Design, Control, And Failure Mitigation For Segmented Space Telescopes
Future astrophysics missions will require the construction of larger space telescopes, which poses numerous technical challenges including: mirror stability, packaging and deployment, active mirror control, and segment actuator failure mitigation. To address these challenges, this dissertation presents the results of four research projects. The first project considers the problems of packaging and deployment and mirror stability, presenting a segmented mirror architecture in which the segments are connected edgewise by mechanisms analogous to damped springs. For low to intermediate stiffnesses, the stiffness and damping contributions from the mechanisms improve both the natural frequency and disturbance response of the segmented mirror. At higher stiffnesses, the segmented mirror performs comparably to or better than a monolith, with the modular design enabling on-orbit assembly and scalability. The second project addresses the mirror stability challenge for cryogenic mirrors in particular, presenting flux-pinning mechanisms designed to increase the mirror stiffness and damping. These mechanisms consist of a collection of magnets and superconductors, and like flexures, preferentially allow motion in specific degrees of freedom. With typical stiffness and damping values on the order of 5, 000 N/m and 5 kg/s, respectively, these mechanisms provide modest improvements to the mirror performance. The third project investigates improvements to mirror control algorithms for active space telescopes at L2. I show that the wavefront for these telescopes can be controlled passively by introducing scheduling constraints and describe the implementation of a predictive con- troller designed to prevent the wavefront error from exceeding a desired threshold. This controller outperforms simpler algorithms even with substantial model error, achieving a lower wavefront error without requiring significantly more corrections. The fourth project discusses how the effects of actuator failures can be mitigated by using the remaining segment actuators to optimize the pose of each affected segment. When one actuator fails, the affected segment can still attain a pose with zero wavefront error by exploiting the rotational symmetry of the primary. Monte Carlo simulations of many failures randomly distributed across an initially well-phased segmented primary show that more than 10% of the actuators must fail before the root-mean-square wavefront error degrades significantly.
Garcia, Ephrahim; Lloyd, James; Stacey, Gordon John
Ph.D. of Mechanical Engineering
Doctor of Philosophy
dissertation or thesis