Automation of the Lock Acquisition of the 3 km Arm Virgo Interferometer

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• Automation of the Lock Acquisition of the 3 km
  Arm Virgo Interferometer

F. Carbognani for The Virgo Collaboration ICALEP
CS - Geneva 14 October, 2005
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Outline

• Virgo Experiment
• Control and Monitoring Automation Layer
• Real-Time and Fast Automation Layer
• Lock Acquisition Procedure
• Automation Performances
• Conclusions

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Virgo Experiment

• Aim Detection of the gravitational waves
  emitted by astrophysical sources
• Consists of a power recycled Michelson
  interferometer with 3 km long Fabry-Perot
  cavities in its arms

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Working Point

• The mirrors must be aligned with a precision of a
  fraction of a micro-radian
• The relative distance of the suspended optics
  must be controlled within a pico-meter
• Working point maintained by using several digital
  feedback loops working at 10 kHz.

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Automation

• Two automation layers
• A control and monitoring automation layer that
  uses a script-like language to monitor the needed
  DAQ channels and to control the first automation
  layer.
• A real-time and fast automation layer implemented
  into the sub-system servers involved in the
  different loops.

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Automation Layers
Control and monitoring automation layer
Automation sequence
Data
Data Acquisition (DAQ)
Commands
Real-time and fast automation layer
Global and Local Control Loops
Data
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Automation of Locking Procedure (ALP)

• Use the data acquired by the DAQ to compute the
  state of any subsystem by processing the
  collected channels related to it.
• According to the subsystem state, actions can be
  performed using the system calls or directly with
  messages in Cm format.

DAQ
AlpRecycled (master)
DAQ Data Display
Action
Action
Sub-systems servers
AlpAli
AlpDet
AlpSa
Action
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ALP Macros

• Set of code related to the same automation phase
  and a script language to define the macros
  content.
• Macro functionalities
• ALP variables declaration,
• new DAQ channels can be created by performing
  arithmetic operation (,-,,/) between input
  channels,
• DAQ channels properties can be computed in the
  time domain (mean,min,max,rms,range) or in
  frequency domain (FFT, bandRMS ) and stored into
  ALP variables,
• Arithmetic operations (,-,,/) between ALP
  variables,
• test and loop conditions on ALP variables,
• commands can be sent to act on given sub-system
  servers,
• direct call of a macro

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Alignment and Longitudinal Controls

• Global Control
• Processes signals coming from the photodiodes and
  sends control signals to suspensions DSP.
• ALP drives
• switch between algorithms (Sensing, Filtering or
  Driving)
• on-the-fly parameters changes
• Gc state transitions

Photodiode Signals Mirror Corrections Locking
Frequency 10 kHz Alignment Frequency 500 Hz
ITF Control
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Local Controls
Mirror Local Control Longitudinal swing reduced
to tenths of mm/s by Inertial Damping. Tidal
effects compensated by acting to the top stage,
Tidal control. Angular mirror displacements
reduced to fraction of mrad by Local
Controls Alp drives switching off the inertial
damping loops swapping from local controls to
angular drift controls switching the coil drivers
to low noise mode
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The Injection System

• First Stage of Frequency Stabilization and
  Automatic Beam Positioner (ABP) control loop
  started by ALP at init time
• Second Stage (SSFS) one ITF error signal as
  frequency reference engaged (and disengaged when
  needed) by the Global Control. In case of
  problems ALP can disengage the loop within few
  seconds delay

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Suspension Control Crate
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Automation Layers (Detailed)
Locking and Alignment Servo-Loops
Timing System
Mirror controls and Injection
LocalServo-loops
Photodiodes Readout
Global Control
Calibration
DAQ
Frame Building Low latency
Alp Automation
Frame Buiding Last stage
Data Archiving Data Processing
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Locking Procedure
The goal of a lock acquisition procedure is to
bring the ITF on its working point, by
controlling its independent longitudinal lengths
4 lengths to be controlled

