Compound Attitude Maneuver Control Of Liquid-filled Flexible Spacecraft Based On Sliding Mode Control

With the continuous development of space technology,space missions are becoming more and more complicated.The spacecraft needs to carry a large amount of liquid boosters and large solar panels to achieve longer on-orbit working hours.During the operation of the spacecraft,the sloshing of the liquid fuel and the vibration of the solar panel will couple with the motion of the rigid spacecraft,which directly affects the stability of the spacecraft attitude control,resulting in the failure of the mission.In addition,the environment in which the spacecraft operates is always subject to unknown disturbances and is also affected by the uncertainty of its own parameters.In this paper,the liquid-filled flexible spacecraft serving in orbit is taken as the research object,and the coupling effect of liquid sloshing and flexible appendage vibration on the spacecraft is considered.Taking sliding mode control as the main control method,a composite control strategy that can not only realize spacecraft attitude maneuver,but also suppress liquid fuel sloshing and flexible accessory vibration is designed.The specific research contents are as follows:(1)Firstly,the sloshing liquid fuel in the spacecraft is equivalent to a spherical pendulum model,and the flexible panel is assumed to be an Euler-Bernoulli beam.The Lagrange method is used to establish the coupling dynamic equation of the liquid-filled flexible spacecraft.The derived rigid-liquid-flexible coupling spacecraft dynamics equation is compared with the rigid spacecraft dynamics equation to qualitatively analyze the influence of liquid sloshing and flexible appendage vibration on spacecraft attitude control,which clarifies the idea for the design of subsequent control strategies.(2)Considering the unknown disturbance torque and uncertain parameters of the spacecraft,a dynamic sliding mode control algorithm is proposed based on the coupled dynamic model of the liquid-filled flexible spacecraft.Firstly,the traditional linear sliding mode variable structure control algorithm is given,and the reason of system jitter under this control law is analyzed.By deriving the switching function in the traditional sliding mode control,a new switching function is constructed to obtain a continuous dynamic sliding mode control.The simulation results show that the dynamic sliding mode control law effectively suppresses the jitter caused by the sign function term,has strong robustness to external disturbances,and has good control performance.(3)Considering that the upper limit of unknown interference in practical engineering is difficult to estimate accurately.In order to eliminate the influence of unknown disturbance torque and its own parameter uncertainty on spacecraft attitude control,a finite-time integral sliding mode disturbance observer is designed to estimate and compensate it.On the basis of traditional sliding mode control,time-varying parameters are introduced to eliminate the system jitter under sliding mode control.The command smoother is designed to suppress the liquid sloshing and the vibration of the flexible attachment,thereby improving the system control performance and control accuracy.The simulation results verify the effectiveness of the composite control strategy.(4)A fuzzy terminal sliding mode control strategy is proposed for the spacecraft control system with unknown external disturbances,parameter uncertainties and some information difficult to measure.Firstly,the disturbance observer is designed to estimate and compensate the lumped disturbance,and the controller is designed based on the terminal sliding mode theory to ensure that the state variables converge in a finite time.Aiming at the jitter problem of terminal sliding mode,the sliding mode surface and control torque are optimized respectively.The numerical simulation results show that the two optimization methods can effectively suppress the jitter of the control system under the premise of realizing the attitude maneuver of the liquid-filled flexible spacecraft.By comparison,it is found that the control effect is better after using fuzzy control technology to optimize the control torque,and the difficulty of parameter debugging is lower.