A 3D modular gripper design tool

Modular fixturing kits are precisely machined sets of components used for flexible, shortturnaround construction of fixtures for a variety of manufacturing purposes. A modularvise is a parallel-jaw vise, where each jaw is a modular fixture plate with a regular grid of precisely positioned holes. A modular vise can be used to locate and hold parts for machining, assembly, and inspection tasks. To fixture a part, one places pins in some of the holes so that when the vise is closed, the part is reliably located and completely constrained. The modular vise concept can be adapted easily to the design of modular parallel-jaw grippers for robots. By attaching a grid plate to each jaw of a parallel-jaw gripper, we gain the ability to easily construct high-quality grasps for a wide variety of parts from a standard set of hardware. Wallack and Canny [WC94, WC96] developed a previous algorithm for planning planar grasp configurations for the modular vise. In this paper, we expand this work to produce a 3-d fixture/gripper design tool. We describe several analyses we have added to the planar algorithm to improve its utility, including a three-dimensional grasp quality metric based on geometric and force information, three-dimensional geometric loading analysis, and intergripper interference analysis to determine the compatibility of multiple grasps for handing the part from one gripper to another. Finally, we describe two applications which combine the utility of modular vise-style grasping with inter-gripper interference: The first is the design of a flexible part-handling subsystem for a part cleaning workcell under development at Sandia National Laboratories; the second is the automatic design of grippers that support the assembly of multiple products on a single assembly line.