ILIASGeneva 29 March 2007

Virgo suspension control progress

E. MajoranaINFN

Mirror Suspension Control workgroup

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Thermal noise

Soft isolator concept:1. very efficient passive

attenuation2. active controls for

normal mode damping

The usual “standard-super-attenuator” suspension …

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The mission of Mirror Suspension Control workgroup: commissioning-oriented activity

Virgo sensitivity (at LF !) Virgo duty-cycle

In fall 2006 the majority of main ITF control issues had been addressed:- lock acquisition strategy - automatic alignment (~)

-suspensions and local controls allowed all above

To start noise hunting, stable operation was needed:=> MSC performance started to be integrated in ITF issues

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Our plan was clear thanks to the previous efforts of MSC Workgroup, during the first part of the commissioning,

and to some ideas matured meanwhile.

Unavoidable acknowledgments to my friends and colleaguesGiovanni Losurdo and Paolo Ruggi

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Disturbance rejection

Sensor blending optimizationGlobal Inverted Pendulum Control

Interface with ITF

Angular control strategies(Local/automatic)

Beam centering

All-the-possible-handles(completion needed!)

Lock force re-allocation (BS)SA-chain damping from ground

Vertical dampingSafe-and-soft operation interfaces

Noise budget

Suspension control chain(read-out,numerical,digital,actuator)

NOW

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Focus on:

- improvement of disturbance rejection - reduction of control noise through the suspension - reduction of ITF coupling with the control noise

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> disturbance rejection: vertical damping needed (1/9)

V-damp ON/OFFvs alignment

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> disturbance rejection: vertical damping needed (2/9)

One-By-One

ALL OFF

ALL OFF

ALL ON

ALL ON

O-B-OOFF

O-B-OOFF

No crucialImprovementduring OBOOFF Test.

Clear ImprovementAll OFF/ON

Relationship withAngular fluctuations

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> disturbance rejection: vertical damping needed (3/9)

TOP stage

Last stage

ITF

One-By-One (detailed)

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> disturbance rejection: top stage sensor blending (4/9)

ACC

LVDT

+

HP

LPcross IP

fcrossover = 50 mHzTrade-off between: 30 mHz (wind disturbance through ACC)and70 mHz (seism disturbance through LVDT)

Winter issues: STD scheme and blending used until Dec 2006

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30 mHz crossover N-arm70 mHz crossover W-arm

cavities locked independently

pulling back the crossover: benefit expected

unexpected !wind-noise

seism (sea) produces angular excitation of the payload.

wind (reasonably through the tilt) fake acceleration re-injected.

Key solutions :- smart experiments to simulate the disturbance ;- tools to optimize the blending on-the-fly ;- other smart ideas ….

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Sensor blending:TWO-SIDE OPTIMIZATION

characterizing Suspension-control_vs_disturbance

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hybrid filters (on-the-fly tuning)

mix =0.5 :“medium” attenuation of seism noise

old new

mix =0 (wind):not worse than old filters against seism + tilt noise attenuation below 50 mHz

old new

mix =1 (seism) :“strong” attenuation of seism noise with slightly worsened tilt noise attenuation.

old new

> disturbance rejection: top stage sensors, first attempts (5/9)

ACC

LVDT

HPs

LPs

IP

HPw

LPwcross

+

+

mix

The answer is worked outthrough a deeper analysis

How much is it enough ?

OUR TARGET

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hybrid filters in use: noise percolation paths

Calm period

mix=1 Ineffective@ 0.45 Hz, (IP notch)

Pitch excitation (payload mode)

yaw bump (due to large zM correction)

Large zM due to tilt disturbance through accelerometers

3 mechanisms (at least): F0_z zCorr, F0_z x, F0_z y

> disturbance rejection: top stage sensors (6/9)

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WSR7, heuristic threshold for mix adjustment in-line

wind (mix 0) sea (mix 1)hic sunt leones

> disturbance rejection: top stage sensors (7/9)

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LP/HP optimization, given the standard actual corrector (cross)

