On the mechanics of natural compliance in frictional contacts and its effect on grasp stiffness and stability

The mechanics of friction and compliance in multi-contact arrangements is key to understanding and predicting grasp stability and dynamic response to external loads. This paper introduces a comprehensive model for the nonlinear force-displacement relationship at a frictional contact. The model is given in an analytic lumped parameter form suitable for on-line grasping applications, and is entirely determined by material and geometric properties of the contacting bodies. The force-displacement law predicts a nonlinear tangential stiffening as the normal load increases. As a result, the composite stiffness matrix of a frictional grasp is asymmetric, indicating that such grasps are not governed by any potential energy. The consequences for grasp stability are investigated. We formulate a rule for preloading frictional grasps which guarantees stable response at the individual contacts. Then we obtain a criterion for selecting contact points which guarantees overall grasp stability. The synthesis rule and its effect on grasp stability is illustrated with a simple 2D example.