Research On The Thrust Allocation Of Dynamic Positioning System Based On The SQP Method

Dynamic positioning system can resist external interferences (wind, wave and ocean current etc) and keep ship steadily in given position and yaw angle in command of control system with its own power. The thrust allocation unit is an important part of the dynamic positioning systems, and the main mission of it is that according to the lateral force, the axial force and the heading rotary torque commands output by the dynamic positioning controller, distribute the reasonable thrust to every thruster. In this paper, based on the sequential quadratic programming method, a rational and effective method of solving thrust allocation is presented for 16000 pipelay crane vessel。Firstly, the mathematical models of wind and motion of ship are given in order to simulation calculation. Afterwards, the PID controllers are designed for them. These controllers calculate the requirement thrust commands which will be transferred to the thrust allocation unit according to heading and positional deviation. Then, this paper give the mathematic model of thrust allocation with fourteen thrusters based on the elementary knowledge of the optimization problem about thrust allocation. Besides, this paper introduce the computational method of propellers thrust and torque, the causes and treatment methods of thruster-thruster interaction, and the basic ideas of the Sequential quadratic programming (SQP) methods.Secondly, a nonlinear objective function about thrust allocation is established in light of the duties and constraints of the thrust allocation. The objective of the thrust optimum allocation is to minimize the energy consumption, including fuel consumption, thrust error etc. Other factors such as thrust limitation, maximum slew rate of thrusters, forbidden zone for the thrust direction angle, maximum thrust change rate of thrust and singular configuration avoidance are also taken into consideration. In the light of the objective function, this paper design an effective method to solve the thrust allocation problem by revising the sequential quadratic programming method with penalty function and one-dimensional search method. Besides, this paper use one convex quadratic programming function instead of the ordinary approximate function in sequential quadratic programming method. The thrust optimum allocation algorithm is implemented by using the C programming language.Lastly, the simulations about the thrust optimum allocation algorithm are carried out. These simulations indicate that the SQP is applied successfully in thrust allocation. Then, this paper carry out the simulations about the thrust allocation problem based on Genetic Algorithm (GA) to show the excellence and defect of sequential quadratic programming method under the same controller and the same simulation environment. In the end, the simulations on thrust allocation problem with different weight matrix parameters are carried out to show the effect about weight matrix parameters in the thrust allocation problem.