[Radhika, Murtaza, Paco]
Today we decided to take a closer look at the demod phases of the T and L error signals for XARM ASS. By eye we tuned the phases to minimize the signal in Q. Here are the new demod phases:
(THE DEMODULATION PHASE VALUE DO NOT RESTORE BACK TO THE ORIGINAL VALUES WHEN DITHER IS TURNED ON.)
C1:ASS-XARM_ETM_PIT_L_DEMOD_PHASE: 15 -> 35
C1:ASS-XARM_ETM_YAW_L_DEMOD_PHASE: 176 -> 180
C1:ASS-XARM_ITM_PIT_L_DEMOD_PHASE: 0 -> -5
C1:ASS-XARM_ITM_YAW_L_DEMOD_PHASE: 10 -> -10
C1:ASS-XARM_ETM_PIT_T_DEMOD_PHASE: 10 -> -3.5
C1:ASS-XARM_ETM_YAW_T_DEMOD_PHASE: -10 -> -5
C1:ASS-XARM_ITM_PIT_T_DEMOD_PHASE: 0 -> -15
C1:ASS-XARM_ITM_YAW_T_DEMOD_PHASE: -5 -> 30
We also noticed that MEDM indicator for dithering on (white --> green LO symbol) for ETM_YAW_L_OSC was tied to the wrong excitation gain channel (C1:ASS-XARM_ITM_YAW_OSC_CLKGAIN instead of C1:ASS-XARM_ETM_YAW_OSC_CLKGAIN). We went ahead and changed this in [insert medm file location]. So now the right green LO symbol appears when the appropriate excitation is turned on.
I manually aligned XARM and calculated the output matrix using scripts/ASS/getASSOutMat.py. This script gives an offset to the ETM/ITM/BS in pitch and yaw and records all error signals to construct a sensing matrix (8 err signals x 6 actuators). Then the ITMX PIT/YAW L error signal rows are removed (as per XASS historical practice) and the resulting square matrix is inverted.
A few times I loaded the output matrix and turned up the ASS servo gain, but this caused error signals to diverge and transmission to drop. I realigned XARM in between each time and recalcuated the output matrix until one looked promising [Attachment 1].
- From a decently aligned state, the servo maintained transmission and controlled error signals began to converge to 0.
- However when I gave an offset to some optic, transmission started to recover but then dropped even as all controlled error signals were minimized [Attachment 2]. A zoomed version of this is in Attachment 3.
- ITMX PIT L (light blue) / ITMX YAW L (light pink) are uncontrolled
- ETMX PIT L (dark blue) has a ~0.2 Hz oscillation, but its mean error is roughly 0. This behavior has been present since XASS stopped working, but it is yet to be understood.
The output matrix is successfully reducing the error signals it controls, but transmission is not maximized. I wonder if it's stuck in a local minimum, but I haven't convinced myself that these error signals can be 0 when the beam spots aren't centeredand cavity/beam axes aren't matched. Maybe we need to use the ITMX PIT/YAW L error signals.
1. Assume pitch and yaw are decoupled and calculate a checkerboard output matrix, and debug each angular DOF at a time. I briefly did this today and got the pitch loop to increase transmission, but not the yaw loop.
2. Altering the sensing matrix to include ITMX PIT/YAW L error signals. Then I'll use the same script to compute the output matrix.
The winning approach was considering T and L loops separately, and adjusting the gain hierarchy. After chatting with Rana, we reconfirmed that the centering length (L) loop should feed back to the cavity optics, and the pointing transmission (T) loop should feed back to the BS. We discussed dithering the BS to generate a pointing error signal, but it turned out this wasn't necessary since a solution was found with just dithering the ITM and ETM. I decided to make the T loop fast and the L loop slow, as was done previously by Anchal.
Attachment 1 shows the final servo gains and output matrix, along with the Striptool showing maximized transmission and suppression of error signals.
The intuition was to use the ITM T signals (transmission demodulated at ITM dither freq) as a proxy for the BS pointing error, as was done previously:
ITM PIT/YAW T --> BS PIT/YAW
Next, ETM T signals were fed back to the ETM to maximize transmission. This has always worked:
ETM PIT/YAW T --> ETM PIT/YAW
The signs were chosen based on what suppressed the error signals.
