J. Kissel

I've performed the same step response system identification (SysID) for the two end station HPI pump systems, as I'd done for the corner station HPI pump system (see LHO aLOG 16447), now that both ends are using the differential, supply - return, pressure signal that close to the chamber (EY switched in Nov 2014 LHO aLOG 14888; EX switched this morning LHO aLOG 16595).

Results: The low frequency response is well-characterized by
EX: a single pole at 12 +/- 1 [mHz] and a DC gain of 0.163 [PSI/ct]
EY: a single pole at 17 +/- 1 [mHz] and a DC gain of 0.141 [PSI/ct]
and for quick reference,
L0: a single pole at 24 +/- 1 [mHz] and a DC gain of 0.0546 [PSI/ct]
I quote an uncertainty on the pole frequency because the fitting method -- matching the step response against data by-eye -- is crude (though sufficiently accurate for my needs). I don't quote an error bar on the DC gain because the fitting method for that is quantitatively robust and the uncertainty is negligible.

The three attachments are the raw data and fitting for EX and EY, and a plot showing the (low) frequency response of all three models together.

I'll speak more on this in a later entry and/or DCC document, but note that this estimation of the plant via step response does *NOT* measure several other parameters that determine the high-frequency (above ~1 [Hz]) response that I've discovered to be relevant to the PID loop design  (in particular, the phase), including
- The second-order, 16 [Hz], Sallen-Key, anti-aliasing filter before the input to the ADC on all pressure sensor channels (see D0901559, pg 2)
- The [clock cycle / sampling rate] time delay
- Any epics smoothing parameters that have been installed on any pressure sensor channels
Including all of these features, I think I'm finally able to predict what we see in the closed-loop amplitude spectral densities (see, e.g. 16474, or 16426). Stay tuned.

Log files of the excitation, including GPS times of step impulses:
/ligo/svncommon/SeiSVN/seismic/HEPI/H1/Common/
2015-02-10_HPIPumpServo_EX_OpenLoopSteps.txt
2015-02-10_HPIPumpServo_EY_OpenLoopSteps_SMOO_ON.txt

Scripts to load and process the data (needs on-site nds access, but could be modified to gather data remotely):
/ligo/svncommon/SeiSVN/seismic/HEPI/H1/Common/
H1HPI_PumpServo_EX_StepResponse_20150210.m
H1HPI_PumpServo_EY_StepResponse_20150210.m