Uncertainty analysis and μ synthesis robust controller design for hypersonic re-entry flight vehicle

The hypersonic re-entry flight vehicle traverses a broad flight envelope in altitude and speed. The mathematical dynamic model used to describe the vehicle in control system design can hardly reflect all complex physical dynamic characteristics, resulting in numerous uncertain parameters in the model throughout the flight envelope. This paper proposes a new sensitivity analysis method based on worst-case gain (WCG) which can get the sensitivity of the control system's WCG to all the individual uncertain parameter's uncertain ranges. The uncertain parameters with small sensitivity values relatively are ignored and kept constant as nominal values in the model. The uncertain dynamic model is thus simplified and is used as the control model for the μ synthesis control law design. Then, on the basis of the requirements of flight performance, a structure of the flight vehicle attitude control system is presented and the corresponding weighting functions are selected. The proposed methods of WCG sensitivity analysis and μ synthesis are used to design the attitude controller of a hypersonic re-entry flight vehicle, and the nonlinear simulation results and their analysis show preliminarily that the μ synthesis control system which is designed based on the simplified control model also has good robust performance to the ignored uncertain parameters, and both under the nominal condition and the worst case, the closed-loop system fulfills the control characteristic requirements.