I have a tentative noise plot for the STACIS that includes accelerometer noise, geophone noise, and platform motion with the STACIS off. (Accelerometer noise was measured for the VEL and NONE setting, which are settings on the accelerometer box which make the accelerometer signal correspond to velocity and acceleration, respectively. ) I'm focusing on sensor noise because this is the variable I am looking at changing, and knowing how the sensor noise translates into STACIS platform motion is therefore important.
The accelerometer and geophone noise I determined as described in my last eLog (http://nodus.ligo.caltech.edu:8080/40m/7027) Along the way I found out several things of importance:
1) Horizontal geophones are ONLY horizontal geophones. This is obvious in retrospect, because the springs supporting the magnet inside must be oriented based on vertical/horizontal operation.
2) The geophones in the STACIS are GS-11D (geospace), with a sensitivity of 32 V/m/s (compared to about 3.9 V/m/s for the accelerometers in VEL setting).
3) The accelerometers have different V/m/s sensitivities. I noticed the voltage output of one was consistently higher than the other, leading to very high noise estimates, but then Jenne showed me the actual calibration factors of the individual accelerometers which differed by as much as 0.4 V/m/s (a few percent difference). Taking this into account made the noise plots much more reasonable, but variations in calibration could still create some error.
The accelerometer noise agrees fairly well with the specs on the Wilcoxon page (http://www.wilcoxon.com/prodpdf/731A-P31%2098079a1.pdf). The geophone noise seems surprisingly low; it is even better than the geophone below about 4 Hz.
To see how this noise translates into actual platform motion, I took PSDs of the STACIS while it was off, on with accelerometer feedback, and on with geophone feedback (the "off" PSD is in the above noise plot). Using this data I'm working on estimating a transfer function that shows how the sensor noise translates to motion so I can come up with a sensor noise budget.
This shows that the geophones are actually doing a better job of isolating than the accelerometers, which is not surprising if the noise plot is accurate and the geophones are actually lower noise. It must be noted, though, that the noise plot was for the horizontal geophones whereas the plot above is for the vertical axis which may have a different noise level. Also, the vertical have some extra isolation by being enclosed in a metal stack with rubber padding at its base.
The problem with the STACIS in the past was the differential motion it introduced. I think this might be because the horizontal isolation was not uniform for each chamber. This means that even what would be symmetric motion (no differential length change) would be translated to differential motion because one end is more fixed than the other. Having accelerometers or better-padded geophones (maybe like the vertical geophones) in the STACIS ought to help with this by making the horizontal isolation more consistent and thus reducing differential motion. So the key may not be the geophone noise as much as varying geophone sensitivities or variation in how well they're mounted in the STACIS. I can test this by swapping out the horizontal geophones with other spares, changing the tightness of the mount, and seeing if either of these changes the horizontal isolation significantly, since these are factors that may differ from unit to unit.
I will also compare horizontal closed loop response with geophone vs. accelerometer feedback to see if the geophones are only doing a good job in the above plot because of their extra padding (the vertical stack).