  We propose, implement and evaluate a natural human-machine control interface
for a variable stiffness transradial hand prosthesis that achieves
tele-impedance control through surface electromyography (sEMG) signals. This
interface, together with variable stiffness actuation (VSA), enables an amputee
to modulate the impedance of the prosthetic limb to properly match the
requirements of a task, 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 hand
impedance is managed through the impedance measurements of the intact upper
arm; 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. The proposed approach is advantageous, since the impedance regulation
takes place naturally without requiring amputees' attention and diminishing
their functional capability. Consequently, the proposed interface is easy to
use, does not require long training periods or interferes with the control of
intact body segments. The performance of the approach is evaluated through
human subject experiments where adequate estimation of references and
independent control of position and stiffness are demonstrated.