On paper I worked through how ITM/ETM misalignment shifts the beam spot on both the ITM and ETM. This was mainly a helpful exercise to gain intuition for the centering. I made small angle assumptions and ended up with:
Here r is the radius of curvature of the ETM (57.37cm); l is the length of the arm cavity (40m); d is the displacement from center of the optic; θ is the angular misalignment of each optic.
In practice, we do not care about the centering of the beam on the ITM. So in reality the useful takeaway was that ETM and ITM angular misalignment both shift the beamspot on the ETM by roughly the same amount.
ETM PIT/YAW L --> ETM PIT/YAW
ETM PIT/YAW L --> ITM PIT/YAW
Again the signs were chosen to suppress the ETM L error signals without the T loop on.
Lastly, I chose the T loop gain 0.1 and L loop gain 0.2.
The servo gains and output matrix have been updated in ASS_Settings_XARM.snap. Just hit "Run ASS".
dfmn = ----- * (m+n) * acos(sqrt(g1*g2))
I pushed the "closed loop" button on PZT2 YAW around 3:40 pm today, then roughly recentered it using the DC Offset knob on the PiezoJena controller and the IP ANG QPD readbacks. There was a large DC shift. We'll watch and see how much it drifts in this state.
Here's the trend.
The transient at ~22:40 is Rob switching to 'Open Loop' on the Piezo Jena PZTs. I don't see any qualitative change in the drift after this event.
At 05:55 UTC, I removed an iris that was blocking the IP POS beam (the sum goes up from 2 to 6.5) without disturbing the mirrors who's oplev beam are on that table. Steve has conceded one sugar Napoleon after betting against my ninja-like iris skills.
We should recenter the beam on IP POS now that its unclipped - I'll let it sit this way overnight just to get more drift data.
I wanted to see how long our IP POS beam has been badly clipped - turns out its since April 1, 2007.
Steve's April Fool's joke is chronicled then. The attached trend shows that the drop in IP POS is coincident with that event.
In trying to align IPPOS, I noticed that someone has placed a ND2.0 filter (factor of 100 attenuation) in front of it. This is kind of a waste - I have removed IPPOS to fix its resistors and avoid this bad optic. Also the beam coming onto the table is too big for the 1" diameter optics being used; we need to replace it with a 2" diamter optic (Y1-2037-45P).
IP ANG dropped by a factor of 2 back in early August of '08.
We need this guy on the investigation:
Tonight I aligned the IFO by running the scripts one by one.
SRC was far off and I had to align SRM by hand before the script could work. SPOB is still low when DRM is aligned.
I'm restoring the full IFO now that I'm taking off.
I have resurrected the MC WFS on Friday night.
I have uncommented the WFS part of the MC autolocker.
The WFS total gain was empirically set to 0.1 such that the loops have no instability.
The loops somewhat worked through the weekend although they seemed to have the drift of the operating points
in accordance with the WFS spot.
According to c1scy.mdl, OL signals should be connected to adc_0_24 to adc_0_27 but they were connected to adc_0_16 to adc_0_19 which are assigned to QPD signals.
Actually cable connections were messed up. One ribbon cable was connected from QPD driver and ADC ports assigned for OL, and another ribbon cable was connected from the board combining the signals of oplev and QPD to ADC port assigned for QPD.
Now ETMY oplev is working well and aligned to center.
I found that the ADC channels for IP_ANG had been assigned to a wrong machine.
IP_ANG is supposed to be acquired at c1auxey (east end), but actually it had been at c1auxex (south end).
This is the reason why we couldn't see any signals from IP_ANG.
So I fixed it by editing the db files (i.e. ETMXaux.db and ETMYaux.db). Now it seems working fine.
This mistake obviously came from the X-Y name swapping business. Something else might be still wrong.
It turns out there are no reasonable signal from the segment 1 on the IP_ANG QPD.
For right now I can still use it as a funny QPD, but I absolutely need somebody to check and fix it in a daytime.
Rana and I found that the QPD for the optical lever at X end are showing small signals.
At this moment each of the segments exhibits approximately 200 counts when the oplev beam is centered.
These small numbers may be due to the coating of ETMX, but we are not sure.
Probably we have to increase the gain of the QPD depending on situations.
So a set of the tomorrow's daytime task is:
1. check the trend data of the QPD outputs to see how much signals were there in the past.
2. check the whitening filters to make sure if it's on or off.
3. If it's necessary, increase the gain of the QPD to have reasonable readouts.
I am going to ask somebody to do this task.
