Externally-Powered
Prostheses
photo source: http://www.rehabtech.org.uk/r&d.html
Description
Electric motors are used in externally-powered prostheses to operate hand
function The three main types of control of an externally-powered prosthesis
are:
• Harness Switch Control (on/off)-
shrugging of shoulders pulls on the switch
• Servo Control- force sensor placed
in harness
• Myoelectric control- one of the newest
and most popular ways to control an electrical prosthesis.
In the myoelectric control system there is an electrode attached to the surface of the skin to record the electromyograph (EMG) signal. The recorded signal is amplified and then processed by a controller that switches the motor on or off in the hand, wrist or elbow to produce movement and function. The system can have a one or two muscle control circuit. The user learns to select and control the muscle(s) required to operate the arm and hand. With a two muscle control sites, one is used for opening and one for closing the hand. If only a single muscle can be used, than the size or speed of the contraction can be used to indicate either the open or close signal.
photo source: http://pubweb.acns.nwu.edu/~rfw586/Interests.htm
Advantages
Electric motors in externally-powered prostheses, increase the grip force
of the hand significantly––often in excess of 20-32 pounds.
For persons with severe scar tissue, or the inability to generate sufficient
force, an externally-powered device can a viable alternative to a body-powered
prosthesis. Myoelectric prostheses transmit electrical activity detected
by surface electrodes on the residual limb muscles to activate electric
motor and so simply flexing a muscle can control a myoelectrically powered
prosthesis. The need for tight harnesses and cables, as used in body-powered
prostheses, is eliminated. Furthermore, the operational range is greatly
increased. It can be used out to the sides of the body, down by the feet
and even over the head.
Disadvantages
One of the major inconveniences of the externally-powered prostheses is
the required battery system. The systems must be charged, maintained,
and sometimes replaced. Electrically-powered prostheses also tend to be
heaver than other options because of the combined weight of the motor
and the battery. Damage from water exposure may result in expensive repairs,
adding to the already costly system.
References
1) Bodeau, Valerie S. “Upper Limb Prosthetics.” 2002. eMedicine:
Instant Access to the Minds of Medicine. 23 Apr. 2003. <http://emedicine.com/pmr/topic174.htm>.
2) Dailami, Jeremy. “The Myoelectric Arm: It’s Electrifying.”
2002. Illumin: A Review of Everyday Life and Engineering. 10 Apr. 2003.
<http://engrwp.usc.edu/illumin/article.print.php?articleID=6>.
3) “Externally Powered Prostheses.” The Catholic University of America. 10 Apr. 2003 <http://engineering.cua.edu/biomedical/courses/be524/powered/>.
