To calibrate the measured TFs and characterize the thermal actuator for the FOL loop, we [ Me, Eric Q, Koji ] made the TF measurements of PZT response by giving a disturbance to the position of each of X and Y arm ETM and ITM.
In order to make reasonable conclusions, the measurements were done at frequencies greater than 20 Hz (assuming the PZT response to be flat till a few KHz), which is out of the bandwidth of the control loops operating for other noises at low frequencies, so that we can get the response only( mainly) due to the disturbance of the masses.
For this measurement , a Sine sweep excitation was given as an input to one of the test mass and PZT actuation signal was monitored. The channels used for the measurement are:
Input( Mirror displacement):
Output ( PZT Response):
The units of the TF of these measurements are not calibrated and are in count/count. For this I will use the ITMX and ITMY calibration values from Izumi's Elog. I will also make some calculations and post in the calibrations of ETMX and ETMY in a separate elog.
I am now estimating the calibrated Thermal Actuator TF and will estimate the location of poles and zeroes to build the PID loop. I will elog the final calibrated TFs in my next elog.
The attached are the Bode Plots for ETM and ITM for X and Y arms.