28 July 2010 Improvement of the SOFIA secondary mirror controller
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The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a 2.5m infrared telescope build into a Boeing 747SP. During observations the telescope will not only be subject to aircraft vibrations and maneuver loads - by opening a large door to give the observatory an unhindered view of the sky, there will also be aerodynamic and aeroacoustic disturbances. A critical factor in the overall telescope performance is the SOFIA Secondary Mirror Assembly. The 35cm silicon carbide mirror is mounted on the Secondary Mirror Mechanism, which has five degrees-of-freedom (rotation about line of sight is blocked) and consists of two parts: The slow moving base for focusing and centering, and on top of that the Tilt Chop Mechanism (TCM) for chopping with a frequency of up to 20Hz and a chop throw of up to 10arcmin. A new controller for the TCM is introduced in this paper in order to meet the stringent performance requirements for the chopper. A state space controller is chosen that combines a feedback path for steady state behavior with a model-based feed forward controller for improved settling time performance. The paper explains the modeling of the TCM via a grey box model approach optimized with system identification data and compares simulated with measured data. Then the structure of the controller is explained and Matlab/Simulink simulations are presented. The simulation results are compared to measurements taken with the real system on ground and finally flight test results with open and closed door are discussed.
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Andreas Reinacher, Emmanuel Onillon, Hans-Peter Roeser, "Improvement of the SOFIA secondary mirror controller", Proc. SPIE 7733, Ground-based and Airborne Telescopes III, 77330Q (28 July 2010); doi: 10.1117/12.856554; https://doi.org/10.1117/12.856554

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