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Häuser, Marco; Richter, Josef; Kriel, Herman; Turbyfill, Amanda; Buetow, Brent and Ward, Michael (2016): Upgrade of the HET segment control system, utilizing state-of-the-art, decentralized and embedded system controllers. Ground-Based and Airborne Telescopes VI, Edinburgh, United Kingdom, 26 June - 1 July 2016. Hall, Helen J. (ed.) : In: Ground-Based and Airborne Telescopes VI : 26 June-1 July 2016, Edinburgh, United Kingdom, UNSP 990602 Bellingham, Washington: SPIE.

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Together with the ongoing major instrument upgrade of the Hobby-Eberly Telescope (HET) we present the planned upgrade of the HET Segment Control System (SCS) to SCS2. Because HET's primary mirror is segmented into 91 individual 1-meter hexagonal mirrors, the SCS is essential to maintain the mirror alignment throughout an entire night of observations. SCS2 will complete tip, tilt and piston corrections of each mirror segment at a significantly higher rate than the original SCS. The new motion control hardware will further increase the system's reliability. The initial optical measurements of this array are performed by the Mirror Alignment Recovery System (MARS) and the HET Extra Focal Instrument (HEFI). Once the segments are optically aligned, the inductive edge sensors give sub-micron precise feedback of each segment's positions relative to its adjacent segments. These sensors are part of the Segment Alignment Maintenance System (SAMS) and are responsible for providing information about positional changes due to external influences, such as steep temperature changes and mechanical stress, and for making compensatory calculations while tracking the telescope on sky. SCS2 will use the optical alignment systems and SAMS inputs to command corrections of every segment in a closed loop. The correction period will be roughly 30 seconds, mostly due to the measurement and averaging process of the SAMS algorithm. The segment actuators will be controlled by the custom developed HET Segment MOtion COntroller (SMOCO). It is a direct descendant of University Observatory Munich's embedded, CAN-based system and instrument control tool-kit. To preserve the existing HET hardware layout, each SMOCO will control two adjacent mirror segments. Unlike the original SCS motor controllers, SMOCO is able to drive all six axes of its two segments at the same time. SCS2 will continue to allow for sub-arcsecond precision in tip and tilt as well as sub-micro meter precision in piston. These estimations are based on the current performance of the segment support mechanics. SMOCO's smart motion control allows for on-the-fly correction of the move targets. Since SMOCO uses state-of-the-art motion control electronics and embedded decentralized controllers, we expect reduction in thermal emission as well as less maintenance time.

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