A Time-Domain Hybrid Control Architecture Design for Spacecraft in Accelerometer Drag-Free Mode

For high-precision drag-free space missions, the accelerometer drag-free mode (ACC-DFM) is a critical transitional phase with stringent time-domain control requirements, enabling spacecraft to enter the scientific measurement mode. However, achieving such performance is constrained by disturbances, measurement noise, control-input saturation, and coupling between the drag-free (DF) and electrostatic suspension (SUS) control loops. To address these challenges, a time-domain hybrid control architecture is developed for the ACC-DFM. A decoupling-oriented DF–SUS controller design strategy is first proposed by reconstructing the coupled hybrid architecture into a sequential design framework, where the influence of the previously designed DF controller is explicitly incorporated into an equivalent plant for SUS controller design. Then, a filter-based cascaded observer (FCO) framework is meticulously designed such that system states and low-frequency disturbances are accurately estimated while measurement noise is simultaneously suppressed. Building upon the FCO, an observer based flexible prescribed-performance control scheme is finally formulated for the SUS controller to maintain prescribed transient and steady-state performance under long-term SUS input saturation. Numerical simulations and hardware-in-the-loop experiments on a drag-free spacecraft platform demonstrate the effectiveness of the FCO and the hybrid control architecture.