Design of a reduced order H∞ robust controller for an expendable launch vehicle in the presence of structured and unstructured parameter uncertainty

Abstract In order to improve the robust stability and robust performance of an expendable launch vehicle control system in which a PID controller was employed, an H∞ optimal robust controller has been successfully designed. The only weakpoint is the higher order of the H∞ controller as compared with that of the PID controller. When the system model will involve the flexibility of the first and second transverse bending modes and liquid fuel sloshing, the order of the full order H∞ optimal robust controller will reach 26. No doubt, this will create many difficulties for implementation. In this paper the design of the reduced order H∞ robust controller will be studied. The goal of the design not only requires the reduced order controller to be robust against the unstructured and structured parameter uncertainty, but also requires the controller to have a high accuracy and an optimal exogenous disturbance attenuation. After analysis, design and simulations, a 7th order (reduced order) H∞ robust controller has been synthesized. Two model reduction methods have been considered for finding the reduced plant design model: the State Space Truncation and Balanced Truncation. The design validity is certified by simulations of the control process for an expendable launch vehicle. The simulation results indicate that both the robust stability and the robust performance of the 7th order H∞ robust controller are all better than that of the classical PID controller.