Variable Stiffness Transradial Hand Prosthesis


We have developed a low-cost, customizable, easy-to-use transradial hand prosthesis capable of adapting its stiffness. Variable stiffness actuation (VSA) of the prosthesis is based on antagonistically arranged tendons coupled to nonlinear springs driven through a Bowden cable based power transmission. Bowden cable based antagonistic VSA can, not only regulate the impedance and the position of the prosthetic hand, but also enable a light-weight and low-cost design, by opportunistic placement of motors, batteries and controllers on any convenient location on the human body, while nonlinear springs are conveniently integrated inside the forearm. The transradial hand prosthesis features tendon driven underactuated compliant fingers that allow natural adaption of the hand shape to wrap around a wide variety of object geometries, while the modulation of the stiffness of their drive tendons enables the prosthesis to perform various tasks with high dexterity. The compliant fingers of the prosthesis are custom built from polyurethane materials utilizing a low-cost manufacturing process and adds inherent robustness and flexibility, even under impacts.

Control of the variable stiffness transradial hand prosthesis is achieved by a surface electromyography (sEMG) signals based natural human-machine interface, called tele-impedance control. This interface, together with variable stiffness actuation, enables an amputee to modulate the impedance of the prosthetic limb to properly match the requirements of a task at hand, while performing activities of daily living. Both the desired position and stiffness references are estimated through sEMG signals and used to control the VSA hand prosthesis. In particular, regulation of finger impedance is managed through the impedance measurements of the intact forearm; this control takes place naturally and automatically as the amputee interacts with the environment, while position of the hand prosthesis is regulated intentionally by the amputee through the estimated position of the shoulder, extracted from sEMG signals of the agonistic and antagonistic muscles placed under shoulder and chest. The proposed approach is advantageous, since the impedance regulation takes place naturally  from task to task or during execution of a single task without requiring amputees’ attention and diminishing their functional capability. Consequently, the proposed interface does not require long training periods or interfere with the control of intact body segments, and provides the amputee with ease of use. The performance of the tele-impedance control of the VSA hand prosthesis is experimentally evaluated through human subject experiments where adequately estimation of position and stiffness references using sEMG signals and independent control of position and stiffness of the prosthesis are demonstrated. Furthermore, further tests with volunteers provide evidence that the proposed interface is easy to use and improves dexterity and performance while interacting with unstructured environments.