Reports until 13:57, Friday 06 February 2015
H1 ISC
kiwamu.izumi@LIGO.ORG - posted 13:57, Friday 06 February 2015 - last comment - 18:25, Friday 06 February 2015(16522)
DRMI lock acquisition study (round 2)

Alexa, Kiwamu

In response to the commissioning alog from the last night (alog 16501), we spent an hour or two to study the DRMI lock acquisition with ETMs miasliged in this morning. The key item we checked this time was the BS limiter. We took a few lock acquisition samples with and without the BS limiter which was suggested by Ryan.

It seems that the limiter improved the locking time. We will tentatively keep the limiter on.

The alignment was adjusted initially so that we observed consistently high build up in each lock. We tried repeating the same test with the arm cavities aligned, but we ran into some aligment issues which prevented us from completing the test with the arm cavities aligned. Note that this limiter method is something we used to use in the past, but  apparently we decided not to use it for some reason at some point.

 

(The test without the limiter)

  time to lock [sec]
attempt #1 10
#2 120
#3 540
#4

270

(The test with the limiter)

  time to lock [sec]
attempt #1 5
#2 120
#3 60
#4 80
#5 90

 

(The limiter)

The limiter resides in BS_M3_ISCINF_LIMIT and the value was set to 5x105 cnts.

Comments related to this report
alexan.staley@LIGO.ORG - 14:07, Friday 06 February 2015 (16524)
Link

This limiter was being engaged during lock aquisition and then turned off when lock was aquired during the MICH_DARK_LOCKED state of the LSC_CONFIGS gaurdian. However, none of this was translated over to the ISC_DRMI guardian. I have added this to the ISC_DRMI guardian now.

lisa.barsotti@LIGO.ORG - 18:25, Friday 06 February 2015 (16531)
Link 

Attaches is a plot corresponding to the test that Kiwamu and Alexa did. Once the SWSTAT is around 46000, the limit on the BS correction was turned on at 500,000, and the DRMI started acquiring lock more frequently (examples of locking attempts with this limit OFF and ON are attached as well).

Since this setting was not in the guardian managing the DRMI lock (and the MICH guardian used in the initial alignment sequence turns this off once lock is acquired), the BS correction limit was typically off, unless someone was intentionally turning this on (which I am pretty sure happened at some point in the past, as this limit is a known important variable for the locking sequence).

How critical this BS limit is for lock acquisition it depends on the seismic input. So, even assuming comparable good alignment states, the easiest explanation for "DRMI lock sometime works, sometime it doesn't" is that even when the seismic noise is not outrageously bad and it is obviously recognized like a problem, it might still be higher than "normal" and make the locking more sensitive to gain settings, and marginal.

Here is some (now old) systematic study that Den did  sometime ago  at LLO changing the locking thresholds, also known to be critical parameters. 

It would be good to do similar tests with arms off-resonant. I think Alexa and Kiwamu tried by they didn't have enough time today, we should keep this in mind and use any available "free" time to make the acquisition time as short as possible ( < 1 min). Also, it would be good if changes to whatever "nominal" settings were clearly highlighted and somewhat motivated, so we can try to make sense of what's happening.
Images attached to this comment