Study On Path Following Control For Underactuated Ships By Using Analytic Model Predictive Control

The rapid development of modern shipbuilding and shipping has brought higher and higher requirement of ships'control performances. The conventional course control is far from satisfying the practical requirements. Therefore, the research on more precise ship motion control techniques has received increasing attention in recent years. The problem of path following control for underactuated surface ships is studied in this thesis. For the different control aims, path following controllers are designed to drive the underactuated ships to follow and stabilize onto the desired reference path. The control system is a typical nonlinear control system which has characters such as underactuated, strong nonlinear, susceptible to varying parameters and environmental disturbances. The study of the subject is contributive to study the general nonlinear system control problem and has great significance in academic study. It will benefit the practical engineering applications such as manoeuvring motion control, automatic mooring and unmooring of ships in complicated environment.A new advanced nonlinear control method, analytic model predictive control, is proposed to study the problem of path following control for underactuated ships. With respect to the nonlinear, underactuated characters, environmental disturbances and uncertain parameters of ship control system, some applicable algorithms are proposed which have the performances of both robustness and adaptiveness on path following control problem for underactuated ships. The main results achieved in this thesis are as follows:1. Mathematical models of path following system for underactuated ships are established. Modeling plays a key role in controller design. Based on the study of other researchers, this thesis has established some kinds of mathematical models of path following system for underactuated ships, including straight-line and curve path following control models, path following control models with environmental disturbances and uncertain parameters, path following control model based on nonlinear filter. These models are the base of the controller design.2. With respect to the straight-line path following control system based on first-order nonlinear K-T model for underactuated ships, the path following control algorithms are studied by using state space nonlinear (discrete) generalized predictive control (GPC) and analytic model predictive control methods respectively. In the control algorithm by using (discrete) GPC, environmental disturbance is not considered in the control model. The proposed control algorithm can asymptotically stabilize the underactuated ship onto the desired straight-line reference path, and is robust to the outputs'changes. In the control algorithm by using analytic model predictive control, the straight-line path following system for underactuated ships without environmental disturbance is considered first. By introducing the output-redefinition, the original SIMO system is transformed into an equivalent SISO system. Based on analytic model predictive control technique, the global asymptotically convergent straight-line path following algorithm is proposed. This control algorithm can guarantee the output-redefinition and its compositions all asymptotically converge to zero. The underactuated ship can be driven onto the desired straight-line reference path. And then, the high gain observer technique and the analytic model predictive control are combined, and the straight-line path following control system with environmental disturbance is studied. The proposed path following control algorithm can make the underactuated ship converge to the desired straight-line path in spite of environmental disturbance. Based on the study, the relations and differences of analytic model predictive control, (discrete) GPC, and feedback linearization techniques are systematically summarized.3. The problem of ill-defined relative degree is studied which occurrs in the curve path following control for underactuated ships. A path following control algorithm for underactuated ships is proposed by using non-switch analytic model predictive control method. This control algorithm is continuous, non-singular, and the system fluctuation is avoided. The problem of ill-defined relative degree is solved. The proposed path following control algorithm can drive the underactuated ship converge to the desired curve path.4. The curve path following control for underactuated ships is studied in the Serret-Frenet frame. By introducing the output-redefinition, the SIMO system is transformed into an equivalent SISO system which simplifies the controller design. In the Serret-Frenet frame, the curve path following control problem with fixed parameters and without environmental disturbances is studied first. Based on analytic model predictive control, Serret-Frenet frame and output-redefinition techniques, the proposed control algorithm can drive the underactuated ship converge onto the desired path. Path following errors including position error and orientation error all converge to zero; and the corresponding output-redefinition also asymptotically converge to zero. And then, robust path following control algorithms for underactuated ships are proposed with respect to the bounded disturbances induced by wind, waves and disturbance due to uniform current. Analytic model predictive control and nonlinear disturbance observer techniques are used to design the path following controllers. With help of the control algorithms, the underactuated ships can be driven asymptotically onto the desired path in spite of the environmental disturbances. Finally, aiming at the model with uncertain parameters, a joint control algorithm is adopted to get an adaptive path following controller of underactuated ships. The controller is made up of two parts: one is analytic model predictive controller with respect to the nominal system, the other is model reference adaptive identification algorithm with respect to the uncertain parameters. Even if the parameters change abruptly, the proposed controller can stabilize the underactuated ships onto the desired path. The estimation of uncertain parameters and the control input can change smoothly. The system's fluctuation induced by the parameters'abrupt changes is avoided.5. With respect to the random disturbances and the parameters'uncertainty of underactuated ships, UKF (Unscented Kalman Filter) is used to estimate the states and uncertain parameters. By combining the analytic model predictive control technique with UKF, the adaptive path following controller is proposed which is robust to random disturbances and uncertain parameters.Comparing with the previous studies by other researchers, the main innovation points of this thesis can be summarized as follows:1. Analytic model predictive control method is used for the first time to study the path following control of underactuated ships.2. With respect to the problem of ill-defined relative degree which occurrs in the curve path following control for underactuated ships, a path following control algorithm for underactuated ships is proposed by using continuous non-switch analytic model predictive control method.3. The path following control problem is studied in the Serret-Frenet frame. By introducing Serret-Frenet frame and output-redefinition, the original SIMO system is transformed into an equivalent SISO system which simplifies the controller design. By combining analytic model predictive control with nonlinear disturbance observer and model reference adaptive identification, the path following control algorithms are proposed which are robust and adaptive to disturbances and uncertain parameters.4. The path following control algorithm is proposed by combining analytic model predictive control and UKF, where UKF is used to estimate the states and uncertain parameters. The proposed control algorithm is robust and adaptive to random disturbances and uncertain parameters.The study of this thesis has demonstrated that the proposed path following control algorithms by using analytic model predictive control and other methods are effective. It provides a new nonlinear control method to study the precise ship motion control technique.