Statics of Power Grasps with a Multifingered Hand

Abstract

A multifingered hand can manipulate an object skillfully with fingertip contacts. It can also grasp an object stably with finger link contacts. Such grasp is called power grasp. When Coulomb friction is assumed at contact points and the number of contacts is greater than the number of joint torques, the contact forces balancing the joint torques are not necessarily determined uniquely. This paper investigates how indeterminate grasp forces of power grasps are distributed assuming that both the finger links and grasped object are rigid bodies. We show that the sliding motion between a grasped object and finger links is restricted when the grasp forces are indeterminate. In general, a frictional force can occur only in the reverse direction of sliding. Thus the indeterminate grasp forces of power grasps are also restricted. This analysis leads to the condition that they are uniquely determined. This paper also discusses form closure of power grasps which can generate unique grasp forces against any disturbances.