Design and analysis of combined nonlinear dynamic inversion and H_infty-based flight contol laws

There is a significant interest in developing flight control laws which need minimum gain scheduling across the whole flight envelope while providing required robust performance and stability properties to the closed-loop system. Dynamic inversion and/or feedback linearization technique are considered by many authors as candidates to achieve these objectives. A common feature of proposed solutions is that dynamic inversion via nonlinear control feedbacks compensates coupled nonlinear aircraft dynamics and then the desired command response is prescribed by external robust linear feedback controller. The main advantage of the method is that control laws automatically achieve decoupling of command variables and avoid extensive gain scheduling. In this paper a combined two-loop design approach is implemented. An inner loop NDI-based controller is designed to linearize the dynamics and then a outer loop feedback controller designed using H_infty-based methods is applied to the resulting system to ensure robustness to model uncertainties. To achieve a more practical design solution both stability and performance characteristics are analyzed depending on roles assigned for inner and outer loop controllers. Robustness stability analysis is performed using mu-analysis, LPV (Linear Parameter Varying) methods and classical stability margins. The ultimate goal is to achieve a relatively simple design process providing robust performance and stability guarantees.