Download Electrostatic actuators: from the GEO experience to the 2nd

Electrostatic actuators: from the
GEO experience to the 2nd
generation requirements
Harald Lück
Perugia, 21.9.2005
GEO600 Length Control Systems
25 MHz
13 MHz
37 MHz
Michelson
Interferometer
15 MHz
10 MHz
Laser
Mode Cleaners
Output Mode Cleaner
Hierarchical Control
Test Mass Actuators
<0.1Hz
3mm
<10Hz
100 µm
>10Hz
4 µm
HV
Operating Principle
Dielectric in an
inhomogeheous
E-field:
Force along
need bias voltage
for bipolar
actuation
Reaction Mass
Mirror
E
Segmented ESD
ESD connections
HV
HV
ESD connections
HV
LV
HV
LV
~3 mm gap
Bare copper wires
Reaction mass
Mirror
GEO600 ESD data
• ~3 mm gap between reaction mass and
mirror
• 630 V bias, range 0-900V
900 V → 350µN force
→ about 3.5µm @DC
• F = U2 ε0 εr d3/2 A
Sqrt circuits in MI loop
ESD: F  U2
Sqrt circuits are
necessary to give
full linear force
range for
acquisition.
Drawback: sqrt
circuits are noisy
(1µV/sqrt(Hz)
=100µV @HV)
Sqrt circuits in MI loop
ESD: F  U2
Sqrt circuits are
necessary to give
full linear force
range for
acquisition.
Drawback: sqrt
circuits are noisy
(1µV/sqrt(Hz)
=100µV @HV)
Bypassing sqrt
circuits after lock
is acquired.
Noise in MI loop
HVA noise =
100nV/sqrt(Hz) =
10µV @ HVA
HV-amplifier noise
can be reduced by
decreasing bias
voltage or active
noise suppression.
Suppressing noise
introduced by loop
electronics needs
whitening
Whiten
MI loop whitening / dewhitening
dewhiten
Whitening right
after mixer:
zero 3.5 Hz
pole 35 Hz
Dewhitening for
both split paths
dewhiten
dewhiten
Whiten
Passive dewhitening done in HV
path
Future needs/options
• Two (eight) segments to give different actuation
ranges for acquisition- and run mode
(in prelim. design doc. for Advanced LIGO)
Segments for different actuation
ranges
Future needs/options
• Two (eight) segments to give different actuation
ranges for acquisition- and run mode
(in prelim. design doc. for Advanced LIGO)
• Change actuation range (noise) by changing
bias voltage (nonlin. effects)
Future needs/options
• Two (eight) segments to give different actuation
ranges for acquisition- and run mode
(in prelim. design doc. for Advanced LIGO)
• Change actuation range (noise) by changing
bias voltage (nonlin. effects)
• Investigate problems associated with charged
mirrors. Can be minimized by using AC (8192 Hz
in Ad. LIGO?) actuation and using symmetrical
voltages.
Bias Reduction
F
U
Hierarchical Control
Test Mass Actuators
<0.1Hz
<10Hz
>10Hz
Reaction Pendulum:
3 coil-magnet actuators at
intermediate mass, range ~ 100µm
Electrostatic actuation on test mass
bias 630V, range 0-900V= 3.5µm