A local control system for the test masses of the Virgo gravitational wave detector

Astroparticle Physics 20 (2004) 617–628

www.elsevier.com/locate/astropart

A local control system for the test masses ofthe Virgo gravitational wave detector

Virgo CollaborationF. Acernese a, P. Amico b, N. Arnaud c, D. Babusci d, R. Barill�e e, F. Barone a,L. Barsotti f, M. Barsuglia c, F. Beauville g, M.A. Bizouard c, C. Boccara h,F. Bondu i, L. Bosi b, C. Bradaschia f, L. Bracci j, S. Braccini f, A. Brillet i,

V. Brisson c, L. Brocco k, D. Buskulic g, G. Calamai j, E. Calloni a,E. Campagna j, F. Cavalier c, G. Cella f, E. Chassande-Mottin i, F. Cleva i,

T. Cokelaer i, G. Conforto j, C. Corda f, J.-P. Coulon i, E. Cuoco j, V. Dattilo e,M. Davier c, R. De Rosa a, L. Di Fiore a, A. Di Virgilio f, B. Dujardin i,

A. Eleuteri a, D. Enard e, I. Ferrante f, F. Fidecaro f, I. Fiori j, R. Flaminio g,J.-D. Fournier i, S. Frasca k, F. Frasconi e,f, L. Gammaitoni b, A. Gennai e,

A. Giazotto f, G. Giordano d, G. Guidi j, H. Heitmann i, P. Hello c, P. Heusse c,L. Holloway f, S. Kreckelbergh c, P. La Penna e, V. Loriette h, M. Loupias e,G. Losurdo j,*, J.-M. Mackowski l, E. Majorana f,*, C.N. Man i, F. Marion g,F. Martelli j, A. Masserot g, L. Massonnet g, M. Mazzoni j, L. Milano a,

J. Moreau h, F. Moreau g, N. Morgado l, F. Mornet i, B. Mours g, J. Pacheco i,

A. Pai i, C. Palomba k, F. Paoletti e,f, R. Passaquieti f, D. Passuello f, B. Perniola j,L. Pinard l, R. Poggiani f, M. Punturo b, P. Puppo k, K. Qipiani a, J. Ramonet g,P. Rapagnani k, V. Reita h, A. Remillieux l, F. Ricci k, I. Ricciardi a, G. Russo a,

S. Solimeno a, R. Stanga j, A. Toncelli f, M. Tonelli f, E. Tournefier g,F. Travasso b, H. Trinquet b, M. Varvella a, D. Verkindt g, F. Vetrano j,

O. Veziant g, A. Vicer�e j, J.-Y. Vinet i, H. Vocca b, M. Yvert g

a INFN, Sez. Napoli and/or Universit�a di Napoli ‘‘Federico II’’, Complesso Universitario di Monte S. Angelo Via Cintia,

I-80126 Napoli, Italy and/or Universit�a di Salerno Via Ponte Don Melillo, I-84084 Fisciano (Salerno), Italyb INFN Sezione di Perugia and/or Universit�a di Perugia, Via A. Pascoli, I-06123 Perugia, Italy

c Laboratoire de l’Acc�el�erateur Lin�eaire (LAL), IN2P3/CNRS-Universit�e de Paris-Sud, B.P. 34, 91898 Orsay Cedex, Franced INFN, Laboratori Nazionali di Frascati Via E. Fermi, 40, I-00044 Frascati (Roma), Italye European Gravitational Observatory (EGO), Via E. Amaldi, I-56021 Cascina (Pi), Italy

f INFN, Sezione di Pisa and/or Universit�a di Pisa, Via Filippo Buonarroti, 2 I-56127 Pisa, Italy

* Corresponding authors. Tel.: +39050752587; fax: +39050752550.

E-mail addresses: [email protected] (G. Losurdo), [email protected] (E. Majorana).

