Evaluation of longitudinal desired dynamics for dynamic-inversion controlled generic reentry vehicles

Dynamic inversion is a control synthesis technique in which the inherent dynamics of a dynamical system are canceled out and replaced by desired dynamics, selected by the designer. The output of such an inner-loop controller is the control input, which produces the desired closed-loop response. The desired dynamics essentially form a loop-shaping compensator that affects the closed-loop response of the entire system. This paper attempts to quantify the effect of different forms of desired dynamics on the closed-loop performance and robustness of a dynamic-inversion flight controller for reentry vehicles. Proportional, proportional-integral, flying-quality, and ride-quality forms of desired dynamics are evaluated using time-domain specifications, robustness requirements on singular values, quadratic cost, and a passenger ride comfort index. Longitudinal controllers are synthesized for a generic X-38 type crew return vehicle, using a set of linear models at subsonic, transonic, and hypersonic flight conditions. For the candidate forms of desired dynamics and inversion controller structures evaluated here, results indicate that the form used impacts closed-loop performance and robustness and more so for some inversion controller structures more than others. The ride-quality dynamics used with a two-loop angle-of-attack inversion controller provide the best overall system performance, in terms of both time-domain and frequency-domain, specifications, and the evaluation criteria.