We have been investigating the saturations of the BSC-ISI occurring during the locking process of MICH. Kiwamu has set MICH in a state that triggers and drives the BS but don't lock, so I can investigate the effect of the triggers and transients. 

Some observations and preliminary results:

- The beam Splitter M2 longitudinal drive saturates the ISI Stage 2 actuators as soon as the locking filter triggers and the OSEM start driving the SUS mass. An example is shown in first page. The yellow line in the top left corner shows the OSEM drive. The purple line in the top right corner shows the ISI DAC saturation. 

- It also saturated the GS13s, though the actuators always trigger the watch dogs trip. I turned the GS13 analog electronics in low gain mode. It reduces the GS13 signal to a few thousands counts (Red line, bottom right figure in the first page)

- This is happening even with Level 1 controller that has less loop gain at high frequency than level 2 or 3. An example is shown in first page of the document attached: Orange lines is for the compensator, Dark blue lines is for the Open loop, Dash lines show the boost (low frequency integrator). Turning off the boost doesn't change anything (it's really a high frequency problem, but I wanted to double check)

- After disabling the locking filter, I took transfer functions from OSEM actuator to ISI actuator output signal (SUS DAQ to ISI DAQ). It is shown by the black line in page 3. The ratio is near or above unity at many frequencies, meaning 1 count on the SUS DAQ output produces roughly 1 count at the ISI DAQ output. The SUS drive spectra is pretty white. The SUS DAQ is 18 bits, the ISI is 16 bits. Those three factors combined lead to the saturation of the ISI DAC.

- I added significant filtering in all six dofs of Stage 2 ISI loops to reduce the ISI controls reaction. The new SUS DAC to ISI DAC transfer function is shown in page 3.

- With this controller, the ISI remains on much longer while the BS OSEMs attempt to lock, but it eventually saturates (see p. 4 just before it trips). In the five tests I did, the ISI stayed on 40 seconds minimum, and about 2.5 mn max (versus tripping on the first rising edge with the standard level 1 controller). We'll do more analysis and full lock tests tomorrow.