Selection of desired dynamics for Dynamic Inversion controlled re-entry vehicles

Abstract

Dynamic Inversion is a design technique used to synthesize flight controllers whereby the set of existing plant dynamics are canceled out and replaced by a designer selected set of desired dynamics. 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 particular form of desired dynamics that produces the best overall closed-loop performance and robustness in a Dynamic Inversion flight controller. Four candidate forms of desired dynamics are evaluated in a multiple time scale approach to dynamic inversion. These four consist of a proportional, proportional integral, flying qualities, and ride qualities forms of desired dynamics. Both longitudinal and lateral/directional controllers are synthesized for the prototype X-38 Crew Return Vehicle using a linear model at a selected point in the flight envelope. Linear Quadratic techniques are used to synthesize a robust outer-loop around the dynamic inversion inner-loop to provide closed-loop stability. The resulting closed-loop performance is then evaluated in the time domain, in terms of singular values in the frequency domain, a quadratic cost, and a passenger ride comfort index. A set of guidelines is then proposed to facilitate the selection of the desired dynamics for this re-entry application.