Design, implementation, and evaluation of a backstepping control algorithm for an active ankle–foot orthosis

Active ankle–foot orthoses are used to assist patients suffering from stroke, multiple sclerosis, cerebral palsy etc. through providing them an external force supply to track normal gait cycles. In this paper, an active ankle–foot orthosis prototype is developed and an adaptive backstepping control algorithm is proposed for tracking desired gait trajectories while reducing the effects of unknown disturbances. A prototype of the orthosis, which is composed of a series elastic actuator, a lever mechanism and an orthotic shoe, is developed. The prototype is mathematically modeled as a two-degree-of-freedom mass–spring system. The unknown disturbances are modeled as a finite sum of sinusoidal signals with unknown amplitudes, frequencies and phases, and an unknown constant. The backstepping control algorithm is designed for the force input supplied to the system, and the stability of the equilibrium point is proved. The proposed algorithm is implemented in a real-time operating system to control the developed ankle–foot orthosis prototype. Numerical simulations and physical experiments on healthy human subjects are performed to show the superiority of the proposed design over a PID controller in trajectory tracking of the prototype. As a further development, an embedded system is designed to bring portability to the ankle–foot orthosis prototype. Experiments are performed to observe the execution performance and power consumption of the embedded system.