In a previous paper, we have proposed to implement a type-II controller in NFIRAOS, the Narrow Field Infra Red Adaptive Optics System for the Thirty Meter Telescope. Type-II control enables increased tip-tilt rejection, which, for a given error budget, translates into increased sky-coverage. Our proposed type-II controller is a cascade of two integrators, a gain and a lead filter. The correction is then split between the tweeter (the deformable mirror surface) and the woofer (a tip-tilt stage that holds the deformable mirror) using high and low pass filters. So far, we had only characterized this controller in the continuous domain, where the discrete nature of the real-time computer part is approximated by continuous functions (Laplace analysis). In this paper, we discuss the discrete implementation, with particular focus on a) anti-windup, to robustly deal with temporary saturations, and b) low sampling rates, where frequency warping and aliasing may occur in the discretization process. The implementation is tested in a hybrid Simulink model, where continuous and discrete processes are properly implemented using continuous or discrete blocks, respectively, and the performance is compared with the performance predicted by the continuous domain analysis. An effective saturation handling strategy is also proposed. Finally, we analyze the implementation of dedicated algorithm to further attenuate narrow band vibrations. These techniques include a traditional notch filter, whose performance is compared to a more advanced adaptive vibration cancelation algorithm (AVCA). We find that the AVCA can correctly reject large amplitude vibrations, even when the AO sampling frequency is low.

Date Published: 12 July 2010
PDF: 14 pages
Proc. SPIE 7736, Adaptive Optics Systems II, 77364I (12 July 2010); doi: 10.1117/12.857604

Published in SPIE Proceedings Vol. 7736:
Adaptive Optics Systems II
Brent L. Ellerbroek; Michael Hart; Norbert Hubin; Peter L. Wizinowich, Editor(s)