CL TF

ACC

LVDT

HPLP mix

cross IP

seism

mix=0.70mix=0.90 mix=0.95 mix=1

LP/HP ratio versus CL TF

Hz

Two regions can be distinguished:A) LP/HP ratio plays a role

attenuating LVDT sensor disturbance (seism)

B) No effect of LP/HP attenuation

LVDT noise projection into zCorr (NE@step 1)

Hz

V/s

qrt(

Hz)

> disturbance rejection: top stage sensors (8/9)

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WSR7 => WSR8 (optimization example)

Hz Hz

Hz Hz

ms-2

HP: anti-wind (

previous WSR7

WSR8 LP: accel. comparison

note: cleaner (LP+HP=1) blending allows to re-tune HP to reduce tilt re-injection

patches added to LP to reduce mirror tilt excitation due to SA seismic noise disturbance

(=> heavier computation…)

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WSR7 => WSR8 optimization

Hz Hz

Hz Hz

ms-2

LP: anti-seismprevious WSR7

WSR8 accel. comparison

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WSR7 => WSR8 overall tuning (mix 0-1)

LP/HP

mix 0 => 1wind => sea

mix 1 => 0sea => wind

A) assessing how much it is enough to enhance LP/HP to beat tilt re-injection is not easy. B) more data needed in windy-only conditions… C) optimization not significantly efficient around the crossing frequency.

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Marionette locking force can be distributed to both input and End mirrors

Since WSR8 the lock correction is applied to four marionettes (NI,NE,WI,WE) instead of two (NE,WE): A) reduction of direct locking force budget and B) non-linear torque negligible in case of wind (=> reduction of large zM by a factor 2 lightens gain request to AA).

> disturbance rejection and noise budget: 4-marionette (9/9)

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Novel strategy (…one smart idea):GLOBAL OPTIMIZATION

anti-seism plus coherent wind compensation

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GIPC (Global Inverted Pendulum Control)

benefits: wind disturbance rejection, lock acquisition, duty-cycle.

Once the ITF is locked, the mirror position, provided by the Global Control, can be used instead of LVDTs, referred to the ground.

Features: - automatic engagement (soft!)- anti-wind blending in NE-WE-BS-PR- anti-seism in NI-WI

GC (reconstructed z)

LVDT

LVDTSplitted anti-seism /anti-seism

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Example1: NE_GIPC,WE_GIPC only

NE “follows” NI WE “follows” WI

step9

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Example 2: NE_GIPC, WE_GIPC only

step9

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Example 3: NE_GIPC, WE_GIPC, BS_GIPC, PR_GIPC

step9

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Example3: NE_GIPC,WE_GIPC only

Thanks toGIPC

GIPC benefit toAutomaticAlignment

step9

Short suspension (i.e.beam) improvementnecessary ?

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A major effortnecessary at LFduring stormy weather!

Next improvements and studies

EM-MSC-041206

A curiosity: in November GIPC had already been tested but we needed to improve our knowledge and to optimize the blending

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Example 4:environmental (central Bld. shock absorption)

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> noise budget: actuation noise & reallocation

-Suitable resistors are used to reduce coil-drivers noise (ln=lownoise/HP=highPowe to accomplish sensitivity/locking)-a trade-off with DAC noise using Emphasis/deemphasis was found.

Still some work:

BS requires marionette reallocation.

WE,NE have a seism peak that should be reduced by further force reallocationto the marionette.

Too large correction directly on the mirror

Low frequency component that can be moved upwards (marionette)

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> synthesis: net improvement of recent activity

Much more seism and similar peak at 130 mHz

In this example:- red data (now) show up much larger seism

In spite of this:

=> the force reallocated to the marionetteis smaller

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> conclusions

It was mandatory to accomplish this task by considering the suspension system as a single component of the interferometer

The development was a bit delayed by the requirement of checking the performance under actual environmental disturbance. Tools to emulate crucial situation have been developed.

A large effort was spent on sensing optimization and on “soft operation”

Now it is possible to reduce by a factor 3 the rms disturbance due to the wind, while seism (sea) does not seem to be the main source of problems.

To be done before (possibly before the MegaRun):-further technical noise reduction-improvement of short suspension performance (InjB,MC,OutB)