Determining null-space motion to satisfy both task constraints and obstacle avoidance

It is a common practice to take advantage of redundancy in a manipulator to avoid obstacles while the end-effector is conducting a task-constrained motion, such as compliant motion for assembly. However, existing approaches rely on increasing the distance between the obstacles and the robot arm to avoid obstacles, which may result in a conflict between task motion and obstacle avoidance, i.e., when the rest of the arm avoids obstacles, the end-effector can no longer follow the task-constrained motion. We have observed that, in some cases, the robot arm has to decrease the distance to obstacles purposefully in order to both enable obstacle avoidance and the end-effector task constrained motion. This paper introduces a novel approach of null-space motion control to enable such manipulator motion to achieve both task objectives and obstacle avoidance, which is called the coordination of task motion and self motion (CTS) method. The method is implemented and tested on both a planar 4 degrees of freedom (DOF) manipulator in simulation and a spatial 7-DOF manipulator in real experiments, which show its effectiveness.