Backstepping Sliding Mode Control for Path Following of Underactuated AUV Affected by Ocean Currents

In the marine environment, the motion characteristics of autonomous underwater vehicles (AUVs) are affected by unknown factors such as time-varying currents and waves, which increase the complexity of path following controller design. In order to solve the effects of unknown currents and external disturbances on the path following of AUV, the kinematics and dynamics of AUV in the current environment are firstly modeled, and the following error equations are constructed in the Serret-Frenet coordinate system. The design of the kinematic controller is accomplished based on the line-of-sight and backstepping methods. In terms of the dynamics control, the adaptive law is utilized to estimate the external disturbances and combined with the sliding mode control method to derive the control law. Further, in order to reduce the deviation caused by the current on the AUV navigation, a current observer is designed to estimate the current speed online. Through theoretical analysis, the cascade control system jointly composed of the current observer and the following controller is proved to be globally asymptotically stable. Simulation and comparison results show that the proposed method can effectively control the AUV to track the expected path in the presence of unknown currents and environmental disturbances, proving the effectiveness and robustness of the control strategy.