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Control of the Virgo Superattenuator: Present and Future

This talk discusses the advancements in controlling the Virgo Superattenuator for improved sensitivity at low frequencies. Topics include control noise, hierarchical control, alignment noise, inertial damping, seismic noise reinjection, and the cradle effect.

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Control of the Virgo Superattenuator: Present and Future

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  1. The control of the Virgo Superattenuator:present and future Giovanni Losurdo - INFN Firenze/Urbino on behalf of the Virgo Collaboration

  2. or… catch the Grishchuk’s shark! The way towards thedesign sensitivity@ 10 Hz

  3. WARNING Some of the issues treated in this talk are a bit technical If the displayed slide is marked by this icon: then you can check your email… GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino3

  4. Low frequency sensitivity can be spoiled by control noise Low frequency noise Virgo: first attempt to extend the detection bandwidth down to 10 Hz GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino4

  5. Thermal noise Superattenuator • SA features: • very efficient passive attenuation • active controls for normal mode damping • 3 actuation points 1014 GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino5

  6. >103 amplifier Control noise sources: mirror actuation • The force needed to acquire the lock is much larger than that needed to keep it • “Stronger actuation” means larger electronic noise magnet electronic noise coil correction signal mirror GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino6

  7. Solution: HIERARCHICAL CONTROL DC-0.01 Hz Tide control 0.01-5 Hz 5-50 Hz • Force reallocation over three actuation stages. • Allows strong reduction of the force exerted on the mirror • After reallocation, reduce the actuators gain GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino7

  8. Hierarchical control Hierarchical control allows to reduce the needed force (and thus the electronic noise) by almost 4 orders of magnitude RMS force on the mirror Lock ACQ Tidal control ON ~ 104 Full hierarchical control GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino8

  9. Control noise sources: ALIGNMENT NOISE • Low frequency sensitivity is dominated by ALIGNMENT noise (coupled with longitudinal d.o.f. via bad beam-mirror centering) • The larger the excitation of the payload angular modes the larger the force needed to keep the mirror alignment • Again, larger force (wider bandwidth)  larger control noise • SOLUTIONS: • better centering of • the beam on the mirrors • reduce the angular • modes excitation GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino9

  10. Inertial damping • Active damping of the SA modes, actuation on top stage, 3 d.o.f., DC-5 Hz • Error signal from inertial sensors but… • Position sensors (LVDT) needed for “DC control”: source of seismic noise reinjection GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino10

  11. Sensors Position sensor Inertial sensor “noisy” reference x0 x GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino11

  12. seismic acceleration Seismic noise vs interferometer • The microseismic peak falls in the same as the main angular modes of the payload • If it leaks to the mirror it makes angular control more difficult and the detector less stable The amplitude of the microseismic depends strongly on the weather GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino12

  13. seismic noise reinjection H(s) L(s) L+H = 1 Blending the sensors • ACC and LVDT are blended using two complementary filters • The fraction of reinjected seismic noise depends on L(s) • To reduce seismic noise: • steeper rolloff • lower blending frequency Moving the crossover from 70 to 30 mHz means reducing the reinjected seismic noise by 10 @ microseismic peak MAKE IT AS LOW AS POSSIBLE! GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino13

  14. Cradle effect: due to mechanical imperfections top table tilts as it translates The cradle effect The possibility to reduce the crossover is limited since… An accelerometer cannot distinguish a translation from a gravitational field In presence of tilt a, accelerometer response: GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino14

  15. Cradle effect subtraction Model: tilt depends on displacement only Use displacement sensors to measure and SUBTRACT the cradle effect: Before subtraction: |tij|<0.02 After subtraction: |tij|<10-3 After subtraction it was possible to reduce crossover form 70 to 30 mHz gain 10x @ microseismic peak GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino15

  16. Test Comparing the performance of different crossover in same noise conditions: • Cavities locked independently, 70 mHz crossover on WEST cavity, 30 mHz on NORTH cavity • Compare the correction signals to measure the motion of the mirrors 70 mHz crossover 30 mHz crossover GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino16

  17. Results cavity lock correction signal Reducing the position sensors control bandwidth: ~10x less noise @ microseismic peak AS EXPECTED 70 mHz crossover 30 mHz crossover excess noise below 50 mHz: WHY? GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino17

  18. The role of seismic tilt Extrapolated tilt Extrapolated translation The response of the ACC on the IP can be fully explained only if the seismic noise is tilt-dominated below 100 mHz GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino18

  19. Effects of tilt on the control strategy GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino19

  20. Effects of tilt on the control strategy The feedback will push the table in the wrong direction! If the seism is tilt-dominated at low frequency we are using the wrong control strategy! GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino20

  21. wind speed susp displacement ITF power Wind vs interferometer The low frequency motion of the suspension is strongly correlated with the wind speed The detector duty cycle is affected by the wind windy days GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino21

  22. Control of tilt • To further improve the inertial control we need to get rid of tilt • IP is designed for tilt control • Sensing: an angular accelerometer is needed, decoupled form translations GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino22

  23. LVDT + + ACC WE WI PR NI NE BS f DARM 3f I zCorr f I MICH PRCL I,Q CARM What can be done more now? Even with tidal control engaged LVDTs are ON GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino23

  24. WE WI PR NI NE BS f DARM 3f I f I MICH PRCL I,Q CARM “Removing” local signals Use 4 locking signals for the position control of 4 mirrors in the beam direction LVDT + ACC • GOALS: • reduce use of noisy sensors • do not use ACC where tilt • dominates with no reinjection • of seismic noise GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino24

  25. Summary • Reduce as much as possible the use of local position sensors to reduce the dependence on seismic noise variability: • smarter filtering • use of interferometric signals for position control • Seismic tilt may mess up the control strategy. Active control of tilt can be important for further improvements Extending the detection bandwidth down to 10 Hz is a hard job Control noise reduction is a crucial issue For more details see: http://wwwcascina.virgo.infn.it/suspcon/MSCdocs/notes/tilt.pdf GWADW – Isola d’Elba, May 31st, 2006G.Losurdo – INFN Firenze-Urbino25

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