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EIB seismic noise and mitigation

I.Fiori, Internal Meeting, 26 August 2009, EGO

with E.Genin, F.Frasconi, R.Day, B.Canuel

• (reminder of EIB movement)

• Tentative noise projection (coherence)

• Tests on bench in optics lab.: – stiffening tests– softening tests

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EIB: TF between bench top and ground3

eLog 20687

Horizontal, EW(pendulum modes of bench on legs)

Vertical(vibration mode of legs top plate)

Amplification of Vertical seism4

BENCH topGROUND

• Bench moves more than ground between 20 and 70 Hz• At peak (41 Hz) amplification is about 30 times• Above 100 Hz ground seism is damped on bench

on floor under MC tube

Fans, amplified on bench top, 10 times

Vertical FFT, different positions on bench5

Piezo (SW corner)Pcb 1 (middle S side)Pcb 2 (SE corner)Pcb 3 (middle E side)

• Main resonance at 41 Hz• Other resonances at 45 Hz, 47 Hz, maybe 53 Hz

Vertical mode 41 Hz6

Vertical mode 47Hz:(rough)

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Environmental mitigationmight help?

• Air conditioning? factor 2 for 10-20Hz, small contribution to 40-60Hz noise

>>To be double checked.

• Racks? no significant coherence with seismometer on racks EE room, next to EIB (apart

from lines)

>>To be better checked (old data, racks switch off Oct. 2008...?)

Main contribution seems racks on platform.

>> Understand if seismic isolation can help (investigate seismic path)

Possible that coupling to dark fringe occurs not at EIB, but elsewhere.

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Which coupling to dark fringe ?

10-20Hz: Angular control noise

40-50Hz:

• Beam jitter + long. control noise PRCL (B2).

• Likely control noise was dominant and reduced after PRCL locked on B5_ACq (Gabriele, Commissioning July 20)

>> Reduced coherence: eLog by Eric (23779)

Also consequence of increased IMC transmitted power?

• 220 and 280Hz: resonances of optics mounts? through Beam Jitter?

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Coherence (data of August 1st)11

eLog ... PyyPxx

PxyC

22 ||2CC

Projection of coherence12

Black line = Upper Limit (low coherence)Red dots = good projection, coherence > 0.15 Chprojh _

10-20Hz13

From Air Conditioning

Mounts resonances?14

Projection of Jitter noise from adding noise to BMS FFh, FFv

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eLog 22574

OLD, March 2009

Which mitigation needed?

Tentative:• 10-20 Hz: a factor >=10 • 40-60Hz : a factor >=5• 200-300Hz (just at peaks): a factor >=5

To better quantify, and project possible solutions:• need to measure TF seism dark fringe

– with bench shaking

– with BMS noise injection (sensitive to just beam-jitter?)

– Also good measure of bench mech. vert. TF

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Which mitigation strategy?

• Resonant damper (Frasconi) – Possibly effective for the “High-Q” modes, like the Horizontal modes. – Vertical modes? Test in progress.

• Stiffening vs Softening? (Vertical is major concern)

– Stiffening means moving 40Hz higher frequency • Avantage: no drifts problem

• Possible issue: increased coupling to bench seism above 40Hz.

Need TF to determine consequences– Softening means moving 40Hz down to < 10Hz

• Advantage: good seismic isolation of bench

• Use of Rubber DRIFTS: critical for ITF alignment reference• Use of Springs Large bench motion at low freq.

• Possible issue: alignment control noise, difficult to suppress at low freq.(Maddalena). Align. fears mainly Horizontal modes below 20Hz with Large Q.

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Tests with Optic Lab. Bench

• Stiffening:– Attempts to stiffen supports (presented at June weekly)

– Conical legs

– No legs

• Softening:– Sorbothane (at June weekly meeting)

– Springs

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Possible limitations of these measurements 19

• Rubber pads: seem to have resonant mode around 65 Hz, might mask real vertical mode of legs. Tests of stiffening bench supports might need to be repeated without pads.

• Peaks in TF above 100Hz seem change with position of accelerometer on floor, might be resonances of floor? Measurements above 100Hz cannot be trusted.

Seismic TF (bench top/ ground) Floor shaked with big shaker, vert.

Attempts to Stiffen the supports

• Tests with Optic Lab bench:1) Move the Rigid support to the edge (tried one or two supports)

resonance moved from 67 Hz to 52 and 74 Hz (double peaks)

2) Tried using a more rigid support (filled cylinder) on edge

resonance moved from 67 Hz to about 84 Hz.

3) Removed supports, bench sits directly on plates

result is similar to 1)

>> Fequency seems to depend (15Hz) on good levelling and contact on supports.

When bench has good contact on 4 supports (same load on

each) double peaks disappear.

4) Loosened all screws holding plates on legs and later bars,

then, while loaded with bench weigth, tight all again to assure

a better fit of parts.

>> Levelling and tightness of plates seem to help moving

frequency to some higher values.

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Experiments with supports:21

67 Hz, 15

73 Hz, 652 Hz, 9

Removed supports(bench sits directly on leg plates)

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56 Hz

79 Hz

When screwed platespeak moved from 56 Hz to 79 Hz

and without Rubber Pads

Other stiffening attempts?

• 1) repeat some tests without rubber pads

• 2) thicker plates

•

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Conical legs

• Filled with concrete

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FOTOGRAFIA Con particolare dei tre appoggi sotto E dell’appoggio autolivellante sopra.

Conical Legs25

• VERTICAL mode: 64 Hz, amplitude 25• HORIZONTAL modes: 8 Hz

64.5 Hz

8 Hz

Origin of 64Hz modenot understood.

Standard TMC legs 26

• VERTICAL mode: 73 Hz, amplitude 10-15• HORIZONTAL modes: 13-14 Hz

No legs, bench on 4-point supports27

• No particular feature below 100Hz.• Peak at 110Hz ...but cannot exclude it is a mode of floor

No legs, bench on floor28

• Peak at 115Hz ...but possible it is a mode of lab. floor.• According to Specs. TMC: first mode is above 160Hz.

29Softening experiments: sorbothane

A test has been performed in the optics lab.

0.5 inch thicknessLength 2 inches90 lbs load (40 Kg)Durometer 70

Natural frequency 21.6 HzComputed with Sorbothane Inc. software:

Sorbothane30

• Pads between bench and legs, can move vertical mode down to 15-25 Hz, and well damped.

• LARGE DRIFTS (>100m over --- hours) which cannot be tollerated since EIB position is used as grobal alignment reference

• Which is drift requirement?

Springs• Spares from AirCond. Mitigation works.

• Nominal frequency: 4-7Hz (vert. and hor.)

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FOTOGRAFIA Installazione molle

Springs32

• Difficult to excite with shaker from floor: no good coherence, no good measure of TF• More useful to compare FFT: at resonance the floor motion is amplified by 50-100 times

VERT: 5 HzHoriz.: 4.5 Hz

Springs vs. Standard LEGS33

Accelerometer noise

Acceleration FFT

Displ.0.1 m

Springs?• Large bench displacement at springs mode freq.

possible concern for Alignment noise

... Could we use Res. Damper to damp it sufficiently?• Easy to excite large oscillations: might make too difficult to

work on bench.• Apparently, springs excite easily by air pressure caused by

closing/opening doors.• Drifts? ... First measure gave: 100 m in 24hours, but could be due to assessment

of rubber top and bottom covers .... to be better measured.•

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Other softening tests?

• Try springs with freq. 15Hz, should perform better in terms of displacement

• Damped springs (viscous material inside), commercial products?

•

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