Implementing a force strategy for object re-orientation

This paper describes a method to reorient grasped rigid objects of polygonal cross-section using open loop force control strategies. A representation of forces in a spherical reference frame has been developed that ensures grasp stability and allows simple commands to control object reorientation. A procedure is shown for controlling at which fingers slip or rotation occurs, that can be used for three orthogonal rotations of an object grasped with three fingers. The implementation of a "twirling rotation" on the Stanford/JPL hand is described. A failure analysis is presented for potential causes for the object to slip from the grasp. Object control strategies are developed assuming line contacts and infinitesimal finger sizes. The influence of cylindrical finger tips on the performance of reorientations is described.