Ross, Tega, Evan

Last night we were using the ESD drivers on ETMX to control a mechanical mode at 15217Hz. We used a similar method as mentioned in our previous aLOG 27153 to track the mode but now this signal can now be used to drive the ESD to damp these modes. The ESD quadrants that are used for the damping of this mode are the lower left and upper right and the damping signal is applied positively to the lower left and negatively to the upper right. 

For this particular case the mode that we were trying to damp was within ~1Hz of another line around 15218Hz which may be a mechanical mode relating to another test mass. This makes it difficult for the line tracker to stay locked onto the mode if it’s amplitude is lower than the neighbouring line.

The time series plot shows the amplitude output from iwave line tracking. Before the green line the tracker settles on the pre existing line at 15217Hz. A gain of 1000 is then applied to the damping signal which exites the mechanical mode. After around two minutes a gain of -1000 is then applied to the the damping signal which damps the mechanical mode. However after the mode has been significantly damped the line tracker then changes the frequency that it is monitoring to the neighbouring line so it is no longer damping the same line. The spectrogram of OMC data underneath the time series shows how the line amplitude grows and then declines as the gain is altered. 

We are hoping to repeat this test to identify more of these modes. This seems to be a good indication that this process should work if it was this mode being excited by a parametric instability.