I silently started studying the performance of the HLTSs, i.e. PR3 and SR3. The major questions I want to answer in this study is:
- Are our HLTSs behaving as expected (mainly in pitch) ?
- Why do we need an oplev damping loop in pitch while LLO does not need them ?
I did not get a point where I can conclude something yet, but here are some summary of what I did today for it. To be continued.
(what I did)
To start off the study, I did measurements of the pitch transmission -- a transfer function from the suspension point to the mirror in pitch -- on PR3. Since SR3 did not have a GS13 witness sensor routed to the front end model yet, I was not able to get the same measurement on SR3 this time.
I shook ISO_RY_EXC of the HAM2 ISI in order to have better signal-to-noise ratio in the measurement. Note that a passive measurement without excitations did not give me good coherence and so for the reason I ended up shaking the ISI. I used random noise excitation in a frequency band of 10 Hz and limited the amplitude to be 200 counts in order not to trip the ISI watchdog. Also I have added 3rd order boosts below 1 Hz in the excitation in order to further improve the signal-to-noise ratio at the low frequencies. The dtt file currently resides in /ligo/home/kiwamu.izumi/Public/dtt/PR3_pitch_ISI_excitation.xml
I did the same measurement twice, one with the oplev damping loop running in pitch and the other without the oplev damping. The attached below shows the results. The red traces are the ones with the oplev running and the blue are without the oplev running. As seen, the oplev damping loop suppresses the components in 0.1-2 Hz at a cost of a gain peaking at 3 Hz. Since the rms is usually dominated by the components below 1 Hz, this confirms that the oplev damping loop is doing the right job.
(Next move)
As a next step, I would like to compare the pitch transmission with a model in order to see whethere it is something we expected or not.