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Entry  Thu Oct 9 16:31:53 2014, ericq, Update, LSC, CARM W/N TFs carm2SQRTinv.pdfcarm2REFLDC.pdf
    Reply  Thu Oct 9 18:30:59 2014, Jenne, Update, LSC, CARM W/N TFs 6x
       Reply  Fri Oct 10 00:20:37 2014, rana, Update, LSC, CARM W/N TFs carm40.pngcarm.pdf
          Reply  Wed Oct 15 02:59:04 2014, rana, Update, LSC, CARM W/N TFs spring.pdfantispring.pdf
             Reply  Thu Oct 16 22:35:05 2014, rana, Update, LSC, CARM W/N TFs carm_spring.pdfcarm_antispring.pdf
       Reply  Wed Oct 15 13:38:33 2014, Jenne, Update, LSC, CARM W/N TFs 7x
          Reply  Mon Oct 20 17:50:30 2014, Jenne, Update, LSC, CARM W/N TFs (Others were all wrong!) 7x
Message ID: 10593     Entry time: Fri Oct 10 00:20:37 2014     In reply to: 10591     Reply to this: 10608
Author: rana 
Type: Update 
Category: LSC 
Subject: CARM W/N TFs 

 

 Assuming that these Watts/Newtons TFs are correct, I've modeled the resulting open loop gain for CARM. The goal is to design a loop that is stable under a wide range of offsets and also has enough low frequency gain.

The attached PDF shows this. I used a CARM OLG Simulink model:

carm40.png

I've replaced the 'armTF' block with a digital gain of zero. After measuring the open loop gain of all but this piece, I multiply that 'OLG' with the W/N that Jenne extracted from Optickle for CARM->TR (not sqrtInv)

I plot the resulting estimate of the actual OLG in the following plot. Since the CARM-RSE peak is moving down, we use the LP filter that Den installed for us several months ago. To account for the radiation pressure spring, we use some low frequency boosts but not the crazy FM4 filter.

As you can see, the loop is stable from 500 to 200 pm, but then goes unstable around 110 pm. I expect that we will want to do some fancy shaping there or switch from TRX+TRY into something else.

This assumes we have filters 0, 1, 3, 5, and 7 on in the CARM filter bank - still need to add the digital AA/AI to make the loop phase lag a little more accruate, but I think this is looking promising.

 

Attachment 2: carm.pdf  11 kB  | Show | Show all
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