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EIB seismic noise and mitigation

I.Fiori, Internal Meeting, 26 August 2009, EGO. EIB seismic noise and mitigation. 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 seismic noise and mitigation

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  1. I.Fiori, Internal Meeting, 26 August 2009, EGO EIB seismic noise and mitigation with E.Genin, F.Frasconi, R.Day, B.Canuel

  2. (reminder of EIB movement) • Tentative noise projection (coherence) • Tests on bench in optics lab.: • stiffening tests • softening tests

  3. EIB: TF between bench top and ground eLog 20687 Horizontal, EW (pendulum modes of bench on legs) Vertical (vibration mode of legs top plate)

  4. Amplification of Vertical seism • 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 BENCH top GROUND Fans, amplified on bench top, 10 times

  5. Vertical FFT, different positions on bench • Main resonance at 41 Hz • Other resonances at  45 Hz,  47 Hz, maybe  53 Hz Piezo (SW corner) Pcb 1 (middle S side) Pcb 2 (SE corner) Pcb 3 (middle E side)

  6. Vertical mode 41 Hz

  7. Vertical mode 47Hz:(rough)

  8. 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.

  9. 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?

  10. Coherence (data of August 1st) eLog ...

  11. Projection of coherence Black line = Upper Limit (low coherence) Red dots = good projection, coherence > 0.15

  12. 10-20Hz From Air Conditioning

  13. Mounts resonances?

  14. Projection of Jitter noise from adding noise to BMS FFh, FFv eLog 22574 OLD, March 2009

  15. 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

  16. 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.

  17. 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

  18. Possible limitations of these measurements • 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.

  19. 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.

  20. Experiments with supports: 67 Hz, 15 52 Hz, 9 73 Hz, 6

  21. Removed supports(bench sits directly on leg plates) and without Rubber Pads When screwed plates peak moved from 56 Hz to 79 Hz 56 Hz 79 Hz

  22. Other stiffening attempts? • 1) repeat some tests without rubber pads • 2) thicker plates

  23. Conical legs • Filled with concrete FOTOGRAFIA Con particolare dei tre appoggi sotto E dell’appoggio autolivellante sopra.

  24. Conical Legs Origin of 64Hz mode not understood. • VERTICAL mode: 64 Hz, amplitude  25 • HORIZONTAL modes:  8 Hz 64.5 Hz 8 Hz

  25. Standard TMC legs • VERTICAL mode: 73 Hz, amplitude 10-15 • HORIZONTAL modes: 13-14 Hz

  26. No legs, bench on 4-point supports • No particular feature below 100Hz. • Peak at 110Hz ...but cannot exclude it is a mode of floor

  27. No legs, bench on floor • Peak at 115Hz ...but possible it is a mode of lab. floor. • According to Specs. TMC: first mode is above 160Hz.

  28. Softening experiments: sorbothane A test has been performed in the optics lab. 0.5 inch thickness Length 2 inches 90 lbs load (40 Kg) Durometer 70 Computed with Sorbothane Inc. software: Natural frequency 21.6 Hz

  29. Sorbothane • Pads between bench and legs, can move vertical mode down to 15-25 Hz, and well damped. • LARGE DRIFTS (>100m over --- hours) which cannot be tollerated since EIB position is used as grobal alignment reference • Which is drift requirement?

  30. Springs • Spares from AirCond. Mitigation works. • Nominal frequency: 4-7Hz (vert. and hor.) FOTOGRAFIA Installazione molle

  31. Springs • 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 Hz Horiz.: 4.5 Hz

  32. Springs vs. Standard LEGS Acceleration FFT Displ. 0.1 m Accelerometer noise

  33. 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.

  34. 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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