Ibrahim, Oli, Jeff, Betsy, Joe, Others
Summary:
- The BBSS Pitch Degree of Freedom is slowly drifting downwards in counts. This indicates a pitch instability in M1. We think it's the M1 Blade heights.
- The drift can be seen in Pitch only, showing up on the F1 OSEM only, the only OSEM above the center of mass on the face.
- We know it is not the electronics nor the OSEM itself. We took F1 out for a few days and there was no drift at its OLV.
- We know it's not the diurnal temp-sag which is day-to-day (as some prev. alogs believed)
- This is not a new issue and has happened consistently since the last RAL visit with one configuration exception (see below).
Relevant Alogs:
alog 79079: Recent Post-TF Diagnostic Check-up - one of the early discoveries of the drift and pitch instability.
alog 79181: Recent M1 TF Comparisons. More recent TFs have been taken (found at: /ligo/svncommon/SusSVN/sus/trunk/BBSS/X1/BS/SAGM1/Data on the X1 network). We are waiting on updated confirmation of model parameters in order to know what we should correctly be comparing our measurements to. We just confirmed d4 a few days ago following the bottom wire loop change and now seek to confirm d1 and what that means with respect to our referential calibration block.
alog 79042: First investigation into the BOSEM drift - still operating erroneously under the tmperature assumption.
alog 79032: First discovery of drift issue, originally erroneously thought to be part of the diurnal temperature driven suspension sag (where I though that blades sagging more than others contributed to the drift in pitch).
Hypothesis:
We think that this issue is related to the height of the blades for these reasons:
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The issue was fixed when we lowered all blades from the calibration block's "nominal" or zero by -1.5mm with all 4 blades roughly close to this number (avg -1.5mm)
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The issue came back when we attempted to fix the S-shaped M1 blade tip by correcting the extra swivel it needed to have in order to stay at the same height. (Joe recommendation to Betsy)
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Oli and Jeff have a d1 investigation in alog 76071 overlays different P to P model TFs when the blade heights are above/below their physical D (called FD in the attached plots).
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Interestingly, there is a new mode at roughly 1.9Hz when d is above the model's physical D by +-4mm. This mode is confirmed to not be cross coupling. Our recent TFs don't have them but TFs with the drift earlier do - I think this is a red herring.
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More clearly, the attached file shows overlays from different d1 sizes (Pitch).
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While the F1 blades are at an avg height of -1.5mm below nominal calibration block height, the spread between the individual blades is higher than before, with the problematic "soft/S" blade measuring at only -1mm. There's another blade at -1.8mm. This is the only difference between our current drifty -1.5mm avg and the non-drifty -1.5mm avg is the spread of each indiv. blade height. At this point, I'm interested in seeing how the spread of individual blades affects the drift effect in addition to just an everage d1 drop - could it be a combo of these effects? We can investigate the latter by playing with the model and the former by emperically measuring the drift itself.
Our Units:
We need to know how the calibration block converts to model parameters in d1 and whether that's effective or physical d1 in the model. Then we can stop using referential units.
To further investigate, we have questions:
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What is the "sensor calibration block" calibrated to? Physical D (Center of Mass to blade tip) or Effective D? What are these values? We just want to find a model way to test parameters rather than the cal block or the shim methods to our model since right now we're going off potentially old information.
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Could differences between the 4 individual blades be causing a drift this stark? (i.e. it's not a net d1 height issue but a blade to blade height issue or a combo). I'm thinking this may be the case since we have two equal net heights (-1.5mm avg) with the only difference being the spread of the indiv. heights.
Some Early Observations (attempting to constrain our model to our measurements):
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TFs before and after the F1 drift manifested (now vs 7 days ago) barely change the actual peak locations, but that's expected due to nature of TFs (I think).
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The difference between -1mm and -3mm drastically changes the 1.05Hz peak's position. In general, small mm changes have noticeable decimal freq. changes.
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The shape of the model curve is different for -3mm and -5mm, having positive inflection. Anything higher has our straight/negative inflection shape.
Attachments:
Update to the triplemodelcomp_2024-08-30_2300_BBSS_M1toM1 file Ibrahim attached - there is an update to the legend. In that version I had the description for the July 12th measurement as 'New wire loop, d1=-1.5mm, no F1 drift', but there was actually F1 drift during that measurement - it had just started over a week before so the OSEM values weren't declining as fast as they had been earlier that week. I also want to be more specific as to what d1 means in that context, so in this updated version I changed July's d1 to be d1_indiv to hopefully better show that that value of -1.5mm is the same for each blade, whereas for the August measurements (now posted ) we have d1_net, because the blades heights differ by multiple .1 mms, but they still average out to the same -1.5mm.