the virgo suspensions control system alberto gennai the virgo collaboration
TRANSCRIPT
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The VIRGO SuspensionsThe VIRGO Suspensions Control System Control System
Alberto Gennai
The VIRGO Collaboration
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Australia-Italy Workshop October 4-7 2005
A.Gennai (INFN Pisa) 2
SummarySummary
SuperattenuatorLocal Controls
– Inertial Damping– Payload Control
Global Control– Hierarchical Control– Lock Acquisition
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Australia-Italy Workshop October 4-7 2005
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SuperattenuatorSuperattenuator 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
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Local ControlsLocal ControlsFeedback 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
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Global ControlsGlobal ControlsFeedback 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
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Local Controls: Inertial DampingLocal 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
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Inertial Damping (II)Inertial Damping (II)
Complex transfer functions Diagonal dominance achieved using static sensing and
driving matricies
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I.D. PerformancesI.D. Performances
Fringe signal
Inverted pendulum motion24 hrs
1 um relative displacement 0.25 um/sec relative speed
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I.D. Blending FiltersI.D. Blending Filters
02
aH l L x L x
s
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Local Control: PayloadLocal Control: Payload Optical levers read both the mirror and the marionette Marionette position readout allows larger bandwidth
control loops
CCD
CCD-MIRROR dist ance =1250 mmCCD focal L. = 25 mmApert ure = 18 mm
incidence 35o
(z) beam axisopt ical port s
dif fusive markers
halogenilluminat or
opt ical port s
XY
Err(xy)
Err(xy)
PSD device on t he focal plane
XY
PSD deviceon t he image plane
Err(z)
14 mW red laserdiode - SM f iber
XY
Err(xy)PSD deviceon t he focal plane
1.4 mW red laserdiode - SM fiber
incidence 30o
f =200 mm
f =200 mm
act uat or
act uat or
t o SA’s f ilt er 7 (F7)
(F7)
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Payload Local ControlPayload Local Control Very complex dynamics
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Local Controls: Filter #7 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
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Filter #7 DampingFilter #7 Damping
Open loop gain
Velocity feedback
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Global ControlGlobal Control
Required locking accuracy:
dL 10-12 m Tidal strain over 3 km: dL
10-4 mWide dynamic range to be covered
without injecting actuation noise.Hierarchical Control
– 3 actuation points
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Marionette Transfer FunctionsMarionette Transfer Functions
z
x
x
Fz Mx My
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Lock AcquisitionLock 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)
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Hierarchical ControlHierarchical Control
Tide compensation (low frequency drifts) is applied on Superattenuator top stage
Marionetta and mirror actuation controls payload normal modes
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Tide ControlTide 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
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Marionette and Reference Mass – Mirror TFMarionette 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.
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Force re-allocation Force re-allocation
Anti-Ramp
Ramp 10s
Lockingcompensator
L(s)(s+s0)2
H(s)
In the GC
In the DSP
zCorr
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Marionette – Mirror BlendingMarionette – Mirror Blending L(s) = 3rd 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
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Force on MirrorForce on Mirror Marionette/RM crossover @5 Hz After reallocation zCorr rms reduced by a factor 100
Without marionette700 nm rms
Without marionette7 nm rms
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ConclusionConclusion 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)