Last time we designed MCL loop with UGF ~ 30 Hz and I think, it was hard to lock the arm because of large frequency noise injected to IFO.
This time I made a low bandwidth MCL loop with UGF=8 Hz. MCL error RMS is suppressed by factor of 10 and arms lock fine.
Attached plots show MCL OL, MCL error suppression and frequency noise injection to arms.
It is interesting that spectrum of arms increases below 1 Hz meaning that IMC sensing noise dominates in this range.
I did not include the loop into the IMC autolocker. I think it is necessary to turn it on only during day time activity and when beatnote is moving too much during arm stabilization.