I tried aligning the IR beam axis for the X arm to have good beam centering on ITMX and ETMX.
As a first attempt, I started translating the beam upward by steering PZT1 and PZT2, since the pitch was quite off from the center on ITMX.
As a result I could decrease the pitch off-centering down to about 0.5 mm on ITMY, but on the other hand TRX decreased a lot (by a factor of 4).
I am worrying if something in the central part of IFO might be clipping the beam.
When I was touching PZT1 and PZT2, I payed attention on IP_ANG so that I don't lose a beam spot on IP_ANG.
As long as the beam is on the IP_ANG QPD, the angle of the beam should not be so much different.
Each time after I touched the PZTs, I realigned ITMX and ETMX to maximize the transmitted light.
In this way I proceeded the alignment by changing the PZT offsets little by little while keeping the X arm locked always.
At the beginning, all the PZT offsets were zero. And at the end of this work they became:
C1:LSC-PZT1_Y = 1.880
C1:LSC-PZT2_Y = -1.699
But during this alignment work TRX gradually decreased eventually down to 0.25, which had been 1 at the beginning (TRX is calibrated by dividing it by its maximum power).
Along with this TRX reduction, I found that the optical gain also decreased by a factor of about 5.
This fact has been confirmed by intentionally increasing the filter gain such that the servo oscillates at the UGF.
The amounts of the X arm's beam off-centering have been measured by the A2L technique.
PIT = -1.61 mm
YAW = -0.918 mm
PIT = -3.76 mm
YAW = -2.24 mm
[Valery, kiwamu, Jenne, Suresh]
I first interchanged the two QPD's on the Y end table to see if the problem QPD related. Exchanging the units did not make any difference. The problem therefore had to be in the cables or the circuit boards in 1X4
We traced the signals pertaining to the IP_ANG QPD ( "Initial Pointing Beam") using Jay's wiring diagram (pages 2 and 5 of 7). We noted that while the signals were available on all Segments till the Monitors (Lemo) on 1X4-2-2A card, two of the lines did not reach the output of the cross connect 1X4-B8. We checked card to make sure that the signals were indeed reaching the back plane of the 1X4-2 chassis using a D990612 extension board. The card was found to be okay. We therefore suspected that the cable (CAB_1X4_?) going from the card to the cross connect 1X4-B8 was faulty. Indeed visual inspection showed that the crimping of the connector was poor and weight of the cable had put further strain on the crimping.
I changed the 64-pin connector on the 1X3-2-2A side of the cable.
When I connected everything back together the problems persisted. Namely the lines P1-1A (Segment 1 high) and P1-2C (Segment 2 Low) were floating They were not reaching points 2T and 3T respectively on the output of the cross connect.
I therefore replaced 1X4-B8 with a similar unit which I found in one of the shelves along the East (Y) arm.
I then checked with the StripTool to make sure that all the quadrants are showing similar response to a flashlight on the QPD. All Segments are working fine now. Currently the IR Initial Pointing beam reaches the QPD but is not centered on it.
I did not attempt to center it since the beam appeared to be clipped and may anyway require repositioning.
JD: We need to meditate on where this beam could be getting clipped. Suresh and I checked that it's not on the viewport on the beam's way out of the ETMY chamber by seeing that the beam is far away from the edges of the viewport, and also far away from the edges of the black beamtube between the viewport and the table. Suresh mentioned that the clipping nature of the IP_ANG beam sometimes goes away. I don't know if this is the same clipping that Kiwamu might be seeing with the main beam, or if this is separate clipping just with the IP beam, after it's been picked off. I suspect it's the same as what Kiwamu is seeing....maybe when we move PZT1, we clip on one of the MMT mirrors or PZT2?? If this is true, it's a total pain since we might have to vent if we can't steer around it.
[Jenne and Kiwamu]
This time we aligned the vertical angle (not the translation) of the IR beam so that the transmitted light from BS shoots the center of ETMY.
The idea is to use ETMY as a beam pointing reference instead using IP_ANG, assuming the translation is not so bad.
As a result it looks like we are wining. A quick A2L test on ITMX_PITCH showed a small off-centering at sub-milimeter level.
We are concluding that the initial beam after PZT2 had been pointing downward somehow.
Before doing this whole job, we checked the spot shape on IP_POS to see if the beam is clipped or not. It was a round shape, which means no clipping around MMT.
