Research On Model Predictive Control For The Ship Dynamic Positioning System

Dynamic Positioning(DP)is a kind of complicated ship motion control technology,it comes true mainly dependent on the thrusters to realize the automatic station and heading keeping functions of the ship.With the development of our nation's marine economy,the ship dynamic positioning system becomes more important in the marine engineering equipment.The control method and the control allocation strategy are two main important components of the DP system.Their performances relate to the ship performance and the operations security,meanwhile impact the economic and environmental conservations of ship operations.The Model Predictive Control(MPC)method has the characteristics of handling constraints and optimizing performance.In order to to deal with the concept of Green DP or area positioning proposed recently in the world,so as to reduce the fuel consumption of the DP ship,it is essential to carry out the research on MPC for ship dynamic positioning system.Based on the synthesis of the state estimation method,control method and the control allocation strategy,this thesis focuses on the treatment of optimality and constraint,makes a deep study of the MPC for ship dynamic positioning and realizes fixed point positioning and area positioning for DP ship.Specific research contents include the following aspects:Firstly,a model which unifies the time domain ship maneuvering and seakeeping performance has been established for the DP ship;according to the mechanism of wave force action,the fluid memory effect which changes with the ship speed has been introduced in the ship motion model in time domain.The traditional motion superposition modeling method has been replaced by the force superposition method in the unified model;in addition,the ocean influences which include wind,wave and current have also been modeled.The simulation results show that the simulation system which includes ship and environment can provide relatively accurate simulation for the ship motion in the seaway.By the comparison of fluid memory effect changes with ship speed and the differences between forces,the results show the dissipation effect of fluid memory effect in ship maneuvering and seakeeping performance of the unified model.Secondly,from the point of view of measuring principle of several positioning reference systems(PRS)used in DP system,for the three PRS,taut wire,hydro acoustic and Global Positioning Satellite(GPS),of DP ship,the thesis proposes a nonlinear measurement equation based state fusion federated estimation method.This method considers the influences of the nonlinear relationship between the original measurement of PRS and the ship position to the state estimation,a sub-filter based on Unscented Kalman Filter(UKF)method has been designed for each kind of the positioning reference system,then the federated filter structure is used to realize the multi-sensor data fusion function,in this structure the weight for each kind of PRS is assigned dynamically by the estimated covariance.The simulation results show that the state fusion federated estimation method can effectively deal with the nonlinear effect in the filter models,including ship motion equations and sensor measurement equations.The function of ship state and slowly varying environment influence estimation can be achieved,the federated filter structure can works well for the data fusion function for redundant sensor system.Next,by analysis the existing constraint of position region as well as the terminal inequality constraint to ensure the stability of the closed-loop system,the thesis proposes an elliptic form constraint,which reflects ship real movement characteristic.Within a dual mode MPC method,the area positioning and fixed point positioning can be realized by importing changeable terminal constraint set under the condition of guarantee of the system stability.The fixed positioning can be realized by applying the continuously reduced terminal constraint set.With the dual modes MPC for area positioning,an internal controller is used to compensate for the slowly varying environmental influence forces with the position constraints,in this way to reduce the DP ship energy consumption;when the ship moves outside of the position constraint,an outer mode controller based on constraint MPC method is used to drive the ship back into the region,the energy consumption is reduce by combined use of this two controllers,and the green DP control function is achieved.The simulation results show that,internal controller with slowly varying environment forces can reduce the energy consumption;meanwhile the fixed point positioning and area positioning can be achieved with low energy consumption.Lastly,to make the thrust change more smoothly so as to reduce energy consumption and thruster wear;aim at dealing with DP ship control allocation problem which with constraint,the extended thrust vector method is applied to deal with the nonlinear problem which arises from the azimuth angle of the azimuth thruster.For the nonlinear thrust region,a regular polygon approximate method is used to change the nonlinear constraints to the linear inequality constraints.The receding horizon optimization idea is used in the control allocation problem,the multi control force estimation value in the control horizon gotten from the MPC and the predicted thrust force are used in the optimization objection function and constraint condition,multi-step optimization method is used to replace the tradition single step optimization in the ordinary control allocation problem.Through this method,the optimal thrust solutions not only fulfill the right now objection function,the future control force trend is also included.The simulation results shown that with the application of the control allocation method,the response of the ship position and heading is more smooth and steady than the traditional QP method,the change of the azimuth angle of the thruster is smaller and the response of the thrust is more quickly.Through the smooth change of the thrust,the power consumption and the thruster wear and tear can be reduced.Aim at the problem of MPC for the ship DP system,this thesis propose a method for dealing with constraints and performance optimization which contains state estimation,control method and control allocation strategy three aspects.