12 July 2010 Implementation of type-II tip-tilt control in NFIRAOS with woofer-tweeter and vibration cancellation
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Abstract
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.
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Jean-Pierre Véran, Jean-Pierre Véran, Craig Irvin, Craig Irvin, Anne Beauvillier, Anne Beauvillier, Glen Herriot, Glen Herriot, } "Implementation of type-II tip-tilt control in NFIRAOS with woofer-tweeter and vibration cancellation", Proc. SPIE 7736, Adaptive Optics Systems II, 77364I (12 July 2010); doi: 10.1117/12.857604; https://doi.org/10.1117/12.857604
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