Fault-Tolerant Control Against Actuator Faults for Hydrofoil Attitude Control Systems via a Cascaded Extended State Observer

Due to challenging underwater conditions, hydrofoils serving as actuators within the hydrofoil attitude control system (HACS) are prone to failures that can lead to system instability and the inability to complete the intended mission. Fault tolerance in the presence of both faults and disturbances is crucial to ensure the safety and reliability of the system. This article proposes a fault-tolerant control (FTC) strategy for the HACS under stochastic ocean wave disturbances and actuator faults. To address this, a new cascaded extended state observer (ESO) is first employed to estimate system states and lumped disturbances, including mismatched disturbances and actuator faults. Through this approach, the need for separately deriving extended state dynamics equations is eliminated, reducing design complexity and computational load. Subsequently, by integrating a sliding mode control (SMC) technique with a prescribed performance approach to compensate for faults and disturbances, a fault-tolerant controller is developed to ensure that the target system remains state-bounded in the presence of actuator faults, with tracking errors converging to a small neighborhood. Finally, experimental validation based on a hardware-in-loop platform is conducted to verify the effectiveness of the proposed FTC scheme. Unlike traditional methods, which rely on additional dynamics to address faults and wave disturbances, the proposed ESO treats the disturbances as a lumped entity and utilizes nonlinear gain functions, simplifying the process. The findings show that the proposed method reduces tracking errors by at least 91.43% in the mean absolute error (MAE) metric compared to other methods across different cases.