Something is not healthy on SR2 suspension.
Recently we noticed that some DAC on SR2 suspension saturated intermittently at a rate of roughlu once per 10 minutes or so. At the begginig we thought this was due to some ISC feedback saturating some DACs when we lock the interferometer. However, it turned out that it saturates even when no ISC feedback is sent.
The attached is a one-hour trend of all the saturation motniors of the SR2 top stage actuators from the period when SR2 was aligned and damped without any ISC feedback signals yesterday. As seen in the plot, the RT and LF DACs saturated mutiple times. The LF saturated more frequently than the RF actuator. Looking at the suspension screen, Betsy and I found that the longitudinal dampig showed higher signal level at its output than every one else. It is on the order of 100 counts at the output. Betsy is currently checking the longitudinal damping loop by running a transfer function measurement.
I ran a damped L (longitudinal) loop TF which looks "healthy" when compared to previous TFs - so nothing agregious there. However, we note that we do not have a lot of damping in any of the H1 HSTS L loops as observed from looking at the other HSTS L TFs. Also the L loop output seems to be doing more work (higher numbers rolling through) than other L loops. We started looking at filter diffs in the L loops and see that in some HSTSes we have the FM10 Ellip50 engaged. We engaged this filter and see that the L loop output became much quieter (closer to zero). Kiwamu wants to see if this will improve the saturations of SR2 and imprve locking. Attached is a quick damped TF of SR2 M1 DAMP L with the FM10 engaged, as well as a screen snapshot.
For some reason, the elliptic filter that we installed at FM10 of the longitudinal damping loop was taken out on 2015-June-2 18:00. SR2 seems to be saturating again. Sad.