0927-6505/$ - see front matter � 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.astropartphys.2003.10.001

mail to: [email protected]

618 F. Acernese et al. / Astroparticle Physics 20 (2004) 617–628

g Laboratoire d’Annecy-le-Vieux de physique des particules Chemin de Bellevue-BP 110,

74941 Annecy-le-Vieux Cedex, Franceh ESPCI––10, rue Vauquelin, 75005 Paris, France

i Observatoire de la Cote d’Azur, D�epartement Fresnel, Interf�erom�etrie Laser pour la Gravitation et l’Astrophysique, BP 4229,

06304 Nice Cedex 4, Francej INFN-Sez. Firenze/Urbino, Via G.Sansone 1, I-50019 Sesto Fiorentino and/or Universit�a di Firenze, and/or Osservatorio Astrofisico di

Arcetri Largo E. Fermi 5, I-50125 Firenze and/or Universit�a di Urbino, Via S.Chiara, 27 I-61029 Urbino, Italyk INFN, Sezione di Roma and/or Universit�a ‘‘La Sapienza’’, P.le A. Moro 2, I-00185 Roma, Italy

l SMA, IPNL 22, Boulevard Niels Bohr, 69622 Villeurbanne, Lyon Cedex, France

Received 30 July 2003; received in revised form 4 October 2003; accepted 7 October 2003

Abstract

The mirrors of interferometric detectors of gravitational waves (GW) are suspended in order to be isolated from

external disturbances. A local control system able to keep them correctly aligned and to damp the angular modes of the

suspension is necessary. In this paper we present the solution adopted for Virgo based on a CCD camera sensor and on

digital controls. With this solution the mirrors are kept aligned at the level of less than 1 lrad rms, enough to lock theinterferometer and start the automatic alignment system.

� 2003 Elsevier B.V. All rights reserved.

PACS: 07.05.Dz; 04.80.Nn

Keywords: Gravitational wave detectors; Seismic isolation; Digital control

1. Introduction

New interferometric detectors of gravitational

waves (GW) [1–4] are presently being commis-

sioned. In such interferometers the mirrors aresuspended from a vibration isolator that filters the

natural seismic vibrations at frequencies larger

than those of the normal modes of the suspension

itself. In the operation of a detector one has to deal

with the problem of the resonant motions of the

vibration isolator. Usually such motions are re-

duced by means of local controls (LC), that is,

active control systems properly designed to per-form alignment tasks and damping of mirror sus-

pension resonance peaks by using error signals

derived from local sensors.

The vibration isolator designed for Virgo is

called the super-attenuator (SA) [5]. It is charac-

terized by very low frequency normal modes (all

are in the range 0.04–3 Hz). The SA passive fil-

tering action is such that the expected pendulumthermal noise becomes dominant over the seismic

noise above 4 Hz. Most of the normal modes of

the suspension are damped by a multi-dimensional

active system called inertial damping [6]. Never-

theless, the inertial damping cannot control some

internal angular modes of the suspension involving

the test mass. Therefore, we developed a dedicated

last-stage-suspension control system with the fol-lowing purposes:

• damping the unwanted angular modes involv-

ing the mirror;

• being able to set and recover the reference angu-

lar position of each mirror;

• keeping aligned the interferometer at the level

of about 1 lrad rms for the locking.

The system described in this paper makes use of

ground-based readout and internal-force action,

i.e. forces applied within the mirror suspension,

and it was used during the commissioning of the

central interferometer (CITF) [7]. The CITF was a

6 m arm-length Michelson interferometer with

power recycling, equipped with all the mainmechanical and control elements designed for

Virgo. In the following (Section 2) we describe the

readout system used to provide information about

F. Acernese et al. / Astroparticle Physics 20 (2004) 617–628 619

the position of the suspended mirrors. Then (Sec-

tion 3) we give a schematic description of the

digital control architecture, which is a key feature

of the Virgo design. Hence, we approach the prob-

lem of the control implementation by describing

the mechanical properties of the mirror suspension(Section 4) and the main servo-loop design strat-

egy (Section 5) and briefly discuss the performance

of the system (Section 6).

