We want to compare the model transfer function with the measurements on BS.
We built a model with two BRDs attached to the main mass. Model is provided in the attachment. Details of the computation will be given in a note later. We implemented the model in Matlab.
Back in July (alog 177), we tuned the BRD frequencies right before mounting them on BS. We installed BRD1 and BRD3 (alog 181) with the following properties (alog 177, alog 164, alog 141):
Mode 
BRD1_v4 Bounce 
BRD1_v4 Roll 
BRD3_v4 Bounce 
BRD3_v4 Roll 
Mass (g) 
4.936 
3.063 
4.865 
3.207 
f (Hz) 
16.693 
24.26 
16.66 
24.32 
Q 
100.4 
117.9 
154.4 
137.4 
Accuracy 
0.016% 
0.34% 
0.18% 
0.097% 
Plugging these properties in our new 2 BRDs model, we obtain the transfer function in Figure 1 and Figure 2. For comparison, the 1 BRD model with only BRD1(3) is shown on the same graph. The tables below show a summary of the model outputs.
Bounce 
F1 
F2 
F3 
Q1 
Q2 
Q3 
Model 2 BRDs 
16.530 
16.678 
16.837 
247 
122 
239 
Model BRD1 
16.588 
16.795 

202 
191 

Model BRD3 
16.567 
16.783 

267 
337 

Measurement (#34) 
16.55 
16.64 
16.75 
108 

172 
Roll 
F1 
F2 
F3 
Q1 
Q2 
Q3 
Model 2 BRDs 
24.051 
24.289 
24.583 
231 
128 
274 
Model BRD1 
24.116 
24.485 

193 
283 

Model BRD3 
24.144 
24.518 

257 
275 

Measurement (#34) 
24.034 

24.507 
126 

160 
The 2 BRD model generates three poles but in the present case we observe only "2 peaks", as we have seen in the measurements (see here measurements #33 and #34). We need some larger detuning to see "3 peaks". See for example figure 3 with respectively 0.1Hz detuning for BRD1 and +0.1Hz detuning for BRD3. The Q are ~ a factor 2 lower than expected from the model. 