Mitushahshi

I summarized the tabletop experiments of Roberts' Linkage.

What I did
The followings are what I did.

• I measured the resonant frequency of Roberts' Linkage with changing COM.
• I checked the difference of resonant frequencys between x-axis and y-axis(The definitions of these axes are written on below sentence).

Set up
I attached the picture(Fig1).
The Roberts' Linkage was shaked by coil magnet actuator powered by FFT analyzer.
The displacement of it's was detected by photo sensor.
I measured the transfer function from coil magnet actuator voltage to photo sensor read out voltage.

Measurement results
Firstly I mentioned resonat frequency and COM position. I attached the picture(Fig2).
The results says that the resonant frequency get lower and lower when Roberts' Linkage was tuned at hight COM points.
The gap are there between first mesurement and second measuremt. I thought the reasons are some coupling by different tension on wires and by human error with tuning COM(this is most properly).

Secondary, I mentioned the strange peak. I attached the power spectra density plot.
The peak at 71mHz is the resonant, and harmonic of it also are there(rabeled blue).
On the other hands, the strange peak that it is not reasonable by above mention also are there(label green). 
The paper "F. Garoi et al., Passive vibration isolation using a Roberts Linkage, Rev Sci Instrum 74, 3487-3491(2003)" reported that there are cross coupling.
So I measured the x-axis and y-axis resonat frequency. The definition of x-axis is following on the road of 20m interference site(Fig3).
I attached the results. The red one is x-axis, and the blue one is y-axis.
I realized that these axes have a difference resonat frequencys, and conclued that the strange peak is from y-axis' resonant frequency (cross coupling).