I have been playing with the IFO tonight. Mostly, I wanted to make sure that all of the scripts for the carm_cm_up sequence were working, and they seem to all be fine.
I also turned on all 4 UGF servos. My big ah-ha moment for the night is that the excitation is multiplied by the gain multiplier. This means that if the UGF servo is multiplying by a small number (less than 1), the excitation will get smaller, and could get small enough that it is lost in the noise. Now the error signal for the UGF servo is very noisy, and can dip to zero. Since we can't take the log of zero, there are limiters in the model, but they end up giving -80dB to the error point of the UGF servo. This makes it all freak out, and often lose lock, although sometimes you just get a weird step in the UGF servo output.
Anyhow, we need to be mindful of this, and offload the UGF servos regularly. I think the better thing to do though will be to divide the excitation amplitude by the gain multiplier. This will undo the fact that it is multiplied by that number, so that the number of counts that we put into the excitation amplitude box is what we expect.
After some brainstorming with Jenne and Q, both the model and the medm screen have been modified: the entire block "Test1 - injection of the excitation - Test2" has been moved after the servo output. In this way we avoid completely the multiplication problem without having to perform divisions that could lead to division-by-zero problems. Because of how the logic is done now, one UnitDelay block had to be inserted before each one of Test1 and Test2.
Since the UGF Servo has been heavily modified lately, I'll post the current status of the model (as an attachment, as inpage images lose too much quality).