Virgo Logbook

Today we continued the investigation about the excess noise observable (between 1-10Hz) in Tx and Tz degrees of freedom after the system recovery from the event (loop oscillation for a few hours) of September 27th. This morning the INJ team inspected once again the EIB bench searching for mechanical shorts (rubbing etc); nothing was found. Then we blocked the laser beam and performed further checks on the control signals integrity. Correct operation of the coil drivers was confirmed when looking at the current monitor signals in closed and open loop. Instead a problem was discovered in the three vertical LVDT channels on which a large excess electronic noise is visible above 1Hz. In plot 1, the EIB error signals and the LVDT raw signals are compared in closed-loop before and after the accident of two nights ago. Plot 2 shows how the excess noise persists in the three vertical LVDTs also with all loops open.
The noise seems to be responsible for the contamination of Tx and Tz channels; due to the high blending frequency (s=6) the roll-off of the LVDT blending filter is not sufficient to absorb it. The cause of this malfunctioning is unclear and it will require an intervention. INJ team also attempted to temporarily block the bench to check whether the noise was affected but no effect was observed.
In order to mitigate the problem, additional low pass filtering has been introduced in the standard 5-th order LVDT blending filter; the new filter can be further optimised but some reduction of the excess noise has been achieved (see Plot 3). The laser beam was unblocked around 17:30 UTC.