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SAMS Review

SAMS Review. Povilas Palunas. Outline. Review Specifications SAMS Architecture SAMS Control Theory Effect of Sensor Errors on Control In House Sensor Characterization Efforts Controlling Global Radius of Curvature Technical Plan. Performance Requirements Summary. Resolution

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SAMS Review

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  1. SAMS Review Povilas Palunas

  2. Outline • Review Specifications • SAMS Architecture • SAMS Control Theory • Effect of Sensor Errors on Control • In House Sensor Characterization Efforts • Controlling Global Radius of Curvature • Technical Plan

  3. Performance Requirements Summary • Resolution • 0.06 arcsec Tip/Tilt • 15μm Piston • Alignment Duration • Tip/tilt/GRoC*: 14 days • Piston: 90 days • For Full HET Environmental Specifications • Bandwidth • Sends updates to PMC every 10 seconds • Range: • Tip/tilt +/- 50 arcsec • Piston +/- 0.3 mm • GRoC +/- 6.0 mm *GRoC = Global Radius of Curvature

  4. Delivered Performance • Alignment Duration • Tip/tilt/GRoC: Operationally, 3-4 hours • Piston: Stacking is a primary source piston error. • Environmental Performance Estimates • ~0.06″ RMS / degC tip/tilt degradation (~0.3″ FWMH / degC at CoC) • ~0.3μm / degC Piston degradation • Bandwidth: 90 seconds • SAMS 34 seconds • PMC 60 seconds

  5. passive active Sensor Design • Measure impedance change due to proximity of the coils • Response is nonlinear and temperature dependent. • Nonlinearity modeled and corrected in design(?). • Temperature compensation hardware dependent. • Measure 2 degrees of freedom Shear ~ A- B Gap ~ A+B • “Other DOF’s contribute at higher order.” B A

  6. O X O X X X O O O X6 X1 O X2 X5 O O X O O O X3 X4 Sensor Configuration • 480 sensor pairs X active Y passive

  7. Sensor Readout

  8. y x z Influence Matrix i r a j

  9. Control Matrix To derive the control matrix we need to invert e=Cx However, x is over constrained by e: 480 constraints on 273 DOF and, 4 modes are unsensed by e: GRoC and Global Tip, Tilt and Piston The Optimal Least Squares Solution Minimizes the global error variance GEV=||(eref-e)||2or σ=||(eref-e)||/480 GRoC and Global Tip, Tilt and Piston must be ignored or controlled separately.

  10. Control Matrix • GRoC and Global Tip, Tilt and Piston controlled by setting up boundary conditions. • 4 segments are “fixed” in piston by removing these DOFs from C. • Control by offsetting boundary conditions. • The optimal solution subject to the boundary conditions is:

  11. Sensor Errors: single Bad Sensor Physical Response Unphysical Error Mirror coord

  12. Sensor Errors: Segment Bad Segment Physical Response Unphysical Error Mirror coord

  13. Sensor Errors: actual σunph ~ 100 nm/degC

  14. Sensor Errors: Random Random Errors σ ~100nm Physical Response Minus Global Modes Mirror coord Mirror coord

  15. Sensor Errors: Random FWHM • 100 nm RMS sensor noise • 72nm RMS unphysical sensor noise • Physical Response 0.33” FWMH Tip/Tilt error at CoC -0.044” Global tilt at CoC 0.062” Global tip at CoC 40μm GRoC 0μm Global piston (M43 is fixed) Spot Diagram at CoC

  16. Sensor Errors: Random • 90nm growth in sensor noise per deg C or 0.41″ FWHM per deg C • 0.9″ FWHM initial stack • 130nm growth in sensor noise per deg C or 0.60″ FWHM per deg C • 0.9″ FWHM initial stack

  17. Bandwidth • Disturbance to the primary takes 3-4, 90 second cycles to correct. • Baseline control resolution is 0.25″ FWHM, within specifications at constant temperature.

  18. The problem • Sensor Errors • Characterize the sensors • GRoC control • Get some • Bandwidth • PMC upgrade + SAMS console modifications • More accurate control (sensor gains)

  19. Nominal Sensor Calibration

  20. Sensor Calibration Gain (A) Gain (A) Test Procedure: • Piston segments in three sets, so that no neighboring segments move. • Piston down 75μm then up 6 steps of 25μm each • Record sensor response • Least squares fit to derive A for each sensor. Ignore downward step and first upward step. • Two measurements for each sensor moving active and passive side segments. Average sensor gain:

