DETC2006-99645 JOINT COUPLING DESIGN OF UNDERACTUATED GRIPPERS

The joint coupling necessary to allow for underactuation is often accomplished through compliance in the manipulator structure. Compliance is perhaps the simplest way to allow for coupling between joints without enforcing the fixed-motion coupling relationship inherent with gear or linkage couplings. Compliant couplings are a simple way to allow a joint to passively deflect without causing a fixed-motion proportional change in the joints to which it is coupled. ABSTRACT This paper examines the nature of joint coupling in underactuated grippers for environments where object properties and location may not be well known. A grasper consisting of a pair of two-link planar fingers with compliant revolute joints was simulated as it was actuated after contact with a target object. The joint coupling configuration of the gripper was varied in order to maximize successful grasp range and minimize contact forces for a wide range of target object size and position. A normal distribution of object position was assumed in order to model sensing uncertainty and weight the results accordingly. The results show that proximal-distal joint torque ratios of around 0.6 produced the best results for cases in which sensory information available for the task was poor, and ratios of around 1.0 produced the best results for cases in which sensory information available for the task was good. Compliant underactuated grippers show particular promise for use in unstructured environments, where object properties are not known a priori and sensing is prone to error. Finger compliance allows the gripper to passively conform to a wide range of objects while minimizing contact forces. Passive compliance offers additional benefits, particularly in impacts, where control loop delays may lead to poor control of contact forces In previous work, we examined the optimization of the preshape and joint stiffness of simple two-fingered grippers with passive springs in the joints. This study showed that for a particular set of joint stiffnesses and rest angles, the widest range of range of uncertainty in object size and location could be allowed for. Contact forces were also minimized at approximately the same gripper configuration. In addition to simulation studies, these results were confirmed with experimental tests using a reconfigurable grippe