Nevertheless, we have some extra noise in the band 1-10 Hz. I checked all raw signals (voice coils, geophones, LVDTs) and they seem all correct. I switched off the MotorSAS process in the vpm, since the driver coils can make electronic noise, but that did not change anything. In the evening when Romain gave the green light, I switched off controls, switched on viscous damping, and switched on the full pid. With the full pid the ground motion is damped well below 1 Hz but there is an extra shoulder between 1-10 Hz. I think it arises in Tx and Tz and is coupled to Ty,x,z via the tilt stabilization but this is just conjecture. Suzanne did not find anything rubbing against the bench, but maybe there is some small detail. I did not see much difference in acoustic noise (such an effect on the bench, bypassing the floor, could maybe cause such a shoulder) and I checked that the actuators and pid have the same transfer functions as last week (plotting the transfer function from actuator current to bench motion) so it is difficult to find the cause of the extra noise now; it seems all sensors and actuators are performing as well as last week,
The only "large" difference is that there is now some 10 times more noise at the secondary microseismic peak (0.3 Hz); this peak is damped well. It should not spoil the pid at 3Hz, but we may hope for improvement on a quieter day.
Included are the files of the d.o.f. and time signals of the actuators with actuation off/only viscous (first attachment), actuation off/pid (second attachment), a comparison to last week (3rd file), and the raw signals/seismic motion for today and last week (4th and 5th file). The excess noise shoulder is obvious, but the damping is good at 0-1 Hz and similar at +10 Hz.
At 14:00 LT, we went into the LL to perform a more detailed inspection of the EIB bench. We first put the beam in scan mode and blocked it at the LB output.
We then opened the bench controls and slightly moved the plexiglass tube leading to the SIB1 tower, as it appeared to be in slight contact with the bench. However, a comparison of the FFT signals before and after the intervention showed no significant change, suggesting that the contact was not the cause of the excess noise. No other obvious issues that could explain the increased noise were identified during the inspection.
We then unblocked the beam, re-locked the INJ system, and left the LL at approximately 14:40 LT.
Afterwards, we continued analysing the data. Paolo identified some LVDT signals, particularly the vertical 0 and 2 channels, that are significantly noisier than they were previously (see Fig. 1). A similar issue appears to have occurred in the past (see #57229), where the solution was to implement a low-pass filter to attenuate the excess noise.
It would therefore be useful to investigate whether this filter is still correctly implemented and functioning, or whether another issue is currently responsible for the additional LVDT noise.
