Research On Thrust Allocation Of Dynamic Positioning System Based On Improved Artificial Fish Swarm Algorithm

With the development of ocean engineering, Dynamic Positioning System has become a necessary supporting system for ships operating at sea, and it is wildly used in field of the laying of submarine pipelines, fixed point operation and sea rescue. In spite of variously complex marine environment, the system makes ships a fixed station or direction by adjusting its actuators. Thrust allocation is one of the most important parts of Dynamic Positioning System,which influences system's control accuracy and reliability directly. Focusing on this frontier subject, thrust allocation model and thrust allocation algorithms are researched in this paper.The main contents are as follows:First of all, build the mathematic model, including the 3-DOF kinematics and dynamics models of ship and environmental load models of wind, wave, and current, so that it can truly reflect the response characteristics of ship under the influences of environmental disturbance and thrusters. Thruster model is established depend on the open-water characteristics of propeller, and the reasons of thrust loss are analyzed as well. All these works lay the foundation for the following research.Secondly, thrust allocation can be attributed to optimization problem, which can be divided into two parts including objective function and constraints. In this paper, the general form of optimization objective function is established considering the energy consumption of thrusters,error of allocation and wear of thruster. What's more, in order to improve the stability and security of the ship power system, the thrust allocation model based on power management is proposed by adding the constraint of power change into target function. Constraints of thrust allocation model are established according to the physical constraints of thrust and azimuth angel of propulsion, and also consider forbidden area of thrusters to reduce the thrust loss.Thirdly, on the basis of analyzing the advantages and disadvantages of commonly used thrust allocation algorithms, the artificial fish swarm algorithm is selected to solve thrust allocation. To improve the performance of the algorithm, two improved algorithms are proposed in view of the defects of the basic artificial fish swarm algorithm. One is global artificial fish swarm algorithm, which can improve the global optimization performance and ability of jumping out of the local extreme value. The other one is simulated annealing global artificial fish swarm algorithm, which can improve the local search ability and convergence accuracy.The hybrid algorithm not only retains the global search ability of swarm intelligence algorithm,but also retains the strong local search ability.Finally, controller is designed for the research object by using the classical control method,and closed loop motion model simulation shows that the designed controller has good control accuracy. Thrust allocation is added into the above closed loop model, and the simulations are carried out by using the basic artificial fish swarm algorithm and its two improved algorithms.Simulation results show that three proposed algorithms can solve the thrust allocation problem accurately, and the thrust allocation model based on power management can retard the consumption power change, which effectively avoid the power system security of ship's power system. Through comparing the simulation results obtained by above three algorithm, it can be easily found that simulated annealing global artificial fish swarm algorithm has the best performance, and the performance of global artificial fish swarm algorithm is better than basic artificial fish swarm algorithm.