Contact stability for two-fingered grasps

We distinguish between two types of grasp stability, which we call spatial grasp stability and contact grasp stability. We show via examples that spatial stability cannot capture certain intuitive concepts of grasp stability and hence that any full understanding of grasp stability must include contact stability. We derive a model of how the positions of the points of contact evolve in time on the surface of a grasped object in the absence of any external force or active feedback. From this model, we obtain a general measure of the contact stability of any two-fingered grasp. Finally, we discuss the consequences of this stability measure and a related measure of contact manipulability on strategies for grasp selection.