Synthesis and Evaluation of Robust Dynamic Inversion Flight Controllers for X-38 Class Re-Entry Vehicles

GN&C Design and Analysis Branch, NASA Johnson Space Center
1 May 2000 – 1 May 2001
Total award $97,320

X-38 In Flight Test
As opposed to traditional synthesis techniques, in which the nonlinear plant is separated into several linearized models at discrete operating points, and a closed-loop controller is synthesized for each one, Dynamic Inversion seeks to synthesize a global control law from a single nonlinear model. Two open research issues are the “user friendliness” of designing Dynamic Inversion controllers, and controller robustness and fragility.

A previous Dynamic Inversion study on the X-38 conducted at Texas A&M (see below) partially addressed the first of these open issues by generating a comprehensive design guidelines document complete with tutorials, procedures, tools, and examples.

With regard to the second issue, Dynamic Inversion by itself cannot assure stability and performance robustness to disturbances and perturbations in the plant and controller. Therefore, an additional robust control technique must be married to the Dynamic Inversion controller to ensure robustness. There are several robust control techniques and robustness measures currently available to the control designer. Examples in the current literature show a tendency to use whatever robust control and analysis techniques the designer is most familiar with, as opposed to those which are best for a particular application. H-infinity and Mu-synthesis are two of the more popular techniques.

Specific tasks and research objectives:

  • Demonstrate practical application of the guidelines, procedures, tools, and software previously developed, and validate the design guidelines document. This will be done with a Dynamic Inversion controller design case study for a re-entry vehicle.
  • Identify and evaluate the advantages that European Dynamic Inversion methods have to offer in terms of ease of use, and suitability for implementation, compared to the particular Dynamic Inversion approach commonly used in North America. These advantages will be directly incorporated into the comprehensive design methodology.
  • Develop new, non-conservative robustness measures, and examine the fragility of Dynamic Inversion control laws.

Working with me on this program are Graduate Research Assistants:

  • Jennifer A. Georgie
  • Dai Ito