Research On Path Following Control Of An Under-actuated UUV Based On Adaptive Sliding Mode Control

The path following control provides the important foundation for civilian and military applications of unmanned underwater vehicle (UUV). It is of great significance for control theory and practical application of UUVs to do in-depth research on the issue of the path following. The design of the path following controller of an under-actuated UUV was studied based on adaptive sliding mode control (ASMC) method in this master dissertation. In order to make the path following control system get better control performance and become more in line with constraints, non-holonomic constraint, nonlinearity, coupling, parameter uncertainty,ocean current disturbance, control input saturation, chattering in sliding mode control (SMC)and control precision were considered for this design.Firstly, the six degree of freedom mathematical model of the under-actuated UUV was derived. The impact of non-holonomic constraint, nonlinearity,coupling,parameter uncertainty, and ocean current disturbance on the design of controller was analyzed. The two-dimensional UUV models in horizontal plane and vertical plane with ocean current disturbance could be obtained by simplifying the six degree of freedom mathematical model.Secondly,a control strategy based on decoupling method was selected. Error equations could be established by using Serret-Frenet coordinate system, the tangential velocity of desired path could be considered as a virtual input, and then a formal fully actuated model could be obtained. Furthermore, the entire path following control system could be divided into three subsystems: speed following subsystem,position following subsystem and yaw angle following subsystem. The SMC laws could be designed for the three subsystems based on the two-dimensional models with ocean current disturbance, and then this interference could be eliminated by the SMC laws with the same interference.For path following control system in horizontal plane, adaptive laws of switching gain and boundary layer thickness were designed based on Lyapunov stability theory, control input and output,and then achieved better following precision and less chattering in comparison to the SMC method.A new reaching law was introduced into the path following control system in vertical plane, which could reduce the chattering effectively. The accessibility of the new reaching law within a limited time and steady state boundary during interference were analyzed.Adaptive law of boundary layer thickness was designed for speed following subsystem in vertical plane based on T-S fuzzy model and experience. Aiming at the poor robustness in reaching process of SMC, a parameter-free ASMC control law was proposed based on Lyapunov stability theory, which makes the ASMC control laws completely get rid of the dependence on uncertain parameters in vertical plane. The impact of uncertain parameters on control effect was eliminated.Aiming at the control input saturation problem, an adaptive saturation compensation method was introduced for propeller and steering engines. Then path following control of the under-actuated UUV under the input restrictions could be archived.The stability of the proposed methods in this dissertation has been proved by Lyapunov stability theory.Finally, numerical simulations of ASMC methods in this dissertation were carried out based on MATLAB. The following performance of the under-actuated UUV in various ocean current disturbances and various parameter uncertainties were analyzed, the validity and correctness of the proposed methods were verified.In addition, to ensure the path following control system under the premise of the stability,the proposed ASMC control scheme of the under-actuated UUV in this dissertation could improve the path following precision effectively and maintain less chattering. A compromise between the following precision and the chattering was reached. The impact of ocean current disturbances and uncertain parameters on control effect was eliminated. The constraints of practical application of control inputs were satisfied.