improved by positioning degenerate regions at suitable areas in the workspace. The authors review arguments toward the design of an ideal redundant wrist for pointing tasks and propose an optimal modification of the wrist of an existing PUMA-560 arm. The main considerations in the modified design are existing joint limits and collisions of the total structure with the forearm. Plots of the joint angles and the joint rates in the workspace are presented. The experimental results confirm that the predicted workspace improvements are attained