Research On Control Algorithm For The Drag-Free Satellite

Drag-free satellite can provide an ulti-quiet, ulti-precise and ulti-stable platform. It plays a key role in more and more scientific missions in which a very low disturbance environment need to be achieved such as the ones involving measurements of relativistic effects and gravitational field. On the one hand, drag-free satellite technology can promote the development of related technologies. On the other hand, it can enhance national defense capabilities and basic science research level. On the basis of the background, this paper mainly deals with the controller design of a drag-free system combined with the subject of space research institutes.Firstly, several representations of coordinate frames are introduced. The equations of motion of a drag-free satellite are derived including the respective and relative orbit and attitude dynamics of the spacecraft and the test mass. Main environment disturbances on the spacecraft and coupling disturbance are discussed, which can serve for the design of a control system for the drag-free satellite.Secondly, with consideration of the disturbance and noise on the drag-free satellite, an optimal controller is designed on the basis of Kalman filter theory with discrete model. Both the states and disturbance of the system are estimated by the Kalman filter. The estimated states are feedback to the optimal controller and the estimated disturbance is feedforward to compensate for the outer disturbance. Simulation results show that the controller designed can effectively reduce the influence of the disturbance with good performance.Then, a frequency-domain control design approach-H∞control is applied to the controller design. H∞is appropriate for drag-free control. On the one hand, robustness is an important issue in the design of a drag-free control system because uncertainties due to outer disturbance and unmodeled dynamics are included in the system model. On the other hand, both the performance requirements and the external/internal noises acting on the satellite are strongly dependent on the frequency ranges to be considered. Controller design is performed with the H∞mixed sensitivity control approach which can be transformed to the standard H∞control problem. Appropriate weighting functions need to be selected. Some design results and simulation tests are given to show the system performance using the designed controllers.Finally, Model Predictive Control is used for its advantages in solving the constrained problem considering the fact that propulsion technology for the drag-free satellite limits the amplitude of control force. The stability of the control system is also considered. Simulation results show that the algorithm can reduce the disturbance to very low level and the performance in the frequency domain can be satisfied with robustness which is appropriate for engineering application.