A Switched Lyapunov-Passivity Approach to Motorized FES Cycling Using Adaptive Admittance Control

Abstract

For individuals with neuromuscular disorders (NDs) affecting the coordination and control of their legs, motorized functional electrical stimulation (FES) cycling serves as a rehabilitation strategy and offers numerous health benefits. A motorized FES cycle is an example of a hybrid exoskeleton involving cooperative physical human–robot interaction where both the cycle’s motor and rider’s muscles (through electrical stimulation) must be controlled to achieve desirable performance. A robust sliding-mode cadence controller is developed for the rider’s muscles, while an adaptive admittance controller is employed on the cycle’s motor to preserve rider comfort and safety. A switched systems stability analysis using Lyapunov and passivity-based methods is conducted to ensure global asymptotic stability of the admittance error system and that the cadence error system remains passive. Experiments are conducted on one able-bodied participant and four participants with NDs to illustrate the performance of the control design. For the participants with NDs, the controller achieved an average admittance tracking error of −0.08±1.05 rpm at an average cadence of 47.85±1.13 rpm for the desired cadence of 50 rpm.