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978-3-8439-1485-7, Reihe Raumfahrt
Inflight Commissioning and Performance Improvement of the SOFIA Secondary Mirror Mechanism
209 Seiten, Dissertation Universität Stuttgart (2014), Softcover, A5
The Secondary Mirror Assembly (SMA) is one of the most crucial componentsof the Stratospheric Observatory for Infrared Astronomy (SOFIA). It is a single point of failure component, and its performance has a great influence on the overall observatory performance.
Among other tasks, such as adjusting the collimation and focus position, the main purpose of the SOFIA SMA is to periodically switch the section of the sky visible in the observatory’s focal plane. This technique is called chopping and allows for the determination and subsequent subtraction of the infrared background radiation. This alternating field of view is achieved with the SMM position following a square wave pattern with frequencies of up to 20 Hz and peak-to-peak amplitudes of up to 10 arcmin on the sky.
Originally, the specified, highly ambitious values for settling time and position stability of the SMM were not met. In the framework of this thesis, the control algorithm is redesigned, and different control design concepts, such as a proportional-integral-derivative (PID) controller, a controller obtained via pole placement, and a controller designed with an optimal control approach are applied and compared. For this purpose mathematical models that reproduce the SMM’s dynamic behavior are developed using system identification methods. Then several additions are made to this linear rigid body model to represent a time delay, flexible modes, and nonlinearities.
The most accurate model is then used for the control design. It is demonstrated that a controller placing the system’s closed loop poles on Bessel filter poles as well as a controller designed by minimizing a cost function dramatically improve the SMM’s performance if they are combined with a so called observer.
The observer applies the previously identified model for real-time estimation of the SMM’s reaction to the control input. This approach allows for the compensation of an inherent time delay. By carefully choosing observer gains a crucial reduction of sensor noise in the control loop is achieved.
These performance improvements, which enable SOFIA to perform extraordinary science, are demonstrated with data gathered on the ground and also in flight under stratospheric conditions.
Finally, the hurdles preventing further performance improvements are explained and the efforts under way or in planning to modify the hardware to overcome these hurdles are introduced.