Last Saturday, while damping violin modes, I wanted to check if DHARD_Y noise below 7-8 Hz was limited by test mass M0 Y damping, as suspected (71466).
I tried to reduce all test mass M0 Y damping loop gains from -1 to -0.5, but with that gain the peak at about 1 Hz was slowly rising, so I aborted that test.
I then reduced all M0 DAMP_Y loop gains from -1 to -0.7, and this configuration was stable:
- all DAMP_Y_IN signals had comparable noise levels, except for the 1 Hz line that got larger
- all DAMP_Y_OUT signals reduced as expected by 0.7
- DHARD_Y_IN1 also reduced everywhere between 0.4 and 7 Hz.
You can see the reduction in the spectra clearly for the damping loop signals. It's harder to judge how much DHARD_Y improved, so the second plot shows the ratio of all signals when the gains were -0.7 to when they were -1. The reduction is evident and pretty close to the expected factor 0.7.
So in conclusion: DHARD_Y signal below a few Hz is dominated by test mass damping noise.
