KAGRA Logbook

MIF Calibration Activities in March 22dn-23rd.

*Result
- MI Calibration Factor : 2.3x10^(10) [C/m]
- Actuator efficiency : 1.8x10^(-14) [m/C] @80Hz.
Currently, MI is locked by feeding signal back to ETX and ETY differencially.
This result is factor~6 difference from actuator-efficiency measurement
with a dummy mass at low frequency (<2Hz)(by N.Ohishi).
[Note: Unit 'C' means ADC Count here]

*Some details
- MI Calibration Factor as calculated from MI fringe amplitude.
Without lock of the MI, C_max=4800 and C_min=800.
Contrast = 71.4%
Slope at the mid-fringe:
I= dC/dx = (C_max-C_min)/2 * 4*pi/lambda = 2.3x10^(10) [C/m].

- Open Loop Gain measurement
Inject calibration peak at 80Hz, 10,000 Count after MI filter.
The peak amplitude was estimatied by demodulationg the signal
at the peak frequency (80Hz).
Demod1 (After the injection point) : Real=5,140 , Imag=70.
Demod2 (Before the injection point) : Real= 150 , Imag=70.
G = (Open Loop Gain) = |Demod2/Demod1|= 0.0320 @80Hz
[Demodulator Settng:
4th-order 0.1Hz Butterworth LPF after demod.]
- Actuator efficiency
Filter gain was F=75 @80Hz with DC gain =15.
A = (Actuator efficiency) = G/(I*F)=1.76x10^(-14) @80Hz.
- Consistency check
The calbration peak amplitude is measured from spectrum.
Peak amplitudes at 80Hz were the followings.
MI Error signal : MI_E=6.99189
Before the injection point : P_B=524.593
After the injection point : P_A=16,824
From there numbers,
G = (Open Loop Gain) = |P_B/P_A|= 0.0312 @80Hz
F = (Filter Gain) = |P_B/MI_E|=75.0288
These results are consistent with the results by lock-in demodulation.
- Unity Gain Frequency (UGF)
By Openloop Transfer Function measurement during the lock, UGF
was found to be around 7Hz. From measured openloop gain of
0.032 @ 80Hz and filter transfer function (14Hz zero, 200Hz pole x2),
the UGF is estimated to be 6.73Hz. The agreed well.