Stable Grasping and Manipulation by a Multifinger Hand with the Capability of Compliance Control

Abstract

This paper deals with two fundamental problems concerning a multi-fingered robotic hand with the capability of compliance control. One of them relates to developing a torque sensor useful for the tendon-pulley driving system and the other was a stable grasping and manipulating problem. In order to construct the finger joint actuation in multifingered systems, a tendon-pulley driving system has normally been used. In this kind of driving system a compact joint torque sensor plays an important role in achieving compliant motions at the finger tip and in turn constructing dextrous multifingered hands. It seems, however, that a satisfactory torque sensor has not yet been developed for such a driving system. To cope with this, a Tension Differential type Torque sensor (TDT sensor) is first proposed in this paper and applied to a newly designed robotic hand with two articulated fingers in experiments. Secondly, the stable grasping and manipulation problems in the multifingered hand are addressed assuming the existence of friction at the contact area of each finger tip and the object grasped. To formulate a stable grasping condition, the stiffness matrix of an object grasped by fingers with compliance adjustable joints was introduced. By using the stiffness components, the condition was described in a simple form. The stiffness matrix was resolved to the simpler form at the tip of each finger when the hand was grasping an object. To satisfy the desired stiffness matrix at the tip, the joint stiffness matrix at each finger was adjusted. To manipulate an object, the desired trajectories of the object were converted to joint trajectories using the inverse kinematics equations, and the position reference at each joint servomechanism was adjusted according to them, keeping the stable grasping condition. A robotic hand with two articulated fingers equipped with specially designed small TDT sensors was constructed. Using the hand, various experiments were carried out and the proposed methods were confirmed.