ProPosition™: a proportional myoelectric hand prosthesis control system with internal position feedback

In myoelectric hand prostheses the degree of sensory feedback and the quality of the forward path from the Central Nervous System (CNS) to the prosthesis are important parameters for achieving successful control of the device. Most approaches to artificial sensory feedback have failed to gain acceptance among the users. Proportional open-loop control systems, on the other hand, have significantly improved the forward path and are used widely. A new control system was proposed utilizing a position servo motor and a two-site RMS-based myoprocessor. The servo controller and myoprocessor were implemented in a computer. A commercial hand prosthesis with additional position transducer was connected to the computer via a power amplifier, and the system was tested on-line with EMG from the m. flexor and extensor digitorum of a non-disabled volunteer. The test person was able to control both position and grip force proportionally. It proved possible to tune the system so that the prosthesis imitated the movements of the test person’s hand. A special power saving algorithm significantly reduced unwanted servo fluctuations due to variance in myoprocessor output. The internal position feedback of the ProPosition™ control system effectively eliminates the integral relationship, displayed by variable-speed systems, between the amputee’s command and the resulting position of the prosthesis. Instead it gives a proportional relationship between the steady-state position and the level of muscle contraction, thus further improving the quality of the forward path. The level of muscle contraction is fed back to the CNS by sensory receptors in the stump tissue, providing a virtual position feedback path. It thus gives a pseudo position proprioception, potentially reducing the need for visual feedback and concentration when controlling the device.