J. Kissel, H. Radkins, J. Warner
We're continuing to attack violin modes post Montana EQ. I attach a spectrum of "our" (all Jim and Hugh) progress, where the reference (GREEN) is this morning. We can definitely see that the violin modes begin to get hefty non-linear shoulders by the time the RMS passes ~5e2 ADC counts, which is pretty surprising for a 32e4 count ADC...
New modes that Hugh and Jim have now successfully cooled today:
EX 1475.09 MODE7 YAW FM4(100dB), FM6(1475.09), G = -500
(This mode interacts with the "already solved" 1475.25 EX mode, so that damping must also be ON and some small level while damping this mode)
IY 1472.45 MODE8 PIT FM4(100dB), FM6(1462.45), FM7(-60d1.5k) G = +1000
IY 1470.38 MODE4 PIT FM4(100dB), FM6(1470.38), G = -500
IY 1470.82 MODE5 PIT FM4(100dB), FM6(1470.82), FM7(-60d1.5k) G = +1000
As before -- these are reported for the record, but the Violin Mode Table has been updated and that should be treated as cannon not this aLOG.
I've been continuing to manipulate the damping output of the problematic pair of modes at 1463.09 Hz, including
- Installing narrow bandpass in all test masses, exploring the unit circle with each of the 4 test masses, and all 3 degrees of freedom (L, P, Y)
- Went back to ITMX and focused energy there (since that;s what I was driving when I originally rung it up)
- Adding +/- 30 [deg] filters, and spinning around the unit circle there
- Broadening the band pass, in hopes of changing the relative drive phase between the two modes.
After three hours of failure, I was *just* about to think I saw hints of progress driving the wide-band band pass in YAW, with FM4(100dB) and FM5(1463.09W) and +1 gain, but then we lost lock because the PSL tripped. Guh!
While wasting time staring at StripTool for 3 hours, I grabbed a 0.5 [mHz] resolution spectra to confirm the violin mode table's claimed exact frequency of these modes. Unfortunately, DTT (and foton's) cursor can't resolve past 2 decimal places. But at least zooming in visually, I can confirm that the two modes are:
1463.0961 +/- 0.0005
1463.0982 +/- 0.0005
We were focused on the 1.5k modes so far today, and made no attempts at progress on the remaining high 1.0k pair of modes at 1008.45 / 1008.49 Hz.
After careful analysis of high resolution spectrums of the "pair" of modes at 1463.098 and 1463.101 Hz I think I understand why commissioners are having issues damping them. In conclusion, the two modes have swapped order after the misalignment caused by the Earthquake, If this has not been taken into consideration then the damping filters are being applied to the wrong mode.
This problem serves as justification of the importance of violin mode identification not only by mass but also by fibre.
Yesterday I commented on another alog saying that this "pair" while associated to the same mass (ITMX), they most probably were associated to different fibres (more concretely to FR and BL fibres), due to the fact that this pair was an order of magnitude closer than any other pair. A confirmation that this assumption is correct is shown next by looking at the pair of modes at 4 different times; (red) is during O1 Dec 2015, (blue) is a couple of days before the Earthquake during Observing mode, (green) is hours after the Earthquake and (purple) is about the time of the entry above.
Looking at red and blue plot we see how the change in frequency of both modes occurs in opposite direction which could not be possible if it was related to the same fibre, basically one is increasing in tension and the other one decreasing, probably due to change on suspension pitch confirming that one is a front fibre and the other one is a back fibre. Furthermore, notice the small change on frequency over 1.5 years, and the huge change caused by the Earthquake, presumably due to the considerable change in alignment caused by the Earthquake and consequent change in the fibres' tension. The Earthquake changed considerably the ground where the detector sits. But most importantly notice that the change in tension is so high that the position of the frequencies associated to the 2 modes switched such that now the lower frequency mode is actually the one that before was the higher frequency one and viceversa. If this has not been taken into consideration then the damping filters are being applied to the wrong mode, which may explain why it is getting hard to damp them.
Notice that the frequency change is once again decreasing as per the purple plot, presumably after ground relaxation after the Earthquake and mass pitch alignment going back to previous values.
Just to confirm this further I looked at spectrums (at the same times as above and same color coding) for the other two pairs of 3rd harmonic modes that have been identified as being associated to the same mass. We can see how for each pair the change in frequency takes place in same direction between them (so they are associated to the same fibre) and same ammount as above, but both pairs change frequency in opposite direction showing that these two fibres are again front and back.
First pair at modes 1456.18 and 1456.84 Hz:
Second pair at modes 1467.47 and 1467.96 Hz:
