Implications of Wind Testing
Results on the GSMT Control
Systems
David R. Smith
MERLAB, P.C.
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.
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.
Errors
• Large, slow errors (m-mm, <0.01-0.1 Hz)
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Gravity
Thermal
Mechanical misalignments
Wind
• Medium-sized, rate (<~10 m, <~10 Hz)
– Wind
– Vibrations
• Small, fast errors (<1 m, >~10 Hz)
– Wind
– Vibrations
– Atmosphere
Controllers (example)
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Main Axis
M1 Gross/Fine Position
M1 Segment warping
M2 Positioner
M2 Fast tip/tilt/position
M2 Deformation
Downstream AO
Assumptions
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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 (cont.)
• 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.)
• 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
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
Gemini Data
• First round data (CD produced)
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Modal Test
Operating Data
Wind pressures
DOE results
• Second round data (analysis beginning)
– Wind speed and pressure only
– Better coverage of parameter space
Nobeyama Data
• Goal was to investigate pointing
– Pointing data analyzed
– Structural data quick-look only
• Deformations relevant to GSMT
– Similar size
– Similar natural frequencies
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
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.
Nobeyama Results
• Deformation of the primary
– Motion normal to surface
– Rigid body tilt removed
• Motion of the secondary
– X,Y,Z of typical point
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
Deformations of the Primary
• Raw acceleration signal
• Removal of rigid body tilt
• Comparison of RMS deformation at/above a
given frequency
Parked Telescope, Calm Wind
Tracking Telescope, Calm Wind
Parked Telescope, Wind 6-8m/s
RMS Comparison
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
Motion of the Secondary
• Accelerations in X, Y, Z
• RMS comparisons at/above a given
frequency (X, Y, Z)
Parked Telescope, Calm Wind
Tracking Telescope, Calm Wind
Parked Telescope, Wind 6-8m/s
RMS Comparison, X
RMS Comparison, Y
RMS Comparison, Z
Implications: Secondary
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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
Conclusions
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Data indicate likely size of errors
Frequency range includes structural modes
Seems to support hierarchical approach
Interaction problem at M2