A predictor-based compensation for electromechanical delay during neuromuscular electrical stimulation-II

Neuromuscular electrical stimulation (NMES) is a promising technique to restore functional mobility in persons with paraplegia. Closed-loop control of NMES allows precise and accurate limb control in critical tasks such as gait restoration or gait retraining. However, a major cause of degraded performance and instability during NMES control is electromechanical delay (EMD). Uncertainty, exogenous disturbances (e.g., spasticity), and particularly, unknown nonlinear muscle force-length and muscle force-velocity relationships in the musculoskeletal system complicate control design to compensate for EMD. Despite these difficulties, a predictor-based control method that compensates for EMD in the uncertain musculoskeletal system is developed in this paper. The developed control method is an improvement over our previous work in which only proportional-derivative (PD)-type delay compensating controller was developed for a completely unknown nonlinear musculoskeletal system. The result incorporates integral control in the previous PD-based control design (i.e., a PID-type delay compensating controller is developed) and is shown to achieve uniformly ultimately bounded tracking.