Switched Motorized and Functional Electrical Stimulation Cycling Controllers for Power Tracking

Motorized functional electrical stimulation (FES) induced cycling is a rehabilitation technique, where lower-limb muscles are artificially activated and an electric motor provides assistance to achieve cadence (speed) and torque tracking objectives. In this paper, cadence and torque controllers are designed based on a cycle-rider model with switched muscle and motor inputs computed based on state-dependent switching. Cadence tracking is accomplished by switching across lower-limb muscles (within the rider's kinematic efficient regions of the crank cycle) and an electric motor (within the rider's inefficient regions of the crank cycle). The position and cadence reference trajectories are generated by a target impedance model yielding bounded trajectories. A robust sliding-mode torque controller is designed for the electric motor to track a desired interaction torque, when the muscles are stimulated within the kinematic efficient muscle regions. A passivity-based analysis is developed to ensure stability of the closed-loop torque subsystem and a Lyapunov-based stability analysis ensures exponential cadence tracking.