An Improved and More Versatile Myoelectric Control

Yves Lozac'h


The use of myoelectrically controlled prostheses has been spreading throughout the world and experience with existing systems has provided information which points the way to further developments. Diversification of externally powered prostheses and orthoses to include pneumatic, electric and electrohydraulic types made necessary the design of a control unit with increased versatility and flexibility, since past systems had been designed to meet specific and unique requirements.

Following clinical experimentation with the Soviet myoelectric hand, workers at our Institute decided to produce a system which emphasized improved performance and miniaturization. Our first unit, completed in 1966, was described in an article entitled "Helping Hands." This on-off control device has since been applied routinely to our patients. In 1968, continued design efforts resulted in the development of our first proportional control unit. The operation of this system is such that any increase in the rectified myoelectric signal produces a proportional increase in the DC power available to the motor.

Clinical results obtained with this control unit show that the time delay between the command input signal and the actuator movement has been considerably reduced as compared to the on-off system. This reduction has permitted a more dynamic response of the controlled devices, effecting a closer physiological link between the user and his prosthesis. Furthermore, it is now possible for the user to manipulate fragile objects and to perform fine work with pliers or tweezers.

The weaknesses of this particular item are high-energy dissipation in the output transistors and low efficiency in the vicinity of the neutral resting zone. To overcome these inconveniences a new system based on pulse-frequency modulation was devised.

Before describing some important characteristics of this new control unit, it should be emphasized that all current prosthetic and orthotic devices require that the patient accept some inconveniences and discomfort. To offset these considerations, the device must significantly improve his functionality or appearance, or both. Some factors which generally contribute to the rejection of a myoelectric prosthesis by the patient are: poor prosthetic performance, unreliability, difficulties in obtaining service, battery maintenance, excessive weight or dimensions, and high cost. The prosthetist may also reject a device for various reasons such as its complexity and difficulties with fitting or repairs.

In the light of this knowledge and drawing upon past experiences we have included the following characteristics in our most recent unit (Fig. 1 ):

  • Modular construction-so that it can be assembled easily by any qualified prosthetist.
  • Electrodes and myoelectric amplifier encapsulated in a single block. The unit is now disposable and, in the event of failure, may be replaced by a new one at reasonable cost. Thus, the prosthetist may proceed rapidly with repairs without having to consult an electronics technician. Furthermore, repairs can be made at predetermined costs. With this arrangement, a better separation of the signal from background interferences has been obtained and, indirectly, a more dynamic response from the controlled device.
  • Reduced overall dimension of the assembly so that it can be incorporated easily into the laminated socket of any type of prosthesis.
  • Power supplied by a single battery.
  • Negligible standby power requirements. This factor is very important since it is, of course, necessary to provide the energy consumed during both activity and rest.
  • Versatility and flexibility of utilization. The unit can proportionally control an electric prosthesis, and, with the aid of pulsed electromagnetic valves, a hydraulic or pneumatic prosthesis. Moreover, it accommodates either a myoelectric control or a displacement transducer adapted to some anatomical movement.
  • Proportional control based on pulse modulation. One of the advantages of this technique is the possibility of adjusting the minimal pulse width applied to the motor so that each of the pulses corresponds to a certain motor rotation. In this way efficiency about the neutral resting zone is improved. Besides, a discrete tremor generated by the pulses informs the patient that his prosthesis is working, thus avoiding unnecessary power dissipation. However, the acoustic level of the tremor is such that it is not noticeable to the ear. The second advantage is the low energy dissipation in the output transistors, thus allowing for even further economy of battery power and higher efficiency.

Summary

The degree of clinical acceptance of a myoelectrically controlled prosthesis depends on both the patient and the prosthetist. With this in mind, a myoelectric control unit has been designed with the following characteristics: proportional control of both force and speed, versatility of application, modular construction which facilitates repairs and installations, small size, and low standby power requirements.

Acknowledgements

The author gratefully acknowledges the many helpful suggestions from Messrs. C. Corriveau, C.P.O., J. P. Courtemanche, C.P.O., and R. Adam, CO., concerning the integration of the unit into the prosthesis.

Department of Research Rehabilitation Institute of Montreal Montreal, Quebec, Canada

References:
Bottomley, A. H , A. B Kinnier-Wilson, and A. Nightingale, Muscle substitute and myoelectric control. J. Brit. I.R.E., 26:439-48, 1963.
Sherman, E. D., A. L. Lippay, and G. Gingras, Prosthesis given new perspectives with external power. Hospital Management, 100:44-49, November 1965.
Lozac'h, Y., A. L. Lippay, E. D. Sherman, and G, Gingras, Helping hands. Electronics, 124-131, Aug. 7, 1967.
Godden, Alex K., The techniques of myoelectric control of prostheses and the prospects of this type of control for thalidomide casualties. Engineering Lab. Rept. No. 1, 048,68. University of Oxford, England, March 1968.