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Entry  Wed Oct 5 22:59:11 2011, Koji, Frank, Notes, Seismic, mechanical properties of new stack  
    Reply  Thu Oct 6 03:37:44 2011, Koji, Notes, Seismic, Modeling of the vertical isolation of the stack vertical_isolation.pdfvertical_isolation.zipvertical_isolation_plot.pdf
       Reply  Tue Oct 25 22:03:32 2011, frank, tara, Notes, Seismic, Modeling of the vertical isolation of the stack nb.png
          Reply  Wed Oct 26 18:05:49 2011, frank, tara, Notes, Seismic, Modeling of the vertical isolation of the stack 6x
             Reply  Thu Oct 27 20:45:58 2011, frank, tara, Notes, Seismic, ring down measurement for rubber spring fit4.pngfit1.pngcomparefit.pnghist.png
                Reply  Thu Nov 3 01:55:03 2011, Frank, Notes, Seismic, ring down measurement for rubber spring 
             Reply  Fri Oct 28 14:04:22 2011, frank, tara, Notes, NoiseBudget, Noise budget for unfloated table beat_27.pngbeat_27.fig
                Reply  Wed Nov 16 19:11:38 2011, tara, Notes, NoiseBudget, seimic coupling to beat noise 
                   Reply  Thu Nov 17 23:30:19 2011, tara, Notes, NoiseBudget, vertical seismic coupling to beat noise SETUP_2011_11_17.pngfreq_resp.pngfreq_resp.fig
                      Reply  Wed Nov 30 19:00:42 2011, tara, Notes, NoiseBudget, vertical seismic coupling to beat noise Seismic_resp.pngSeismic_resp.figSNR.png
Message ID: 717     Entry time: Thu Oct 27 20:45:58 2011     In reply to: 716     Reply to this: 721
Author: frank, tara 
Type: Notes 
Category: Seismic 
Subject: ring down measurement for rubber spring 

We did ring down measurement of the rubber cone used for supporting the seismic isolation stacks. The resonant frequency and Q of the spring will be used for better TF of the stack later. The measured values are f =46.7 Hz, Q = 14.67. (mass will be added later)

[details about the measurement will be added later]

These are the results from the data Frank gave me.

There is a problem in the result. The frequency of the ring down response can vary from 42 Hz to 47 Hz. So when I fit it I cannot really get the "best" frequency fit from the whole train of data.

From figure (1)

  • At point A) The data and fit seem to be well matched.
  • At B) The phase of the fit curve advances the measurement ( the fit frequency is too large)
  • At C) The phase of the fit curve lags the measurement (The fit frequency is too small)
  • At D) The frequency totally changes causing the fit and the measurement to be almost out of phase

 fit4.png

fig1: showing fit with the data. Non-linearity behavior of the spring can be seen clearly.

 

There are 5 sets of measurement. I notice that the nonlinear behavior was very small in the first data set which has its initial amplitude smaller than 0.4 V

fit1.png

fig2 Fit from data TEK00000. The initial amplitude is less than 0.4 V.

 

For other data sets, where their initial amplitude exceed 0.4, it does not matter where I analyze the data, fitted frequency still varies a lot. I take the data set #4, and  fit the data at high amplitude and low amplitude (when it is less than .15V). The frequency still varies. See fig3 below.

comparefit.png

fig3: fit from data TEK00003. Even when I use data at smaller amplitude, frequency still varies from the fit, similar to what happens in fig(1).

 

The problem is, the fitted frequency from fig(2) is ~ 46 Hz, while the fitted frequency (at low amplitude) from fig(3) is ~ 43 Hz. The difference is too large. So we decided to sample the frequency by averaging the frequency over 3 adjacent cycles of the ring down data, ignoring the first few peaks. Then histogram the samples.

hist.png

Most of the counts are around 46.5 Hz, which is contributed mostly from the first data set where the variance is very small. I think it is ok to pick the value from that data set

f =46.7Hz, Q = 14.67.

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