A general framework for open-loop pivoting

Pivoting is the rotation of an object between two fingers using gravity and inertial forces to impart angular momentum. We present an analysis of the mechanics of pivoting and a framework for planning and execution. Extrinsic dexterity was defined by Chavan-Dafle et al. [1] as the use of external forces, such as gravity and inertial forces in post grasp manipulation. We analyze one such regrasp termed “pivoting” by Rao et al. [2]. We find a grasp and arm trajectory which can rotate an object between stable poses, if any. We demonstrate an implementation of pivoting with an ABB industrial arm and a two fingered gripper.

I. Introduction

Humans have a huge repertoire of regrasps. Some regrasps employ coordinated motions of many hand freedoms, dubbed ‘intrinsic motions’ of the hand. However humans also use the environment, like gravity or momentum, as an additional resource. For example, we find it quite natural to roll an object from our fingertips to our palm or to reorient chopsticks in our hand by pressing them against the table. Chavan-Daile referred to these actions as “extrinsic dexterity”. Much robotics research has focused on how to manipulate an object with the hand, using only the hand: intrinsic dexterity. But robots, lacking the dexterity of humans, should use the strategies provided by extrinsic dexterity, if anything, more often than humans. Chavan-Dafle describes several examples of extrinsic regrasps. This paper will explore, in detail, one of those regrasps: extrinsic pivoting. Pivoting is defined to be the rotation of an object about an axis determined by two contacts between the effector and object.