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George Angeli 11 September, 2001
Current Concepts and Statusof GSMT Control Systems
Introduction
Feasibility of GSMT depends on its controllability
Segment alignment maintenance
Wind effect compensation (structure correction)
Whatever is designed, it must be observable and CONTROLLABLE!
How does the boot get on the dinner table?
Why do we care about the control system in this early phase of design?
Frequency Band Separation of Subsystems
0.001 0.01 0.1 1 10100 Bandwidth [Hz]
Ze
rnik
e m
od
e s
~100
~50
~20
~10
2
aO (M1)
AO (M2)
Main Axes
LGS MCAO
Secondary rigid body
temporal avg
spatial & temporal avg
spatial & temporal avg
spatial & temporal avg
spatial avg
spatial avg
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
Active Optics
Initial phasing in open loop
Low spatial frequencies barely observable by edge sensors
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)
Phasing Maintenance
Static influence function
actuatoredge
actuatoredge
BACG
Guy1
Edge detector / actuator modes by SVD
actuatoractuator
edgeedge
actuatoredge
uVu
yUy
uGy
VUGG
~
~
~~
T
Control Configuration for Phasing Maintenance
TUVGG
From phasing
Edge sensors
GK(s)
<1 Hz
n(s)
r(s) y(s)u(s)R=G†
d(s)
estimator controller
Actuator space
Pseudo-inverse:
Adaptive Optics Adaptive (deformable) secondary
Atmospheric correction
r0 0.5 m @ 1.2 ~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)
Deformable Secondary
Face-sheet mass is negligible
No interaction with telescope structure
Face-sheet motion is over-damped No local dynamics
Secondary AO system is static
Wavefront correction with deformable secondary
Temporal average (0.1 Hz) off-loaded on primary
Frequency Band Separation of Wavefront Correction and Tracking
2
3
20
50
0.1 10 1001
Bandwidth [Hz]
Zer
nik
e m
odes
0.01
AO (M2)
Secondaryrigid body
Main Axes
temp.avg.
temp.avg.
temp.avg.
temp.avg.
aO (M1)
Control Configuration for Wavefront Correction and Tracking
Measurement noise Wind, Gravity, Heat Atmosphere
Offset due to:• telescope aberration• off-axis guide star
A
C
D1B1
B2
d(s) a(s)n(s)
r(s) y(s)xu(s)
G(s)
Ksec(s)<10 Hz
{Z1... Z2}
{Z1... Z3}
{Z1... Z50}
Kmain(s)<0.5 Hz
KAO(s)<20 Hz
D2
KaO(s)<0.1 Hz
D3
{Z3... Z50}
Physical Configuration
Segmented PrimaryMirror
Actuators
EdgeSensors
ActiveOptics
AdaptiveOptics
SecondaryControl
Main Axes
TelescopeStructure
Secondary Mirror
DeformableFacesheet
MainAxes
WFS
WindGravityHeat
Atmosphere
1 Hz .1 Hz 20 Hz 10 Hz .5 Hz
Z3... Z50 Z1... Z50Z1... Z3 Z1... Z2
Computational Load Static active optics
Deformable secondary
1 Hz bandwidth 10 Hz sampling rate Reconstructor matrix [1800 x 3600]
2 sensors on each edge 230 GFLOP/s
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
System Modeling and Simulation
Investigate telescope behavior
Validate design assumptions
Observability (sensor choices, placement) Controllability (actuator choices,
placement)
Allows modal-based feedback design (Linear Quadratic Gaussian, H, etc.)
Performance
Validate model reduction for simulation and control
Current Model Modal based state space representation of
the structure, based on FEA (20 modes)
Primary mirror as a surface fit on raft support nodes
Line-of-sight equation for rigid body motion of primary and secondary
Force actuators at weakened or completely opened degrees of freedom
Zernike representation of wavefront quality
Integrated structure FEA
Redefined base for OPD as a linear combination of Zernikes linked to structural modes
Primary Mirror Truss Structure
Structural Mode Shapes on the Primary Mirror
Zernike Content of the Structural Modes
0 1 2 3 4 5 6 7 8 9-0.25
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1Z
ern
ike
coef
fici
ents
[ar
bit
rary
un
it]
Zernike modes
#1 2.169Hz#2 2.248Hz#3 2.754Hz#4 3.207Hz#5 3.259Hz#6 3.908Hz#7 4.376Hz#8 4.559Hz#9 4.777Hz#10 4.783Hz
Zernike Content of the Structural Modes
0 1 2 3 4 5 6 7 8 9-0.25
-0.2
-0.15
-0.1
-0.05
0
0.05
Zernike modes
Zer
nik
e co
effi
cien
ts [
arb
itra
ry u
nit
]
#11 4.792Hz#12 5.396Hz#13 5.404Hz#14 5.952Hz#15 6.000Hz#16 6.091Hz#17 6.208Hz#18 6.274Hz#19 6.553Hz#20 7.635Hz
Zernike Content of Secondary Rigid Body Motion
0 2 4 6 8-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
Zernike modes
Zer
nik
e co
effi
cien
ts [
arb
itra
ry u
nit
]
TxTyTzRxRy
Wind Load (X Direction)Bode Magnitude Diagram
Frequency (Hz)
Mag
nit
ud
e (d
B)
100
101
102
-350
-300
-250
-200
-150
-100From: Wind (x)
piston (0)
y tilt (2)
y astig (5) x coma (6)
x tilt (1)
x astig (4)
y coma (7)
focus (3)
spherical (8)
Wind Load (Y Direction)Bode Magnitude Diagram
Frequency (Hz)
Mag
nit
ud
e (d
B)
100
101
102
-350
-300
-250
-200
-150
-100From: Wind (y)
y tilt (2)
piston (0)
x astig (4) y coma (7) focus (3)
spherical (8)
y astig (5)
x tilt (1) x coma (6)
Wind Load (Z Direction)Bode Magnitude Diagram
Frequency (Hz)
Mag
nit
ud
e (d
B)
100
101
102
-350
-300
-250
-200
-150
-100From: Wind (z)
piston (0)
x tilt (1)
y tilt (2)
focus (3)
x astig (4)
y astig (5)
x coma (6)
y coma (7)
spherical (8)
Future Path
Wavefront control (wind buffeting)
Segmented primary model (edge detectors, detector and actuator mode spaces)
Verification of ‘static phasing maintenance’ hypothesis
Primary control
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
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
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