A hybrid FES-motor cooperative control over a knee joint movement: A feasibility study

The integration of functional electrical stimulation (FES) with active motorized exoskeletons is gaining interest in the motor rehabilitation of spinal cord injury (SCI) patients. FES-based hybrid robotic solutions would enable SCI subjects, suffering a partial or total loss of sensation and muscle function, to combine the advantages of limb mobilization provided by exoskeleton motors with the typical therapeutical benefits of FES. However, the development of a FES-motor shared control still represents a challenging problem because of actuation redundancy and FES non-linearity. In this work, a FES-motor cooperative control is proposed and tested over a single-joint movement, using an iterative learning control (ILC) for FES and an impedance-based control for the motor. The system includes a lower-limb motorized exoskeleton and a commercial electrical stimulator. Four healthy subjects were involved in a testing session during which conditions with a different motor contribution were compared in terms of tracking accuracy and motor torque contribution. Results evidenced that FES, beyond providing physiological benefits, is able to support movement execution when the input from the motor is reduced. This represents a promising starting point in the perspective of reducing motor's power with advantages for exoskeletons usability.