Robust compensation of electromechanical delay during neuromuscular electrical stimulation of antagonistic muscles

Neuromuscular electrical stimulation (NMES) to extend as well as flex a limb joint requires stimulation of an antagonistic muscle pair. This is due to the fact that muscles are unidirectional actuators. The control challenge is to allocate control inputs to antagonist muscles based on the system output, usually a limb angle error. Further, NMES input to each muscle is delayed by an electromechanical delay (EMD), which arises due to the time lag between the electrical excitation and the force development in a muscle. EMD is known to cause instability or performance loss during closed-loop control of NMES. In this paper, a robust delay compensation controller for EMDs in antagonistic muscles is presented. A Lyapunov stability analysis yields uniformly ultimately bounded tracking for a human limb joint actuated by antagonistic muscles. The simulation results indicate that the controller is robust and effective in switching between antagonistic muscles and compensating EMDs during a simulated NMES task.