Sliding mode control of AUV trajectory tracking in the presence of disturbance

In this paper, trajectory tracking control of AUV in the horizontal plane is investigated. Due to the disturbances in ocean environment, as well as uncertainty of AUV parameters, a two-layer adaptive and robust control is proposed. The conventional sliding mode control as a robust and nonlinear control technique is one of the most applicable methods for AUVs. Unfortunately, it leads to a chattering that is an undesirable phenomenon for driver and motors circuits of AUV. The bound of the switching part of the control signal in the sliding mode control, which is chosen according to disturbances, is a cause of chattering. The greater the gain of the switching control, the lower the tracking error, and the chattering takes place instead. Therefore, a compromise between chattering and tracking error should be made. Combination of adaptive and sliding mode control, while increasing the robustness against disturbances, is also effective in reducing chattering. Furthermore, disturbance is estimated by an adaptive scheme. Using estimationed data, the control effort is calculated and applied to the control system. As a result, chattering decreases and robustness against disturbance is also maintained. In the proposed method, the control objectives are realized by using an adaptive controller based on the conventional sliding mode control; which is robust against the uncertainty and disturbance caused by ocean waves, assuming that the disturbance and its derivative are bounded by unknown boundary values. This methodology is based on a two-layer adaptive law which is not dependent upon the prior knowledge of disturbance boundary and its derivative. The stability of the proposed adaptive-robust control law is proved by the use of Lyapunov theory and the controller's performance is verified through the simulation results. The proposed controller performance is evaluated, in terms of error and control effort. According to the results of comparison and investigation in various disturbance scenarios, the tracking error in the proposed control is found much less than the conventional sliding mode control. In addition, the amplitude of control effort is greater in conventional sliding mode control and is accompanied by chattering, while no chattering is observed in the proposed adaptive-robust control, when the required amplitude of the control effort is estimated. The study is verified through analytical approach as well as simulations.