Today I think that we had 3 locklosses due to a DARM crossover instability, although I am not entirely sure that is what happened.  

Daniel had suggested leaving the PUM out of the DARM loop would be a way to simplify things.  The last lock tonight I tried to transition DARM to the EY UIM +EX ESD with a crossover around 7 Hz, which worked.  I think that this would have been acceptable for range on the ITM ESD if I had increased the crossover to about 10Hz and added a boost to the UIM.  The crossover should be unconditionally stable and the partial measurement I got matched the model well.  

This crossover required turning off the bounce mode notch on the UIM which I thought might be OK now that we have bounce roll dampers, I had tried turning off the notches friday while we were locked with the usual DARM actuator which was OK.  Tonight after about 15 minutes with the higher UIM cross over the bounce mode rung up, I transitioned back to EX UIM with the lower crossover.  When I then tried the ESD transition as it is in the guardian I lost lock.  

It would be helpful the next time people lock with DARM on the low noise ESD ETMX to measure the PUM crossover to lower frequencies than I have. There is a template with references /sheila.dwyer/LSC/DARM/PUM_crossover.xml     It would be nice to get this measurement down to 3-4 Hz to see the instability frequencies.