Disturbance observer based fractional-order sliding mode control for underwater vehicle with variable center of mass

To address the challenges of complex underwater environments and model parameter uncertainties, this paper proposes a fractional-order sliding mode control (FOSMC) algorithm integrated with a disturbance observer (DO) for an underwater vehicle-manipulator system (UVMS) with a variable center of mass. First, a conventional dynamic model of the underwater vehicle is established. Considering the motion of the underwater manipulator, an extended dynamic model incorporating the variable center of mass is further derived. A disturbance observer is then designed to estimate both external disturbances and parameter uncertainties in the dynamic model, with a theoretical analysis conducted to ensure its stability. Based on this foundation, a fractional-order sliding mode controller is developed, and a rigorous proof of closed-loop system stability is provided. Finally, numerical simulations are carried out to validate the effectiveness of the proposed control scheme. The results demonstrate that, compared to traditional sliding mode control, the proposed approach achieves superior trajectory tracking accuracy and enhanced robustness under complex hydrodynamic disturbances and parameter uncertainties.