Fractional-order nonsingular terminal sliding mode control of hydraulic manipulators using time delay estimation

For the trajectory tracking control problem of rigid hydraulic manipulators under heavy uncertainties and nonlinearities, a novel fractional-order nonsingular terminal sliding mode (FO-NTSM) control method based on time-delay estimation (TDE) is proposed. The proposed control scheme mainly contains two parts: a TDE term and a FO-NTSM term. The TDE term is applied to approximately cancel the complex system dynamics using the intentionally time-delayed information leading to an attractive model-free nature. Meanwhile, the FO-NTSM term, based on a novel fractional-order terminal sliding surface, is designed to stabilize the tracking error to zero in finite time. Stability of the closed-loop control system is analyzed based on the Lyapunov stability theory. Finally, comparative degrees of freedom (2-DOF) practical experiments are performed and the results show that the new proposed method can ensure faster convergence rate and higher tracking precision under heavy lumped uncertainties and nonlinearities compared with its integer-order counterpart.