Research On Control Methods Of Ship Fin Stabilizer Svstem

Rolling motion with large amplitude is a hazardous phenomenon and affects the stability and safety of the vessel in a heavy weather condition.Moreover,it will affect the health of the crew and reduce the economic efficiency of the vessel.Therefore,how to reduce the vessel rolling motion as much as possible is an important issue.To reduce the effect of roll motion on stability of ships,from the 19th century,many devices have been used to reduce vessel rolling,such as bilge keels,antiroll tanks,rudder stabilizers,etc.In the navigation process,due to the complexity of the ship’s movement,traditional methods can hardly achieve a good roll reduction effect.Due to the adaptive,robustness,simple and easy to realize of the active fin,therefore,fin stabilizer became a device that is widely applied to ships today,the roll reduction performance of this method has been proven to be very effective.However,its efficiency is closely related to the control algorithm of the system.Nowadays,many control approaches have been proposed and studied for the ship fin stabilizer,and they have their own advantages and disadvantages.In this thesis,four control schemes are proposed for the ship fin stabilizer system.The specific design process is as follows:(1)A Linear Quadratic Gaussian(LQG)optimal controller is developed for the ship linear fin stabilizer system.The LQG control method is combined with the linear quadratic regulator(LQR)method and linear quadratic estimator(LQE)method.In this study,LQG method is conducted by using the separation principle,which means that the LQG controller can be derived by solving the predestined optimal control problem and the optimal state estimation problem.The effectiveness of the proposed LQG controller is validated by conducting contrast simulations with the LQR controller,and the results indicate that LQG controller has a better performance than LQR.(2)A model predictive control(MPC)method for ship nonlinear fin stabilizer system based on recurrent neural network is proposed.MPC is an effective method in process control,which can be used to improve the efficiency of the control system.However,one of the constraints of MPC is the heavy computational burden when solving the optimization problem.To overcome this problem,the recurrent neural network(RNN)is introduced to solve quadratic programming(QP)problem so that a higher convergence rate of the method can be achieved.In this study,the ship’s nonlinear fin stabilizer system is firstly linearized by means of the exact feedback linearization method,and then MPC is applied to the linearized model to derive the control strategy for the fin stabilizer system.(3)A robust controller for the ship nonlinear fin stabilizer system is proposed based on sliding mode control(SMC)method with a PID sliding surface.The SMC method is an effective robust control method used to improve the performance of the control system,namely,to reduce the ship roll motion to the lowest level.However,one of the constraints of SMC is the phenomenon of chattering around the sliding surface when the amplitude of the control law varies greatly.To overcome chattering problem,a novel robust SMC method with a PID sliding surface is applied to the ship nonlinear fin stabilizer system with time-varying parameters and external disturbances.The Lyapunov stability theory is utilized to analyze the closed-loop stability of the fin stabilizer system.(4)The adaptive dynamic programming(ADP)control method is studied for the ship fin stabilizer system.ADP control approach is a well-known method that can be used to obtain the optimal control policy without the knowledge of system dynamics,and the characteristic makes it suitable for ship nonlinear fin stabilizer system with complex and unknown dynamics.In this study,an online optimal control scheme for the ship fin stabilizer system is developed,and the proposed algorithm using current and recorded data to obtain the optimal controller without knowing the precise system dynamics.The Lyapunov technique is utilized to guarantee the stability of the closed-loop system.The four control methods proposed in this paper,the first method is to use the ship linear fin stabilizer model is utilized to design the controller and achieve a sound effect when the ship roll angle is small.In the following three methods,the ship’s nonlinear fin stabilizer model is utilized to design the controller and the methods can achieve high efficiency even when the ship roll angle is significant and nonlinear variation.The stability of closed-loop systems in the methods is always guaranteed.Finally,all the simulations are carried out in a MATLAB environment and show the effectiveness of the methods.