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Entry  Wed Aug 4 01:44:56 2010, Koji, Configuration, PEM, Accelerometer located on and below the PSL table IMG_2675.jpgIMG_2678.jpgIMG_2682.jpg
    Reply  Tue Aug 10 22:40:55 2010, Koji, Summary, PEM, Accelerometer located on and below the PSL table PEM_100809.pdf
Message ID: 3395     Entry time: Tue Aug 10 22:40:55 2010     In reply to: 3355
Author: Koji 
Type: Summary 
Category: PEM 
Subject: Accelerometer located on and below the PSL table 

Result of the accelerometer measurement

Introduction

We wanted to characterize the PSL table before the work before its lifting up.
We put a set of three-axis Wilcoxon accelerometers on the ground and another set on the PSL table through the weekend.

Result

- The data at 9th Aug 00:00(UTC) is used. This was Sunday 5PM in the local time.
- The freq resolution was 0.01Hz. The # of avg was 50.

- The accelerometer signals were calibrated by the value 1.2e-7 V/(m/s^2). We use this absolute value of the spectrum for the comparison purpose.

- The accelerometers were aligned to North(X), East(Y), and Up(Z). There was the coherence observed from 2~20Hz.
  The transfer functions are valid only this frequency region although we still can set the lower bound of them.

- The transfer functions in the horizontal directions show huge peaks at around 20Hz. The Q of the peaks are ~30 to ~100.
  The vertical transfer function shows somewhat lower peak at around 50Hz with Q of ~10.

Some thoughts

- The low resonant freq and the high Q of the horizontal mode comes from the heaviness of the table.

- We are going to raise the table. This will usually mean that we get the lower resonant freq. This is not nice.

- So, the decision to use 6 tripods rather than 4 was right.
- The steel tripods are expected to give both more rigidity and more damping than the chep-looking hollow Newport legs.
- Concrete grouting of the tripods will also lower the effective height and will benefit for us.

 

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