H-infinity control design specifications applied to the Hubble Space Telescope

Preliminary redesign efforts for the Hubble Space Telescope Pointing Control System are presented. These and previous efforts by NASA and Lockheed are motivated by pointing errors caused by the effects of unanticipated thermally induced solar array flexing. The design philosophy for the results presented here is based primarily on the loop shaping approach of traditional H-infinity theory. However, the actual controller designs presented are the results of a hybrid of Multiple Input Multiple Output (MIMO) H-infinity specifications and Single Input Single Output (SISO) specifications. This hybrid approach allows the specification of simultaneous robustness (H-infinity) and performance (SISO) design criteria. A numerical approach to calculating controllers that meet these hybrid specifications is taken, due to numerical problems with standard H-infinity design software and the inability of such software to deal with the imposition of additional SISO constraints. The paper includes performance and robustness analyses of two flight controllers and of the controller designed as a result of the present redesign effort. Performance comparisons are made based on the results of NASA-supplied simulations derived from flight data and robustness results are based on MIMO singular value analyses and classical phase and gain margins. It is shown that the present controller exhibits significant improvements in robustness, peak pointing errors in each axis, and root mean square (RMS) pointing errors in each axis. Plans for continuing the redesign efforts are presented. The issues of simulation and model enhancements are addressed, as is that of the need for a reliable and consistent set of performance and robustness specifications that future controllers must meet.