Extrinsic dexterity: In-hand manipulation with external forces

“In-hand manipulation” is the ability to reposition an object in the hand, for example when adjusting the grasp of a hammer before hammering a nail. The common approach to in-hand manipulation with robotic hands, known as dexterous manipulation [1], is to hold an object within the fingertips of the hand and wiggle the fingers, or walk them along the object's surface. Dexterous manipulation, however, is just one of the many techniques available to the robot. The robot can also roll the object in the hand by using gravity, or adjust the object's pose by pressing it against a surface, or if fast enough, it can even toss the object in the air and catch it in a different pose. All these techniques have one thing in common: they rely on resources extrinsic to the hand, either gravity, external contacts or dynamic arm motions. We refer to them as “extrinsic dexterity”. In this paper we study extrinsic dexterity in the context of regrasp operations, for example when switching from a power to a precision grasp, and we demonstrate that even simple grippers are capable of ample in-hand manipulation. We develop twelve regrasp actions, all open-loop and hand-scripted, and evaluate their effectiveness with over 1200 trials of regrasps and sequences of regrasps, for three different objects (see video [2]). The long-term goal of this work is to develop a general repertoire of these behaviors, and to understand how such a repertoire might eventually constitute a general-purpose in-hand manipulation capability.

I. Introduction

In the robotics research literature “dexterity” often refers to the manipulation of an object in the hand, with the hand. By “extrinsic dexterity” we mean the manipulation of an object in the hand, using resources extrinsic to the hand. For example, if you tilt your hand so that an object rolls from your fingertips to your palm, you are using gravity, not just your hand. If you press two chopsticks against the tabletop to even them up, you are using the surface of the table and arm motions, not just your hand. If you toss an object upwards from your palm and catch it in your fingertips (Fig. 1), you are using object inertia and arm acceleration, not just your hand. In each case, you can do more, better, or faster by exploiting resources extrinsic to the hand.