sheila.dwyer@LIGO.ORG - posted 01:53, Thursday 20 November 2014 - last comment - 11:10, Friday 21 November 2014(15193)
sensor correction turned off, back on now.
Nic ran Jim's script.
Earlier tonight, we had a large drift in ETMX pitch (1 urad over about 7 minutes). I turned off the sensor correction (ETMX ISI X only) for the last several hours, this seemed to help. Now I've turned it back on so we can see if this continues to happen overnight.
We are leaving the arms locked in green, and the mode cleaner locked. This should give a chance to see if the mode cleaner is really stable when there is no actuation on it.
Thanks Sheila, for reporting this.
I looked for minutes long features into the ISI CPS, at 0am and 4am, but did not find anything obvious over such low frequencies. However, I don't understand why the vertical Stage 1 CPS signal is so low. The local CPS signals should be quite larger if the platform was well isolated. I am getting inertial sensors and optical levers data to look into more details.
[Rich, Fabrice]
At 0am, the time series show a large drift in ETMX SUS top mass, not only in pitch but also in longidudinal, see first plot (the ylabel should say "nm"). According to the Stage 1 T240s (second plot), the ETMX ISI was performing as well as the other units, but this doesn't tell us anything about slow drift, so we need to get back to CPS data (maybe look at the rotations in the cartesian basis for the non-corrected CPS signals).
At 4am, ETMX SUS drift has stopped (third plot), though the sensor correction seems to be on (fourth plot).
K. Venkateswara
One thing to note is that turning sensor correction on is somewhat equivalent to blending at still lower frequencies than the normal 45 mHz. This makes it very sensitive to ambient ground tilt (caused by human activity or wind). It should not be used if there are people/activity near the ground seismometer.
Using tilt-subtraction makes sensor correction less susceptible to wind-induced tilt but unfortunately BRS is very sensitive to gravity gradients from people. So walking near the BRS when it is being used can cause very large low frequency signals to show up in the super-sensor.
Should we ask the operators to turn off the sensor correction at End X if they know someone is headed down there?
Did you and Evan go there around 6 PM? The attached plot shows a spike at ~6:20 PM
In general, yes, it would be better to turn off sensor correction along the horizontal directions during the day if there is a chance of someone being close to the ground seismometers.
Yes Sensor correction should not be on if there is any activity in the VEA.
Sensor correction is a form of feed forward so it assumes that the chamber and ground sensor are seeing the same thing. So someone walking by the ground sensor, making a large local disturbance can produce large motions in the platform
[Rich, Seb, Fabrice]
We are now looking into HEPI to see if it caused the drifts that was observed on the ETMX suspension. Sebastien is processing the data.
Please let us know if this is not anymore of interest because the cause of problem is now known (for instance related to John's comment on temmperature, or Nic's comment on suspension control).
A 1ºC temperature excursion was reported in alog 15212.