An Avoidance Control Strategy for Joint-Position Limits of Dual-Arm Robots

The joint position constraints of two cooperative robotic manipulators is a critical issue, especially when both manipulators grasp and move the same object. When two robots cooperate to move a grasped object from one point to another one, following a desired path, the general solution adopted in the literature is to use the kinematic redundant motions possessed by a single manipulator, in order to avoid joint constraints. However, this method requires a number of redundant motions at least equals to the number of joint constraints to satisfy. The study proposed in this paper is focused on developing a joint position limits avoidance strategy, which is able to satisfy all joint limits even when the number of redundant motions are no longer sufficient to ensure them with a classical approach. This is achieved by means of a supervisory control system, which allows to locally and temporary change the desired end-effectors motion, when the redundant motions are not available. The supervisory control sacrifices the path following task in order to ensure joint position limits avoidance, while preserving at the same time the relative end-effector motion, such that the stability of the grasped object is still ensured. The proposed strategy has been implemented on a commercial robot, namely Baxter, which possesses two anthropomorphic arms having one degree of redundancy. The obtained results prove the effectiveness of the proposed supervisor controller, which can be easily extended and applied in manufacturing scenario. Whereas path following can be temporary sacrificed (e.g., the grasped object can be moved from one point to another in different ways), the joint constraints must be strictly preserved for safety reasons.