Disturbance observer-based predefined-time sliding mode attitude control for rigid spacecraft

The study tackles how to keep a rigid spacecraft’s orientation stable when unknown external forces act on it, and it must do so within a time that the user sets in advance. Older methods that guarantee convergence in a finite or fixed time often have limits that depend on the starting state or are hard to adjust directly. To avoid these issues, the paper proposes a predefined‑time stability condition based on a bounded function. It also builds a continuous predefined‑time disturbance observer that estimates the unknown forces. This observer removes the need to know the maximum size of the disturbance, but it still requires a known upper limit on how

fast the disturbance can change (its time derivative). Using the observer’s estimates, the authors design a nonsingular terminal sliding‑mode controller. A piecewise‑continuous function is added to the controller to get rid of mathematical singularities. The final control scheme ensures that the spacecraft’s attitude reaches the desired state within the user‑specified time, no matter what the initial condition was. For M.Tech researchers, this approach offers a solid way to simulate high‑precision attitude‑control systems that face bounded, time‑varying disturbances.

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