An Exploration of Nonprehensile Two-Palm Manipulation

When two hands manipulate a part, they perform three basic opera tions : holding or rotating the part in an equilibrium grasp, allowing the part to fall in disequilibrium, and sliding one hand or the other relative to the part. This paper discusses a method for represent ing these primitive operations, focusing in particular on the sliding motions. The basic idea is to partition the combined configuration space of the part and palms into volumes of invariant contact me chanics. Within each volume, the possible motions of the part and palms are qualitatively equivalent. The precise accelerations may vary, but the contact mode is invariant. The possible part and palm motions are a function of the overlap of the contact friction cones with the line of gravity acting through the part's center of mass. A critical event analysis of this dependence allows the robot to parti tion the combined configuration space into the volumes of invariant contact mechanics. This system generates plans for manipulating the part by searching the resulting graph.