Tracking Control of 7-DOF Redundant Manipulators with Enhanced Null Space Compliance

Abstract

In this paper, the problem of controlling the 7 degree-of-freedom (DOF) redundant manipulator accurately executing tasks along a desired trajectory with time-varying position and orientation while addressing a constrained compliant behavior within the null space is considered. The objective of this work is to extend null space impedance control from the traditional fixed point to any pose to meet the human-robot physical interaction in practical applications, such as service and medical robotics. To track the desired trajectory, a Cartesian impedance controller containing desired task variables is derived. Redundancy is then exploited to handle human-robot interaction behavior by using the designed null space impedance controller while constraining the range of elbow motion. In addition, an analytical inverse kinematics (IK) solution is employed to guarantee the compliant behavior of the null space is balanced to any elbow configuration. Finally, the performance of the proposed approach is verified through various experiments on a torque-controlled 7-DOF redundant manipulator.