Key Technologies Of Bionic Mechanism And Control For Zero Speed Fin Stabilizers

The unavoidable motion in six degrees of freedom of ships in underway can be induced by the action of environmental disturbances such as winds, waves and currents. Among all motions disturbed including the rolling-pitching-yawing-swaying-surging-heaving, the roll motion is the most serious so as to influence the ship navigability, security of equipments and comfort of the crew, therefore it is of crucial importance to improve the roll stabilization. Common active fin stabilizers have been considered as the most widely used and effective anti-roll equipments with ninety percent of roll reduction, however they have the poor performance at anchored due to their force generation mechanism. Although anti-roll tanks have the capacity of damping the rolling under any speed, they are not perfect options with several drawbacks such as large volumes and much space occupied in the cabin.As a first step nonlinear models coupled describing the ship motion in three degrees of freedom and generalized wave disturbance models are analyzed according to the Newton law and wave theory, then a platform simulating horizontal motions in three degrees of freedom is established. The principle of roll stabilization and drawbacks existing in common fin stabilizers are presented, and zero speed fins with single and double wings are discussed in detail around problems such as the compatibility, mode switch, cost etc. Their advantage and disadvantage are evaluated impersonally.After a detailed investigation on the force generation mechanism employed by appendages of aquatic and semi-aquatic animals in the nature, the bionic mechanism and working mode for zero speed fin stabilizers are proposed. With analysis of hydrodynamic components and defects from existing models and their modeling methods, models describing hydrodynamics and driving are established with the use of advanced hydrodynamics theory and a composite method of the analytical approach and numerical simulations for the purpose of enough accuracy without loss of engineering practicability. The fluid dynamics is simulated with the computational fluid dynamics (CFD) software FLUENT, then the pressure and velocity distribution is analyzed. The modeling precision is evaluated in comparison with the data from FLUENT and main reasons inducing errors are discussed. The experiment published by the academic authority to test the load characteristic for zero speed fin stabilizers is performed, and their results show good agreement with each other.In previous researches the selection of fin sizes is more optional due to the lack of theoretical basis, and the adverse influence of the low aspect ratio on the conventional working mode such as lift characteristic has been not taken into consideration. With analysis of the hydrodynamic performance, a method on fin size estimation and integrated evaluation for zero speed fin stabilizers is proposed using the linear systems theory according to the detail requirements for the common and the zero speed modes, and then the optimal range of aspect ratio for non-retractable fins is obtained. Based on conclusions of bionic mechanism and hydrodynamic modeling, the principle for optimizing the leading and trailing edges is established to make a compromise between the two working modes. From the viewpoint of quality or quantity, several morphing design proposal both here and abroad are demonstrated on the basis of research findings in the bionics field obtained by the team.The characteristics of the zero speed fin stabilizer system and nonlinear input are explained in detail including the memory, monotonicity and saturation constraints. The structural configuration of the controlled system and design philosophy as well as applicable premise of the opposed PID controller are analyzed, and the essential drawbacks of opposed PID and integrated design is shown. Then the shape filters for roll disturbance moments and augmented state space model are established using the random signal analysis and the spectrum estimation theories. According to the special structure (i.e. dynamic nonlinear subsystem in series with dynamic linear subsystem), a two-step master slave control law consisting of a variable constrained model predictive controller and a numerical iterative inverse controller is proposed by means of the nonlinear removal strategy, and its convergence is proved with the use of the passivity theory. The results from simulations are given to show the good performance of the controller designed, but due to hard constraints induced by physical reasons the anti-roll efficiency under zero speed presents the nonlinear degression with increased sea conditions.As a matter of fact that zero speed fin stabilizers are also used as common fins to reduce the roll motion at the moderate or the high sailing speed, however this problem has been not paid any attention to. For solution of the nonlinear lift characteristic induced by the low aspect ratio, a robust adaptive neuron control law adequate for the conventional anti-roll mode is developed, and its Lyapunov stability is proved. The results from simulations performed show that this design with the better adaptability can obtain the satisfying effect for roll stabilization under any sea conditions. The whole attitude control system is evaluated using the sailing resistance model established, and the fact is testified that damping the rolling is of great advantage to reduce the sailing resistance and save the energy for ship propulsion.