The condition for contact grasp stability

The author distinguishes between two types of grasp stability, called spatial grasp stability and contact grasp stability. The former is the tendency of the grasped object to return to an equilibrium location in space; the latter is the tendency of the points of contact to return to an equilibrium position on the object´s surface. It is shown, 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. A model of how the positions of the points of contact evolve in time on the surface of the grasped object in the absence of any external force or active feedback is derived. From this model, a condition is obtained which determines whether or not a two-fingered grasp is contact stable