Research On Thrust Estimation And Thrust Allocation For Dynamic Positioning System Of Ships

With the deepening of China's deep-sea exploration and development,marine vessels become increasingly dependent on dynamic positioning system,and the control precision,reliability and adaptability of dynamic positioning system are also put forward higher requirements.High performance control of dynamic positioning ship must be achieved in the various sea states which comply with the design requirements.But,when the propeller ventilation,in-andout-of water effects,thruster-thruster interactions or low efficiency zone is presented,severe thrust loss of propeller is occured as a result.This not only brings some difficulties to the control of the ship,but also the propeller's service life is greatly reduced.So it is very necessary for dynamic positioning system of ship to estimate the actual thrust of propeller and develop the thrust allocation algorithms which considered the thrust loss.Focusing on this frontier subject and related engineering applications,thrust estimation methods and thrust allocation algorithms during the operations of dynamic positioning ships were researched.Works achieved in this paper are present as follows.To truly reflect the response characteristics of ship and thrusters to environmental disturbance and control input during motion control process of ship,the 3-DOF kinematics and dynamics models of ship,the environmental load models,propeller shaft dynamics,motor dynamics,shaft friction torques and 4-quadrant open-water characteristics were established.To validate the proposed method,the simulation parameters of the ship and thrusters were given.To achieve the 4-quadrant thrust estimation,based on 4-quadrant propeller open water characteristics,the thrust estimation method is proposed.Quasi static method was used for the first time,in order to deal with the nonlinear terms in the friction torque.Further more,the propeller torque estimator and torque coefficient estimator were proposed.The stabilities of torque estimator and parameter estimator were analyzed.Through the mapping relationship between the thrust and torque coefficients,as well as the estimation results of torque and torque coefficient,4-quadrant propeller thrust estimation was achieved,and two thrust estimation methods for only the torque estimation results available or only torque coefficient estimation results available were given.Simulation results showed that the both methods were fast,accurate and effective in achieving the estimations on actual torque and thrust of the propeller.To deal with the multi-propeller thrust allocation problem in the complex environment,the affecting factors relate to thrust allocation problem were analyzed in detail from marine environments,operating conditions and thruster constraints these three aspects.Further more,the hierarchical layered idea was introduced,and the algorithms in different operating conditions or sea conditions and switching supervisory control between algorithms were hierarchically abstracted into several level.As a result,thrust allocation layered architecture which includes an application layer,coordination layer,algorithms set layer and physical layer 4 levels of abstraction is established.To improve the performance of thrust allocation under different sea conditions,multiple thrust allocation algorithms were proposed.According to the thrust allocation problem in moderate sea state,power penalty term was corrected by an approximate method which can make power penalty more accurate,and the cost function which includes error,power consumption and maximum thrust was built.As a result,the quadratic programming algorithm of thrust allocation was obtained.According to the thrust allocation problems in calm seas state and high seas state,the quadratic programming algorithm was improved by introduce thruster bias and penalty factor of thrust loss,and the group biasing thrust allocation method and the thrust allocation scheme in harsh sea conditions were proposed respectively,.Simulation results showed that the proposed algorithms can effectively solve the matching problem with high reliability and validity.To realize the switching between algorithms,enhance the adaptability and robustness of the thrust allocation algorithm,multi-model switching control with dwell-time and hysteresis switching was introduced into thrust allocation level.A thrust allocation switching supervisory control strategy was proposed,as well as thrust allocation switching method between different sea conditions or operating conditions and non-convex thrust allocation switching method.Simulation results showed that the control performances of dynamic positioning ship can be improved reliably and effectively by the adaptive switching of thrust allocation algorithms,and the controller did not require any change.