Non-Singular Terminal Sliding Mode Control Algorithm with Nonlinear Disturbance Observer and Adaptive Thrust Allocation Method Applied to an UUV with multimodal motion characteristics

To improve the stability and adaptive ability of underwater robots in dynamic environments, this paper proposes a non-singular terminal sliding mode control method (NTSMC) combining nonlinear disturbance observer (NDO), referred to as NDO-NTSMC. The thrust allocation algorithm based on sequential quadratic programming (TA-SQP) and the adaptive thrust allocation algorithm (ATA-SQP) are designed for fixed and variable propulsion systems. Experimental results show that, under six degrees of freedom external disturbances, the lateral error in vector propulsion mode is significantly reduced compared to quadrotor mode, with RMSE and MAE reduced by 80.08% and 78.38%, respectively. For trajectory tracking, NDO-NTSMC reduces the average RMSE by 77.5%, 81.2%, and 90.5% compared to NTSMC, SMC, and PID. The proposed control method shows robustness under disturbances, with the adaptive thrust allocation algorithm offering higher energy efficiency and adaptability, especially during propulsion system changes. The thrust fault tolerance experiment also confirmed that even when a dual thruster failure occurs, the system can still maintain trajectory tracking and stability.