Active fault tolerant control based on adaptive back-stepping nonsingular fast integral terminal sliding mode approach

In this paper, finite-time active fault tolerant control based on adaptive back-stepping nonsingular fast integral terminal sliding mode control is proposed to control a lower limb exoskeleton in the presence of actuator fault. In order to detect, isolate and accommodate the actuator fault, a third-order super twisting sliding mode observer is used. To eliminate the chattering of conventional sliding mode, supper twisting sliding mode algorithm is applied, which leads to finite-time convergence and high precision in tracking the desired trajectories. Back-stepping term guarantees global stability based on Lyapunov theory. Upper limb motion is used to provide stability to robot's motion based on zero-moment point criterion. In order to attain maximum stability based on zero-moment point, minimize error in tracking the desired trajectories, increase the tolerance of the controller against actuator fault, controller, observer and upper limb trajectory parameters are optimally tuned based on harmony search algorithm. Performance of the proposed controller is compared with the performance of sliding mode controller with/without fault information. Simulation results reveal the effectiveness of the proposed controller in the presence of actuator fault, uncertainty and disturbance in comparison with sliding mode controller.