Page 186 - Handbook of Biomechatronics

184 Georgios A. Bertos and Evangelos G. Papadopoulos

needed hygiene for keeping the control sites functional and free of infec-
tions (see “Cineplasty” section).

1.4.2 Myoelectric
Myoelectric control systems use muscle electricity as the control method for
controlling the prosthesis. Myoelectric control’s distinctive characteristic is
that it uses electromyogram (EMG) signals from the stump as inputs to con-
trol the upper-limb prosthesis. Surface electrodes placed on the skin near a
muscle can detect the electricity produced by contracting muscles at the
nearby area. The intensity of the EMG signal produced increases as muscle
tension increases. The signal is detected from the surface electrodes, ampli-
fied, processed, and then used to control the prosthesis (Fig. 3).
Myoelectric control first appeared in the 1940s but it was only until
1970s that it was broadly used in the clinical environment. Today myoelec-
tric control is a favorite but may not be the best way of fitting upper-limb
prostheses. It provides open-loop velocity control which is inferior to posi-
tion control achieved from a position controller like a power-enhanced
extended physiological proprioception (EPP) controller. In myoelectric
control, the input command signal is proportional to the speed of the pros-
thesis. Visual feedback is the only feedback to inform the amputee of the
state of the prosthesis. The advantage of myoelectric control over power-
enhanced EPP control is that myoelectric does not require neither a harness
nor a cineplastic surgical procedure. Myoelectric control disadvantage over
EPP is that it does not provide proprioceptive sensory feedback. In addition,
myoelectric control is velocity control which has been proven to be inferior
to position control in positioning tasks (Doubler and Childress, 1984b).

1.4.3 Extended Physiological Proprioception
The problem of control, or how one interfaces the amputee to the
prosthetic-mechanical arm is one of the most challenging problems. Today’s
externally powered systems for upper-limb prostheses use switch or myo-
electric controllers implementing open-loop velocity control strategies.
Doubler and Childress (1984a,b) have demonstrated that the position
control is superior to velocity control in positioning tasks. In addition, pros-
thesis control techniques that incorporate the body’s own proprioceptive
sensors and actuators (e.g., body-powered systems) seem to be incorporated
easier by prosthetic users and to result in subconscious control (Childress,
1989). Open-loop velocity control implemented by switch or myoelectric
control cannot provide feedback. D.C. Simpson, at Edinburgh, in 1969 first