The VIRGO Suspensions

bustlingdivisionΗλεκτρονική - Συσκευές

15 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

110 εμφανίσεις

The VIRGO Suspensions

Control System

Alberto Gennai

The VIRGO Collaboration


Australia
-
Italy Workshop
October 4
-
7 2005

A.Gennai (INFN Pisa)

2

Summary


Superattenuator


Local Controls


Inertial Damping


Payload Control


Global Control


Hierarchical Control


Lock Acquisition




Australia
-
Italy Workshop
October 4
-
7 2005

A.Gennai (INFN Pisa)

3

Superattenuator


Passive seismic isolation for optical
elements


18 coil
-
magnet pair actuators distributed
in 3 actuation point:


Filter zero (top stage)


Filter #7


Marionette


Recoil Mass


Mirror


Several sensors distributed along the
whole chain:


5 accelerometers on top stage


14 position sensors


Payload coarse local position readout via
CCD camera


Marionette and mirrors fine local position
readout via optical levers


Digital control system using DSP
processors


Australia
-
Italy Workshop
October 4
-
7 2005

A.Gennai (INFN Pisa)

4

Local Controls


Feedback loops using measurements
provided by local sensors


Top stage inertial control (Inertial
Damping)


Reduction of payload free motion


Always active


Payload local control


Positioning along a local reference frame


Damping of payload modes


Active only with interferometer unlocked



Australia
-
Italy Workshop
October 4
-
7 2005

A.Gennai (INFN Pisa)

5

Global Controls


Feedback loops using measurements
provided by photodiodes readout
system


Active when interferometer is locked


Payloads longitudinal position control
(Locking)


Payloads angular position control
(Automatic Alignment)


Position error signals computed by a single
processing unit and distributed to
suspensions using fiber optics connections

Australia
-
Italy Workshop
October 4
-
7 2005

6

Local Controls: Inertial Damping


Inertial sensors (accelerometers):


DC
-
100 Hz bandwidth


Equivalent displacement sensitivity: 10
-
11

m/sqrt(Hz)



Displacement sensors LVDT
-
like:


Used for DC
-
0.1 Hz control


Sensitivity: 10
-
8

m/sqrt(Hz)


Linear range:
±

2

cm



Coil magnet actuators:


Linear range:
±

2

cm


0.5 N for 1 cm displacement



Loop unity gain frequency:


5 Hz



Sampling rate:


10 kHz

7

Inertial Damping (II)


Complex transfer functions


Diagonal dominance achieved using static sensing and
driving matricies

I.D. Performances

Fringe signal

Inverted pendulum motion

24 hrs



1 um relative displacement


0.25 um/sec relative speed

Australia
-
Italy Workshop
October 4
-
7 2005

A.Gennai (INFN Pisa)

9

I.D. Blending Filters

0
2

a
H l L x L x
s
     
Local Control: Payload


Optical levers read both the mirror and the marionette


Marionette position readout allows larger bandwidth control
loops

C
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-
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7
(
F
7
)
(
F
7
)
Payload Local Control


Very complex dynamics

Australia
-
Italy Workshop
October 4
-
7 2005

A.Gennai (INFN Pisa)

12

Local Controls: Filter #7


16 mHz “chain” mode
gets excited when
marionette horizontal
coils are involved.


Long decay time,
large elongation.


Impossible to damp it
from top stage


It can be damped
acting on Filter #7

1
2
3
4
7
Australia
-
Italy Workshop
October 4
-
7 2005

A.Gennai (INFN Pisa)

13

Filter #7 Damping

Open loop gain

Velocity feedback

Australia
-
Italy Workshop
October 4
-
7 2005

A.Gennai (INFN Pisa)

14

Global Control


Required locking accuracy:


dL



-
12

m


Tidal strain over 3 km:





dL



-
4

m


Wide dynamic range to be covered
without injecting actuation noise.


Hierarchical Control


3 actuation points

Marionette Transfer Functions

z


x


x

F
z

M
x

M
y

Australia
-
Italy Workshop
October 4
-
7 2005

A.Gennai (INFN Pisa)

16

Lock Acquisition


Coil
-
magnet pair actuators steering VIRGO
optical elements need a wide dynamical range
due to the big force impulse required for
acquiring the lock of VIRGO optical cavities



The DAC dynamical range (17.5 effective bits, 105 dB
SNR) is not large enough.


Solution


Use of 2 DAC channels


DAC #1 for lock acquisition when the large force impulse is
required


DAC #2 for linear regime when low noise is required


Use of two different coil drivers for the two DAC
channels


HighPower (up to 2A output current)


LowNoise (programmable max output current)


Australia
-
Italy Workshop
October 4
-
7 2005

A.Gennai (INFN Pisa)

17

Hierarchical Control


Tide compensation
(low frequency
drifts) is applied on
Superattenuator
top stage



Marionetta and
mirror actuation
controls payload
normal modes

Australia
-
Italy Workshop
October 4
-
7 2005

A.Gennai (INFN Pisa)

18

Tide Control


Low frequency (< 10 mHz) part of z error signal is sent to
top stage (IP) actuators


Cavity transmission

Correction to the mirror

Suspension point position

24 h

Australia
-
Italy Workshop
October 4
-
7 2005

A.Gennai (INFN Pisa)

19

Marionette and Reference Mass


Mirror TF


Acting on Marionette from Filter #7, Superattenuators modes are excited


Acting on Mirror from Reference Mass only one longitudinal mode is
excited.

Australia
-
Italy Workshop
October 4
-
7 2005

A.Gennai (INFN Pisa)

20

Force re
-
allocation

Anti
-
Ramp

Ramp 10s

Locking

compensator

L(s)(s+s
0
)
2

H(s)

In the GC

In the DSP

zCorr

Australia
-
Italy Workshop
October 4
-
7 2005

A.Gennai (INFN Pisa)

21

Marionette


Mirror Blending


L(s) = 3
rd

order low pass filter, H(s) = 1
-
L(s)


Force applied on mirror from reference mass is high
-
pass filtered
while force applied on marionette from filter #7 is lowpass filtered


Australia
-
Italy Workshop
October 4
-
7 2005

A.Gennai (INFN Pisa)

22

Force on Mirror


Marionette/RM crossover @5 Hz


After reallocation zCorr rms reduced by a factor 100


Without marionette

700 nm rms

Without marionette

7 nm rms

Australia
-
Italy Workshop
October 4
-
7 2005

A.Gennai (INFN Pisa)

23

Conclusion


Superattenuator Controls


Even if basic control strategies has not change
during the last few years, feedback loops
compensators are keeping on changinng to
improve controls performances.


Possibility to adapt compensator to specific
states of the interferometer has shown to be a
key feature. Re
-
allocation of forces along the
chain allows easy lock acquisition and good
performances in linear regime.


Control strategies, expecially for lower stage,
are continuosly upgraded.


Powerful and flexible digital control system is
a must. (See next talk)