Mechanical Implementation of Postural Synergies of an Underactuated Prosthetic Hand

Recent advances in neurology showed that human controls dozens of muscles for hand motions in a coordinated manner. Such coordination is referred as to a postural synergy and synergies could be combined to form various hand poses. Implementing the synergies digitally in the controller, 6 to 12 motors of a robotic prosthetic hand can be controlled by a few synergy inputs from an amputee's bio-signal interfaces. This paper proposes to implement the synergies via a mechanical transmission unit so that two rotation inputs can be scaled, combined and mapped to 13 rotary outputs to enable not only grasping but also manipulation of a prosthetic hand. Synthesis of the postural synergies and design of the prosthetic hand are briefly reviewed, whereas the transmission design is elaborated as the implementation of the postural synergies. Tests were performed to quantify how well the synergies could be reproduced via the transmission. Experiments that follows are expected to demonstrate the effectiveness of constructing a low cost yet versatile prosthetic hand by mechanically implementing the postural synergies.