Active optics and control architecture for a Giant Segmented Mirror - - PowerPoint PPT Presentation

Active optics and control architecture for a Giant Segmented Mirror Telescope

George Z. Angeli, Myung K. Cho, Mark S. Whorton

Overview

• A feasible control architecture

– How to separate and organize control functions

• Supporting simulations

– Proving it’s viability – With real, measured wind data

Physical configuration 1

New challenge – wind:

Increased area Lower resonance frequencies

Integrated aO and AO

Physical configuration 2

Control philosophy

• Forced decoupling of control subsystems
• Allows decentralization
• Improves understanding of underlying

concepts and processes

• Simplifies control laws and cost functions
• Supports detached design, implementation and

troubleshooting of subsystems

• Subsystems are still sophisticated MIMO

(multiple-input-multiple-output) systems

Control architecture

Parallel optical and mechanical feedback

Main axes (tracking) control based on WFS (0.5 Hz) M1 phasing maintenance based

• n edge sensors (0.5 Hz)

M2 rigid body motion control based on WFS (10 Hz) M2 facesheet control based

• n WFS (100 Hz)

M1 low order shape control (aO) based on WFS (0.1 Hz)

Frequency separation of optical subsystems

2 3 20 0.1 10 100 1

Bandwidth [Hz] Zernike modes

0.01

M2 Deformable M2 Rigid Body Main Axes

temp.avg. temp.avg. temp.avg. temp.avg.

M1 Shape

Control configuration

Msec Mpri K(s)sec K(s)pri

BDM Bsec Bpri Ats

x

Telescope dynamics

∫

ADM

x

Deformable M2 dynamics

∫

CDM2 Cpri Csec Cedge Bedge

Redge WFS Hatm Sky motion, Turbulence Wind Phasing reference

Bwind Optics

K(s)edge Rpri Rsec RDM2 Aberration reference

Control system

K(s)sec

Optical system

Fundamental assumption

– Structural interactions avoidable

• Primary mirror phasing maintenance possible

with limited bandwidth loop

– High order, high frequency M1 wind deformations well bounded – Secondary rigid body control only with actuator-structure interaction Whorton et al. 4840-23

• Verifying simulations

– Segment modeling for continuity check (no structural deformation) – Structural modeling for large scale deformations

Model for GSMT structural simulation

• Structure

– Modal description (20 modes) – State-space representation

Bu Φ M Ωq q ZΩ q

m m m T 1

2

−

= + + & & & u B Ω M x ZΩ Ω I x       +       − − =

− T 1 2

2 &

• Wind

– Gemini South measurements – Open dome, slit facing wind – Wind velocity

• ~10 m/s @ dome
• ~4 m/s @ M1
• ~4 m/s @ M2

10

• 2

10

• 1

10 10

10

• 3

10

• 2

10

• 1

10 10

10

Fre que ncy [Hz] PSD [Pa2/Hz] measurement von Karman fit

Cho et al. 4837-40

M1 deformation due to wind

• 20

20

• 20
• 10

10 20 5 10 15 20

Y [m] X [m] RMS deformation [µm]

Zernike expansion of M1 deformation

2 4 6 8 10 12 14 16 18 5 10 15

Zernike term RMS Zernike coefficient [ µm]

wind on secondary wind on primary total wind

PSD of RMS M1 deformation

10

• 2

10

• 1

10 10

1

10

• 1

10 10

1

10

2

Frequency [Hz] PSD of RMS primary mirror deformation [

µm/√Hz]

Residual M1 deformation

10

• 2

10

• 1

10 10

1

10

• 3

10

• 2

10

• 1

Frequency [Hz] RMS error [µm]

32 nmRMS Zernike terms removed up to #36:

Model for segment control simulation

1 2 3 4 5 6 7 8 0.5 1 1.5 2 2.5 Sens or s pacing, d [m] Structure function, √D [Pa]

( ) ( ) ( ) [ ]

spatial p

p p r D

2

r r r − + =

Wind

– Same as for structural simulation – Cho et al., SPIE 4837-40 – Correlation length < 2m on M1

“Segmented” Gemini mirror

– Segment size 1.152 m edge-to- edge – Actuator stiffness 10 N/µm – No dynamics

Segment continuity control

G K(s) n(s) r(s) y(s)

u(s)

R=G† d(s)

estimator controller

10

• 1

10 10

1

10

2

10

• 4

10

• 3

10

• 2

Frequency (Hz) PSD of Edge Sens or Nois e (µm/√Hz)

( )

T i

diag V U G       = σ

T i

1 diag U V G               =

+

σ

( )

1 6 . 1 20 + = s s K

108 84 Band limited proportional From initial phasing Actuator and sensor modes based on SVD:

M1 deformation (ventilation gates open)

µm

RMS edge displacement (ventilation gates open)

Wind velocity: ~10 m/s @ dome t ~4 m/s @ M1 ~4 m/s @ M2

High wind

2 4 6 8 10 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Time [s econd] RMS Edge Reading [µm]

Open loop Closed loop

110 nmRMS open loop 30 nmRMS closed loop

RMS edge displacement (ventilation gates closed)

Wind velocity: ~11 m/s @ dome t ~0.6 m/s @ M1 ~4 m/s @ M2

Low wind

2 4 6 8 10 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Time [s econd] RMS Edge Reading [µm]

Open loop Closed loop

12 nmRMS open loop 6 nmRMS closed loop

Conclusion

• Wind load on a 30-meter class telescope is not

trivial, but manageable with a distributed control architecture

• Further studies necessary

– Integrated structural, optical and control model to – Realize optical feedback – Evaluate performance – Balancing dome seeing and structural deformation effects to find the “optimum” wind inside the enclosure

Frequency bands of actuator groups

2 8 20 50 0.1 10 100 1

Bandwidth [Hz] Zernike modes

0.01

M2 Deformable M2 Rigid Body Main Axes

spatial avg. spatial & temporal avg.

MCAO M1 Actuators

temporal avg. spatial & temporal avg. spatial & temporal avg. spatial & temporal avg.