Force distribution of power grasps based on the controllability of contact forces

An analysis of force distributions for grasping by robot hands requires determination of contact forces between the fingers and the grasped object. This topic has been extensively investigated for fingertip grasps under the assumption that the fingers have enough joints so that contact forces can be actively controlled. This assumption is, however, not valid in power grasps for which the internal forces may have uncontrollable components. To ensure that the contact forces can be obtained by controlling the joint torques, the uncontrollable components must be separated from the internal forces. In this paper, a method is obtained to identify those power grasps for which the internal forces have uncontrollable components. A new formulation of contact forces is developed for the general solution to the force distribution of power grasps and provides a framework for optimization. The advantage of the approach is that the joint torques are explicitly considered, and, therefore, optimal contact forces can be achieved by modifying the joint torques.