Electrostatic actuators: from the GEO experience to the 2nd generation requirements

Harald LückPerugia, 21.9.2005

Mode CleanersOutput Mode Cleaner

GEO600 Length Control Systems

MichelsonInterferometer

Laser

25 MHz13 MHz37 MHz

15 MHz

10 MHz

Hierarchical Control Test Mass Actuators

<10Hz

>10Hz

<0.1Hz 3mm

100 µm

4 µm

HV

Reaction Mass Mirror

Operating Principle

Dielectric in an inhomogeheous E-field:

Force along E

need bias voltagefor bipolar actuation

Segmented ESD

ESD connections

HV

HV

ESD connections

HV

HV

LV

LV

Reaction mass Mirror

~3 mm gapBare copper wires

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)

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.

Sqrt circuits in MI loop

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 / dewhiteningWhitening right after mixer:zero 3.5 Hzpole 35 Hz

Dewhitening for both split paths

Passive dewhit-ening done in HV path

dewhiten

dewhiten

dewhiten

Whiten

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.

U

F

Bias Reduction

Reaction Pendulum:

3 coil-magnet actuators at intermediate mass, range ~ 100µm

Electrostatic actuation on test mass bias 630V, range 0-900V= 3.5µm

Hierarchical Control Test Mass Actuators

<10Hz

>10Hz

<0.1Hz