Slosh Suppression And Robust Attitude Control Of Liquid Filled Spacecrafts

Liquid slosh can cause the instability of a spacecraft attitude control system. With more and more requirements of the spacecraft performance such as payload, maneuvering ability and long-term operation, more liquid propellant is carried on spacecrafts, the adverse effect also increases. Hence, the liquid slosh problems are inevitable in attitude control system design of a spacecraft.The dynamic model of the liquid slosh is a set of nonlinear partial differential equations. Such a model cannot be directly utilized to attitude controller design before model simplification. In this thesis an equivalent mechanical model which consists a set of pendulum dynamic equations is adopted to approximate the slight liquid slosh dynamics. On basis of the approximation, the attitude dynamic model of the liquid filled spacecraft is established. After analysis and simulation it is found that liquid slosh can lead to system instability in certain situations.Both of the attitude manuvering and the attitude stablizing of the liquid filled spacecraft can result in liquid slosh. In the case of spacecraft attitude manuvuering with liquid slosh, time optimal control is adopted to fulfill the attitude maneuvering. Combining the vibration suppression performance of the input shaper, the order moment of time optimal control is solved based on that liquid slosh is linear and the control moment is less than the maximal output of the actuator. The simulation results show that the input shaper can enlarge the order moment of time optimal control, accelerate attitude manuvuering, increase attitude stability after attitude manuver, meanwhile liquid slosh is suppression effectively. In the case of spacecraft attitude stablizing with liquid slosh, a state feedback controller and a reduced-order observer are designed in this thesis. Then the matrices are parameterized to introduce performace indices. In order to solve the gain matrices of the controller and the observer, three performance indices are introduced which are used to restrict the maximal state variable errors, the maximal control moments and reduce the sensitivity to the perturbation of the model parameters. The simulation results show that the attitude of state feedback control system based on reduced-order observer achieve desired attitude precision in short time, as well as the liquid slosh is also effectively suppressed. Moreover, the closed-loop system is robust to the uncertainty of the model parameters.