ilias – wg1 hierarchical suspension control g.losurdo infn firenze

ILIAS – WG1Hierarchical suspension control

G.Losurdo

INFN Firenze

ILIAS-WG1– July 7th, 2004 G.Losurdo – INFN Firenze-Urbino

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


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


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 sec

– Max. sampling freq. 160 kHz (used at 10 kHz)

– Frequency accuracy at 10 kHz: f=2.5 Hz


Inverted pendulum

• Gravity as antispring: low resonant frequency

• Pre-isolation effect

• Low control forces:

– f0=40 mHz, m=1 ton, l=6 m, x =1 cm

F = 0.6 N !!

• Ideal as control platform: soft actuation

lg

mk

f 21

0

xmF 20


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


Control strategy

• From a MIMO to 3 SISO systems:diagonalization with respect to IP modes


Inertial Damping performance

Fringe signal

Inverted pendulum motion24 hrs

Rms over long periods 1 m

d/dt(L2- L1) ~ 0.25 m/s


• 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

Blending the sensorsx

xAccel.

LVDT0x x

dt xHighpass

Lowpass

+x

Highpass + Lowpass = 1


• The seismic noise filtering depends on L(s)

• The loop design is independent on the L(s) cutoff

02

aH l L x L x

s


Local control setup

• Optical levers read both the mirror and the marionette

• Marionette control allows larger bandwidth

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)


• Marionette error signal allows a bandwidth of 2-3 Hz

• Uncontrolled resonance (1.2 Hz) exists: needs blending with mirror error signal


Hierarchical control

• Required locking accuracy: L 10-12 m

• Tidal strain over 3 km: L 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…


SA local sensing

Mirror PSDMarionette PSD

F7 LVDTs

IP LVDTs/ACC


SA sensing

• IP and F7 diagonalized with respect to VRS (P.Ruggi, S.Braccini, F.Frasconi)

P.Ruggi


RM actuation

• RM actuators can compensate up to 100 m (in high power/high noise configuration)

• Tidal strain can be larger

• Locking is lost

Power in the cavity IP position Correction to mirror

mic

rons


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


C4 run

• Tide control: data vs prediction


16 mHz – the problem

• Main rotational mode of the SA

• Long decay time, large elongation.

• Hard to be controlled from the marionette

1

2

3

4

7

1

2

3

4

7

1

Braccini, Vicerè


16 mHz – solution

• Damp it off using F7 actuators!


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

Open loop gaincorrection

error signal


Locking from the RM: noise

• Reference mass actuators dynamics: 100 m

• DAC noise: 300 nV/Hz1/2

10-1

100

101

102

103

10-20

10-15

10-10

10-5

Actuators noise: current status

Frequency (Hz)

m/H

z1/2

Reference Mass - Mirror Actuators NoiseFilter #7 - Marionetta Actuators NoiseVIRGO Sentivity

A.Gennai


Standard design

DAC noise: 300 nV/sqrt(Hz) (17.5 effective bits) Coil Driver noise: 80 nV/sqrt(Hz)

10-1

100

101

102

103

10-20

10-15

10-10

10-5

Actuators noise: current status

Frequency (Hz)

m/H

z1/2

Reference Mass - Mirror Actuators NoiseFilter #7 - Marionetta Actuators NoiseVIRGO Sentivity

LCoil3mH

1

2

RCoil10

Ref. Mass Coil

+

-

OUT

R2

R

R1R

Coil Driver

DAC


Noise Reduction

LCoil3mH

1

2

RCoil10

Ref. Mass Coil

+

-

OUT

R2

R

R1R

Coil Driver

DACRN

500

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?)


New Solution

LCoil3mH

1

2

RCoil10

Ref. Mass Coil

+

-

OUT

R2

R

R1R

Coil Driver

DAC 2RN

500

DAC 1Transconductance Amplifier We supply the required

additional force for lock acquisition with a transconductive amplifier.

During lock acquisition phase only DAC 1 is used.

During linear phase DAC 1 output set to zero and DAC 2 is used to keep the lock.


Basic Equations

• Lock Acquisition: g1 = 1, g2 = 0

• Linear Regime: g1 = 0, g2 = 75

CoilNDAC

CoilN

NDACm ZR

VZR

RVgForce

121

zCorrgV

zCorrgV

ssLRZ

R

g

DAC

DAC

CoilCoilCoil

N

m

2

1

3

2

1

10310

500

2

3.0

753.0 21 ggzCorrForce

Note: coil pole shifted above 20 kHz

LCoil3mH

1

2

RCoil10

Ref. Mass Coil

+

-

OUT

R2

R

R1R

Coil Driver

DAC 2RN

500

DAC 1Transconductance Amplifier


High power – low noise switch

0 10 20 30 40 50 60-8

-6

-4

-2

0

2

4

6

8

Time (sec)

Cur

rent

Mon

itor

Coil UpCoil Down

switch

0 10 20 30 40 50 601.94

1.95

1.96

1.97

1.98

1.99

2

2.01

2.02

2.03

2.04x 10

-4

Time (sec)

Tra

nsm

itted

Pow

er


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


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

To be re-allocated to marionette

Vp=0.1 V


Mechanics of the last stages

• Complicated dynamics, important couplings…


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

SA mode tuning: I.Fiori, A.Vicerè

LC tuning: S.Avino, E.Calloni, I.Fiori


Marionette TF matrix

Fz FTx FTy

z

Tx

Ty

I.Fiori

Strongly coupled sy

stem!


• 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


Mechanics

• The two mechanical TF are different– For the structure around 1 Hz – For the asymptotical slope

1/f2

1/f4


“Modified” marionette

• Adding two zeroes makes the marionette TF “very similar” to the RM one

1/f2


1st scheme: composed lock ACQ

• Advantage: simpler, no need of transition

• Drawback: marionette control bandwidth limited by higher ITF noise (no SSFS)

PHD Lockingcompensator

L(s)(s+s0)2

H(s)

In the GC

In the DSP

zCorr

cavity power RM correction marionette correction


2nd scheme: re-allocation

• Advantage: allows wider marionette bandwidth

• To be tested with AA and SSFS

PHD

Anti-Ramp

Ramp 10s

Lockingcompensator

L(s)(s+s0)2

H(s)

In the GC

In the DSP

zCorr

cavity power RM correction marionette correction


Filters

• To blend the two systems use usual strategy

• L(s) = 3rd order low pass filter, H(s) = 1-L(s)


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

1997 !

mic

rons

DC-0.01 Hz

0.01-1 Hz

1-50 Hz


Performance with no AA/SSFS

Mirror displacement correction over the two stages


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


• One should consider peak values of zCorr instead of rms. In C4, over 18 hrs:

zCorrpeak ~ 10 zCorrrms

• 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)

Peak correction rms correction

C4 data


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


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 hierarchical suspension control g.losurdo infn firenze

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