  21. Sensor Calibration Gain (A) Actuator errors • Measure as RMS deviations from fit. • 0.37μm RMS, 1.5% per move

  22. Sensor Calibration Gain (A) Actuator/Sensor repeatability • Compare Gain fits from different trials. • Short term (3hours) 0.4% • Long term (1month) 1% • With a few bad cases. • Actuator errors • Sensor electronics

  23. Sensor Calibration Gain (A) Actuator Accuracy • Compare active and passive side gain measurements. • δA/A= 4% RMS

  24. Sensor Calibration Gain (A) Source of Range in A • The Segment electronics • Binning the gains by segment • δAseg/Aseg= 1.6% RMS

  25. Sensor Calibration Gain (A) We need to measure this and/or keep the array flat!

  26. Sensor Calibration Gain tempcomp (α) Test procedure • Repeat Gain measurements at different temperatures. • Fit (A-Aref)/Aref vs. T • Average sensor tempcomp • Individual fits

  27. Sensor Calibration Gain tempcomp (α) Segment α • Bin αby segment

  28. Sensor Calibration Gain tempcomp (α) • Avs.α • Ignoring outliers • Slope -0.0046 • Zero compensation when A=0.828

  29. Sensor Calibration Gain tempcomp (α) In closed loop the sensor error due to α goes as Gain tempcomp is not the dominant source of sensor error

  30. Sensor Calibration Zeropoint tempcomp (β) • Zeropoint calibration is more difficult. It requires being able to set and maintain or measure accurate absolute offsets at the sensors. • We are pursuing 3 strategies: • Modeling eunph • Setting partial constraints (Tip/Tilt with HEFI/MARS) • Direct measurements with fixturing • Sandwich • Interferometer

  31. Sensor Calibration Zeropoint tempcomp (β) Model Averageβ Mirror coord Error Physical Response Unphysical Error

  32. Sensor Calibration Zeropoint tempcomp (β) Model Averageβ Delete sensors 69-2, 78-4 Gain Corrected βav = 51nm/degC σeunph~ 112 nm/degC 96 nm/degC 79 nm/degC

  33. Sensor Calibration Zeropoint tempcomp (β) Model Segmentβ [(I-CK)Xseg] is invertible after deleting 2 waffle like modes. Beginning on-sky verification

  34. Sensor Calibration Zeropoint tempcomp (β) Reducing DOFs • Tip/Tilt • Measure segment Tip/Tilts under closed loop operation. This will allow us to correct for physical Tip/Tilt DOFs before solving for individual sensor β’s. • Errors in the derived β’s result in piston only modes. • Sandwich Test • Fix relative motion of the active and passive sides of a sensor with a sandwich fixture with a fixed gap spacer. Measure shear over a range of temperatures. • Test a minimum of one sensor per segment at several gap spacings. • The compensated response of these sensors will allow a measurement of the piston error remaining from the Tip/Tilt calibration.

  35. Sensor Calibration GAP Measurement of Gap provides an independent sensor diagnostic and it can be a better predictor of the growth of errors than temperature.

  36. Sensor Calibration GAP Gap vs T • Measure Gap values as a function of Temperature. • Fit to get Gap gain or “effective cte”. • Scatter in fits likely due to segment rotation, which we can model. • Gap transfer function is nonlinear • Distribution for all sensors: 11.7±1.2 μm/degC

  37. Sensor Calibration GAP Shear Gain vs Gap Gain • Outliers due to disturbance to the Truss.

  38. Sensor Calibration Full Sensor Characterization • We have the tools in hand to characterize the SAMS sensors. However, • The SAMS sensor transfer function is complex and inadequately known. • The nominal sensor configuration in the truss keeps changing. We need To characterize a set of sensors/electronics with full and accurate control of shear/gap/(other sensor DOF) and temperature.

  39. X O X O Global Radius of Curvature • Initiating technical study for GRoC control system • Current Options • Offset current sensors • GAP based dihedral measurement • Additional sensor plane

  40. Technical Plan • Contract with Blue Line receive spares/software/consulting. • Tip/Tilt + Sandwich Characterization of SAMS sensors. • Extend piston testing to measure sensor nonlinearity. • Evaluate nominal sensor calibration formula (gap ?). • GRoC control • Control System Modeling (Mode based error analysis, predictor/corrector filtering)

  41. Technical Plan Software • Document Code • Console Level Calibration • Evaluate and reduce Overheads • Compute on demand interface with new PMC • GRoC control / Filtering upgrades • Improved Graphical Feedback

  42. Budget

  43. Conclusions • SAMS is telling us a good part of what is wrong. • The average sensor error rises predictably with temperature. • Tip/Tilt + Sandwich will constrain all the DOF’s to determine zeropoint compensation.

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