Myoelectric control

Englehart, K. and B. Hudgins, 2003, A robust, real-time control scheme for multifunction myoelectric control, IEEE Trans. Biomed. Eng. 50(7) 848-854. 

Shannon, G.F., A myoelectrically-controlled prosthesis with sensory feedback. Med. Biol. Eng. Comput., 1979.17 73-80. 

Externally powered prostheses use electrical power to provide function. The electrical power is applied via motors located in the terminal device (hand or hook), wrist, and elbow. The grip force of the hand can be in excess of 100 N. Command signals are generated either by voluntary contraction of muscles, so-called myoelectric control, or by using switches of different kinds. For apphcations that are more complex, both the command signals are used for different operations (e.g., control of several degrees of freedom). 

FIGURE 70.6 The Utah arm is a self-contained battery powered artificial arm-hand complex. The system comprises myoelectric control of the elbow, wrist, and grasping movements. 

Scott, T.R.D., Peckham, P.H., and Kilgore, K.L., Tri-state myoelectric control of bilateral upper extremity neuroprosthesis for tetraplegic individuals, IEEE Trans. Rehab. Eng. TRE-4 251-263, 1996. 

FIGURE 6.18 A method for processing EMG data for a myoelectric control signal, (a) the raw EMG data and (b) the signal after being rectified and low pass filtered (10 Hz cutoff). 

Evans, H., Pan, Z., Parker, P., and Scott, R. (1994). Signal processing for proportional myoelectric control, IEEE Transactions on Biomedical Engineering, 41 207-211. 

Finally, it is possible to have a combination of PWM and PPM where one has variable width pulses and variable periods. In all cases, the output PWM stream is a series of pulses in which the dc voltage level is proportional to the ratio of the amount of on time with respect to the amount of off time for the pulses in the stream. Myopulse modulation, as implemented by Childress (1973) in his myoelectric controller, is an example of this form of pulse modulation (Fig. 32.25). This is an elegant proportional controller that can be implemented with a minimum of components. Because percentage myoelectric signal on time (EMG signal level above an arbitrary threshold) is monoton-ically related to muscle output force level, a proportional signal can be achieved very simply. This... 

FIGURE 32.25 Myo-pulse modulation (Childress, 1973)—the voltage level seen at the motor is proportional to the ratio of ON time of the pulses to Ol time. The mean of this ratio is in turn proportional to the intensity of muscle contraction and as such is a very simple means of providing proportional myoelectric control, (a) The trace y(f) shows the switching response of a comparator in response to a time-varying EMG signal e(r) and is given by the expression ... 

Myoelectric control derives it name from the electromyogram (EMG), which it uses as a control input. When a muscle contracts, an electric potential (the EMG) is produced as a by-product of that contraction. If surface electrodes are placed on the skin near a muscle, they can detect this signal (Fig. 32.26). The signal can then be electronically amplified, processed, and used to control a prosthesis. While the intensity of the EMG increases as muscle tension increases, the relationship is a complex nonlinear process that depends on many variables, including the position and configuration of the electrodes (Heckathome and Childress, 1981). Although the EMG is nonlinear it is broadly monotonic, and the human operator perceives this response as more or less linear. 

The first externally powered prosthesis was a pneumatic hand patented in Germany in 1915. Drawings of this hand and possibly the first electric hand were published in 1919 in Ersatzglieder und Arbeitshilfen (Borchardt et al., 1919). The first myoelectric prosthesis was developed during the early 1940s by Reinhold Reiter. Reiter published his work in 1948 (Reiter, 1948), but it was not widely known, and myoelectric control had to wait to be rediscovered during the 1950s. Reiter s... 

The Russian hand was the first semipractical myoelectric hand to be used clinically. This hand also had the distinction of being the first to use transistors (germanium) to process the myoelectric control signal (Childress, 1985). In this country, following World War II, the Committee on Artificial Limbs contracted with IBM to develop several electrically powered limbs. These were impressive engineering feats in their day but never found use outside the laboratory (Klopsteg and Wilson, 1956). 

Myoacoustic signals are auditory sounds created as a by-product of muscle contraction. A myoacoustic control system is very similar in structure to a myoelectric control system, but the elimination of unwanted acoustic noise appears to be a bigger problem than the elimination of unwanted electrical noise in myoelectric systems (Barry and Cole, 1990). It has not gained widespread use. 

Taylor, D. R., and Wirta, R. W., (1970) Development of a myoelectrically controlled prosthetic arm. In Advances in External Control on Human Extremities, Proceedings of the Third International Symposium on External Control of Human Extremities, Dubrovnik, Yugoslavia, Aug. 25-30,1969, Yugoslav Committee for Electronics and Automation (ETAN), Belgrade, Yugoslavia. 

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