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Filter #7 control April 18 , 2016 – , Cascina

Filter #7 control April 18 , 2016 – , Cascina. Paolo Ruggi. GENERAL MOTIVATION.

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Filter #7 control April 18 , 2016 – , Cascina

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  1. Filter #7 control April18, 2016 – , Cascina Paolo Ruggi

  2. GENERAL MOTIVATION Filter #7 is the suspension last stage before the payload.A sensor-actuator system at this level allow us to see and control some suspension degrees of freedom, which are not controllable from the standard controlled stages.Moreover, it can be used to compensate an unbalanced back action of the locking force.

  3. The entire suspension is a long and soft torsion pendulum. In VIRGO the first normal mode was at 15mHz. In AdvV it is at 11 mHz. It is not easy to set correctly that d.o.f. in air; when the suspension goes in vacuum the rest position could be much different then the expected one. Filter #7 monitor fix the problem. TORSIONAL D.O.F.(theta-y)

  4. The suspension torsion modes can be easily excited a lot during normal operations. In that condition, the accuracy of mirror alignment cannot be the best possible one.Generally the natural damping of that oscillations takes a long time.Filter #7 control system fix the problem.

  5. LONGITUDINAL D.O.F. There is a pendulum mode which cannot be controlled through the top stage, but is well visible by the filter #7 monitor, as pitch motion. This is the payload longitudinal oscillation against the backward motion of filter #7 center of mass, which is located much below its suspension point. The excitation of this mode can worsen the accuracy of marionette pitch control.

  6. Last stage pendulum mode (0.42 Hz)

  7. LOCKING PHASE Filter #7 is the stage which supports the coils used for marionette and mirror driving. It has been observed that the longitudinal force applied to the marionette for the ITF locking, even if well balanced in order to avoid yaw action on the marionette, produce a yaw back action on filter #7. The effect worsen the alignment accuracy of ITF. It can be compensated sending a component of the locking signal to the filter #7 yaw actuation.

  8. POSSIBLE ADDITIONAL USE A combination of filter #0 and filter #7 sensing system can produce seismic free signals, able to see the residual motion of the pendulum chain due to filter #0 control lacks of accuracy. Therefore, in principle, an advanced control strategy has more powerful if the filter #7 sensor-actuator system is installed.

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