Smoothly Switched Adaptive Torque Tracking for Functional Electrical Stimulation Cycling

Abstract

Motorized functional electrical stimulation (FES) cycling can serve as a rehabilitation strategy for individuals affected by neurological injuries. It is unique among human-robot interaction tasks because the cycle's motor must be simultaneously controlled alongside the rider's leg muscles (using neuromuscular electrical stimulation). In this letter, two tracking objectives are proposed for the FES cycle: 1) stimulate the rider's leg muscles to track a desired cadence, and 2) use the cycle's motor to indirectly track a desired torque with an adaptive admittance controller. A combined Lyapunov and passivity based switched systems stability analysis is conducted to prove the cycle's motor is able to globally exponentially track the admittance trajectory and stabilize the overall system. Additionally, this letter showcases a method for the rider to smoothly enable and disable torque tracking. Experiments are presented on four participants to show the efficacy of the controllers.