Predefined-time robust attitude tracking control of flexible spacecraft with continuous and nonsingular performance

To address the predefined-time robust attitude tracking control of flexible spacecraft with both external disturbances and model uncertainties, this paper proposes an adaptive terminal sliding mode controller. A modal observer is designed to estimate the unmeasured modal information of flexible attachments, and the estimation error is regarded as a part of the lumped disturbance. The term with a power index less than one in the terminal sliding mode is replaced by a piecewise function such that the controller is global nonsingular. To ensure the continuity of the applied control torque to avoid chattering phenomenon, the signum function in the robust term is replaced by the hyperbolic tangent function and its influence on control accuracy is analyzed quantitatively. The Lyapunov stability analysis shows that the attitude tracking error can be drawn into a small neighborhood near zero before the specified time which can be easily adjusted during the controller design without initial conditions constraint. Finally, we evaluate the predefined-time convergence performance by numerical simulations.