Nonlinear Sliding Mode Control For Dynamic Positioning Of Ships

This dissertation takes the dynamic positioning ship as the research object,and employs the iterative sliding mode method,the nonsingular fast terminal sliding mode method,Backstepping technique,the observer technique and LS-SVM nonlinear function approximation theory to carry out intensive studies on dynamic positioning control of ships.It is dedicated to addressing five issues: ship model uncertainties,unknown environmental disturbances,thruster faults,unknown velocities of the ship and environmental optimal heading.The major work includes the following several aspects:1.In view of serious chattering and large system static errors caused by the dynamic positioning sliding mode control scheme,an iterative sliding mode-based state feedback control scheme is proposed,which consists of proportional and differential term of tracking errors,integral term of sliding mode,known state of the ship and integral term of sliding mode switching.It is able to copy with problems of the sliding mode control scheme.Furthermore,in the presence of unknown velocities of ships,the finite-time state observer independent of the ship mathematical model is addressed to observe velocities of the ship,and guarantees observation errors to converge to a small region around zero in the finite time.And then,the iterative sliding mode-based output feedback control scheme is constructed based on the iterative sliding mode surface and the state observer and Lyapunov is adopted to prove the system stability.Final simulation studies demonstrate strong observation capability of the proposed finite-time state observer and superior advantages of the proposed output feedback control scheme.2.In light of the case that the dynamic positioning sliding mode control scheme is inclusive of several demerits,such as complex formula,excessive design parameters and heavy reliance on the ship mathematical model,a novel incremental feedback control scheme is deduced,which is independent of the ship mathematical model and takes into account the thruster model.Simulation studies demonstrate that the proposed scheme facilitates good stability,strong robustness and adaptability.What’s more,in order to satisfy requirements of reducing the thruster wear and energy consumption,and preventing the saturation or even overload of the thruster,a nonlinear iterative sliding mode control scheme based on environmental optimal heading is proposed.It’s easy to change the output of the control scheme through parameters adjustment of the sliding mode surface.Besides,an improved“ZPC-W” heading optimization algorithm is presented.It adds the differential term of transverse control force to promote the convergence rate of the environmental optimal heading without the system overshoot.Finally,simulation results verify good advantages of the proposed control scheme and the improved “ZPC-W” algorithm.3.In consideration of the issue that both sliding mode control scheme and Backstepping control scheme just guarantee asymptotic convergence of the system,a Backstepping-based nonsingular fast terminal sliding mode control scheme is proposed.It combines Backstepping technique with the nonsingular fast terminal sliding mode surface to improve system performance,involving convergence rate and stability.And adaptive method is introduced to compensate the unknown environmental disturbances,which reduces the stability errors and enhances stability of the system.4.In view of ship model uncertainties,a LS-SVM-based nonsingular fast terminal sliding mode control scheme is designed based on the nonsingular fast terminal sliding mode surface,where LS-SVM is employed to adaptively estimate model uncertainties.The“minimum parameter technique” is introduced to reduce the number of adaptive parameters to one,simplifying complexity of the control algorithm.Simulation results show that the proposed control scheme covers great advantages of fast convergence rate,small stability errors and strong robustness in comparison with sliding mode control scheme.5.In consideration of unknown environmental disturbances and thruster faults,a disturbance observer-based finite-time fault-tolerant control scheme is constructed.The proposed scheme adds integral term of sliding mode,which is helpful to improve transient response and steady state performance of the system.In order to tackle unknown environmental disturbances and thruster faults,a finite-time disturbance observer is presented to observer unknown environmental disturbances and thruster faults,guaranteeing estimation errors to converge to a neighborhood around zero in the finite time.And then,it is incorporated into the construction of the control scheme.Finally,simulation studies are carried out among the proposed scheme,the sliding mode control scheme and the nonsingular fast terminal sliding mode control scheme,and final results show that the proposed scheme gains the fastest convergence rate,the smallest stability errors and the strongest robustness.To sum up,this dissertation designs an improved nonlinear sliding mode control scheme for dynamic positioning of ships.It combines with the observer technique,LS-SVM approximation theory and the heading optimization algorithm to settle several issues,including unmeasured velocities of the ship,unknown environmental disturbances,environmental optimal heading,etc.The results are valuable for theoretical research and engineering application of nonlinear control for dynamic positioning of ships.