Robust Drag-Free Attitude Control System Design for Low-Earth Orbit Satellites with Two Test Masses

This paper discusses the drag-free attitude control problem of two low-earth orbit drag-free satellites with two test masses, which are used for the detection of space gravitational waves. The control schemes of the science mode are designed, and a robust controller capable of resisting disturbances are proposed. First of all, the whole system dynamics model of the satellite with two test masses is established. Decoupling the complex system dynamics model based on characteristics of dynamic coupling and control time-frequency, three loops called the spacecraft attitude control loop, the drag-free control loop and the suspension control loop are produced. Then, taking consideration of the control requirements in frequency domain and the spectral models of various external disturbances and sensor noises, constrains for sensitivity function and complementary sensitivity function of each control loop are derived utilizing the mixed-sensitivity method of $H_{\infty}$ robust control theory. With selecting appropriate weight functions, an $H_{\infty}$ robust controller is designed. Finally, simulation results indicate that not only the inter-satellites pointing error but also the pose errors and residual acceleration of test masses satisfy the science performance specification, which verifies the effectiveness of the designed controller.