Active Fault Diagnosis for Spacecraft Attitude Control Systems Using Set-Theoretic Unknown Input Observers

This letter proposes an active fault diagnosis method for spacecraft attitude control systems with disturbances by utilizing the unknown input observer. Firstly, the spacecraft attitude control system is formulated as a linear time-invariant model under diverse faulty conditions. A general unknown input observer framework is constructed to decouple process disturbances by designing optimal gain matrices, which are analytically obtained by solving an unconstrained optimization problem. Then, zonotopes are utilized to characterize the dynamics of state estimation error, and a set-theoretic unknown input observer is built. To achieve the guaranteed active fault diagnosis, a quadratic program and a mixed-integer quadratic optimization program are formulated to obtain the separating input that can maximize the separation degree of all zonotopes under fault isolation and input saturation constraints. Finally, an illustrative example is provided to verify the effectiveness of the proposed active fault diagnosis method.