A backstepping control design for an active ankle-foot orthosis

Active ankle-foot orthoses are used to assist patients suffering from stroke, cerebral palsy etc. through providing them an external force supply to track normal gait cycles. In this paper, we propose a novel backstepping control design for active ankle-foot orthoses to track desired gait trajectories and to reduce the effects of unknown disturbances such as ground reaction force and weight of the foot. The actual system is modeled as two-degree-of-freedom mass-spring system. Disturbances are modeled as a finite sum of sinusoidal signals with unknown amplitudes, frequencies and phases, and an unknown constant. An adaptive backstepping control law is designed for force input to the system. It is proved that the equilibrium of the closed loop system is stable, and vertical position of the patient's heel tracks the desired trajectory. Finally, simulations are performed to show superiority of the proposed control design over a PID control.