I've made v1 of of a blend for the SR3 Pitch Estimator. It is in the SUS SVN as described below

Design 
This time I designed the OSEM path and set the MODEL path to be 1-OSEM_path.
This design is very simple to implement, hopefully it will work well and we can use this approach for other suspensions.
I designed a basic low-pass for the OSEM so that the DC position is maintained (same as yaw). This is probably not necessary because the damping filters are AC coupled, however it was useful to be able to compare the time traces when we were doing the development here, so I've included it for in the new one also. It may also be useful in case we try to do something more interesting than just AC coupled damping. The low-pass is a zero-pole pair so that it levels off to a transmission of 0.1. This is so the gap between the two resonant peaks will be well behaved. Those will cross at about 180 degrees of phase shift and make a sharp zero, and deep zeros seemed to be causing numerical issues when Oli was implementing the yaw blends. The flat bottom at 0.1 prevents that.

The next thing is to include OSEM signal at the pitch peaks. This is to help deal with errors in the models and with unmodeled inputs. At this point, the models Ivey has done for Oli's data look remarkably clean and tight, so the unmodeled inputs are my main concern. The various TFs of SR3 show that there are really just 2 modes which are mostly Pitch and those are the ones I've targeted. These blends are all based on the fits by Ivey (LHO log: 86430) and the data from Oli (LHO log 86203)

Figure 1: pitch blend filters

Figure 2: zoom in on pitch blend filters, the dashed lines are peaks of the 2 modes we are letting the OSEMs damp directly

Figure 3: OSEM path vs. the M1 plant, showing the overlap of the narrow peaks in the OSEM path with the lightly damped pitch plant modes

Figure 4: MODEL path vs. the M1 plant, showing the overlap of the very narrow notches in the MODEL path with the lightly damped pitch plant modes

Figure 5: Fit for the OSEM pitch Response of M1 to OSEM drive of pitch. The length damping is nominal (gain of 0.5) and the Pitch damping is set to 'light damping', ie 0.2 of nominal (0.1 overall)

figure 5 shows there are 4 modes visible in the M1 pitch -> pitch TFs. The first 2 are pretty well damped by the length damping control, so I'm going to let the estimator get those. I'm hoping that any unmodeled inputs will be damped by the length control, and any plant errors are small because of the length damping.

I've attached a pdf with a few other plots for reference.

Design script: blend_SR3_pitchv1.m 
in the SUS_svn at  .../HLTS/Common/FilterDesign/Estimator/   (revision 12601)

make_SR3_Pitch_blend.m: run this script to install the filters into the foton file for SR3.
This is a modification of the script make_SR3_yaw_blend.m

some other notes:

modes of the L/P plant for SR3, there are 2 combined modes, 2 mostly Pitch, and 2 mostly Length
L damping at nominal (0.5 overall)
P damping at 'light' (0.1 overall)
(Aug 5, 2025, Oli Pantane) 

for the M1 L drive to M1 L OSEM, 4 peaks are visible
black looks like the nominal damped 
peak #
1: 0.617
2: 0.734
3: 1.585
4: 2.977

in the M1 Pitch drive to M1 pitch OSEM, (L damping nominal, P 'light damping')
(to the fit)
1: 0.649  (data peak closer to 0.641)
2: 0.749  (data closer to 0.75)
3: 2.09
3: 3.44

The poles and zeros for the blend filters. Some of the Q's are pretty high, but our conversion to foton seems to be fine.

>> blend_SR3_pitchv1;
 
The OSEM filter is
 poles:
ans =
  -0.3142 + 0.0000i
  -0.4377 +13.1246i
  -0.4377 -13.1246i
  -0.3602 +21.6112i
  -0.3602 -21.6112i
 
 zeros:
ans =
  -2.0779 +20.2143i
  -2.0779 -20.2143i
  -3.6907 + 0.0000i
  -4.0312 +12.2400i
  -4.0312 -12.2400i
 
The MODEL filter is
 poles:
ans =
  -0.3142 + 0.0000i
  -0.4377 +13.1246i
  -0.4377 -13.1246i
  -0.3602 +21.6112i
  -0.3602 -21.6112i
 
 zeros:
ans =
   0.0000 + 0.0000i
  -0.0803 +21.5959i
  -0.0803 -21.5959i
  -0.0970 +13.1319i
  -0.0970 -13.1319i