Abstract
Functional electrical stimulation (FES) has been considered to be an effective technique for aiding quadriplegic persons. However, the musculoskeletal system is highly nonlinear and varies with time; moreover, it is subjected to fatigue and has variable reflex characteristics. It is also difficult to stably and accurately control the limbs using FES. In this paper, we propose a simple FES method that is consistent with the motion control mechanism observed in humans. We focus on joint motion by a pair of agonist-antagonist muscles of the musculoskeletal system, and define the “electrical agonist-antagonist muscle ratio (EAA ratio)” and “electrical agonist-antagonist muscle activity (EAA activity)” in the light of the agonist-antagonist muscle ratio and agonist-antagonist muscle activity, respectively, proposed to extract the equilibrium point and stiffness from Electromyography (EMG) signals. The agonist-antagonist muscle ratio and agonist-antagonist muscle activity are based on the theory that the equilibrium point and stiffness of the agonist-antagonist motion system controlled by the central nerve system. We derived the transfer function between the input EAA ratio and force output of the end-point; this transfer function model is expressed as a cascade-coupled dead time system and secondary delay system. Through the agonist-antagonist muscle stimulation pattern determined by this transfer function model, high-speed, high-precision, and smooth control of the hand force were achieved.
