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Current Concepts and Status of GSMT Control Systems

This article provides an overview of the current concepts and status of control systems for the Giant Segmented Mirror Telescope (GSMT). It discusses the importance of control systems in the feasibility and design of GSMT, as well as specific areas of control such as wind effect compensation and segment alignment maintenance. The article also covers topics like frequency band separation, adaptive optics, deformable secondary systems, measurement noise, computational load, system modeling and simulation, and future paths for GSMT control systems.

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Current Concepts and Status of GSMT Control Systems

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  1. Current Concepts and Statusof GSMT Control Systems George Angeli 11 September, 2001

  2. Introduction • How does the boot get on the dinner table? • Feasibility of GSMT depends on its controllability Why do we care about the control system in this early phase of design? • Wind effect compensation (structure correction) • Segment alignment maintenance • Whatever is designed, it must be observable and CONTROLLABLE!

  3. Frequency Band Separation of Subsystems ~100 ~50 ~20 ~10 2 LGS MCAO spatial & temporal avg Zernike modes AO (M2) spatial & temporal avg aO (M1) spatial avg temporal avg spatial avg Secondary rigid body spatial & temporal avg Main Axes 0.001 0.01 0.1 1 10 100 Bandwidth [Hz]

  4. MCAO • MCAO can be separated from telescope control • First MCAO sensor is behind the last telescope control actuator in the light path • MCAO is fed with a wavefront corrected up to 30-50 Zernike @ 20 Hz BW on tracking guide star

  5. Active Optics • Initial phasing in open loop • Phasing maintenance in closed loop with edge sensors • Assumption: wind buffeting has negligible high spatial frequency effects on primary mirror Continuity maintenance system is static (no interference with structural resonances) • Low spatial frequencies barely observable by edge sensors

  6. Phasing Maintenance • Static influence function • Edge detector / actuator modes by SVD

  7. Control Configuration for Phasing Maintenance From phasing Edge sensors Actuator space Pseudo-inverse:

  8. Adaptive Optics • Adaptive (deformable) secondary • Atmospheric correction r0 0.5 m @ 1.2 m, ~7000 actuators for 30 m MMT 1200 actuator/m2 on secondary@ 0.6 m GSMT 2200 actuator/m2 on secondary@ 2 m • Telescope deformation correction max. 1800 actuators for 600 segments 570 actuators/m2 on secondary • In the close future atmospheric correction is not feasible in the NIR (maybe in midIR)

  9. Deformable Secondary • Wavefront correction with deformable secondary • Face-sheet mass is negligible • Temporal average (0.1 Hz) off-loaded on primary • No interaction with telescope structure • Face-sheet motion is over-damped • No local dynamics Secondary AO system is static

  10. Frequency Band Separation of Wavefront Correction and Tracking

  11. Control Configuration for Wavefront Correction and Tracking Measurement noise Wind, Gravity, Heat Atmosphere • Offset due to: • telescope aberration • off-axis guide star

  12. Physical Configuration

  13. Computational Load • 1 Hz bandwidth  10 Hz sampling rate • Static active optics • Reconstructor matrix [1800 x 3600] 2 sensors on each edge • 230 GFLOP/s • Deformable secondary • 20 Hz bandwidth  200 Hz sampling rate • Reconstructor matrix [1800 x 1000] • 360 GFLOP/s Texas TMS320C64x 4.8 GFLOP/s Intel P4 1.4GHz 5.6 GFLOP/s

  14. System Modeling and Simulation • Investigate telescope behavior • Observability (sensor choices, placement) • Controllability (actuator choices, placement) • Performance • Validate design assumptions • Allows modal-based feedback design (Linear Quadratic Gaussian, H, etc.) • Validate model reduction for simulation and control

  15. Current Model • Modal based state space representation of the structure, based on FEA (20 modes) • Zernike representation of wavefront quality • Primary mirror as a surface fit on raft support nodes • Line-of-sight equation for rigid body motion of primary and secondary • Redefined base for OPD as a linear combination of Zernikes linked to structural modes • Integrated structure FEA • Force actuators at weakened or completely opened degrees of freedom

  16. Primary Mirror Truss Structure

  17. Structural Mode Shapes on the Primary Mirror

  18. Zernike Content of the Structural Modes

  19. Zernike Content of the Structural Modes

  20. Zernike Content of Secondary Rigid Body Motion

  21. Wind Load (X Direction)

  22. Wind Load (Y Direction)

  23. Wind Load (Z Direction)

  24. Future Path • Primary control • Segmented primary model (edge detectors, detector and actuator mode spaces) • Wavefront control (wind buffeting) • Verification of ‘static phasing maintenance’ hypothesis • Feedback design and simulation • Deformable secondary ‘surface fit’ model • Primary ‘surface fit’ model on actuator nodes • Wind load definition (on structure and primary) • Feedback design and simulation

  25. Future Path (cont’d) • Tracking • Actuator definition • Nonlinear (large signal) and linearized (small signal) models • Gravitational load definition • Feedback design and simulation • Integration • Structural model integration • Optical model integration • Feedback integration and simulation

  26. Modeling Issues • Structural model • Integrated structure versus interfaced subsystems • Boundary value problems • Optical model • Refined ‘fitted surface’ model with ray tracing and fitting each structural modes, i.e. building a new orthogonal basis for OPD which is characteristic to the telescope • Segmented mirror optical response • Load model • Wind power spectral density and spatial distribution • Wind-to-force conversion

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