Linear Scale Based Vibration Measurements
To double check the results of the laser interferometer measurements, I temporarily installed a linear glass scale.
This is a RS422 quadrature encoder with a step size of 1 micrometer. Thus, I could just use a uMD2 board to connect and take data:
I set the board to take data measurements at the same speed as uMD1 board that took the laser measurements (~610 Hz) and captured them from a serial port. Then, I could compare the vibration data. Here a representative sample from the laser measurement (units in y are nanometers):
And here a representative sample from the glass scale measurement (units are in micrometers):
The results are pretty similar. There is a big fundamental vibration in the 20-25 micron range or so. And there are high frequency vibrations in the 5 micron or so range. These seem stronger with the glass scale. So which one is right? I think they both are. To explain this, we have to go back and ask what it is that we are actually measuring. In the case of the glass scale, we measure the vibration of the read head compared to the mount point of the glass scale body (yellow arrows in the following picture. Any movement in the loop of structural members mounting scale and readhead (red arrows) will will me measured too.
This 'structural loop' for the laser interferometer measurement is different. We measured the distance/vibration between the interferometer optics and the corner-cube reflector. any flex in the concrete floor or movement/vibration of the machine as a whole would be included in this measurement. In conclusion, I think the measurements yield roughly the same information. Being able to draw the same conclusion from two different data sets increases my confidence in the findings.