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

F. Carbognani for

The Virgo Collaboration

ICALEPCS - 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

• 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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Real-time and fast automation layer Real-time and fast automation layer

Control and monitoring automation layer Control and monitoring automation layer

Automation Layers

Global and Local Control LoopsGlobal and Local Control Loops

Data A

cqu

isition(D

AQ

)D

ata Acq

uisition

(DA

Q)

Automation sequenceAutomation sequence

Data

Commands

Data

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AlpAli AlpDet AlpSa

AlpRecycled(master)

Sub-systems servers

Action

Action

Action

DAQ DAQData Display

– 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.

Automation of Locking Procedure (ALP)

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ALP Macros

Set of code related to the same automation phase and a script language to define the macro’s content.

Macro functionalities:• ALP variables declaration,

• new DAQ channels can be created by performing arithmetic operation (+,-,*,/) between input channels,

• DAQ channel’s 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

ITF Control

Photodiode Signals

Mirror Corrections

Locking Frequency: 10 kHz

Alignment Frequency: 500 Hz

Global ControlProcesses 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

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Mirror Local Control Longitudinal swing reduced to

tenths of m/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 controlsswitching the coil drivers to low noise mode

Local Controls

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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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DAQ DAQ

Automation Layers (Detailed)Locking and Alignment Servo-LoopsLocking and Alignment Servo-Loops

LocalServo-loops

LocalServo-loops

Mirror controlsand Injection

Mirror controlsand Injection

PhotodiodesReadout

PhotodiodesReadout

GlobalControl Global

Control Calibration Calibration

Timing SystemTiming System

Alp Automation Alp Automation

Frame BuildingLow latency

Frame BuildingLow latency

Frame BuidingLast stage

Frame BuidingLast stage

Data ArchivingData ProcessingData ArchivingData Processing

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Locking ProcedureThe goal of a lock acquisition procedure is to bring the ITF on its working point, by controlling its independent longitudinal lengths

• MICH = ln-lw

• PRCL= lrec+(lN+ lw)/2

• CARM= LN+LW

• DARM= LN-LW

4 lengths to be controlled:

LW

LNlrec 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

5 mins

Lock to DF variable finesse

OMC lock

Low noise

SSFS

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PerformancesCommissioning run C6:Jul 29 – Aug 13, 2005

Step1 to Step15

duration distribution

Percentage of time spent on the Step1 – Step 15 sequence: 5.29%

Duty cycles: Locking : 89%Science mode (locked and stable ITF) : 86%

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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.