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ILIAS – WG1
Hierarchical suspension control
G.Losurdo
INFN Firenze
Virgo Superattenuator
•
PASSIVE isolator
•
Designed with 3 points of actuation:
– Inverted pendulum
– Marionette
– Recoil mass
•
Local controls
– Inertial damping of internal modes (IP)
– Pre-alignment/damping of the payload modes
(optical levers on marionette/mirror)
•
Global control: locking correction distributed
hierarchically over the three actuation points
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
Digital Controls
•
•
DSP 1: sends correction for inertial damping and
tide control to IP actuators
DSP 2: sends correction for local
controls/AA/locking to marionette/RM actuators
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
Control electronics
•
Digital electronics (16 bit ADC – DSP – 20 bit DAC)
•
DSP characteristics:
– Clock frequency 60 MHz
– 2 poles/2 zeroes filter in 330 nsec
– 3x3 matrix-vector product in 1 msec
– Max. sampling freq. 160 kHz (used at 10 kHz)
– Frequency accuracy at 10 kHz: df=2.5 mHz
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
Inverted pendulum
•
Gravity as antispring: low resonant frequency
f0 
•
Pre-isolation effect
•
Low control forces:
1
2
k
g

m l
F  m0 x
2
– f0=40 mHz, m=1 ton, l=6 m, x =1 cm
 F = 0.6 N !!
•
Ideal as control platform: soft actuation
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
Sensors/Actuators
•
Inertial sensors:
– DC-100 Hz bandwidth
– Equivalent displacement sensitivity:
better than 10-11 m/rt(Hz)
•
Displacement sensors:
– Used for DC-0.1 Hz control
– Sensitivity: 10-8 m/rt(Hz)
– Linear range: few cm
•
Coil magnet actuators:
– Linear range: few cm
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
Control strategy
•
From a MIMO to 3 SISO systems:diagonalization with respect to IP modes
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
Inertial Damping performance
Inverted pendulum motion
24 hrs
Rms over long
periods  1 mm
Fringe signal
d/dt(L2- L1) ~
0.25 mm/s
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
x
Blending the sensors
•
Low frequency position control is needed because:
– Inertial sensors do not provide DC error signal
– Inertial sensors response at f<40 mHz can be spoiled by tilt
•
Problem: blend the sensors
– dominating the tilt effect
– minimizing the seismic noise re-injection
– Simplyfing the control strategy
Accel.
LVDT
x
x
dt

