This page is for writing explanations or plans which we are not confident about and may be subject to change. For instance, a possibe course of action to carry out a particular type of measurement or the plan to follow to identify a noise source.

Noise hunting in SR3 optics' residual motion

Date: 05-08-2019.

Author: Fabian.

Context: entry from 03-08-2019 and klog entries 9702 and 9755.

Following the plan for noise source identification I measured the coherence between the following degrees of freedom:

• TM-L, TM-P ⇔IM-L, IM-P.
• TM-L, TM-P ⇔ IP-L.
• IM-L, IM-P ⇔ IP-L.

The highlights for the ALIGNED state are

• Directory: /kagra/Dropbox/Subsystems/VIS/TypeBData/SR3/Noise/Measurements/20190805/
• File: SR3_ALIGNED_TM_OPLEV_ASD.xml

• The seismic environment seems noisier today. See klog entry 9702 for reference. The integrated RMS values are

  • TM longitudinal: 0.55 µm,

  • TM longitudinal speed: 0.62 µm/s,

  • TM pitch: 0.21 µrad.

  • TM yaw: 0.28 µrad.

  • Additionally there is a large peak in TM-L, TM-P and TM-Y at 0.158 Hz.

• There is high coherence between the following degrees of freedom and frequency bands:

  • TM-L and TM-P, 0.150 Hz and 0.9 Hz. This is the reason I suspect of IM-L to TM-P coupling. See SR3_ALIGNED_coherence_TM.png.

  • IM-L and TM-L and IM-L and TM-P, 0.35 Hz to 0.8 Hz. See SR3_ALIGNED_coherence_TM-IM.png.

  • IM-P and TM-P, 0.15 Hz and 1 Hz. See SR3_ALIGNED_coherence_TM-IM.png.

  • IM-P and TM-L, 0.15 Hz and 0.8 Hz. See SR3_ALIGNED_coherence_TM-IM.png.

  • IP-L and each of IM-P, TM-L and TM-P, 0.135 Hz and 0.4 HZ. See SR3_ALIGNED_coherence_TM-IM-IP.png.

  • IP-L and each of IM-L, IM-P, TM-L and TM-P, above 0.4 Hz has many peaks of coherence up to above 1 Hz. See SR3_ALIGNED_coherence_TM-IM-IP.png.

• The conclusions of these measurements are:
  • The motion of the IP in the longitudinal direction induces motion in the optic in pitch and longitudinal in the frequency band from 0.135 Hz to 0.4 Hz. This should be fixed at the IP.
  • The motion of the IM in the longitudinal direction induces motion in the optic pitch and length in the frequency band from 0.3 Hz to 0.8 Hz. This should be fixed with L2P and L2L control loop.
  • The motion of the IM in pitch introduces motion in the optic in pitch and logitudinal in the frequency band from 0.135 Hz to 1 Hz. The motion in the band from 0.135 to 0.4 Hz is likely ground motion coming from the IP, however, the motion from 0.4 Hz to 1 Hz should be fixed with P2L and P2P filters.
  • Motion in optic yaw still has to be investigated. There are outstanding features between 1 Hz and 2 Hz.

The highlights for the FLOAT state are

Date: 03-08-2019.

Author: Fabian.

Context: klog entries 9702 and 9755.

On Friday afternoon I found high coherence between SR3 L and P degrees of freedom in at least two frequency bands, as measured by the oplev. High coherence is observed in both states ALIGNED and FLOAT.

• Directory: /kagra/Dropbox/Subsystems/VIS/TypeBData/SR3/Noise/Measurements/20190802

• Files: SR3_ALIGNED_TM_OPLEV_ASD.xml, SR3_FLOAT_TM_OPLEV_ASD.xml

Possible reasons for the high coherence are:

• The oplev was not successfully diagonalized in those frequency bands. In this case the high coherence would be a consequence of measurement coupling between L and P and not to a real motion of the optic. In this case we would have to identify which one of the two degrees of freedom exhibits real motion. I have the impression thisis unlikely.

• Lack of balance in optic coil-magnet actuators. Although the actuators have not been balanced yet, it's unlikely this is the main cause becasue the high coherence is also observed in FLOAT state, in which the control system is off.

• Pitch motion happens as a consequence of the longitudinal motion in upper stages. I tend to think this is the most likely cause of high coherence.

• There is an external environmental excitation. This may be acoustic noise or ground motion leaking through the suspension into the optic. It may be something affecting the oplev optics directly, but it's unikely because the effect is seen at low frequencies.

Plan for noise source identification.

• Find out the frequencies in which the oplev was diagonalized. Review the diagonalization procedure.

• Balance the coils. Excite the optic using the saddle configuration and change actuator gains until the motion of the optic dissapears. Use the same frequency as in the oplev diagonalization.

• Look for coherence between the motion of the optic and upper stages:

  • TM-L, TM-P ⇔IM-L, IM-P.
  • TM-L, TM-P ⇔ IP-L.
  • IM-L, IM-P ⇔ IP-L.

• Change and monitor enviromental conditions:

  • Ask Yokozawa-san whether he has an ASD of the acpustic noise inside the clean rooms and compare.

  • Turn off the filter fan units (FFUs) inside the clean room and repeat the measurements.
  • Place a microphone close to the oplev optics and another one close to the IP, measure the amplitude spectral density of the acoustic excitation with the fans off and on and search for coherence with the TM/IM/IP motion.
  • Place an accelerometer on each oplev plaforms and repeat previous item.

Plan for reducing control noise

• Reduce gain of control signal. This is in principle something quick and easy to do.

• Check the values of the unity gain frequencies (UGF) in SR2 optics control loops.
  • In case the control noise is higher than the UGF roll-off the filter.
  • In case the control noise is within the control frequency band check whether reducing the gain worked. If not continue with the noise hunting.