Research On Nonlinear Robust Control Strategies Of Underactuated Autonomous Underwater Vehicle

Autonomous underwater vehicles (AUVs) have recently become an intense area of domestic and international research because of their potential military and civil applications. As the application range of AUVs expands, it becomes more and more important to develop AUV autonomy. One of key technologies is to improve its control performance. However, the problem of AUV control continues to pose challenges to system designers since many AUVs are underactuated systems, with fewer control inputs than the degrees of freedom, out of the need to reduce the actuator cost and weight. In addition, the dynamics of AUVs are highly nonlinear, the hydrodynamic coefficients are not precisely known, and the vehicle is often disturbed by the ocean environments. As such the development of nonlinear robust control strategies for underactuated AUVs is of considerable important.In this dissertation, nonlinear robust control strategies are deeply studied for the depth, planar trajectory and position tracking of underactuated AUVs with model uncertainties.Control models are built for underactuated AUVs. The six degrees of freedom kinematic and dynamic equations of AUV motion are given. According to that the translational velocities relative to the Earth or surrounding water are measured, and whether the nominal values of the viscous damping coefficients are known or not, four horizontal motion models are built for underacutuated AUVs with parameter uncertainties and external disturbances. A vertical motion model is built using a small disturbance analysis. Some related control theories and methods are introduced, and some related control concepts and theorems are given. To solve the depth control problem of underacutuated AUVs with model uncertainties, a fuzzy sliding mode controller is presented. The design method of the controller offers a systematical means of constructing a set of shrinking-span for fuzzy inputs and dilating-span membership functions for fuzzy output. To avoid chattering better, two control strategies are proposed. One is to design some fuzzy rules to adaptively tune scaling factors of the fuzzy controller. Another is to optimize parameters of the fuzzy controller by genetic algorithm. Numerical simulations are presented for the underactuated AUV systems with control input delay, large parameter uncertainties and unmodeled dynamics to illustrate the effectiveness and robustness of the proposed control strategies.To solve the trajectory tracking control problem of underacutuated AUVs with parameter uncertainties and external disturbances in the horizontal plane, three control strategies are proposed according to different cases. The first control strategy is developed using the backstepping approach for the problem of small curvature trajectory tracking where the sway velocity is relatively much smaller than the surge velocity. An adaptive control strategy is proposed for the unknown constant viscous damping coefficients. For compensating other parameter uncertainties and external disturbances, different control laws based on sliding mode and fuzzy sliding mode control methods are presented. Stability proof of the closed-loop control system is given by the Lyapunov stability theory. The second control strategy is developed by using the cascade approach where the reference yaw velocity satisfies the persistence of excitation condition. The tracking error system is formed, and then decoupled into two separate subsystems. A feedback linearization and sliding mode controller are presented respectively to stabilize the two subsystems. The closed-loop control system is proved to be globally K exponentially stable by stability criteria for cascade system. The third control strategy is developed by using the backstepping approach for the general trajectory tracking control problem. For compensating parameter uncertainties and external disturbances, different control laws based on classical and quasi SMC methods are presented. The relation is derived between parameter uncertainties, external disturbances, system states and sliding mode gains. Different effects of positive and negative mass uncertainties on the control systems are analyzed. Stability proof of the closed-loop control system is given by the Lyapunov stability theory. Numerical studies for different cases are presented to demonstrate the effectiveness and robustness of the above proposed three control strategies.To solve the position tracking control problem of underacutuated AUV with parameter uncertainties and external disturbances in the horizontal plane, four control strategies are proposed respectively for the above four horizontal models. The robust position tracking controllers are proposed by the backstepping approach. If the translational velocities relative to surrounding water are measured, two observers are proposed to estimate unknown constant ocean current velocities, which can ensure that estimation errors are globally exponentially stable. It is proved that the above proposed four control strategies are globally stable, and system states and control inputs are bounded, by the Lyapunov stability theory and stability criteria for cascade system. Simulation results show that the proposed control strategies are effective, adaptive to unknown ocean current velocities and viscous damping coefficients, and strongly robust against parameter uncertainties and external disturbances.