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A fixed-parameter, discrete-time, first-order, feedback control system is described for regulating grasp during electrical stimulation of paralyzed muscles of the hand. The stiffness of the grasp (relationship between grasp force and grasp opening) is kept constant by linearly combining force and position feedback signals. Thus, a single continuous command signal can control the size of the grasp opening prior to object acquisition and both grasp force and opening after contact. The controller achieves this change in controlled variables by scaling and summing the force and position feedback signals, rather than by a discrete switch in control strategy. Experimental tests of the control system in quadriplegic subjects show that control can be obtained over conditions ranging from unloaded position regulation to isometric force regulation, as well as in the transition between these conditions. The robustness of the control system was evaluated during force regulation with isometric loads. Step response rise time and overshoot were much more dependent on system gain than on the location of the controller zero. Responses with rise time less than two seconds and overshoot less than 30% were obtained over a gain range up to ten, indicating good robustness to muscle gain reductions such as might be caused by fatigue.
Crago et al. (Tue,) studied this question.