A Single-Actuator Prosthetic Hand Using a Continuum Differential Mechanism
December 2014 to September 2016
Substantial progresses have been obtained towards versatile anthropomorphic prosthetic hands in the past years using emerging technologies. However the trade-offs between functionality, reliability, affordability, appearance, etc. have not been fully settled. Prosthetic hand designs have spanned a wide spectrum of varieties.
The human Central Nervous System (CNS) controls dozens of hand muscles in a coordinated manner. This coordination is referred to as a postural synergy. A fully actuated anthropomorphic robotic hand can then be controlled to achieve dexterous grasps via two to three channels of bio signals (e.g., electric myography). Although the synergy-based control has been implemented in a number of research prototypes, this approach might not be completely practical due to the concerns on a hand’s complexity, cost, weight, battery life, etc., associated with the use of ten or more servomotors. Despite the fact that mechanically implemented synergies have been proposed, the complex structures still limit their practical uses.
Postural synergy provides a continuous description of the hand motion atlas that can also be described by the discrete grasp taxonomy. Many prosthetic hands designs with underactuated structures and three to six motors often refer to such a grasp taxonomy in order to ensure the hands’ capabilities of performing various grasps. Besides the research prototypes, quite a few high-end commercial prosthetic hands also adopted such underactuated structures and five to six actuators, such as the Vincent hand (Vincent Systems), the iLimb and iLimb Pulse hands (Touch Bionics), and the Bebionic hands (RSL Steeper). These fancy prosthetic hands even support reprogramming of the controllers to achieve various distinct grasping postures. Even with the impressive functionalities, concerns might still stem from the affordability and durability of these hands.
One-actuator prosthetic hands are still widely used in clinics due to the structural simplicity and low cost, such as the SensorHand from Otto Bock. This company seems to prefer fewer motors. Even its latest product, the Michelangelo Hand, only has two actuators. A simple, robust and cheap hand design could be beneficial for its business success. With a similar belief, many researchers developed single-actuator prosthetic hands, using stacked lever linkages, differential pulleys, or compliant structures.
This project proposes the design of a single-actuator prosthetic hand using a continuum differential mechanism as shown in Fig. 1. Structure of the continuum differential mechanism is simple enough to allow all the components, including the actuator and a battery pack, to be packed into the palm.
Continuum differential mechanisms (CDMs) generate differential outputs via redistributions and/or deformations of their own materials and structures. Their working principle is fundamentally different from the existing differential mechanisms which generate differential outputs from the motions of the kinematic pairs.
The proposed continuum differential mechanisms have the planar and the spatial forms. They consist of a rigid base link, a flexible input backbone, a rigid end link, and two or three output backbones. The output backbones could be arranged arbitrarily around the input backbone in the spatial CDM while the backbones are in a plane for the planar CDM. All the backbones are attached to the end link and can slide in holes in the base link.
The CDM can provide pushing as well as pulling outputs. For the planar CDM, a force fa acts on the input backbone to generate two outputs to push external objects. When the load on the left is bigger (indicated by the longer arrow), continuing to drive the input backbone would bend all the backbones to generate differential outputs. The object on the right will be continuously pushed. Similarly for the spatial CDM, a force fa on the input backbone could generate three pulling outputs (external objects are pulled). When the load on one backbone is bigger (indicated by the longer arrow), continuing to drive the input backbone would bend the spatial CDM for differential outputs.
The single-actuator prosthetic hand has eleven joints, including ten active joints and one passive joint (the rotation joint of the thumb). The passive joint could be adjusted by the healthy hand, whereas the active joints are actuated by the outputs from a two-stage planar continuum differential mechanism.
Two prosthetic hand wwas designed, fabricated and assembled. Various grasps could be formed as follows.