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

2. 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 %

3. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 3 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

4. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 4 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 bandwidth Reduce the actuation noise

5. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 5 Inertial damping  Inertial sensors: –DC-100 Hz bandwidth –Sensitivity: a few 10 -9 m/s 2 /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

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

7. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 7  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 sensors Accel. LVDT Highpass Lowpass + Highpass + Lowpass = 1

8. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 8  The seismic noise filtering depends on L(s)  The loop design is independent on the L(s) cutoff

9. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 9 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:

10. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 10 Cradle effect subtraction Before subtraction: (20 mHz tilt-hor crossing) After subtraction: (5 mHz tilt-hor crossing) Drifts on timescale of tens of minutes are of the same order

11. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 11 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 subtraction After tilt subtraction

12. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 12 30 mHz crossover All towers with 30 mHz crossover after subtraction of intrinsic tilt Factor 10 gain achieved Reinjected seismic noise

13. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 13 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

14. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 14 Results  NORTH vs WEST with mid-intensity seism –A factor 10 gained at the microseismic peak! –Noise reinjected below 50 mHz.

15. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 15 Low frequency performance Stronger wind means larger motion (obvious, maybe…) Just Earth shaking or control noise effects? (i.e. residual cradle effect or seismic tilt?)

16. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 16 IP response: LVDT response: ACC response: Simple IP model

17. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 17 Tilt or translation? Use the IP as a seismometer (open ID) Is it possible to understand if the larger noise is due to tilt or translational seism?

18. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 18 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)

19. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 19 Estimate the input through the model. To reproduce the sensor output one should assume: ~10 2 -10 3 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

20. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 20 Model 1: translation only measurement Extrapolated translation Unable to fit the dip

21. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 21 Model 1: translation + tilt Extrapolated translation Extrapolated tilt

22. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 22

23. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 23 If the seism is tilt-dominated at low frequency we are using the wrong control strategy! The feedback will push the table in the wrong direction!

24. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 24 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

25. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 25 ACC noise Equivalent tilt sensitivity: 6 10 -10 rad/√Hz If the seism is tilt-dominated at low frequency we could use our accelerometer to sense it

26. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 26 NINE WE WI BS PR 3f I f I,Q CARM MICH PRCL f I DARM LVDT ACC + + zCorr What can be done more? Even with tidal control engaged LVDTs are ON

27. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 27 Removing local signals LVDT ACC + zCorr NINE WE WI BS PR 3f I f I,Q CARM MICH PRCL f I DARM Use 4 locking signals for the position control of 4 mirrors in the beam direction

28. Collaboration meeting – Cascina, Feb. 6, 2006 G.Losurdo – INFN Firenze-Urbino 28 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