Today we measured one of RCAV's suspension modes, The mode @ 3.73 Hz, Q = 190+/- 10 is probably tilt.
This can be excited by shaking the curtain frame above the table.
Yesterday we measure several eigenmodes of the suspension on a model. So now
we measure the real suspension used in the experiment.
1) First we tried to use shadow sensing technique on the copper plate for eddy current damping.
I used the lower edge instead of the side edge because I expect to see large signal from vertical mode.
The 3.73 Hz mode can be excited by shaking the metal frame above the table.
Q from this measurement varies alot, depending on where we start counting.
It can be ~ 70 just after the excitation or upto 180 when the motion damped down for awhile.
2) we used reflected beam from the mirror. The reflected beam is then used
for shadow sensing technique, this can be used for determining tilt/yaw and longtitudinal motion.
Note: there might be dust on the mirror, we can see it with HeNe laser.
We use a PZT set for pushing the table for seismic feedback control. With 3.73 Hz
signal, we can drive the tilt motion of the cavity (the reflected beam clearly move up and down)
Result for ringdown measurement. Q is 200. f0 = 3.73 Hz.
We can excite 2 modes, one is 3.73 Hz (tilt), another is 4.5 Hz (yaw). From the motion
of the reflected beam, we can roughly identify what mode we excite. We will try to excite another modes
and see the actual motion of the cavity.
SO if I replace f0 and Q in the noise budget, and remove the horizontal seismic for now( I'm not sure how it couple into frequency change)
The peak seems close to the measurement data