Slip Avoidance in Dual-Arm Manipulation

In multi-finger or multi-arm grasping with friction contacts, maintaining force closure during motion is critical. Violation of this condition would cause contact slippage and possibly loss of grasp. This issue is of particular importance in space robotics, where the loss of grasp could lead to catastrophic consequences. There has been ample literature on stable grasp and force closure under static conditions. This paper investigates multi-arm grasping during motion, where the inertial force from the load could adversely affect grasp stability. Our approach dynamically adjusts the squeeze force and commanded robot/load motion to maintain a safe force closure condition. For a specified motion trajectory, the squeeze force is updated to prevent slippage based on the estimated inertial force. When the required squeeze force is beyond what the manipulators can safely apply, the trajectory will be scaled to reduce the inertial force component. In addition to motion-induced disturbance force, contact between the load and other objects in the environment can also cause slippage. The slip prevention strategy is extended to this case as well. The application scenario is based on the dual-arm transportation and berthing of a load in a micro-gravity environment. For laboratory testing, we use a fixed-base dual-arm robot to grasp, transport, and berth an object on a planar air bearing table. We also extend the transportation tests to a more general spatial setting, and use the dynamic squeeze adjustment to grasp, lift, and transport an object. Experimental results show the proposed method is effective at avoiding contact slippage during motion and when the object is in contact with the environment.