J. Kissel

More interesting data on the HEPI Pump Servo. Looking to ID the response of the HEPI Differential Pressure "Plant," i.e. from the control drive output to differential (supply - return) pressure change, I turned OFF the servo and put small steps in the control output of varying sizes (see pg 1 of 2015-02-03_H1HPI_PumpServo_SysID.pdf). First linearly increasing step amplitudes to determine the DC gain (see pgs 2 and 3). Then, to determine the frequency response, I put in many steps of the same size to fit the impulse response (see pg 4).

The results: As it stands, the HEPI pump servo plant can be well-approximated by a single pole at 24 +/- 1 [mHz] and a DC gain of 0.0546 [PSI/ct]. Note, E1100508 suggests that the plant is a single pole a 7 [mHz], with a DC gain of 0.087 [PSI/ct]. Looks like not.

With this information, and the current PID settings of (35,0.45,0) respectively, I can predict the current open loop gain (see pg 5). I model a UGF of 39 [mHz], with a suppression of 0.37 @ 10 [mHz], 0.2 @ 1 [mHz], below which the 1/f integrator kicks in. Note, E1100508 suggests using PID params of (20, 0.07, 0), which would result in even lower a UGF and less suppression.

With such a simple plant, it's really tough to find PID parameters that aren't stable, so I also show a toy set of parameters with much higher values, (P,I,D) = (300,30,0), where the integrator dominates. With these parameters, we get a UGF of 390 [mHz], with suppression of 0.24 @ 100 [mHz], 0.035 @ 10 [mHz], and so on.

Why not increase the UGF to infinity? 
Because there're worries out there that above a few [Hz], poorly-matched impedance, transmission-line-like effects take over the frequency response, which may cause sharp, phase-full features in the plant that are not characterized by a simple pole.

Why does this not-at-all match what's measured when comparing Closed vs. Open. vs. No Sensor ASD spectrum?
(see 2015-01-27_H1HPI_DifferentialPressure_Open_v_ClosedLoop_Comp.pdf, and 2015-02-02_H1HPI_PumpServo_ADCNoiseCharacterization_DiffASD.pdf, which is not new data, but copied to this aLOG for convenience)
Open vs. Closed loop ASDs show that there is suppression of 0.1 @ 10 [mHz], not the modeled 0.37 @ 10 [mHz]. Further, the measured spectra shows some gain peaking of a factor of 2, where the model predicts no region where the suppression is above unity. It really does seem like the ASD behavior of the signal is totally disconnected from the time series behavior of these sensors. I really hope this whole saga isn't some false alarm from EPICs vs DTT nonsense...

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Raw Data lives here:
/ligo/svncommon/SeiSVN/seismic/HEPI/H1/Common/2015-02-03_H1HPI_PumpServo_StepResponse.mat

Data Processing Script lives here:
/ligo/svncommon/SeiSVN/seismic/HEPI/H1/Common/H1HPI_PumpServo_StepResponse_20150203.m