Synthesizing two-fingered grippers for positioning and identifying objects

Most industrial grippers now in use are two-fingered. Among them the parallel-jaw gripper is the simplest. It can partially remove the pose uncertainty of an object through grasping, such as the orientation uncertainty. This paper addresses a new type of grippers with the finger configuration of four circles instead of two parallel lines. It has a number of important advantages. Especially, it achieves form-closure and confines the object to a locally unique pose, so as to remove the pose uncertainty completely. It allows the gripped object to reach this pose freely without loss of required friction in the direction perpendicular to the grasping plane. More information can be acquired for identifying the object and its grasp mode. As a result the identification can be performed of one grasp. The key parameter of a symmetric four-spin gripper is the distance (span) between two spin centers on each finger, which depends upon the object shape and impacts the closure property. Based on a new approach to the grasp geometry, selection and limitations of the span are illustrated.