We tried to close a global alignment control loop for PR3 PIT, using an ADS output as a sensor, and succeeded.

= Dither =
Dithering line is sent to PR3 TM PIT, with amplitude of 100 counts at 4.0 Hz.
Infrared transmission of X arm (AOS-TMSX_IR_PD_OUT) is then demodulated at 4.0 Hz, and used for an error signal of PR3 PIT.

= How it works =
See the following figure.
I have not measured the OLTF, but we should be able to get the UGF and phase mergin from this step response. Target UGF is 0.03 Hz.