Robust optimal control of a flexible spacecraft

Linear Quadratic Gaussian (LQG) and mixed sensitivity H-2/H infinity robust controllers are designed for a flexible spacecraft's attitude control. H infinity norm bounded reduced order models are constructed using both additive (balanced and Hankel realizations) and multiplicative unstructured uncertainty models for the modeling error. Alternative model reduction is made by reducing the number of finite-elements in the structural model. Results show that the reduced order controllers derived from infinity norm bounded model reductions fail to stabilize the full-order plant within a prescribed bandwidth. Ln contrast, the model reduction of the same order achieved by a crude finite-element model produces controllers with drastically improved closed-loop characteristics, when applied to the full-order plant.