J. Kissel, A. Pele, A. Staley

All HSTS and HSTS at both sites have incorrect M2 and M3 coil/magnet COILOUTF gain signs.

While once again banging our heads against the IMC crossover measurements against Alexa's discrepant ALS COMM model, we had one more epiphany regarding the SUS -- RESPECT THE PHASE. Her lowest frequency cross-over, the M1-M2 crossover, which should have a phase of -180 [deg], was measured to have a phase of 0 [deg], but otherwise the magnitude matches exquisitely. Then I remembered her mentioning a month ago that LLO had to account for an unknown minus sign on there lower stages of MC2 to get models to match measurements. Having just recently found an AOSEM vs. BOSEM COILOUTF gain sign bugs on the QUADs during the coil balancing, it occurred to me to check that the M2 and M3 stages of the MC2 matched the PUM stage of a QUAD. They did not, and are indeed incorrect. BIFF!

Worried that it was a systematic flaw copied and pasted everywhere (and because green team need mode cleaner, but not PRM), I checked PRM. Also incorrect. To confirm that that sign orientation *is* in correct, we drove a DC offset on the lower stages of PRM, and indeed, a +L requested drive moves the suspension in -L (as measured by the OSEM sensors). I asked Arnaud to quickly plot the phase of the acceptance measurements that he had for one of these suspensions. ZLONK! +180 [deg] measured phase at DC, where we should expect (from any suspension on the planet) 0 [deg] phase at DC.

Sad about this, I then modified the acceptance measurement scripts to plot the phase of a random smattering all of the M2 and M3 acceptance measurements that have been taken of all HSTS at both sites. All have a phase of +180 [deg] -- see attached. POWIE!

The reason for the flaw in magnet compensating gains? AOSEMs and BOSEMs have coils wound in exactly opposite direction. So a positive current sent to a BOSEM coil will result in magnetic fields opposite in direction than the AOSEM coil. We realized this halfway through defining all of our suspensions' lower stage magnet signs, as is reflected by the poor quality of information in our sign convention documentation, T1200015, specifically the Cheat Sheet, where it shows a table of COILOUTF gains for "Any 4 OSEM Stage" as
    UL   -
    LL   +
    UR   +
    LR   -
which is the current sign distribution on the M2 and M3 stages for HSTSs. However, this table was created / determined assuming a BOSEM winding. You'll notice that the difference between -v2 and -v3 is "adding a AOSEM vs. BOSEM push/pull table," and the (unfinished!) statement that "AOSEM and BOSEM coils are wound exactly opposite in direct[[ion]]." CLANK-EST!

Naturally, because the HLTS EPICs values began as a copy-and-paste of the HSTS, any HLTS has this same flaw of its lower stages, but we haven't used them enough to notice.

Conclusions:
- All HSTS and HSTS at both sites have incorrect M2 and M3 coil/magnet COILOUTF gain signs. That's MC1, MC2, MC3, PRM, PR2, PR3, SR3, SR2, and SRM.

There signs should be:
    UL   +
    LL   -
    UR   -
    LR   +

- I blame myself.

- I clearly need to create a version 4 of T1200015.

- Of course this affects all IFO loops that use the HSTS, so I have not and will not change the signs unless I have whomever designed the ISC loops sitting next to me.