A Hybrid Systems Approach to Dual-Objective Functional Electrical Stimulation Cycling

Motorized functional electrical stimulation (FES) cycling can serve as a physical rehabilitation strategy for individuals whose lower limbs are affected by neurological injuries. Motorized FES cycling is unique among human-robot interaction tasks because the cycle's motor and rider's leg muscles must be simultaneously controlled. In this letter, two tracking objectives are proposed for the combined cycle-rider system. First, the rider's leg muscles are stimulated to pedal the cycle at a desired cadence, and second, the cycle's motor is used to regulate the interaction torque between the cycle and rider using an admittance controller. A novel hybrid systems analysis using Lyapunov- and passivity-based techniques is conducted to prove that the cycle's motor can globally exponentially regulate the admittance error system and prove that the cadence error system is output feedback passive. Experiments conducted on five participants illustrate the efficacy of the controllers with an average admittance error of -0.01 +/- 0.70 RPM at an average cadence of 45.51 +/- 1.77 RPM for a desired cadence of 50 RPM.