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.

Date Published: 28 July 2010
PDF: 9 pages
Proc. SPIE 7733, Ground-based and Airborne Telescopes III, 77330Q (28 July 2010); doi: 10.1117/12.856554

Published in SPIE Proceedings Vol. 7733:
Ground-based and Airborne Telescopes III
Larry M. Stepp; Roberto Gilmozzi; Helen J. Hall, Editor(s)