40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
  40m Log  Not logged in ELOG logo
Entry  Fri Aug 1 12:49:06 2014, Koji, Summary, IOO, MC servo analysis MC_OLTF.pdfMC_Error_Common.pdfMC_Crossover.pdfMC_CLTF_Fast.pdf
    Reply  Thu Aug 7 11:57:59 2014, Koji, Summary, IOO, MC servo analysis MC_OLTF_Fit.pdfIMC_OLTF.zipMC_OLTF_estimated.pdf
       Reply  Fri Aug 8 12:39:19 2014, ericq, Summary, IOO, MC servo analysis mcLoopAug8.zipMCloopAug8.pdf
          Reply  Fri Aug 8 15:57:29 2014, ericq, Summary, IOO, MC servo analysis OLTFs.pdfCLTFs.pdfmcLoopAug8.zip
             Reply  Fri Aug 8 18:08:12 2014, Koji, Summary, IOO, MC servo analysis 
             Reply  Sat Aug 9 14:35:28 2014, Koji, Summary, IOO, MC servo analysis MC_OLTF_Fit.pdfliso.zipMC_CLTF_Fit.pdfMC_CLTF_new.pdfMC_OLTF_new.pdf
Message ID: 10322     Entry time: Fri Aug 1 12:49:06 2014     Reply to this: 10343
Author: Koji 
Type: Summary 
Category: IOO 
Subject: MC servo analysis 

Reasoning to choose the current parameters:

FSS Common: 18dB
FSS Fast: 20dB

Attachment 1:
Openloop transfer function of the IMC loop with the nominal gain setting. The UGF is 176kHz and the phase margin is 48 deg.
This is about 3 time more bandwidth than the previous setting. (Good)

It is visible that the TF has sharp roll off around 1MHz. I wonder if this comes from the demodboard LPF and/or the PMC cav pole.
In fact, according to Manasa, the PMC has the ringdown of 164.6ns which corresponds to the cavity pole of 967kHz. So this must
be there in the OLTF.

From the plot, the order of the low pass is about 5. Subtracting the slope by the cavity pole, the order is four. If I look at the TF of the minicircuits
LPFs (this entry), the phase delay of the filter at 1/10 of the cut off freq is ~30deg. And the order of the filters are maybe 6th elliptic?
So it's not yet clear if the LPF is causing a significant phase delay at 180kHz.

More significantly, the gain margin at ~1MHz is way too small. This is causing a big servo bump at that frequency as seen in Attachment 2.

In total, my recommendation is to move the LPF freq up by x2 or x3, and give a mild LPF above 500kHz.
This requires some modeling as well as try and error.

Attachment 2:

This figure is to explain how the common FSS gain was set. By increasing the gain, the UGF is increased and we can enjoy more supression (from red to purple).
The more gain, however, the more servo bump we observe above the UGF. The gain was chosen so that the total PC feedback does not exceed 3V.

Attachment 3/4:

This figure explains how the fast FSS gain (namely crossover frequency between fast and PC) was set. When the fast is low (red) the phase margin between two loops
are plenty and therefore the openloop TF is smooth. But the PC's frequency domain is large and has to work more (in rms). As the fast gain is increased, the actuation
by the PC is offloaded to the fast PZT (that's good). But eventually the phase margin is not enough and the dip start to show up (purple). This dip cause worse closed loop TF,
as seen in Attachment 4, or even an instability of the loop eventually. So the fast gain was set somewhere in between (green).

Attachment 1: MC_OLTF.pdf  55 kB  | Hide | Hide all
MC_OLTF.pdf
Attachment 2: MC_Error_Common.pdf  65 kB  | Hide | Hide all
MC_Error_Common.pdf
Attachment 3: MC_Crossover.pdf  84 kB  | Hide | Hide all
MC_Crossover.pdf
Attachment 4: MC_CLTF_Fast.pdf  44 kB  | Hide | Hide all
MC_CLTF_Fast.pdf
ELOG V3.1.3-