Limited mobility grasps for fixtureless assembly

Abstract

A novel approach to grasping with an end effector for the purposes of fixtureless assembly is presented. The grasping strategy is based on having the final position of the contacts determined by specific regions of the object geometry. The fingers are placed within limited spaces of the object and moved using frictionless contacts until motion ceases. The limited spaces usually take the form of concave edges or holes in the object. This strategy allows the positioning error to be determined by the accuracy of the part and is independent of the accuracy of the robotic manipulator. A new method for finding form closure is introduced based on maximizing the distances between contacts. The grasping strategy allows deterministic positioning of the object and also provides a means of convergence to these holding points. Testing was done in the plane with three fingers for several cases to show the sensitivity of the grasps to part geometry. The results show the position error is dependent on local shape and was reduced from 1 mm to 0.1 mm for several cases.