2. Position readout system

The scheme of position measurement designedto provide the error signals through a CCD-cam-

era as sensor is shown in Fig. 2. Six error signals, x,y, z, hx, hy , hz are referred to the mechanical ground

of the CCD-camera and reconstructed in a left-

handed reference frame with its origin in the mir-

ror barycenter and the z-axis oriented outward.

Similar schemes had been already successfully

tested and operated in the context of Virgo [8]. Theoverall local control position sensing system is

located in air, sitting on customized optical

benches rigidly fixed at the base of the vacuum

chamber. The mechanical stand of the CCD-

camera was designed in order to allow removal

and repositioning of the device within 20 lm.We used a Super Photon CCD-camera model

EEV-17CAM. A dedicated read-out board wasdeveloped in order to use this CCD-camera [10]

for real-time applications in the standard VME

crates of Virgo [9]. The CCD-chip is a scientific

grade square pixel matrix 512 · 512 with 7.7 · 7.7mm2 surface. The pixel width is 15 lm. The pixel,clock is set at 20 MHz and the system provides a

digital signal through a 8-bit ADC. The image is

gathered and converted in 16 ms. Thus, the max-imum speed is 60 frames/s. The frame grabbing,

triggered by a external timing control, is a key

feature of this system integrated in the real-time

local control standard crate of Virgo.

The accuracy of the CCD position sensing is

roughly 10 nm rms, on the surface of the CCD

chip, with a spot diameter of 30–300 lm.According to the theoretical and experimentalperformances of the CCD-camera [10], this sensor

is suitable for our local control applications. In

Fig. 1 a typical spectrum of the overall sensor

noise of the camera system is shown.

The single-device readout system described

above was designed to accomplish two main tasks:

(A) the control of large angular offsets or oscilla-

tions in the pre-alignment phase (coarse-mode);(B) control during the locking and operation

phases (fine-mode).

The CITF mirrors were smaller (diam¼ 50 mm)than those designed for Virgo (diam¼ 300 mm). Inorder to operate the control in a mechanical con-

figuration similar to the final one, they were

housed in aluminum holders providing the overall

weight and aspect ratio of Virgo mirrors. Hence,we exploited such a design to simplify the scheme

of camera detection for the CITF. In Fig. 2 two

sets of spots observed by the camera imaging

system are shown. The first set was used for the

coarse-mode of operation and consisted of four

diffusive markers glued onto the mirror holder and

illuminated by means of a halogen lamp. The

second set, is used in fine-mode of operation and ismade of just one single laser spot diffused by the

mirror holder. This solution allowed us to use the

same camera optics adopted to provide the imag-

ing of the full surface of the holder, not optimized

for detecting a single (and small) spot with high

sensitivity since its focal length was rather short

(25 mm).

The markers for the coarse-mode were ceramic(MACOR) disks with 2 cm diameter, with a chro-

mium coating, etched by a lithographic-processed

mask in order to obtain a matrix of nine diffusive

spots per marker. These markers were glued onto

the mirror holders by means of UHV compatible

glue. This system was designed to provide a good

contrast when illuminated along the camera view-

side. The other spot, used for the fine-mode, wasprovided by a compact laser-diode system (OZ

Optics) with 633 nm wavelength through a polar-

ization-maintaining optical fiber with 1.3 mW

output power. The system was a simple optical

lever in which after the first reflection onto the

suspended mirror, the beam was folded back to-

ward the mirror holder. The size of the spot pro-

duced by the diffusion onto the mirror holder wasroughly 3 mm. The full path of the optical lever

was 3.9 m. The movement of the two sets of spots

f [Hz]

µrad

/Sqr

t(H

z)

10

1

0.1

0.010.01 0.1 1 10

Fig. 1. Linear spectral density of the angular accuracy due to

the position noise on the camera chip. In this table-top mea-

surement an incoherent source illuminates a pinhole fixed on

the camera frame and we have about 0.7 lrad rms.


(markers and laser) in the field of view of the

CCD-camera was used to produce suitable error-

signals for the local control system.

Fig. 2. Schematic side-view of the Virgo payload, lumped from the su

The coils providing marionetta control actuation are attached to the

themarionetta and the position readout layout. The mirror holder is su

the interferometer. The optical lever plane is rotated with respect to th

mirror is roughly 35�.

