Adaptive Fixed‐Time Disturbance Observer Based Optimal Sliding Mode Control for Dynamic Positioning Ships With Uncertainties and External Disturbances

ABSTRACT In this paper, an adaptive fixed‐time disturbance observer based optimal sliding mode control (AFTDO‐OSMC) strategy is presented for dynamic positioning ships affected by uncertainties and external disturbances. First, a control Lyapunov function based inverse optimal control (CLF‐IOC) law is designed for the nominal ship model. Then, to ensure the robustness of the designed CLF‐IOC law under uncertainties and external disturbances, an OSMC law is proposed by combining the SMC and CLF‐IOC, and an AFTDO is integrated to attenuate the effect of uncertainties and external disturbances. The developed AFTDO can guarantee the practical fixed‐time stability of estimation errors and needs no prior knowledge of the bound value of lumped uncertainties. With the aid of the developed AFTDO, the gain of discontinuous control part is reduced remarkably, and therefore the control chattering is alleviated. Finally, the effectiveness of the proposed control strategy is demonstrated by numerical simulations.