implications of wind testing results on the gsmt control systems

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)

– 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)

• Main Axis

• M1 Gross/Fine Position

• M1 Segment warping

• M2 Positioner

• M2 Fast tip/tilt/position

• M2 Deformation

• Downstream AO

Assumptions

• 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)– 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

• Twist motions much smaller• Tracking and wind cause same scale errors• Lateral and focus/tilt motions: 10’s of m• Most effects (>1m) below 3 Hz• M2 probably must correct ~3Hz effects

– Deformation– Position/tilt– Implies interaction with structure

Conclusions

• Data indicate likely size of errors

• Frequency range includes structural modes

• Seems to support hierarchical approach

• Interaction problem at M2