The mechanical adjustment range of the pay-

load system (see Fig. 2) (marionetta + reaction

mass +mirror) under pure rotations and transla-

tions was Dhx ¼ Dhz ’ �18 mm rad, Dhy ’ ½�18;þ40� m rad, and Dx ¼ Dz ’ �7 mm, Dy ’ �10mm where, according to the local left-handed ref-erence frame, z is the beam direction leading out-

ward from the mirror and hx and hy are the pitch

and yaw angular degrees of freedom.

The software dedicated to the real-time treat-

ment of CCD-camera data [11] operates hierarchi-

cally providing error signals for both coarse- and

fine-modes. It works as follows:

• first, the coarse computation makes independent

telemetric measurements, typically affected by a

small coupling among the mirror d.o.f. (few %)

[12]. Differential signals are derived from these

measurements at 50 Hz and passed to a digital

signal processing unit (DSP) until the control

accuracy provides 100 l rad rms and 100 lmrms. The range of linearity in the response of

per-attenuator (SA) and frontal-view of the mirror (top-right).

last mechanical filter (SA-F7). The mirror is suspended from

rrounded by the reaction mass, whose coils were used for locking

e horizontal one by 23�. The incidence angle of the laser on the


this signal is large enough (tens of degrees) to

cover completely the range of angular variations

allowed by the mechanics. In the CITF setup

transversal (x) and longitudinal (z) signals wereonly provided in coarse-mode and their usewas limited to pre-alignment tasks and to the

damping of major excitations of the system

(dxrms ’ 2 l m, and dzrms ’ 80 lm).• Then, the fine error-signal computation of hx

and hy , obtained by a linear combination of

the variations along the two coordinates of

the CCD-camera chip (with a coupling of about

10%), automatically replaces the coarse compu-tation and the halogen illuminator is switched

off.

• If during the operation the control is switched

back from the fine to the coarse-mode, the cal-

ibration and offsets of coarse-mode are auto-

matically updated by matching the last (more

accurate) values provided by the fine measure-

ment.

The local control system cannot fulfil the stric-

ter alignment specifications for the operation of

the detector: once the interferometer is longitudi-

nally locked, the differential angular motion of the

mirrors results in higher order TEMik modes at the

detection port and can be detected by using

demodulated asymmetry signals from a quadrantphotodiode. We used this signal to actuate an

angular feedback on the marionetta of one curved

mirror of the CITF in order to keep the dark-

Fig. 3. A schematic view of the digital control chain of mirror susp

shown (For interpretation of the references in colour in this figure le

fringe stable [7]. In such a configuration, the

angular local control of that mirror was no longer

active. In Virgo, by means of a complete mapping

all the mirror misalignments, should be possible in

principle to switch from local to automatic angular

control of all the mirrors [13,14]. However, it iscrucial to reach angular local control performance

with accuracy allowing rms residual motion

smaller than 1 lrad for three main reasons:

• the longitudinal locking strategy is strongly

facilitated by the performance of the local con-

trol;

• the measurement of the automatic alignmentsignals matrix requires a good accuracy;

• the fully automatic alignment is a challenging

task. A good backup solution consists in having

one mirror under local control.

3. Basic scheme of the digital local control chain

In Fig. 3 the main elements of the local control

chain are shown. The VME crate is divided in two

sections, each equipped with a dedicated CPU

board (coded as A and B) [15]. The main process

running in the CPU-A is the DSP-server, a soft-

ware program designed to allow real-time com-

munication between two digital signal processing

boards (DSP-A and B) and a software programthat controls a set of ADC and DAC boards [5].

Both DSP units are also programmed to send data

ended mirrors. Actuation (blue) and read-out (red) points are

gend, the reader is referred to the web version of this article.).


to the data acquisition system (DAQ) running

elsewere and to receive signals from the Global

control system [14] through digital optical link

boards (DOL) providing signals to glass fibers.

