Robot Hand based on a Spherical Parallel Mechanism for Within-Hand Rotations about a Fixed Point

Rotating a grasped object about all three spatial axes is challenging, because kinematically redundant robot hands require complex control schemes for within-hand rotations, and simple parallel grippers require inefficient whole arm motions. We present a novel 3-finger robot hand design inspired by a spherical parallel mechanism that achieves these rotations with just 3 actuators. The hand is designed such that at every hand-object configuration, the object pose moves along a sphere with a fixed center, which is determined by the intersection of the fingers’ revolute axes and is independent of the object shape, pose, and the initial grasp configuration. We optimize the hand based on 3-RRS spherical manipulator to maximize both its rotational workspace size and manipulation motion quality. From these parameters, we implement and experimentally evaluate the hand design through grasping tests, manipulation characterization, and real-world task scenarios, which show that the hand is able to grasp a variety of object geometries and accomplish precise single and multi-DOF rotations about a fixed point. We believe this design can remarkably improve robustness and simplify control for dexterous within-hand rotations, which finds utility in augmenting the capabilities of low-DOF robot arms without an active wrist.

I. Introduction

The hand plays an integral role in functional robot manipulation by physically interacting with the robot’s environment. While a simple gripper on a standard 6-7 degree of freedom (DOF) robot arm may be sufficient for many constrained applications like manufacturing, a dexterous end effector capable of precisely reorienting and repositioning grasped objects can be critical in overcoming practical shortcomings such as avoiding obstacles and arm singularities, improving safety and speed, and increasing energy efficiency [1]. In-hand manipulation of this nature has been previously achieved through anthropomorphic hand designs with serial, fully actuated fingers that often require accurate knowledge of object shape and contact locations for their complex redundant control schemes [2]–[5].