H∞ flight control design with large parametric robustness

The H^∞ optimal control theory represents an important advance in control technology. In recent literature promising results have been reported involving the design of H^∞ robust control systems with frequency dependent unstructured uncertainties. However, poor structured parametric robustness often occurs in this standard "mixed-sensitivity" approach due to the LHP pole-sero cancellation property of H^∞ controllers. This issue becomes critical when designing H^∞ flight control systems for modern supermaneuverable fighters, because these fighters have an expanded maneuver envelope that is characterized by models with lightly damped poles having significantly large levels of structured parametric uncertainty. This paper analyses this problem and proposes several enhancements to the standard H^∞ approach. A flight control design case study is employed to show that the parametric robustness can be greatly increased without invoking other complicated design procedures such as diagonally-scaled H^∞ synthesis.