Grasp and manipulation analysis for synergistic underactuated hands under general loading conditions

In dexterous grasping, the development of simple but practical hands with reduced number of actuators, designed to perform some manipulation tasks, is both attractive and challenging. To carefully synthesize inter- and intra-finger couplings a rigorous way to establish grasping and manipulation properties of an underactuated hand is of paramount importance. In this paper, we propose a general approach to characterize the structural properties of underactuated hands focusing on their kinematic and force analysis. A complete kinostatic characterization of a given grasp (pure squeeze, spurious squeeze, kinematic grasp displacements and so on) is introduced. The analysis is quasi-static but it is not limited to rigid-body motions, encompassing also essential elastic motions, statically indeterminate configurations, and pre-loaded initial conditions. The introduction of generalized compliance at contacts and in the actuation mechanism is included, as it is an essential feature of safe and dependable modern hands. Efficient algorithms to characterize the system behavior are presented and applied in two different numerical examples.