Evaluation of grasp stiffness in underactuated compliant hands

Underactuation represents a solution to reduce the number of Degrees of Freddom (DoF) of robotic hands. Altough reducing the number of DoFs in general limits the ability to perform many and different types of grasp, the use of springs in the structure improves the compliance of the grasp and mitigates the loss of generality due to the reduction of DoFs in the mechanical structure. The use of active and passive elastic elements improves robustness of the whole grasp. In this paper we evaluate the grasp stiffness which depends on the structural compliance of hand links and joints and of the contacts, and on the gains of impedance controllers. A quasi-static model of the grasp for underactuated hands is presented and used to explicitly compute the grasp compliance.