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Implications of Wind Testing Results on the GSMT Control Systems

This study examines the implications of wind testing results on the control systems of the Giant Segmented Mirror Telescope (GSMT), highlighting the hierarchical approach used to manage errors. The study analyzes different types of errors induced by wind and their impact on the control systems of the telescope. The results suggest that a hierarchical approach is necessary to effectively manage and correct errors, especially for systems with large payloads and long strokes. The study also identifies the interaction problem at the M2 stage as a key challenge in the control systems of the GSMT.

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Implications of Wind Testing Results on the GSMT Control Systems

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  1. Implications of Wind Testing Results on the GSMT Control Systems David R. Smith MERLAB, P.C.

  2. Hierarchical Approach • If errors can be arranged hierarchically, then the control system can be as well. • Large, high payload, long stroke systems can be slow and less precise. • Higher bandwidth systems can be smaller stroke and capacity.

  3. Hierarchical Approach (cont.) • Keeping high-bandwidth control on smaller systems eliminates control-structure interactions. • Intent is to keep cost/risk low by combining simpler and more standard control systems and components.

  4. Errors • Large, slow errors (m-mm, <0.01-0.1 Hz) • Gravity • Thermal • Mechanical misalignments • Wind • Medium-sized, rate (<~10 m, <~10 Hz) • Wind • Vibrations • Small, fast errors (<1 m, >~10 Hz) • Wind • Vibrations • Atmosphere

  5. Controllers (example) • Main Axis • M1 Gross/Fine Position • M1 Segment warping • M2 Positioner • M2 Fast tip/tilt/position • M2 Deformation • Downstream AO

  6. Most systems don’t interact Separated physically and in bandwidth Final image corrected by AO Each previous system used to offload mean positions. E.g., M2 offloads AO to ~5 Hz M1 fine offloads M2 to ~1 Hz M1 gross offloads M1 fine to ~0.001 Hz Assumptions

  7. Separability of systems has limits Motion of slow systems may induce vibrations Some systems are partially redundant, so must ‘agree’ on how to remove certain errors (e.g., pointing) Some systems can’t avoid interaction M2 fast positioner Assumptions (cont.)

  8. Assumptions (cont.) • Input must allow hierarchical approach • Roll-off of errors must allow separation of high-bandwidth control from large structures. • Wind is a key unknown • Magnitude of errors • Frequency content

  9. Wind Data • Gemini South 8m (Optical) • Structural (modal and operating) • Pressure on primary • Wind speed (on structure and dome) • Nobeyama 45m (mm-Wave) • On-sky pointing • Structural (operating) • Controller

  10. Gemini Data • First round data (CD produced) • Modal Test • Operating Data • Wind pressures • DOE results • Second round data (analysis beginning) • Wind speed and pressure only • Better coverage of parameter space

  11. Nobeyama Data • Goal was to investigate pointing • Pointing data analyzed • Structural data quick-look only • Deformations relevant to GSMT • Similar size • Similar natural frequencies

  12. Wind Effects • Generally assumed to be low frequency • For 10m/s wind at 10m height • Davenport Spectrum peaks at ~0.01 Hz • Antoniou spectrum peaks at ~0.1 Hz • Roll-off is slow • Slope of -2/3 in typical approach to plotting • Vortex generation from structure • All frequencies are affected

  13. Wind Effects (cont.) • All structural frequencies excited • Amplitude drops as 1/² • If a specific mode isn’t driven by a vortex, then deformations are unimportant above some frequency.

  14. Nobeyama Results • Deformation of the primary • Motion normal to surface • Rigid body tilt removed • Motion of the secondary • X,Y,Z of typical point

  15. Conditions of Tests • Parked, calm (<2 m/s wind) • Benchmark case • Tracking, calm • Effects of controller and motion • Parked, windy (6-8 m/s) • Effects of wind • No data tracking in wind

  16. Deformations of the Primary • Raw acceleration signal • Removal of rigid body tilt • Comparison of RMS deformation at/above a given frequency

  17. Parked Telescope, Calm Wind

  18. Tracking Telescope, Calm Wind

  19. Parked Telescope, Wind 6-8m/s

  20. RMS Comparison

  21. Implications: Primary • Total RMS error can be 10’s of microns • Tracking is as important as wind • Hydrostatic bearings • Motion planning essential • After ~3-4 Hz, residual is <1 m • Control of M1 would interact with structure • Low spatial frequency errors: M2 correction

  22. Motion of the Secondary • Accelerations in X, Y, Z • RMS comparisons at/above a given frequency (X, Y, Z)

  23. Parked Telescope, Calm Wind

  24. Tracking Telescope, Calm Wind

  25. Parked Telescope, Wind 6-8m/s

  26. RMS Comparison, X

  27. RMS Comparison, Y

  28. RMS Comparison, Z

  29. Implications: Secondary • Twist motions much smaller • Tracking and wind cause same scale errors • Lateral and focus/tilt motions: 10’s of m • Most effects (>1m) below 3 Hz • M2 probably must correct ~3Hz effects • Deformation • Position/tilt • Implies interaction with structure

  30. Conclusions • Data indicate likely size of errors • Frequency range includes structural modes • Seems to support hierarchical approach • Interaction problem at M2

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