Model-Free High-Order Sliding Mode Control for Stabilization of Underwater Vehicles

Station-keeping for underwater vehicles remains a challenging control problem due to nonlinear dynamics, uncertain hydrodynamic effects, and external disturbances. Many existing solutions rely on accurate system models or disturbance observers and are often validated only in simulation. This paper proposes a Model-Free High-Order Sliding Mode Controller (MF-HOSMC) for the stabilization of an 8-thruster underwater vehicle, addressing vertical position and angular motions (roll and pitch). The controller requires no prior knowledge of the system model and utilizes only measured outputs and their finite-time estimated derivatives. A high-order sliding surface combined with adaptive gain tuning ensures finite-time convergence, robustness, and reduced chattering. The proposed approach is validated through both numerical simulations and real-world experiments under unmodeled disturbances, demonstrating reliable and effective station-keeping performance. Experiments show a settling time of 14.32s and 4.35% overshoot, compared to 28.3s and 30% with a PID controller, demonstrating superior precision and faster response.