Advanced Attitude and Position MIMO Robust Control Strategies for Telescope-Type Spacecraft with Large Flexible Appendages

With extraordinary high priority science objectives to break the current barriers of our knowledge of the universe, and dealing with significant weight limitations of launch vehicle for cost-effective access to space, several NASA and ESA missions will involve both formation flying technology and satellites with large flexible structures in the next few decades: Terrestrial Planet Finder, Stellar and Planet Imager, Life Finder, Darwin and Lisa missions, etc. This chapter deals with the design of multi-input multi-output (MIMO) robust control strategies to regulate simultaneously the position and attitude of a telescope-type spacecraft with large flexible appendages. Section 2 describes the main control challenges and dynamic characteristics of a MIMO system in general, and a spacecraft in particular; Section 3 presents advanced techniques to design MIMO robust controllers based on the quantitative feedback theory (QFT); and Section 4 shows some illustrative results achieved when applying the MIMO QFT control methodology to one of the telescope-type spacecraft (a 6inputs/6-outputs MIMO system) of a multiple formation flying constellation of a European Space Agency (ESA) cornerstone mission (Fig. 1). Control of spacecraft with large flexible structures and very demanding astronomical performance specifications, as the telescope-type satellite mission, involves significant difficulties due to the combination of a large number of flexible modes with small damping, model uncertainty and coupling among the inputs and outputs. The scientific objectives of such missions require very demanding control specifications, as micrometer accuracy for position and milli-arc-second precision for attitude, high disturbance rejection properties, loop-coupling attenuation and low controller complexity and order. The dynamics of such spacecraft usually present a complex 6-inputs/6-outputs MIMO plant, with 36 transfer functions with high order dynamics (50th order models in our example), large model uncertainty and high loop interactions introduced by the flexible modes of the low-stiffness appendages. This chapter presents advanced tools and techniques to analyse and design MIMO robust control systems to regulate simultaneously the position and attitude of telescope-type spacecraft with large flexible appendages.