Virgo suspension control progress

1. Virgo suspension control progress E. Majorana INFN Mirror Suspension Control workgroup ILIAS Geneva 29 March 2007

2. The usual “standard-super-attenuator” suspension … Thermal noise • Soft isolator concept: • very efficient passive attenuation • active controls for normal mode damping

3. The mission of Mirror Suspension Control workgroup: commissioning-oriented activity Virgo sensitivity (at LF !) Virgo duty-cycle • In fall 2006 the majority of main ITF control issues had been addressed: • lock acquisition strategy • automatic alignment (~) • suspensions and local controls allowed all above To start noise hunting, stable operation was needed: =>MSC performance started to be integrated in ITF issues

4. Our plan was clear thanks to the previous efforts of MSC Workgroup, during the first part of the commissioning, and to some ideas matured meanwhile. Unavoidable acknowledgments to my friends and colleagues Giovanni Losurdo and Paolo Ruggi

5. NOW Noise budget Suspension control chain (read-out,numerical,digital,actuator) Interface with ITF Angular control strategies (Local/automatic) Beam centering Disturbance rejection Sensor blending optimization Global Inverted Pendulum Control All-the-possible-handles (completion needed!) Lock force re-allocation (BS) SA-chain damping from ground Vertical damping Safe-and-soft operation interfaces

6. Focus on: - improvement of disturbance rejection - reduction of control noise through the suspension - reduction of ITF coupling with the control noise

7. > disturbance rejection: vertical damping needed (1/9) V-damp ON/OFF vs alignment

8. > disturbance rejection: vertical damping needed (2/9) ALL ON ALL OFF O-B-O OFF ALL OFF ALL ON O-B-O OFF One-By-One No crucial Improvement during OBO OFF Test. Clear Improvement All OFF/ON Relationship with Angular fluctuations

9. > disturbance rejection: vertical damping needed (3/9) One-By-One (detailed) TOP stage Last stage ITF

10. > disturbance rejection: top stage sensor blending (4/9) ACC HP cross IP + LP LVDT Winter issues: STD scheme and blending used until Dec 2006 fcrossover = 50 mHz Trade-off between: 30 mHz (wind disturbance through ACC) and 70 mHz (mseism disturbance through LVDT)

11. pulling back the crossover: benefit expected unexpected ! wind-noise cavities locked independently mseism (sea) produces angular excitation of the payload. wind (reasonably through the tilt) fake acceleration re-injected. 30 mHz crossover N-arm 70 mHz crossover W-arm • Key solutions : • smart experiments to simulate the disturbance ; • tools to optimize the blending on-the-fly ; • other smart ideas ….

12. Sensor blending: TWO-SIDE OPTIMIZATION characterizing Suspension-control_vs_disturbance

13. > disturbance rejection: top stagesensors, first attempts (5/9) old old old new new new ACC HPw LPw cross IP mix HPms LPms LVDT + + OUR TARGET How much is it enough ? mix =0 (wind): not worse than old filters against mseism + tilt noise attenuation below 50 mHz mix =1 (mseism) : “strong” attenuation of mseism noise with slightly worsened tilt noise attenuation. mix =0.5 : “medium” attenuation of mseism noise hybrid filters (on-the-fly tuning) The answer is worked out through a deeper analysis

14. > disturbance rejection: top stagesensors (6/9) Pitch excitation (payload mode) yaw bump (due to large zM correction) Large zM due to tilt disturbance through accelerometers Ineffective @ 0.45 Hz, (IP notch) mix=1 3 mechanisms (at least): F0_z zCorr, F0_z qx, F0_z qy hybrid filters in use: noise percolation paths Calm period

15. > disturbance rejection: top stagesensors (7/9) WSR7, heuristic threshold for mix adjustment in-line wind (mix 0) hic sunt leones sea (mix 1)

