Design of an underactuated anthropomorphic hand with mechanically implemented postural synergiesby Kai Xu, Huan Liu, Yuheng Du, Xiangyang Zhu

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Design of an underactuated anthropomorphic hand with mechanically implemented postural synergies

Kai Xua, Huan Liua, Yuheng Dua & Xiangyang Zhub a UM-SJTU Joint Institute, Shanghai Jiao Tong University, Shanghai, China b School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, China

Published online: 14 Nov 2014.

To cite this article: Kai Xu, Huan Liu, Yuheng Du & Xiangyang Zhu (2014) Design of an underactuated anthropomorphic hand with mechanically implemented postural synergies, Advanced Robotics, 28:21, 1459-1474, DOI: 10.1080/01691864.2014.958534

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Design of an underactuated anthropomorphic hand with mechanically implemented postural synergies

Kai Xua*, Huan Liua, Yuheng Dua and Xiangyang Zhub aUM-SJTU Joint Institute, Shanghai Jiao Tong University, Shanghai, China; bSchool of Mechanical Engineering, Shanghai Jiao Tong

University, Shanghai, China (Received 7 April 2014; revised 5 July 2014; accepted 15 August 2014)

Neurology shows that human controls dozens of muscles for hand poses in a coordinated manner and such coordination is referred as to a postural synergy. The concept of postural synergies was recently adopted in the control of robotic hands. With the synergies implemented digitally in a controller, all the motors in an anthropomorphic robotic hand can be controlled via a few synergy inputs. Aiming at exploring alternative approaches for synergy realization, this paper proposes to implement the postural synergies using a mechanical transmission unit. Two rotation inputs can be scaled, combined, and mapped to the rotary outputs to drive an anthropomorphic hand, enabling not only various grasping tasks but also a manipulation motion. Synergy synthesis and design of the anthropomorphic hand are firstly presented. The transmission unit as the implementation of the postural synergies is then elaborated. Tests were performed to quantify how well the synergies could be reproduced via this transmission unit. The results suggest it might be promising to construct a low cost yet versatile prosthetic hand by implementing the postural synergies mechanically.

Keywords: anthropomorphic hand; robotic hand; underactuation; postural synergy; planetary gear 1. Introduction

It is a challenging task to construct an anthropomorphic prosthetic hand that can reproduce the delicate motions of the biological original. An ideal prosthetic hand is expected to be versatile for various daily tasks and controllable through a biosignal interface, such as electromyography or electroencephalography. However, limited bandwidth of these interfaces used to prevent fully actuated robotic hands from being applied as prostheses if each joint needs individual control to perform grasping tasks, even though many designs were absolutely the state-of-the-art (e.g. the ones in [1–4]).

Many prosthetic hands hence followed an underactuated design approach. Researchers often refer to grasp taxonomy in order to assure the prosthetic hand motions are functional enough for various daily tasks. Study on the grasp taxonomy can be traced back to the 1950s [5] with milestones marked in the 1980s.[6,7] The taxonomy of discrete hand postures was shown useful by the development of several prosthetic hands [8–11] and it was still being expanded by recent results.[12,13]

However, in the design of underactuated prosthetic hands, referring to the discrete grasp taxonomy does not directly indicate how to assign actuation and coupling relations among the hand joints. Alternative approaches might be needed to better guide the development of underactuated prosthetic hands.

Findings in neurology suggested a possible way of achieving dexterous control of a multi-Degree of Freedom robotic hand via a few inputs. It was showed that

Central Nervous System (CNS) controls dozens of muscles for hand poses in a coordinated manner. Such coordination is referred as to a postural synergy, which corresponds to the coupled flexion and/or extension actuation statuses of the involved muscles.[14] CNS combines postural synergies, adjusting each synergy’s coefficient (weight), to realize various hand motions.

Combination of two primary postural synergies accounts for about 84% of the variance of many grasping postures.[15] What’s more, CNS switches between different sets of postural synergies for distinct grasping and manipulations tasks.[16]