A novel grasping strategy and gripper for fixturing in 3D is presented for the robotic fixtureless assembly application. The goal of the strategy is to accurately immobilize a part in the presence of initial robot and part positioning errors. The grasping strategy expands a previously developed 2D theory into 3D and is implemented on two automotive parts using a multi-degree of freedom gripper. The gripper is able to fixture a variety of parts and the only change is reconfiguration of the computer controlled axes. To fixture a sheet metal part, the fingers are placed within holes of the part and moved until the desired set of contact locations is achieved. The fingers are grooved at fixed angles such that the edge of the sheet metal part can be held within the grooves. Three fingers and six frictionless point contacts are used for each part. A computer algorithm is described that solves for suitable contact locations based on the part geometry. The algorithm was implemented and tested on two Buick sheet metal parts from the front fender assembly. Twenty five trials were performed for each grasp. The standard deviation of the part location prior to being grasped was 0.43 mm. After being grasped, this was reduced to 0.01 mm.

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.