Nonlinear Control Of Underactuated Surface Vessels

The underactuation of mechanical systems means that the dimension of the control vectors of systems is less than that of the configuration vector of systems. The feature of underactuated mechanical systems is that the motion of mechanical systems in configuration space can be controlled by the fewer independent control inputs than the dimension of configuration space. It implies that underactuated mechanical systems can reduce the cost and the weight of systems. Moreover, the idea of underactuation can be used to control systems in which some components are failed during operation so that the system is also be controlled by the unfailed components effectively and the influence on the system performance caused by failures of system components can be decreased. Therefore, the more and more attention has been paid on underactuated mechanical systems in recent years.The focused object is surface vessel. It is pointed out that the underactuate characteristic is emerged under special control aims. So for the different control aims, the problem on nonlinear control of the underactuated surface vessel is focused in this dissertation.The 6 degrees of freedom dynamic and kinematic nonlinear models of surface vessels are derived based on Lagrangian energy method and some assumptions, the dynamic and kinematic models of surface vessels in horizontal plane are obtained. It is proved that the motion control system of underactuated surface vessels in horizontal plane belongs to the second-order nonholonomic system. The strong accessibility and small time local controllability of the system are proved. From the energy view point, the passivity of the system is revealed.For the stabilization control law design using on the dynamic positioning and position mooring of underactuated surface vessels, based on the method of σ -process, a convergence control item is added in the original equations to control the converge rate of the whole system. A smooth time-varying feedback stabilization law with exponentially convergence rate is obtained. The aim of stabilization control of underactuated surface vessels is achieved. The method canalso be used for the second-order state equations which obtained by diffeomorphism transforming from the dynamic and kinematic models of surface vessels in horizontal plane. The proposed method is superior to the other methods which stabilized the system with discontinuous control law and some limitation on the variation of states.For the direct path tracking control of underactuated surface vessels in horizontal plane, the system is proved to be a differential flatness system. The method of dynamic extended feedback linearization is adopted. Then the control law is derived such that the tracking error and heading error with respect to the planned path could be stabilized exponentially asymptotically and globally. The method relaxes the assumption of persistent exciting in yaw rate required in other methods.The controllability of heading-drift system of underactuated hovercrafts with two aft propellers is proved. Then adopting the concept of desired speed vector field in homogeneous approach, combining the backstepping approach, a new method for hovercrafts heading-drift control law design is proposed that keeps hovercrafts heading constant as well as minimizes the lateral movement of hovercrafts.Simulation tests are carried out to illustrate the effectiveness of the proposed control laws for different control aims. At the same time, the validity of the proposed control law is verified by the simulation tests.