Research On Robust Control For Trajectory Tracking And Area-keeping Of Underactuated Surface Vessel

In recent years,with the rapid progress of marine science and technology,the study on underactuated surface vessels has received more and more attention.As an autonomous intelligent maritime motion platform,the underactuated surface vessel is able to carry out various tasks in the marine environment.Because of its strong autonomy,high reliability and so on,the unmanned underactuated surface vessel has broad application prospects in civil,commercial and military fields.But,the underactuated surface vessel is not only underactuated,highly nonlinear,strongly coupled but also susceptible to the internal parameter uncertainty,the external environmental disturbance and the input characteristics of actuators,which undoubtedly puts forward higher requirements for the reliability,safety and robustness when designing the underactuated surface vessel nonlinear motion control system.Good motion performance is the basic premise for the completion of different tasks,which can ensure that the underactuated surface vessel undertake long-term,low-cost and high-efficiency marine research and engineering tasks.Based on the civil research project “Research on the sailing control technology of an unmanned surface vessel”,this dissertation carries out research on tracking and area-keeping robust control theory for an underactuated surface vessel performing the marine environment monitoring mission in shallow water areas.The main research contents are as follows:(1)The characteristics of underactuated surface vessel are analyzed.Firstly,a motion mathematical model of the underactuated surface vessel is established,and simulation tests verify the controllability and maneuverability of the established model.Secondly,on the basis of the motion mathematical model,the underactuated surface vessel is proved underactuated and non-accumulative to the acceleration,so the underactuated surface vessel motion system is a typically underactuated system with second-order nonholonomic constraints.Then,the reachability and small time local controlability of underactuated surface vessel are proved.Finally,the theoretical basis for the controller design and stability analysis of underactuated surface vessel is introduced.This chapter lays the theory and model foundation for the controller design and stability analysis in the following chapters.(2)Considering the system uncertainties of underactuated surface vessel(uncertainty of the internal parameters and external unknown time-varying environment disturbances),robust backstepping tracking controllers based on the disturbance observer and online construction fuzzy approximator are proposed respectively for the underactuated surface vessel to track the trajectory.Firstly,the unknown external time-vary disturbance is estimated by designing an exponential convergence disturbance observer,and a robust backstepping tracking controller based on the disturbance observer is proposed to compensate the influence of environment disturbances,combining with the SFLOS time-vary guidance algorithm which can improve the tracking accuracy.Then,further considering the uncertainty of internal parameters,an online construction fuzzy approximator,whose structure can be adjusted dynamically,is designed to estimate the system uncertainties.Besides,a first-order low-pass filter is introduced to avoid the “differential explosion” phenomenon of traditional backstepping technology,and an online constructive fuzzy robust dynamic surface tracking controller is proposed,which can not only handle the system uncertainties but also remove the dependence on prior knowledge of fuzzy system.Finally,the stability of the whole closed-loop system is proved by the Lyapunov stability theory,and the effectiveness and robustness of the proposed tracking controller are verified by comparative simulations.(3)Considering the input saturation and time delay of underactuated surface vessel,a finite time robust sliding mode saturated tracking controller with time-delay sliding mode term is proposed to track the trajectory.Firstly,a continuous differentiable saturation function based on the Gauss error function is proposed to deal with the saturation constraints and a robust sliding mode saturated tracking controller is designed based on the proposed saturation function when the unknown time-varying environment disturbances are considered.Besides,the simulation results show the proposed tracking controller's robustness and effectiveness in dealing with saturation problem.Then,the time delay is further considered: firstly,the actuator time delay is simplified as the corresponding control input time delay;secondly,the input time-delay system is transformed into a state time-delay system;and a finite time robust sliding mode control law with time-delay sliding mode term is designed,whose parameters are selected according to the upper bound of time delay;by combining the designed smooth saturation function before,the proposed tracking controller can not only deal with the system uncertainties but also reduce the influence of input saturation and time delay.Finally,the finite time convergence characteristics of the tracking error are proved by the Lyapunov-Krasovskii theory,and the effectiveness and robustness of the proposed tracking controller are verified by comparative simulations.(4)Considering the input saturation and time delay of underactuated surface vessel,area-keeping robust sliding mode controllers based on event triggering mechanism and environment optimization strategy are proposed respectively to keep the underactuated surface vessel in allowed region.Firstly,an area-keeping robust sliding mode controller is designed by introducing the barrier Lyapunov function to realize the position restriction for the underactuated surface vessel with unknown time-varying environment disturbances.Meanwhile,the proposed smooth saturation function before is considered to reduce the influence of input saturation.Secondly,an event triggering mechanism is designed to update the control input when the triggering condition is met and an area-keeping robust sliding mode saturated controller based on the event triggering mechanism is proposed on the basis of the proposed area-keeping controller before.So the execution time can be reduced while achieving the purpose of area keeping.Then,under the unknown slow time-varying environment disturbances,a virtual suspension point updating law is designed to obtain the weather optimal heading and positions of underactuated surface vessel according to the idea of weather optimal area keeping.And a weather optimal area-keeping robust sliding mode saturated intermittent controller with time-delay sliding mode term is designed by combining with the proposed intermittent control strategy.This proposed area-keeping controller can not only handle the uncertainty of the internal parameters,unknown slow time-varying environment disturbances,input saturation and time delay but also reduce the energy consumption of underactuated surface vessel while achieving the purpose of the area keeping.Finally,the stability of the closed-loop control system is analyzed by using the Lyapunov stability theory,and the effectiveness and robustness of the proposed region keeping controller are verified by comparative simulations.