• MICH ln-lw
• PRCL lrec(lN lw)/2
• CARM LNLW
• DARM LN-LW

By using a carrier beam phase modulated at 6 MHz
and the Pound-Drever-Hall technique all the four
lengths can be reconstructed by mixing the
signals coming from photodiodes placed at
different output ports of the interferometer.
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Locking Procedure

• For the ITF locking a novel strategy, called
  Variable Finesse Locking has been adopted
• Basic Idea the ITF is locked on the half (gray)
  fringe, then brought sequentially to the dark
  fringe through several steps. During those steps
  the control scheme is changed.
• The lock acquisition procedure embeds this
  strategy and consists in two main sequences
• Pre-alignment Sequence
• Locking Sequence

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Pre-alignment sequence

• This sequence, not always executed at each lock
  acquisition attempt, is implemented by the
  following three macros
• Direct_Beam_Alignment Alignment of the direct
  beam into the North and the West arms.
• Cavities_Alignment North and West arm cavities
  independent locking, their non-linear alignment
  (when really needed) and their linear alignment.
• PR_Coarse_Alignment Non-linear alignment of
  the PR mirror, with respect to the arm cavities
  mirror alignment performed in the previous macro.

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Locking sequence

• Implemented by a single macro, called
  Lock_Step_request.
• STEP 1 Lock acquired with the PR mirror
  misaligned by 10 mrads and the ITF on the grey
  fringe (i.e. Dark Fringe at 50)
• STEP 2 A boost filter added to the PRCL and
  MICH loops. Dark Fringe from 50 to 40.
• STEP 3 CARM loop controlled by the SSFS.
• STEP 4 PR mirror aligned and consequently the
  power stored in the ITF increases
• STEP 5 Dark Fringe from 40 to 20. A boost
  filter added to DARM loop.
• STEP 6 Dark Fringe from 20 to 8.
• STEP 7 Dark Fringe from 8 to 5.
• STEP 8 Final step to Dark Fringe. Transition of
  the MICH loop error signal from the B1p DC signal
  to the B5 demodulated. Angular drift control
  switched on.

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Locking sequence

• STEP 9 Transition of DARM loop error signal
  from the noisy B8 to the less noisy B1p
  demodulated.
• STEP 10 Output Mode Cleaner (OMC) put on
  resonance, transition of DARM loop error signal
  from B1p to B1. After the transition all the
  noisy mirror motion dampers are switched off.
• STEP 11 A filter having a reduced band and a
  high roll-off is added the MICH loop.
• STEP 12 Activation of the tidal control,
  swapping to low-noise coil drivers.
• STEP 13 Re-adjustment of demodulation phase and
  gain of PRCL loop.
• STEP 14 A fraction of the MICH correction
  signal is sent in counter-phase to the end
  mirrors.
• STEP 15 Permanent lines are added to the
  different mirror corrections for ITF calibration.

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Automation Client
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Performances

• Lock Acq procedure fully automated

Low noise
OMC lock
Lock to DF variable finesse
SSFS
5 mins
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Performances

• Commissioning run C6Jul 29 Aug 13, 2005

Duty cycles Locking 89 Science mode (locked
and stable ITF) 86
Percentage of time spent on the Step1 Step 15
sequence 5.29
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Conclusions

• The automation has allowed to define a
  reproducible lock acquisition sequence, thus to
  stabilize the commissioning environment
• Automation has showed all his effectiveness in
  supporting the ITF operations during
  commissioning and run periods
• The Virgo machine is being provided with a tool
  allowing the operator to easily, reliably and
  quickly drive the machine into the working state.
  
• The planned future improvements are
• Adding more controls and checks inside macros
• Monitoring of all Servers status and check of
  correct handling of the requested commands.
• Automated subsystem failure recovering plus
  automated re-locking.
• Performance optimisation on critical servers
  currently generating latency peaks.