Sharing inertia load between multiple robots with active compliant grippers using trajectory pre-shaping

In this paper we propose a cooperative motion scheme that is able to control the sharing of gravitational and inertia load (forces required for load acceleration/deceleration) among multiple position-controlled robots. We adopt a clamped-free configuration, where a leader robot grasps the load directly and a follower robot grasps the load through a spring mounted on its end effector. While the leader travels the designated trajectory, a delta trajectory is added to the follower that supplies its share of inertia force by stretching/compressing the spring. The trajectory of the follower is pre-shaped to cancel out vibrations, therefore reducing the impact force on the robot mechanism. The spring also provides compliance in non-force-controlled directions to reduce internal forces. We analyze the influence of temporal error on both load sharing and internal force and propose a synchronization method to reduce this error to less than 1 ms. Every aspect of our scheme can be implemented with commercial robots without any modification of controller parameters or knowledge of the manipulator dynamics. Being able to execute our cooperative motion at a speed of 0.5 m per sec, it is much faster than impedance/hybrid control based approaches, suggesting the ability to satisfy the cycle time requirement for industrial applications such as material handling.