Robust longitudinal control design using dynamic inversion and quantitative feedback theory

The objective is to design low-bandwidth control laws that yield robust performance over a wide flight envelope in the presence of 20% model uncertainties. This is achieved by using a hybrid controller in which a second order dynamic inversion transforms the nonlinear angle-of-attack dynamics to linear tine-invariant form, while an outer leap, designed using quantitative feedback theory (QFT), ensures robust performance. The dynamic inversion reduces the plant variations due to nonlinearity, which are traditionally treated as uncertainty in QFT. The reduced uncertainty allows lower controller bandwidth for a given level of performance. The hybrid controller design is applied to the unstable, nonlinear, short-period dynamics of the F-16 aircraft and compared with a baseline QFT controller designed without dynamic inversion. Both designs provide satisfactory performance, but the baseline QFT controller requires a bandwidth of around 4-8 rad/s, whereas the hybrid design has a bandwidth of only about 1 rad/s, leading to reduced demands an the control hardware. The success of the dynamic inversion depends strongly on the desired linear dynamics, and so recommendations are given on their selection.