Computing and Controlling the Compliance of a Robotic Hand

Compliance is one of the most important quantities characterizing the grasp of a robotic hand on a tool or workpiece. This is particularly true in fine manipulation (as in assembling components) where small motions and low velocities lead to dynamic equations that are dominated by compliance, friction, and contact conditions. In this paper we express the compliance of a grasp as a function of grasp geometry, contact conditions between the fingers and the grasped object, and mechanical properties of the fingers. We argue that the effects of structural compliance and small changes in the grasp geometry should be included in the computation. We then examine factors that can lead a grasp to become unstable, independently of whether it satisfies force closure. Finally, we examine the reverse problem of how to specify servo gains at the joints of a robotic hand so as to achieve, as nearly as possible, a desired overall grasp compliance. We show that coupling between the joints of different fingers is useful in this context.