ADC-A receives analog signals from inertial

damping sensors at 10 kHz [5], DSP-A analyzesthem and computes digital correction signals sent

to DAC-A, whose outputs are finally sent to SA

actuator coil drivers.

CPU-B runs the GxServer [11]. This software

sets up the timing board [16] synchronization with

respect to the main Virgo timing system. GxServer

deals with the CCD-camera system [10], analyzes

its images, sampled at 50 Hz, reconstructs errorsignals and sends them to DSP-B. The latter board

is controlled by the DSP-server in order to com-

pute digital correction signals sent to DAC-B,

whose outputs are finally sent to the coil drivers of

the actuators acting at the level of the last sus-

pension stages to steer the mirror. Hence, in the

CITF we could distinguish among three different

sets of error signals which can be accessed by thedigital control program, downloadable by the

DSP-server, according to their source device: local

sensors of the SA (accelerometers and LVDT po-

sition sensors), local sensors for the mirror posi-

tion readout (CCD-camera), global control signals

(locking photodiodes and linear alignment quad-

rant photodiodes).

Fig. 4. Torque-to-angle transfer function of the Virgo payload

for the pitch and yaw modes. The torque is applied on the

marionetta. The motion is sensed at the mirror level.

4. Mechanical transfer functions

The last-stage of the Virgo mirror suspension,

which includes a steerable stage (the marionetta)

suspending the mirror and the reaction mass, is a

double pendulum with two parallel stages [17].

This system is suspended from the last filter of thesuper-attenuator (so called steering filter or F7)

[18]. The mirror position can be controlled either

from the reaction mass (by acting directly on it) or

from the steering filter (by acting on the marion-

etta). According to the configuration designed for

Virgo we used to apply local control angular cor-

rections between the steering filter and the mari-

onetta stages. The advantage of this solution isthat one further stage of mechanical filtering is

applied to the local control actuator noise at the

cost of a narrower control bandwidth. The main

duty of this system is to perform alignments of

suspended optics, without exciting the mechani-

cal modes of the mirror suspension last stages, in

order to set the detector in the interference con-

dition. Sometimes the local control is also useful tomisalign one mirror if only part of the interfer-

ometer is to be used (this may happen if one wants

to use the interferometer in simple Michelson

configuration).

Hence, in order to be effective the system has to

control the DC position of the mirrors and damp

its normal modes in two d.o.f.: hx (pitch) and hy


(yaw). The simulated force-to-displacement trans-

fer functions for these modes are shown in Fig. 4.

The transfer functions, in very good agreement

with the simulated data, have been measured using

as sensor both the CCD-camera and a small

interferometer built on purpose [19]. The maincharacteristics of these transfer functions are:

structures: the yaw d.o.f. is characterized by five

modes which are relevant in the digital filter de-

sign. Three low frequency resonances respectively

involve the torsional motion of the overall SA

chain (14 mHz), that of the payload in opposition

of phase opposition with respect to the rest of the

SA chain (33 mHz), and an internal mode of theSA chain with small recoil on the marionetta (43

mHz). Two further resonances, at 0.85 and 1.2 Hz,

are due to internal modes of the payload.

asymptotic behavior: slope f �4 above the reso-

nances, as expected for a double pendulum system.

5. Control filters

The local control should be designed in order to

keep the mirror to a reference position within the

required accuracy and to damp the resonance,

limiting the re-injection of noise as much as pos-

sible. Therefore, the compensator CðsÞ should

have three main characteristics (in Fig. 5 a block

diagram of the servo is shown):

• keep the DC position: high DC gain is needed

(integral action);

M(s)

C(s)

x(s)

n(s)

o(s)

Fig. 5. Block diagram of the servo-loop: MðsÞ is the force-to-displacement transfer function of the mechanics, CðsÞ is thetransfer function of the digital compensator. xðsÞ is the outputof a noiseless sensor reading the angular position of the mirror,

nðsÞ is the sensor noise, oðsÞ is the resulting sensor output.