x  x0
Highpass
x
+
Lowpass
Highpass + Lowpass = 1
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
•
•
The seismic noise filtering depends on L(s)
The loop design is independent on the L(s) cutoff
a
 H  l  L  x  L  x0
2
s
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
Local control setup
•
•
Optical levers read both the mirror and the marionette
Marionette control allows larger bandwidth
t o SA’s f ilt er 7 ( F7 )
( F7)
act uat or
CCD-MIRROR dist ance =12 5 0 mm
CCD f ocal L. = 2 5 mm
Apert ure = 1 8 mm
incidence 3 0 o
1 .4 mW red laser
diode - SM f iber
Err( x y )
opt ical port s
CCD
halogen
illuminat or
act uat or
XY
1 4 mW red laser
diode - SM f iber
f =2 0 0 mm
incidence 35 o
( z) beam axis
Err( x y )
opt ical port s
f =2 0 0 mm
PSD device on t he f ocal plane
XY
dif f usive markers
XY
Err( x y )
PSD device
on t he f ocal plane
Err( z)
PSD device
on t he image plane
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
•
•
Marionette error signal allows a bandwidth of 2-3 Hz
Uncontrolled resonance (1.2 Hz) exists: needs blending with mirror error
signal
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
Hierarchical control
•
•
Required locking accuracy:
Tidal strain over 3 km:
dL  10-12 m
dL  10-4 m
The required dynamic range can be covered by two stages. The third one helps for
bandwidth/noise issues…
Tide/drifts compensation
Control of the resonances
Widening the bandwidth…
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
SA local sensing
IP LVDTs/ACC
F7 LVDTs
Marionette PSD
Mirror PSD
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
SA sensing
•
IP and F7 diagonalized with respect to VRS (P.Ruggi, S.Braccini, F.Frasconi)
P.Ruggi
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
RM actuation
•
•
•
RM actuators can compensate up to
100 mm (in high power/high noise
configuration)
Tidal strain can be larger
Locking is lost
Power in the cavity
ILIAS-WG1– July 7th, 2004
IP position
Correction to mirror
G.Losurdo – INFN Firenze-Urbino
Tide Control
•
Re-allocation of the low frequency (<10 mHz) correction to the IP
Cavity transmission
Correction to the mirror
Suspension point position
24 h
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
C4 run
•
Tide control: data vs prediction
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
16 mHz – the problem
•
•
•
Main rotational mode of the SA
Long decay time, large elongation.
Hard to be controlled from the marionette
Braccini, Vicerè
1
1
2
2
3
3
4
4
1
7
ILIAS-WG1– July 7th, 2004
7
G.Losurdo – INFN Firenze-Urbino
16 mHz – solution
•
Damp it off using F7 actuators!
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
F7 control
•
•
•
•
Hardware/software for F7 control implemented on the NE tower
16 mHz resonance control activated
Used either with or without LC
Possibility to control other “dangerous” SA modes to be studied
error signal
Open loop gain
ILIAS-WG1– July 7th, 2004
correction
G.Losurdo – INFN Firenze-Urbino
Locking from the RM: noise
•
•
Reference mass actuators dynamics: 100 mm
DAC noise: 300 nV/Hz1/2
Actuators noise: current status
-5
A.Gennai
10
Reference Mass - Mirror Actuators Noise
Filter #7 - Marionetta Actuators Noise
VIRGO Sentivity
-10
m/Hz 1/2
10
-15
10
-20
10
-1
10
ILIAS-WG1– July 7th, 2004
0
10
1
10
Frequency (Hz)
2
10
3
10
G.Losurdo – INFN Firenze-Urbino
Standard design
Coil Driver
Actuators noise: current status
-5
10
Ref. Mass Coil
DAC
RCoil
10
+
OUT
Reference Mass - Mirror Actuators Noise
Filter #7 - Marionetta Actuators Noise
VIRGO Sentivity
LCoil
3mH
R2
R
-10
10
m/Hz 1/2
1
-
R1
R
2
-15
10
-20
10
-1
0
10
10


1
10
Frequency (Hz)
2
10
3
10
DAC noise: 300 nV/sqrt(Hz) (17.5 effective bits)
Coil Driver noise: 80 nV/sqrt(Hz)
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
Noise Reduction
Coil Driver
Ref. Mass Coil
DAC
+
RCoil
10
RN
OUT
500
-
1
LCoil
3mH
R2
R
R1
R
2

To reduce the DAC noise we should insert a resistor in series with coil driver. To
get closer to VIRGO specs, the resistor value should be about 500 ohms. Bigger
values could be used if force will be enough to keep the cavities locked.
 The resistor limits the maximum force we can apply and therefore makes lock
acquisition very difficult (impossible?)
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
New Solution


We supply the required
additional force for lock
acquisition with a
transconductive amplifier.
Transconductance Amplifier
DAC 1
During lock acquisition phase
only DAC 1 is used.
Coil Driver

During linear phase DAC 1
output set to zero and DAC 2 is
used to keep the lock.
Ref. Mass Coil
DAC 2
+
RCoil
10
RN
OUT
500
-
1
LCoil
3mH
R2
R
R1
R
ILIAS-WG1– July 7th, 2004
2
G.Losurdo – INFN Firenze-Urbino
Basic Equations
Transconductance Amplifier
DAC 1
Force   g mVDAC1
RN
1
   VDAC2
RN  Z Coil
RN  Z Coil
Coil Driver
 g m  0.3
  2

