JeffK HughR
Jeff will likely have more coherant things to say but the engagement of the X & Y HEPI loops rings up around 8 or 9 Hz. They may each have there own of these--too tired to be sure. Will continue looking tomorrow.
The ground STS2 has some motion at these frequencies and the tall BSC SEI Pier Resonance is in this area so, it may be the peak on the ground or the X & Y loops themselves or in combination with the ground and Pier.
The attached spectra shows the peak rung up on the Y DoF. The reference is with both X & Y off and all other SEI loops are closed (ISI HighIsolated & rest of HEPI closed loop). The plots of the ground STS2 with references shows how this power changed within a short time. The lower left shows the peak in all Horizontal IPS sensors suggesting it is indeed coming from the ground.
I was just out to the EndY this afternoon and things are pretty quiet. However, given that this HEPI has been sort of locked for a few months, I can't say that the position loops have ever been robust at these frequencies.
More study tomorrow.
Hugh sums up what we've found nicely; not much more to add. I attach a time series of the actuator trip. Hugh showed me the loop design plots, and they don't look like a bad design. There may be a sharp feature in the plan around 8 [Hz] that's not resolved, and around 8 [Hz] is where the gain peaking is the largest, but it's predicted to amplify the motion no more than a factor of 1.585 at 8.35 [Hz]. Strangely, I didn't find nearly as many loop design plots for X as for Y in the /ligo/svncommon/SeiSVN/seismic/HEPI/H1/ETMY/Data/Figures/Transfer_Functions/Simulations/Isolation/ but the MIMO plot I did find seems to confirm that the gain peaking should be similarly small. Remember, though -- all of these models are based on ONE local-to-local measurement. Perhaps the HEPI plant (including the state of the ISI) has changed since these were last measured... There's certainly been a whole lot of action with the position locking stops recently, see LHO aLOGs 13070, 13077, 13123, but I'm not sure how stops would create any sharp feature that would create a slowly evolving positive feedback loop at 8/9 [Hz] especially -- but as Hugh points out, this feature is right-around the HEPI pier resonances.
Has anyone run a HEPI valve check recently?