Fractional order adaptive fuzzy terminal sliding mode controller design for a knee joint orthosis with nonlinear disturbance observer

Rehabilitation and assistive robots have drawn a large amount of interest, related to the increase of the elderly and the increase in diseases such as stroke and spinal cord injuries as well as the high cost of rehabilitation. In this paper, a new fractional order adaptive fuzzy terminal sliding mode control is proposed for a knee joint orthosis. This robot is used for rehabilitation and assistive purposes. A model integrating the human lower-limb and orthosis based on the Lagrange equations is used. A sliding mode control is designed to overcome the uncertainties and external disturbances. Then an adaptive fuzzy controller is designed to remove the undesirable chattering phenomenon in control signal caused by sliding mode control. Then a nonlinear disturbance observer is combined with fractional order terminal sliding mode control to improve the precision and speed of tracking and to decrease the effect of the uncertainties in muscular torque modeling on the system control. The stability of the closed loop system is proved by Lyapunov theorem. The PSO algorithm is used to determine the coefficients of the adaptive fuzzy fractional order terminal sliding mode control and the coefficients of the fuzzy membership functions. Finally, the performance of the proposed controller is compared with conventional sliding mode control.