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Message ID: 246     Entry time: Fri Oct 23 12:25:13 2020
Author: Marie K. 
Type: Progress 
Category: BS BRDs 
Subject: BS BRDs for LLO are assembled and tuned 

The 5 BS BRDs for installation at LLO are now assembled and tuned in the LLO Optics Lab. The target frequencies are 16.846 Hz and 24.672 Hz (measurements from A. Effler, I will post an alog in the LLO logbook about it).
As per the LHO ones, I tuned them at lower frequencies (-1 or -2%) in order to anticipate the frequency drifts.

For the LHO & LLO bounce modes and for LLO roll modes, I had to put two copper blocks per end at the blade tips in order to reach the target frequency.

There are several factors to take into account:

  1. The measured mass for each type of the copper mass is lighter than what was expected from the design D1500429-v4 (about 0.2 to 0.3 g difference, see table in figure 2)
  2. Somehow I have to use heavier masses than expected from the experiments at CIT (between 0.7 and 1.5 g, see below)
Target Freq [Hz] 16.64 16.85 17.79
Mass [g] 9.89 10.43 10.150
k [N/m] 108.11 116.81 126.82
Expected mass* [g] 9.89 9.65 8.65
Diff 0.00 -0.78 -1.50


Target Freq [Hz] 24.34 24.67 26.06
Mass [g] 7.32 7.876 7.247
k [N/m] 171.20 189.27 194.29
Expected mass* [g] 7.32 7.12 6.39
Diff 0.00 -0.75 -0.86

* Extrapolated from the k CIT experimental k.

I would have expected smaller variations in the spring constant.


The resulting masses are:

  • H1 Bounce: lightest bounce type (type 8, ~8.6g) + spare mass of 0.595g + washer+screw
  • L1 Bounce: heaviest bounce type (type 14, ~8.8g) + spare mass of 0.595g + washer+screw
  • H1 Roll: heaviest roll type (type 7, ~6.4g) + washer+screw
  • L1 Roll: heaviest roll type (type 7, ~6.4g) + spare mass of 0.55g + washer+screw

Even if the mass kit had the expected weight, I would have not been able to build the BRDs with the masses from the kit (or use several washers of 0.1g).

Attachment 1: LLO_BRDs.jpg  681 kB  Uploaded Mon Oct 26 15:03:28 2020  | Hide | Hide all
Attachment 2: Table1__MassKit.png  328 kB  Uploaded Mon Oct 26 15:32:24 2020  | Hide | Hide all
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