A Nonprehensile Method for Reliable Parts Orienting

Abstract

Prehension may be defined as "The act of taking hold, seizing or grasping, as with the hand" (Webster's 3rd International Dictionary). Nonprehensile manipulation, then, can be defined as the manipulation of objects without grasping them. Manipulation without prehension is a natural way of handling objects for both humans and machines. The ability to manipulate objects which may not be graspable increases the flexibility of a robot interacting with its environment, without adding complexity to the mechanical design. This research analyses the mechanics of nonprehensile contact between a simple, two degree of freedom manipulator and a part. The intent is to develop reliable but sensorless manipulation routines for use in an automated assembly environment. While nonprehensile, sensorless devices are in common use in such environments, existing parts orienting devices, such as bowl feeders or the SONY Automatic Parts Orienting System, must be custom designed for each specific task. To decrease the setup or changeover time for an assembly line, what is needed is a simple but more general device, which can be easily modified or reprogrammed in response to a change in tasks. We present a planning algorithm for sensorless parts orienting in the plane with two one degree of freedome palms. Our method finds feasible paths through the space of equivalent state configurations of the object in the palms, without requiring that the palms maintain stable support of the object over the entire path. We show that such a device can reliably orient parts in the plane. Planning reorientations requires the geometric descriptions of the parts, the part's center of mass, and an upper bound on the coefficient of friction between the part and the palms. The plans produced by our algorithm are robust to uncertainties in the part's initial state and in the coefficient of friction, as well as to small inaccuracies in manipulator calibration.