Thoughts on short term improvements for Mirror Suspension Control G.Losurdo - P.Ruggi.

Thoughts on short term improvements

for Mirror Suspension Control

G.Losurdo - P.Ruggi

Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 2

Seismic noise vs Duty cycle

C6

C7

C6 duty cycle:89 %

C7 duty cycle:70 %


Seismic noise variability

Seismic noise may vary by ~100 at the microseismic peak

A large soil tilt can be indiced by the action of wind on the buildings

The contribution of tilt is hard to measure (no proper sensing)

Seismometers are not good below 0.1 Hz


Main goal of the recent activity

Many lock losses are associated to angular motion of the mirror

The larger the motion the wider the required control bandwidth

We have worked to reduce the residual angular motions of the mirrors along two paths:

1. Reducing the re-injected seismic noise

2. Reducing the translation-to-angle couplings

Increase the ITF robustness Reduce the control bandwidthReduce the actuation noise


Inertial damping

Inertial sensors:– DC-100 Hz bandwidth– Sensitivity: a few 10-9 m/s2/rt(Hz)

above 1 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



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

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


Tilt of the IP

Ideal IP: no tilt of the top table.

In presence of tilt, accelerometer response:

Tilt induced by cradle effect is proportional to displacement:

a x g

0 0

0 0

( ) ( )

( ) ( )x

z

a x A x x B z z

a z C x x D z z

0 0

0 0

( ) ( ) ' '

( ) ( ) ' 'x x z

z x z

a x A x x B z z x A l B l

a z C x x D z z z C l D l


Cradle effect subtraction

z

x

z

x

l

l

zs

xs

a

aT

2

2

Before subtraction: (20 mHz tilt-hor crossing) 02.0|| ijt

After subtraction: (5 mHz tilt-hor crossing) 310|| ijt

Drifts on timescale of tens of minutes are of the same order


Effect of tilt and its correction

Before: IP translations are coupled to ACC DC signals

After: the effect is cancelled by proper LVDT subtraction

Before tilt subtractionAfter tilt subtraction


30 mHz crossover

All towers with 30 mHz crossover after subtraction of intrinsic tilt

Factor 10 gain achieved

Reinjected seismic noise


Test on NE/NI

Comparing the performance of different damping configs in same noise conditions:– Cavities locked, standard config on WEST cav.ty, new config. on

NORTH cavity– Compare the zCorr signals to measure the motion of the mirrors

30 mHz crossover

70 mHz crossover


Results

NORTH vs WEST with mid-intensity seism– A factor 10 gained at the microseismic peak!– Noise reinjected below 50 mHz.


Low frequency performance

Stronger wind means larger motion

(obvious, maybe…)

Just Earth shaking orcontrol noise effects?

(i.e. residual cradle effect or seismic tilt?)


IP response:

LVDT response:

ACC response:

Simple IP model

2

02 2 2 20 0

g

l x

22 20

0 02 20

a x g l

2

002 2 2 2

0 0

g

x x


Tilt or translation?

Use the IP as a seismometer (open ID)

Is it possible to understand if thelarger noise is due to tilt or translational seism?


ACC-LVDT TF: - very good coherence with strong wind- TF = 0

2, as expected

The sensors are good also at very low frequencies, (at least in strong wind conditions)


Estimate the input through the model.To reproduce the sensor output one should assume:

~102-103 m/√Hz @10 mHz in case of translation noise

~10-2-10-1 rad/√Hz @10 mHz in case of tilt noise

Strong wind

Weak wind


Model 1: translation only

measurement

Extrapolatedtranslation

Unable to fit the dip


Model 1: translation + tilt

Extrapolatedtranslation

Extrapolatedtilt



If the seism is tilt-dominated at low frequencywe are using the wrong control strategy!

The feedback will push the table in the wrong direction!


Getting rid of tilt

Achieve an angular sensor with sensitivity:

Use it:

– To subtract the tilt from the accelerometer signal

OR

– As error signal for tilt servo (need mechanics modification)

~10-9 rad/√Hz @10 mHz


ACC noise

Equivalent tilt sensitivity:6 10-10 rad/√Hz

If the seism is tilt-dominated at low frequencywe could use our accelerometer to sense it


NI NE

WE

WI

BSPR

3f

I f

I,QCARM

MICHPRCL

f

I DARM

LVDT

ACC+

+

zCorr

What can be done more?

Even with tidal control engagedLVDTs are ON


Removing local signals

LVDT

ACC+

zCorr

NI NE

WE

WI

BSPR

3f

I f

I,QCARM

MICHPRCL

f

I DARM

Use 4 locking signals for theposition control of 4 mirrors

in the beam direction


Summary

The decoupling of the detector from seismic perturbation can be pursued in two directions

1. The removal of the position sensors from the loop, replacing them with the interferometric signals

2. The correction of the seismic tilt, either by subtraction from the ACC signals or by active control of the top stage

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Thoughts on short term improvements for Mirror Suspension Control G.Losurdo - P.Ruggi.

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