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8/13/2004 Stefan Ballmer, MIT / LIGO Hanford 1
Length and angularcontrol loops in LIGO
Stefan Ballmer
Massachusetts Institute of Technology
LIGO Hanford Observatory
8/13/2004 Stefan Ballmer, MIT / LIGO Hanford 2
Goal
Overview of the LSC and ASC control loop fabric Find corresponding data channels (error / control signal) Find corresponding filters of the control system
8/13/2004 Stefan Ballmer, MIT / LIGO Hanford 3
Reading the signalsReading the signals
Laser
antisymmetricsignal
symmetricsignal
pickoffsignal
transmitted powerphotodiodes
Ly
Lxlx
ly
Frequency Response of the LIGO InterferometerT970084-00.pdf
8/13/2004 Stefan Ballmer, MIT / LIGO Hanford 4
Length Sensing and Control
4 loops: DARM CARM/CM MICH PRC
8/13/2004 Stefan Ballmer, MIT / LIGO Hanford 5
The DARM loop
Sensing Cavity pole (~90Hz,~180Hz for H2)
Error signal AS_Q
Control filter bank H1:LSC-DARM
Control signal DARM_CTRL
Actuation calibration (S3)
(H1)
~1.7nm/ct, Pendulum at 0.76Hz, Q=10
8/13/2004 Stefan Ballmer, MIT / LIGO Hanford 6
The common mode servo
Is an analog servo…
Sensing Double cavity pole (~1Hz, ~2Hz for H2)
Error signal REFL_I
Actuation Laser frequency
8/13/2004 Stefan Ballmer, MIT / LIGO Hanford 7
The common mode servo
8/13/2004 Stefan Ballmer, MIT / LIGO Hanford 8
The PRC loop
Sensing flat (due to high CM gain)
Error signal POB_I
Control filter bank H1:LSC-PRC
Control signal PRC_CTRL
Actuation calibration (S3)
(H1)
~3nm/ct, Pendulum at 0.76Hz, Q=10
Acting on recycling mirror (RM)
8/13/2004 Stefan Ballmer, MIT / LIGO Hanford 9
The MICH loop
Sensing flat
Error signal POB_Q
Control filter bank H1:LSC-MICH
Control signal MICH_CTRL
Actuation calibration (S3)
(H1)
~3nm/ct, Pendulum at 0.76Hz, Q=10
Acting on beam splitter (BS)and recycling mirror (RM)
8/13/2004 Stefan Ballmer, MIT / LIGO Hanford 10
Known couplings 1
MICH->DARM AS_Q 140 times less sensitive to
BS than to differential ETM’s (build-up) MICH loop acts on BS MICH loop shot noise limited above ~50Hz
2 solutions: A) until end of S3 (mid S3 at LLO): run MICH loop at low BW
(UGF ~11Hz) & filter MICH_CTRL B) now: run MICH loop at high BW (UGF >50Hz), send MICH_CTRL
to ETM’s to cancel known BS motion reduces coupling by ~40, but MICH_CTRL still significant noise source
8/13/2004 Stefan Ballmer, MIT / LIGO Hanford 11
Known couplings 2
PRC->DARM Coupling due to arm imbalance (~1/400 for H1), has zero at cavity pole PRC loop shot noise limited above ~50Hz (same diode as MICH!)
Again same 2 solutions: PRC correction (send PRCH_CTRL to ETM’s) implemented after S3
reduces coupling only by ~2 since arm imbalance is modulated (microseism)
8/13/2004 Stefan Ballmer, MIT / LIGO Hanford 12
Adaptive Input Matrix
Arm power and sideband buildup do fluctuate (~10% LHO, ~20%LLO) Front-end code calculates new input matrix element on the fly (since S3) Both arm powers and NSPOB are filtered before used
(Modules: H1:LSC-NPTRX, H1:LSC-NPTRY, H1:LSC-NSPOB)
DARM H1:LSC-ICMTRX_01
MICH H1:LSC-ICMTRX_23
PRC H1:LSC-ICMTRX_12
CM MCL path (static at LLO during S3)
H1:LSC-ICMTRX_34
8/13/2004 Stefan Ballmer, MIT / LIGO Hanford 13
Angle Sensing and Control
Per optic: Optical lever
– Error signal: H1:SUS-ETMX_OPLEV_PERROR H1:SUS-ETMX_OPLEV_YERROR
– Control Signal: H1:SUS-ETMX_OPLEV_POUT H1:SUS-ETMX_OPLEV_YOUT
Local Damping– Error signal: H1:SUS-ETMX_SENSOR_UL H1:SUS-ETMX_SENSOR_UR
H1:SUS-ETMX_SENSOR_LL H1:SUS-ETMX_SENSOR_LR Wave front sensors (WFS)
– Error signal: H1:ASC-WFS1_QP H1:ASC-WFS2_IP H1:ASC-WFS2_QP H1:ASC-WFS3_IP H1:ASC-WFS4_IP (and …1_QY etc. for yaw)
– Control signal H1:ASC-ETMX_P H1:ASC-ETMX_Y etc.
8/13/2004 Stefan Ballmer, MIT / LIGO Hanford 14
Angle Sensing and Control
Transmission QPD’s Error signal: H1:ASC-QPDX_P H1:ASC-QPDX_Y etc.
Beam splitter centering servo Slow servo
8/13/2004 Stefan Ballmer, MIT / LIGO Hanford 15
Where are the filter files?
Foton readable files at http://blue.ligo-wa.caltech.edu/hanford1/cvs/lho/chans/ http://london.ligo-la.caltech.edu/cds/llo/chans/
Matlab function to read filters: http://blue.ligo-wa.caltech.edu/hanford1/j/dtt/sballmer/matlabfilter/
