1. VIRGO–KAGRA Meeting about bottom filter damping
Design of the filter 7 damping
V. Dattilo (EGO),
R. Passaquieti (INFN and Pisa university)
VIRGO–KAGRA Meeting about bottom filter damping
Cascina, 19 April 2016

2. VIRGO–KAGRA Meeting about bottom filter damping Cascina, 19 April 2016
Outline
Design for Virgo Upgrade ( )
Design for AdvancedVirgo
VIRGO–KAGRA Meeting about bottom filter damping Cascina, 19 April 2016

3. Part 1
Design for Virgo ( )

4. Design for Virgo Upgrade: overview
During the first months of 2003 the Filter #7 of long suspensions have been equipped with a 6 d.o.f. position monitoring system
This system was required as a mean to ensure the correct positioning of the Filter #7 around its working point.
In particular, the horizontal position (z, x, ty) is measured by a set of 3 LVDTs
These LVDTs are part of 3 contact-free actuator/sensor devices that offer also the possibility to implement a new actuation point in 3 d.o.f. (z, x, ty) at level of Filter #7.
Indeed, in view of the locking force reallocation from ref-mass to marionette, it was also foreseen an actuation at level of Filter #7 to damp payload excitations generated by locking attempts.
VIRGO–KAGRA Meeting about bottom filter damping Cascina, 19 April 2016

5. Design for Virgo Upgrade: overview
Vertical (y):
Sum of all the 6 vertical LVDTs of the filter chain. (pre-existing in the chain)
Roll and pitch (tz and tx):
Bi-axial Tilt-Meter (commercial):
vacuum compatible, low weight, dynamical range of ± 16 mrad and resolution of 0.1 microrad.
Horizontal (x and z) and yaw (ty):
3 horizontal special LVDTs (purposely realized)
This system is also able to implement a new actuation point in 3 horizontal d.o.f at the level of F7.
VIRGO–KAGRA Meeting about bottom filter damping Cascina, 19 April 2016

6. Idea of a contact-free actuator/sensor: the two starting devices
Secondary coils
displacement sensor
(LVDT)
Primary coil
Coils
actuator
(Coil/magnet system with
a push-pull configuration in order to minimize the force gradient)
Magnet
VIRGO–KAGRA Meeting about bottom filter damping Cascina, 19 April 2016

7. VIRGO–KAGRA Meeting about bottom filter damping Cascina, 19 April 2016
Idea of a contact-free actuator/sensor: their merging in a single device
Secondary coils
Primary coil
Magnet
actuator/sensor
The secondary coils (and their cable) are shared
 Saving in space and costs
Sensing point coinciding with the actuation point
 It is useful for the control loop implementation
VIRGO–KAGRA Meeting about bottom filter damping Cascina, 19 April 2016

8. the realized actuator/sensor
Magnet
(L=20 mm, =10 mm)
Primary coil
Secondary coils
voltage/displacement: mV/mm
force/current:  = 73 mN/A

9. integration in Virgo
Top view of the actuator/sensor installed on Filter #7 :
- Secondary coil solidal to ground.
- Magnet and primary coil on Filter body, at the same altitude of the center of mass.

10. Electrical connections
Matching of the device with the existing LVDT drivers and actuator drivers.
It is based on the fact that the two drivers work at different frequency bands. TX RX
LVDT
driver
Secondary
coil
Primary
out
actuator
driver

11. Interaction between sensing and actuation
Effects of the sensing on the actuation: no detectable effects
Effects of the actuation on the sensing: a lowering in the
calibration factor voltage/displacement of about 25%. It is an attenuation setted by the LVDT oscillator frequency and by the passive components used for the matching.
It doesn’t depend on the
current supplied by the
actuator driver. Therefore,
this lowering is constant
and can be recovered
by increasing the LVDT gain.

12. Interaction between sensing and actuation

13. Interaction between sensing and actuation

14. LVDT calibration and linearity
Calibration factor voltage/displacement: 245 mV/mm
Linearity within 5% over a range of ± 15 mm (axial) x 15 mm (radial)
Linearity within 2% over a range of ± 15 mm (axial) x 10 mm (radial)

15. Actuator calibration and linearity
Calibration factor force/current:  = 73 mN/A
Linearity (i.e.  constant) within 1% over a range of ± 15 mm (axial) x 15 mm (radial)

16. Block diagram of the control system using the actuator/sensor devices
LVDT
Coil
driver
LVDT
driver 3 3
ADC
DSP
DAC

17. Considerations on control noise
Displacement sensitivity:
1·10-9 m/Hz0.5 measured with a HP 3562 analyzer
2·10-6 m/Hz0.5 measured with a Virgo ADC
Actuation noise with the control OFF
The main contribution is due to the DAC noise floor (300 nV/Hz0.5 ),
that generates on Filter #7 a white force of 1 nN/Hz0.5 .
The induced mirror motion along z is mechanically filtered and
decreasing with the frequency. Its value is about m/Hz0.5
@ 10 Hz .
Actuation noise with the control ON
The main contribution at frequencies in the control bandwidth is due to the max between the seism directly reinjected from ground and the sensor noise.

18. VIRGO–KAGRA Meeting about bottom filter damping Cascina, 19 April 2016
Part 2
Design for AdvancedVirgo
VIRGO–KAGRA Meeting about bottom filter damping Cascina, 19 April 2016

19. Motivations
In the framework of the new filter #7 and payload system for AdV, a new F#7 actuation/sensing system was required.
Main motivations/specs are:
to be extended to 6 DOFs (3 DOFs in Virgo)
to be adapted to the new space constraints dictated by the new separating roof.
to minimize (or better: to lead to zero) the device conductors at level of F#7. This is dictated by the complication in the cable passage through the new F#4-F#7 conductance pipe

20. Solution under development
Motivations
A standard ‘Virgo-like’ solution would have been to install 6 LVDTs and 6 magnet/coil actuators. This would have resulted in 12 boxes, 12 conductors at F#7 level, 24 conductors at bottom ring level.
To fulfill the aforementioned specs, we tried to develop a device that:
- has shared the actuation coils and LVDT secondary coils, both fixed on bottom ring
primary coil fixed on bottom ring (instead of F#7)
The advantages with respect to a standard solution are better highlighted in the following table:
Standard solution
Solution under development
n. of boxes 12 6
conductors at F#7 levels
conductors at bottom ring level 24

21. Horizontal device

22. Vertical device

23. Integration of the 6 devices in the new Filter #7
- stringent space constraints dictated by the new separating roof and to avoid conductors at of F#7.

24. Effect of the ferrite on sensing

25. Effect of the ferrite on actuation

27. Current status in AdV
a full set of the original system has been realized and installed on Beam Splitter suspensions
during the validation in the tower, it turned out that a better linearity of the sensing would be advisable;
the electrical matching with the new electronics of AdV would have required additional boards and tests;
due to time constraints, we decided to proceed we a change in the design, in order to simplify the device and avoid the above mentioned issues, in particular:
- primary on F#7
- separation between actuation coil and secondary coils