J. Kissel, H. Radkins, J. Warner

To help better characterize and model the HEPI system (and to get some gratification out of using M*g = k*x), Hugh, Jim, and I measured the vertical spring rate / spring stiffness of several H1 HEPI systems in the corner station. 

Here're the results:
Chamber       Spring Rate (+/- Uncertainty) [N/m]
ITMX (BSC1)       7.52e6 (4.22e4)
ITMY (BSC3)       7.75e6 (8.09e4)
HAM4              7.55e6 (1.19e5)
HAM5              8.21e6 (3.23e5)

Method:
(1) Turn OFF HEPI and ISI feedback control for chamber (brought OFFLINE by guardian)
(2) Measure raw V1-V4 IPS signals (waiting for signals to settle enough that the one's digit is all that's changing, and then just caget the H1:HPI-${CHAMBER}_IPSINF_${DOF}_INMONs)
(3) Place 10 [kg] load on all four corners of the suspended payload (on the blue cross-beams directly vertical of the vertical actuators on the BSCs, and on the "do not step" cover for the vertical L4C, also directly vertical of the vertical actuators, on the HAMs)
(4) Measure raw V1-V4 IPS signals
(5) Remove masses
(6) Measure raw V1-V4 IPS signals
(7) Re-place masses
(8) Measure raw V1-V4 IPS signals
... repeat of all four chambers.
(9) "Offline" convert raw data to [m] of displacement, x,
    (a) convert raw IPS signals to [m], with 3.87e-08 [m/ct]
    (b) take average of V1 - V4 to get Z position
    (c) create three Z displacement measurement trials, (4) - (2), (6) - (4), and (8) - (6)
(10) Calculate spring rate for each trial, k = F / x, where F = m g, with m = 40 [kg] and g = 9.81 [m/s^2].
(11) Take the mean and std / sqrt(3) of three trials, to report spring rate mean and uncertainty on the mean in the above table.

Raw notes from data taking (includes load on each spring, which was also measured)
/ligo/svncommon/SeiSVN/seismic/HEPI/H1/Common/2015-02-03_HPI_SpringRates.txt


Script to process data:
/ligo/svncommon/SeiSVN/seismic/HEPI/H1/Common/HEPI_Spring_Rate_20150203.m