Are nonfragile controllers always better than fragile controllers in attitude control performance of post-capture flexible spacecraft?

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This paper addresses the attitude stabilization and vibration suppression problem for post-capture flexible spacecraft subject to external disturbances, model parameter uncertainty, measurement errors, actuator faults, unknown and uncertain inertia, coexisting controller's perturbations of additive and multiplicative types and input constraints. The hybrid nonfragile controller considering the aforementioned multiple complex factors has rarely been reported. In addition, although the nonfragile controller can to some extent tolerate the fragility, the question whether nonfragile controllers are always better than fragile controllers in attitude control performance of flexible spacecraft has never been answered. With these motivations, the aim of this work is to design a hybrid non-fragile controller such that the closed-loop combined attitude system is stabilized, and to test whether nonfragile controllers are always better than fragile controllers in attitude control performance of post-capture flexible spacecraft. Based on the Lyapunov stability theory, the existence condition of such controller is derived in terms of linear matrix inequality. It is worth mentioning that the controller's additive and multiplicative perturbations are accounted for simultaneously. Finally, an illustrative example is given to test the effectiveness of the proposed hybrid non-fragile controller and corresponding fragile controller for comparison.