FEXO Knee: A rehabilitation device for knee joint combining functional electrical stimulation with a compliant exoskeleton

This paper presents the design and control of a novel assistive system, FEXO Knee, which combines functional electrical stimulation (FES) with a compliant exoskeleton for better physical rehabilitation of knee joint. The exoskeleton and FES work together in a synergetic manner that attempts to allow arbitrary torque allocation via regulating a tunable gain. The study focuses on controlling human rhythmic movements, i.e., the swing of shank, to demonstrate the assistance efficiency of the hybrid FES-exoskeleton rehabilitation. Two muscle groups (Vasti and Hamstrings) are stimulated to produce active torque for knee joint. The reference trajectories of the exoskeleton and FES are provided by central pattern generator that acts as a phase predictor to deal with unexpected phase confliction between human shank and exoskeleton. The modulated pulse width of FES stimulator is controlled by a model-based feed-forward controller. The elastic cable-driven actuator of knee exoskeleton allows safe interaction with the patients and avoids abruptly large torque shocks, which is more important than pure position tracking in robotic-assisted rehabilitation. The motion of the knee exoskeleton is controlled by a proportional-integral-derivative controller. The joint angle is the only feedback signal that needs to be measured in the control frame. The mutual torque is also measured during the swing but it is merely for the purpose of performance evaluation. Four healthy subjects participate in the initial evaluation experiments and the results show good performance of the hybrid FES-exoskeleton system.