 RN  500

3
Z

R

L
s

10

3

10
s
Coil
Coil
Coil

VDAC  g1  zCorr
1

VDAC2  g 2  zCorr
•
•
Lock Acquisition: g1 = 1, g2 = 0
Linear Regime: g1 = 0, g2 = 75
ILIAS-WG1– July 7th, 2004
Ref. Mass Coil
DAC 2
+
RCoil
10
RN
OUT
500
-
1
LCoil
3mH
R2
R
R1
R
2
Force  0.3   zCorr  g1  g2 75
Note: coil pole shifted above 20 kHz
G.Losurdo – INFN Firenze-Urbino
High power – low noise switch
8
Coil Up
Coil Down
6
2
-4
2.04
x 10
2.03
0
2.02
2.01
-2
switch
Transmitted Power
Current Monitor
4
-4
-6
2
1.99
1.98
1.97
1.96
-8
1.95
0
10
20
30
Time (sec)
ILIAS-WG1– July 7th, 2004
40
50
60
1.94
0
10
20
30
Time (sec)
40
50
G.Losurdo – INFN Firenze-Urbino
60
Noise figures
•
DAC noise expected (?) @100 Hz:
3 10-16 m/Hz1/2
•
Virgo design sensitivity @100 Hz:
2 10-19 m/Hz1/2
•
Required noise reduction @100 Hz:
1500
•
•
Using tide control allows to reduce the required correction by a factor 100
Re-allocating locking to the marionette in the SA resonance region should
provide the residual attenuation
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
Correction to the mirror in C4
•
•
Marionette control with 3 Hz
bandwidth allows to reduce the
correction by 50 (Vp= 2 mV)
Coil driver gain could be reduced
by a factor 5000
50
Vp=0.1 V
To be re-allocated
to marionette
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
Mechanics of the last stages
•
Complicated dynamics, important
couplings…
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
Use of SA simulation
•
SA simulation has been important to design the marionette control strategy:
– Tuning of SIESTA to reproduce the measured TF
– Use of tuned simulation to estimate and subtract the couplings due to
the sensing
– Calculation of a filter to compensate for x marionette motion induced
by longitudinal forces
LC tuning: S.Avino, E.Calloni, I.Fiori
SA mode tuning: I.Fiori, A.Vicerè
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
Marionette TF matrix
Fz
FTx
FTy
I.Fiori
z
Tx
Ty
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
•
•
Using 4 coils to move the
marionette along z: reduce
the z-x coupling
Good data-simulation
agreement
I.Fiori, A.Gennai
I.Fiori, S.Avino
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
Mechanics
•
The two mechanical TF are different
– For the structure around 1 Hz
– For the asymptotical slope
1/f2
1/f4
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
“Modified” marionette
•
Adding two zeroes makes the marionette TF “very similar” to the RM one
1/f2
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
1st scheme: composed lock ACQ
•
•
Advantage: simpler, no need of transition
Drawback: marionette control bandwidth limited by higher ITF noise (no
SSFS)
In the DSP
In the GC
PHD
L(s)(s+s0)2
zCorr
Locking
compensator
H(s)
cavity power
ILIAS-WG1– July 7th, 2004
RM correction
marionette correction
G.Losurdo – INFN Firenze-Urbino
2nd scheme: re-allocation
•
•
Advantage: allows wider marionette bandwidth
To be tested with AA and SSFS
In the DSP
Anti-Ramp
L(s)(s+s0)2
Ramp 10s
H(s)
In the GC
zCorr
Locking
compensator
PHD
cavity power
ILIAS-WG1– July 7th, 2004
RM correction
marionette correction
G.Losurdo – INFN Firenze-Urbino
Filters
•
•
To blend the two systems use usual strategy
L(s) = 3rd order low pass filter, H(s) = 1-L(s)
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
Hierarchical control
•
The north cavity has been locked by distributing the forces over the three SA
stages: the controllability of the SA has been demostrated
0.01-1 Hz
microns
DC-0.01 Hz
1-50 Hz
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
Performance with no AA/SSFS
Mirror displacement correction over the two stages
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
C4 data - extrapolation
•
•
C4 data (noisy stretch) have been filtered with current hierarchical control TFs
to predict the correction one expects on the RM when SSFS is ON
Expected zCorrrms= 3 mV.
L.Holloway
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
•
One should consider peak values of zCorr instead of rms. In C4, over 18 hrs:
zCorrpeak ~ 10 zCorrrms
Peak correction
•
•
rms correction
With hierarchical control in the present configuration one can assume:
zCorrpeak ~ 30 mV
Therefore, the coil driver gain can already be reduced by ~ 300
We are not far from Virgo sensitivity…
New promising design is being tested in MATLAB (L.Holloway)
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
Interaction with angular control
•
•
•
The alignment/power fluctuations are larger when hierarchical control is ON
This is a concern: to be tested with AA
Larger statistics needed, analysis going on
Standard locking
Hierarchical locking
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino
Next steps
•
•
•
Test hierarchical locking with linear alignment
Widen the bandwidth of marionette control
Switch to low noise coil driver after re-allocation
ILIAS-WG1– July 7th, 2004
G.Losurdo – INFN Firenze-Urbino