But on the other hand, the spot on IP_ANG had been clipped more than half of its bottom as Suresh reported on his elog (see here).
I found that this clipping is able to be fixed by moving the beam angle upward. I guess the clipping happened at one of the steering mirror in the ETMY chamber.
According to these information, we imagined that the beam was somehow pointing downward after PZT2.
So we started aligning the beam by touching only PZT2 for vertical direction. Then we found a beam spot on ETMY's suspension frame, and brought it to the center.
Then we aligned BS and X arm for this new beam axis. The it resulted a small off-centering on pitch.
Once the MC fully gets back, we will examine the TRX degradation with this configuration.
I made several scripts to handle the mcass configuration and sensing measurements:
- The scripts and data are in the scripts/ASS directory
- The mcassUp script restores the settings for the digital lockins: oscillator gains, phases, and filters. The MC mirrors are modulated in pitch at 10, 11, 12 Hz and in yaw at 10.5, 11.5, and 12.5 Hz. The attached plot shows the comb of modulation frequencies in the MCL spectrum.
- The mcassOn and mcassOff scripts turn on and off the dither lines by ramping up and down the SUS-MC1_ASCPIT etc gains
- The senseMCdecenter script measures the response of the MCL demodulated signals to the decentering of the beam on the optics by imbalancing the coil gains by 10% which corresponds to the shift of the optic rotation point relative to the beam by 2.65 mm (75mm diameter optic) and allows calibration of the demodulated signals in mm of decentering. The order of the steps was MC1,2,3 pitch and MC1,2,3 yaw. The output of the script can be redirected to the file and analyzed in matlab. The attached plot shows the results. The plot was made using the sensemcass.m script in the same directory.
- The senseMCmirror script measures the response of the MCL demodulated signals to the mirror offsets (SUS-MC1_ASCPIT etc filter banks). The result is shown below (the sensemcass.m script makes this plot as well). There is some coupling between pitch and yaw drives so the MC coils can use some balancing - currently all gains are unity.
- The senseMCdofs scripts measures the response to the DOF excitation but I have not got to it yet.
- The next step is to invert the sensing matrix and try to center the beams on the mirrors by feeding back to optics. Note that the MC1/MC3 pitch differential and yaw common dofs are expected to have much smaller response than the other two dofs due to geometry of this tree mirror cavity. We should try to build this into the inversion.
The alignment of the interferometer goes basically step by step.
Tuesday will be an alignment day.
0. MC beam centering (it's done)
1. F2P to balance the coils on every optics including BS, PRM, SRM, ITMs and ETMs (Kiwamu).
2. A2L and then change the DC bias of ITMY and ETMY to get a perfect eigen axis (VF/Jamie).
3. align input PZT mirrors (PZT1 and 2) to maximize the Y arm transmission (VF/Jamie).
4. do the same things for X arm but using BS instead of the PZTs.
5. Alignment of the central part.
6. Make a script to automatically get those things done.
[Jamie / Valera / Kiwamu]
The incident beam pointing was improved a lot by using C1ASS realtime code.
Some more details will be posted later. The below is the list of the highlights today.
- The Y arm cavity was aligned to have good beam centering on the mirrors.
- The input PZTs were also aligned to the aligned Y arm by hand.
- Automation of the Y arm alignment using C1ASS_LOCKIN got partially functional with two loops closed. C1ASS correctly servos the centering on ETMY
- The amount of the off-centering on ITMY and ETMY look roughly within 1 mm.
- As a result the intracavity power got bigger by a factor of about 3.5
I closed all 8 dither loops for the Y arm initial alignment: 2x2 centering servo (this worked before) and 2x2 input beam servo for both pitch and yaw.
So far it looks pretty good - the error points go to zero and the arm power goes up to 1.
The offloading to the alignment biases and the PZTs is not yet automated.
Today the PMC, MC, and Y arm were very cooperative and a pleasure to work with.
The attached plots show the location of the ~29.5 MHz pole and the 59 MHz notch for each quadrant of the WFS1 Sensor head.
As may be seen from the above table, these frequencies will need to be adjusted in some cases.
From the plots we can see that, when there is no attenuation set on the attenuator AT65-0263 (ref D990249-A), the MAX4107 oscillations are seen in Q2,Q3,Q4 quadrants at around 200 MHz.
Rana suggested, from his previous encounter with this circuit, that the solution is to remove the second MAX4106 and the attenuator on the RF line to avoid this oscillation.
