Combining grasping and rotation with a spherical robot hand mechanism

Abstract

Object reorientation is a key functionality in dexterous manipulation tasks, such as turning a doorknob. This is usually done on robot arms with a simple gripper and a three-degrees-of-freedom wrist. However, wrists are mechanically complex, and the wrist axes are often far away from the grasped object, resulting in coupled translations that need to be compensated with awkward whole-arm motions. We present a robot hand mechanism based on a spherical parallel architecture that can both grasp and rotate a wide range of objects in all three axes, combining much of the function of traditional wrists and grippers. The hand mechanism allows for pure spherical rotations of the grasped object about a known fixed point close to the object, thereby avoiding parasitic translations and inefficient arm motions. This point also stays fixed with respect to the hand, and is independent of the object shape, pose or initial grasp. We detail the spherical parallel design and workspace model of the wrist-like Sphinx hand, validate its performance for lower-degrees-of-freedom robot arms without traditional wrists and show that it can accurately rotate the grasped objects over large angles with basic open-loop control. Developing robot hands for unstructured human environments is a major challenge. A robotic hand that combines grasping and wrist-like rotation in one mechanism for more efficient and versatile object manipulation is presented.