• damp all the modes: the unity gain frequency

must be set above the resonances, were the

mechanical transfer functionMðsÞ s�4. There-fore, the loop stability requires CðsÞ / s3;

• avoid the re-injection of sensor noise: a low passaction has to be provided in order to avoid sat-

uration and to reduce control noise re-injection

on the mirror. This is the main problem to face.

A compensator CðSÞ s3 amplifies the sensornoise. Adding poles to it will certainly filter off

the noise but will also reduce the phase margin

and the robustness of the control loop. A prop-

er compromise has to be found.

Fig. 6. Transfer function of the compensators used for the pitch

and yaw modes.

Table 1

Compensator for the yaw control

Yaw mode f (Hz) Q

Real pole 0

Complex poles 0.016 20

Complex zeroes 0.016 2


Real zero 0.06

Real pole 0.22

Complex poles 0.7 1.08





Complex poles 1.05 6

Real pole 3.08

Fig. 7. Open loop gain functions for the pitch and yaw degrees

of freedom.

Table 2

Compensator for the pitch control

Pitch mode f Q

Real pole 0

Real pole 0.01

Real zero 0.03



Real pole 1.92


The compensator used is shown in Fig. 6 andthe corresponding list of poles and zeroes is pro-

vided in Tables 1 and 2. The open loop gain

function are shown in Fig. 7.

In the case of the yaw mode servo, all the open

loop gain function shows the modes are damped,

but the gain at 10 Hz is less than 10�9 thanks to a

steep low pass filter included in the compensator.

The design of such filters was not straightforward.In order to obtain a stable loop, we used a design

approach that we defined double loop technique

[20]. The description of such technique will be the

subject of a forthcoming paper. Such a method

allows to optimize the design of the low pass fil-

tering action of the compensator. It is based on a

simple idea: the s3 slope of the compensator ex-tends well above the unity gain frequency. This isrequired for stability reasons but is unnecessary

from the point of view of the control force. Then,

just the low passed part of the correction signal

can be sent to the actual mechanical system, while

the high frequency part feeds a branch made of thesimulated suspension mechanics in order to pre-

serve the loop stability. In this way there is more

freedom, with some cautions, in the choice of the

loop gain and bandwidth without saturating

actuator drivers and noise re-injection.

For the pitch mode, we decided not to control

the resonances at 2.4 and 3.2 Hz. The reason for

this choice was that such modes are quite stiff anddifficult to excite. When they were excited for any

reasons (typically because of a mechanical shock),

we had the possibility to damp them by switching

the control authority to a servo noisier but with

Fig. 9. The coherence spectra between the dark fringe signal

and the correction signal for the local control of the hy angle of

one of the mirrors. There is no coherence apart for a few lines.


larger bandwidth. The loop gain at 10 Hz is less

than 10�11.

In the following we explain why the sensor noise

reduction guaranteed by such servos is good en-

ough for Virgo even if some of the mirrors remains

under local control during the data acquisitionphase. Let us consider the case of the yaw servo (the

noisier one). Let oðsÞ ¼ xðsÞ þ nðsÞ be the output ofthe sensor, where xðsÞ is the signal (hy in this case)

and nðsÞ is the sensor noise. Let MðsÞ and CðsÞ bethe transfer functions of the mechanics and the

compensator respectively (see Fig. 5). The closed

loop motion of the mirror due to the sensor noise is:

xðsÞ ¼ hyðsÞ ¼MðsÞCðsÞ

1þMðsÞCðsÞ � nðsÞ ð1Þ

At 10 Hz, where MðsÞCðsÞ 1, hy �MðsÞCðsÞnðsÞ � 10�9. The measured sensor noise is

nðsÞ � 10�7 rad/ffiffiffiffiffiffiffi

Hzp

, therefore:

hyðsÞ � 10�16 rad=ffiffiffiffiffiffiffi

Hzp

ð2ÞSuch angular noise will couple to translation if the

laser beam is not perfectly centered with respect to

the mirror. If we assume that the laser is off-center

Fig. 8. The sensitivity of the CITF in the range 1–100 Hz, in a

configuration when all the mirrors were under local controls

and no automatic alignment had been implemented, in two

different cases. The best sensitivity was measured with the local

controls described in this paper. The increase of the noise is due

to local control noise when simpler compensators (less effective

low pass action) are used for the local controls.

