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Message ID: 217     Entry time: Fri Nov 8 13:47:22 2019
Author: Marie K. 
Type: General 
Category: BS BRDs 
Subject: Model with 2 BRDs  

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.

Attachment 1: Model3BRD_bounce_elog217.png  80 kB  Uploaded Fri Nov 8 15:45:13 2019  | Hide | Hide all
Model3BRD_bounce_elog217.png
Attachment 2: Model3BRD_roll_elog217.png  80 kB  Uploaded Fri Nov 8 15:45:22 2019  | Hide | Hide all
Model3BRD_roll_elog217.png
Attachment 3: Model3BRD_roll_detuned_elog217.png  79 kB  Uploaded Fri Nov 8 16:31:23 2019  | Hide | Hide all
Model3BRD_roll_detuned_elog217.png
Attachment 4: BRD_3masses_elog217.m  4 kB  Uploaded Fri Nov 8 16:35:18 2019  | Hide | Hide all
%% Use the results from the Model_with_2parallel_BRDs.nb
% based on the model from Norna T1600259
% MK November 2019

clc; clear all; close all;
 
% BS and damper parameters
% primary oscillator with damping
 
% BS Bounce mode
... 136 more lines ...
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