brett.shapiro@LIGO.ORG - posted 10:20, Tuesday 28 October 2014 (14665)
HSTS model M3 pitch to length coupling
Posting some notes retroactively from an email conversation with Rana last month.
Based on https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=14057, Rana was curious of whether the HSTS model can predict the PR2 M3 stage angle to length coupling down to the mm/rad level. My response was the following:
"The existing models don’t appear to be good enough. See the attached figure of M3 long-pitch coupling. The blue is the generic HSTS model. The black is a (very thorough) fit of the model to H1SR2 I made earlier this year, 10124 You’ll note that these models differ by quite a bit more than 0.001 m/rad.
This plot was made by plotting the TF ratio of (M3 L to L [m/N]) / (M3 L to P [rad/N]).
You should have better luck if we simply replace the HSTS model with the fit to H1SR2. The second attached figure compares top mass P to P measurements of a few suspensions, and all seem to agree with this sus better than the generic model.
However, the errors on mode freqs between HSTSs are still on the order of 1%. Playing with some parameters in the model indicates that this corresponds to a DC error on the M3 m/rad at least as big as the 0.001 m/rad level. So we would basically have to make really good fits of the model to each sus to predict the coupling to that level.
Also, the diagonlization on the M3 OSEM actuators would have to be really good [to not generate additional spurious coupling].
If you are interested in yaw to length as well, the model doesn’t predict anything useful at all for that."
Rana then suggested an alterntive to fitting many models, "that as long as the mechanical TF is stable, we can always use this as a relative reference; once we find a good global alignment, this is a a good technique to recover the alignment."
If we go the route of fitting models for each sus, the best bet will be to use the good H1SR2 fit as a starting point, since this fit incorprates data from double and single hang tests. Then, update the model for each sus based on its measured top mass to top mass resonance frequencies.
Images attached to this report
