Equilibrium Conditions of a Polygonal Object When Grasped by Soft-Rolling Contacts

In this paper, we study two soft spherical fingertips grasping a rigid polygonal object as a kinematically constrained system. We assume that the contact locations of the soft fingertips are not fixed on the object but are able to move due to rolling constraints. A contact motion model is proposed for soft-rolling fingertips based on previously reported experimental findings on the rolling distance for a variety of soft materials. The equilibrium conditions are derived from the system dynamics and depend on the deformation and on the kind of soft material with regard to the fingertip’s rolling distance characteristics. Also, a discussion on contact forces and a grasp analysis at object’s equilibrium is made. It is shown that using the model of point contact with friction in soft-rolling contacts is not adequate for describing the real system with fingertips of low or medium stiffness characteristics. In general, the equilibrium conditions of the three contacted bodies (soft fingertips object) show that no-collinear interaction forces act on the object’s surface and they correspond to the equilibrium of an “internal object” held by two point contacts with friction.