J. Kissel, M. Evans

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These are the results and analysis from the Open Loop Gain TF measurements of H2 SUS ETMY mentioned below. For details of the measurement and setup, please check out the associated comments to that entry. 
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In order to check / confirm that the damping loops were "working as expected," I took Open Loop Gain TFs of the degrees of freedom of interest to the excess motion recently found in H2 SUS I/ETMY; Longitudinal (L), Pitch (P), and Yaw (Y). Attached are the results.

Each collection of plots shows the Open Loop Gain TF (in red), and a reference Plant TF (in black). 
The black traces are what we typically loosely call simply "TFs," which are taken from with the excitation point from the out-of-loop TEST bank. These are in OSEM repsonse [ct]s / COIL drive [ct]s, but because all frequency response in the analog signal chain is compensated, the calibration is merely a collection of scale factors; an even scale factor of 60 -- so we can consider it in [m/N] or [rad/N.m] which is indeed the "plant," P, of the damping control loop. 
The new red traces are taken by exciting through the DAMP filter bank, which means the measurement includes the damping filters and gains, -K, and is hence the Plant times the Filters, P*-K == G, or the Open Loop Gain. Here, because we're sensing and actuating from the same point in the loop (i.e. the input to the damp filters), the transfer function is dimensionless, and no calibration is necessary. Also, because the loop is defined with minus sign explicitly outside of the damping filters, the stability criteria is for Upper Unity Gain crossings to stay away from -180, and Lower Unity Gain crossings to stay away from +180. 

These plots confirm what was mentioned below, that with the gains as is [er was by now], the

- L is basically undamped for both SUS, especially its low frequency modes (where the dominant RMS motion lies), because the Open Loop Gain is below 1,
- P is totally squashed (over-damped), especially the higher frequency modes; not necessarily obvious from these plots -- because it's unclear from the open loop gain how much suppression one would get, it merely shows that you have lost of gain over all resonances -- but it certainly obvious from the closed loop TF plots in the previous log.
- Y could maybe use a little more juice at low frequency, but otherwise looks OK


It's from the combination of these plots and the previous plots our "quick fix" recommendation came: it looks like there was plenty of phase-margin head room to increase the gain of the L loop by 10, and decreasing the P gain by a factor of 3 looked like we could still get some gain at the resonances, without squashing them entirely.

Of course, while writing this log, I found that Keita had remeasured the performance using these new "quick fix" gains, and found that the motion has increased. So, I will tuck my tail between my legs and head back to the drawing board with my thinking cap on. l'*sigh*.