SELECTION OF LONGITUDINAL DESIRED DYNAMICS FOR DYNAMIC INVERSION CONTROLLED RE-ENTRY VEHICLES

Dynamic Inversion is a design technique used to synthesize flight controllers whereby the set of existing dynamics are cancelled out and replaced by a designer selected set of desired dynamics. The output of such an inner loop controller is the control input required to achieve the desired 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 particular form of desired dynamics which produce the best closed-loop performance and robustness in a Dynamic Inversion flight controller. Four candidate forms of desired dynamics which invert the short period dynamics are evaluated. These four include a proportional, proportional integral, flying qualities, and a ride qualities form of desired dynamics. Longitudinal controllers are synthesized for the prototype X-38 Crew Return Vehicle using a linear model at a selected point in the flight envelope. Pole placement is used to synthesize a robust outer loop around the dynamic inversion inner loop in order to provide closed-loop stability. The resulting closedloop performance is evaluated in the time domain, and in terms of frequency dependent singular values, quadratic cost and a passenger ride comfort index. Of the desired dynamics presented here, results indicate that the ride quality compensation dynamics provide the best overall system performance in terms of both time domain and frequency domain responses.