Résumé
This paper presents a new finite-time fractional-order Robust Integral of the Sign of the Error (RISE) control of an actuated ankle-foot orthosis (AAFO) to assist in plantarflexion and dorsi-flexion movements of the ankle joint; within an assistance-as-needed paradigm. The proposed approach incorporates a nonlinear disturbance observer (NDOB) to estimate the human torque alongside other external torques acting on the AAFO-wearer system. In complement to the estimated human effort and the other external torques, the control input torque provided by the AAFO is based on two main terms, (i) a smooth adaptive finite-time fractional-order RISE term built based on fractional integral and derivative calculus to ensure the tracking performance, and (ii) an adaptive feedforward term to compensate for the system nonlinearities and reduce the effect of measurement noise on the control signal. A new augmented fractional-order filtered error is designed to modulate the convergence of the tracking ankle joint position and velocity errors and the adaptive parameters. The adaptation mechanism of the proposed fractional RISE also adopts fractional dynamics. A finite-time stability analysis of the whole proposed control approach is demonstrated. The effectiveness of the whole proposed controller is evaluated through real-time experiments on five healthy subjects. Comparison, in terms of tracking performance shows that the proposed controller outperforms several benchmark approaches and robustness tests are also conducted to validate its effectiveness.