16. > disturbance rejection: top stagesensors (8/9) ACC HPLP mix cross IP seism LVDT LP/HP optimization, given the standard actual corrector (cross) LP/HP ratio versus CL TF mix=0.70 mix=0.90 mix=0.95 mix=1 CL TF Hz LVDT noise projection into zCorr (NE@step 1) Two regions can be distinguished: A) LP/HP ratio plays a role attenuating LVDT sensor disturbance (mseism) B) No effect of LP/HP attenuation V/sqrt(Hz) Hz

17. Hz Hz Hz Hz note: cleaner (LP+HP=1) blending allows to re-tune HP to reduce tilt re-injection WSR7 => WSR8 (optimization example) patches added to LP to reduce mirror tilt excitation due to SA mseismic noise disturbance previous WSR7 HP: anti-wind (tilt) WSR8 LP: accel. comparison mms-2 (=> heavier computation…)

18. Hz Hz mms-2 Hz Hz WSR7 => WSR8 optimization previous WSR7 LP: anti-mseism WSR8 accel. comparison

19. mix 0 => 1 wind => sea WSR7 => WSR8 overall tuning (mix 0-1) LP/HP mix 1 => 0 sea => wind A) assessing how much it is enough to enhance LP/HP to beat tilt re-injection is not easy. B) more data needed in windy-only conditions… C) optimization not significantly efficient around the crossing frequency.

20. > disturbance rejection and noise budget: 4-marionette (9/9) Marionette locking force can be distributed to both input and End mirrors Since WSR8 the lock correction is applied to four marionettes (NI,NE,WI,WE) instead of two (NE,WE): A) reduction of direct locking force budget and B) non-linear torque negligible in case of wind (=> reduction of large zM by a factor 2 lightens gain request to AA).

21. Novel strategy (…one smart idea): GLOBAL OPTIMIZATION anti-mseism plus coherent wind compensation

22. GC (reconstructed z) GIPC (Global Inverted Pendulum Control) benefits: wind disturbance rejection, lock acquisition, duty-cycle. Once the ITF is locked, the mirror position, provided by the Global Control, can be used instead of LVDTs, referred to the ground. • Features: • - automatic engagement (soft!) • anti-wind blending in NE-WE-BS-PR • anti-mseism in NI-WI LVDT LVDT Splitted anti-mseism /anti-mseism

23. Example1: NE_GIPC,WE_GIPC only stormy weather step9 NE “follows” NI WE “follows” WI

24. Example 2: NE_GIPC, WE_GIPC only step9

25. Example 3: NE_GIPC, WE_GIPC, BS_GIPC, PR_GIPC step9

26. Thanks to GIPC GIPC benefit to Automatic Alignment Short suspension (i.e.beam) improvement necessary ? Example3: NE_GIPC,WE_GIPC only stormy weather step9

27. A curiosity: in November GIPC had already been tested but we needed to improve our knowledge and to optimize the blending Next improvements and studies storm+GIPC step 12 A major effort necessary at LF during stormy weather! EM-MSC-041206

28. Example 4:environmental (central Bld. shock absorption)

29. > noise budget: actuation noise & reallocation • Suitable resistors are used to reduce coil-drivers noise (ln=lownoise/HP=highPowe to accomplish sensitivity/locking) • a trade-off with DAC noise using Emphasis/deemphasis was found. • Still some work: • BS requires marionette reallocation. • WE,NE have a mseism peak that should be reduced by further force reallocation to the marionette. Too large correction directly on the mirror Low frequency component that can be moved upwards (marionette)

30. > synthesis: net improvement of recent activity In this example: - red data (now) show up much larger mseism In spite of this: => the force reallocated to the marionette is smaller Much more mseism and similar peak at 130 mHz

31. > conclusions It was mandatory to accomplish this task by considering the suspension system as a single component of the interferometer The development was a bit delayed by the requirement of checking the performance under actual environmental disturbance. Tools to emulate crucial situation have been developed. A large effort was spent on sensing optimization and on “soft operation” Now it is possible to reduce by a factor 3 the rms disturbance due to the wind, while mseism (sea) does not seem to be the main source of problems. • To be done before (possibly before the MegaRun): • further technical noise reduction • improvement of short suspension performance (InjB,MC,OutB)