Grasp Invariance

This paper introduces a principle to guide the design of finger form: invariance of contact geometry over some continuum of varying shape and/or pose of the grasped object in the plane. Specific applications of this principle include scale-invariant and pose-invariant grasps. Under specific conditions, the principle gives rise to spiral shaped fingers, including logarithmic spirals and straight lines as special cases. The paper presents a general technique to solve for finger form, given a continuum of shape or pose variation and a property to be held invariant. We apply the technique to derive scale-invariant and pose-invariant grasps for disks, and we also explore the principle's application to many common devices from jar wrenches to rock-climbing cams.