Research On Longitudinal Motion Of Under-actuated Surface Vessels

In recent years, high-speed marine vehicles have played important roles in both civil and military fields. Presently, most surface vessels have no lateral thrust device. These vessels realize three degrees of freedom motions in horizontal plane, by means of vertical thrust and steering devices which includes screw propeller, water jet propeller or rudder.This is definitely a question of under-actuated control system. It is quite important that we should focus on the research of under-actuated vessels. Planing vessel is a typical kind of high-speed vessels, which is deeply affected by vertical motions. Vertical motion is a huge threat for the performance and security of a running vessel. Therefore,planing vessel is a starting point for this paper to makes a research.We addressed the research of under-actuated surface vessels as follows:Firstly, the development of interrelated theories, the problems of under-actuated vessels and the whole idea of the paper are briefly described. The importance of under-actuated control system for the theoretical research and practical application is defined. The difficulties and the problems to be solved are pointed out in this part. Current situation of under-actuated vessels research , at home and abroad ,is summarized and analyzed. Finally, the researching line of this paper is given, according to the problems that exist in vertical control of under-actuated surface vessels. A periodic force control strategy is proposed to mitigate porpoising of planing vessels.According to the characteristics of vertical motions, six-degrees freedom model is simplified by ignoring rolling, yawing and swaying. Hydrodynamic equations and its moment equations are given, according to the kinematical characteristics. Motion equations and kinematical equations are constructed and analyzed. The physical meanings and expressions of each coefficient are defined. The accomplishment of modeling provides a basis for the following research.After that,a pitch-heave coupled equation is introduced in this paper. Added mass,damping coefficients and restoring coefficients are calculated and simplified. Pitch-heave coupled equations of porpoising are derived by Routh-Hurwitz stability rule. Finally, these coefficients are programmed by GUI of Matlab. The model of the vessel, whose coefficients are given, can be simulated by imputing coefficients in GUI window interface. The relationship between critical speed and the vertical distance of the center of gravity is proved by simulation curves. The curve of stability parameters and beam Froude number is obtained. The rules that how wetted length beam ratio changes with the speed of vessel are shown in simulation curves. The rules above provide theoretical guidance for practical applications of the following chapters.Hereafter, the basic theory of using periodic force to mitigate porpoising is proposed.According to the periodic characteristics of porpoising,a changing force is added to the constant force, in order to mitigate pitch and heave. The open-loop simulations are carried out to obtain the rules that how periodic force affects pitch, heave and speed of planing vessel. Pitch information is detected and feedback to form a close-loop control system. In this metihod, the changing trend of pitch angle can be traced by periodic force, which avoids the interleaving of phase and the divergence of pitch. When the pitch angle decreases and damping into desired range, reduce the magnitude of periodic force to avoid that too strong excitation disturbs stable state.Finally, a diesel engine servo system is designed to turn periodic force control into diesel engine speed control. The engineering application of periodic force theory is proved by this diesel engine control system.The research results of this paper have very important academic value and potential engineering application prospect. Some research results can be generalized to the attitude control field of other high performance vessels.