by an amount d ¼ 1 mm, the noise injected into

the interferometer at 10 Hz will be:

dxðsÞ ¼ d � hyðsÞ � 10�19 m=ffiffiffiffiffiffiffi

Hzp

ð3Þ

to be compared with the Virgo target sensitivity at

10 Hz of 3� 10�18 m=ffiffiffiffiffiffiffi

Hzp

.As a result of this local control upgrade the

CITF sensitivity was no more dominated by con-

trol noise in the 1–10 Hz region (Fig. 8). This was

verified by checking that the coherence spectra

between the photodiode signal and the correction

signals of the local control loops were negligibly

small in this frequency range (Fig. 9).

6. Alignment performance

Fig. 10 shows a typical activation of the servo-

loop. We show as the hy (yaw) d.o.f., intentionally

excited, is driven to the reference alignment by the

hierarchical application of coarse and fine angular

error signals as input of the digital corrector filter.The system appears sufficiently robust provided

that the initial yaw oscillation is rather large. The

correction signal is dominated by a 16 mHz oscil-

lation: this is due to the digital compensation of the

first notch in the yaw transfer function (Fig. 4).

Fig. 10. Activation of hy servo-loop when the modes of yaw

d.o.f were highly excited. The upper line is the correction signal

and the one below the error signal. In the first phase the loop is

open (no correction), then a phase in which the error signal

provided to the corrector is automatically switched from coarse

to fine and backward, then, as the signal is reduced within the

fine-mode range the control is operated by means of only the

fine error-signal and the illumination on the mirror is switched

off.

Fig. 11. Pitch and yaw signals in fine-mode with open and

closed loop for the North Input mirror local control of the

CITF.


The effect of the servo-loop on the spectra of the

fine-mode signals is shown in Fig. 11. The two

signals show a 10% sensing cross-talk, but this

coupling was small enough to operate efficiently

the control. However, we also notice the damping

effect of the last-stage internal modes and anoverall noise reduction in the low frequency range

when the servo-loop is closed.

Further elements for the characterization can

be deduced from the study of the spectral

behavior of the rms. In Fig. 12 hx (pitch) and hy

(yaw) rms are shown for the three suspensions

under angular local control involved in the CITF

operation during the last engineering run (E4).The fourth mirror (West Input) was automati-

cally controlled, i.e. forced to follow the other

curved mirror (North Input), by means of the

global control.

The good stability of the angular control

accuracy is demonstrated by the histograms of

the distribution of the rms angular error signals.

Typical rms distributions (NI mirror), computed

over time-chunks of increasing length are shown

in Fig. 13. The stability achieved for the CITF is

demonstrated by the fact that the median val-

ues of the distributions are not significantly dif-ferent.

7. Conclusions and perspectives

The system we developed and implemented for

the local control of suspended mirrors matched the

requirements for the CITF operation since the

Fig. 12. The local control system provides residual error signals

whose spectral contributes to the overall rms are plotted for

both pitch and yaw of three mirror suspensions during CITF

operation (NI¼ North Input, PR¼Power Recycling, BS¼Beam Splitter).

Fig. 13. Local control accuracy on pitch and yaw of the mirror

using the F7-marionetta actuators. Four-hour time period is

divided in a variable number of chunks and the statistics of the

rms values is reported in the different cases.


interferometer final sensitivity was not limited by

the local control performance.

However, from the experience gained with the

CITF, we learned that some improvements are

needed in view of the transition to the full scale

experiment (Virgo). Above all, a wider control

bandwidth can make the interferometer operationfaster. To achieve such a wider bandwidth without

control noise re-injection one can sense also the

marionetta angular position. Moreover, in this

way the gain of the servo-loop slope will be opti-

mized in order to damp more efficiently the lowest

torsional mode of the suspension. Such a system,

requiring further read-out devices, is currently

being tested.

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A local control system for the test masses of the Virgo gravitational wave detector

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