A look at the circuit board shows that some of the inductors have not been mounted. That explains the presence of only one notch though the schematic shows two.
Last night I aligned the incident beam axis and the Yarm by touching the PZT mirrors and the suspensions.
I didn't estimate how good they were aligned, but I guess the Y arm is now ready for the Y green light.
Next : Y green alignment and the MC spots measurement / alignment.
++ Motivation ++
Prior to the coming vent we want to have the Y arm, incident beam axis and Y green light aligned so that we can align some necessary optics in the chamber.
Also alignment of the incident beam will allow us to re-position the incident beam alignment monitor (i.e. IPPOS and IPANG).
Our plan was to first align the Y arm using the ASS system and then align the Y green light to the Y arm.
++ what I failed ++
First I was trying to measure the spot positions on the MC mirrors to make me sure the beam axis has/hasn't changed.
Also I was going to align the MC suspensions to have nice spot position on each suspension using the MCASS system
because this will help us checking the beam clearance in the Faraday and perhaps re-positioning of the Faraday during the coming vent.
But essentially I failed and eventually gave up because MCASS didn't work. It seems that MCASS needs some modifications in the scripts.
Then, to make me feel better I moved on to the Y arm and beam axis alignment.
++ what I did ++
I tried using C1ASS to align the incident beam and suspensions on the Y arm, but it didn't work.
However the drive signals from ASS and its demodulated signals looked fine. Only the feedback did not work correctly.
Every time I enabled the feedback paths, the arm just lost the lock. Something is wrong in the feedback paths.
Then I started to align the cavity by my hands while looking at the demodulated signal from each LOCKIN module.
I aligned the things until each demodulated signal fluctuates around zero.
At the end the beam spots on the ETMY and ITMY camera looked well-aligned and the transmitted light became larger by a factor of 2ish.
The servos of C1ASS for the Y arm and the beam axis alignments were fixed.
Now we can correctly run the Y arm ASS from the C1IFO_CONFIGURE window as usual.
The sign of some control gains had been flipped for some reasons, so I changed them to the correct signs.
Next : Health-check for the X arm ASS, the loss measurements.
Since we will measure (and hopefully adjust) the spot positions on the MC suspensions prior to the vent, MCASS is necessary for it.
Here is the MCASS status so far:
+ Valera worked on MCASS on the last February, and basically no progress after he left.
+ The MCASS model had been completed in C1IOO.mdl.
+ He made some useful scripts, including mcassup, mcassOn/Off, senseMCdecenter, senseMCmirrro and senseMCdofs.
Summary of those scripts can be found in his entry #4355.
+ We haven't closed the MCASS loops.
+ The control filters are still blank.
+ We haven't put any elements on the input and output matrices.
+ Some parameters for the dithering oscillators and demodulation systems were properly set.
So we can get the demodulated signals by simply running mcassUp and mcassOn. (This essentially corresponds to the A2L measurement.)
+ The PIT motions are driven at 10, 11 and 12 Hz for MC1, 2 and 3 respectively. For YAW, the frequencies were chosen to be 11.5, 12.5 and 13.5 Hz.
+ Some medm windows were prepared but not as refined as that of ASS.
+ Valera performed a measurement of the spot positions by using MCASS. The results are summarized in #4660.
+ We made an estimation about the beam clearance on the Faraday based on the measured spot positions (#4674)
So, it seems we should be able to at least measure the spot positions soon by using his scripts.
The X arm ASS was also fixed. So both X and Y arm ASS are now back to normal.
Now we can align the arms any time from the buttons on the C1IFO_CONFIGURE window.
The reason why the servo didn't work was that the sign of some control gains had been flipped.
This was exactly the same situation as that in the Y arm ASS (#5067).
SUS-ETMY_QPD is not responding. It is reading zero in dataviewer and 4,400 counts on QPD MEDM screen.......must be wrong cable connected
IP-POS is sick. Last time alive 7-19-2011
IP-ANG beam is clipping on pick-up mirror at ETMY chamber. This will have to be fixed at the vent. The qpd itself is responding to light.
The interferometer was coarsely aligned.
Now spatially overwrapped DRMI and FP arm fringes are visible on the AS camera although the incident beam alignment was done only with PZT2.
All the DC biases were saved so that we can go back to this condition any time.
/***** some health checks *******/
[FINE] IPPOS : it looks okay but the spot on the QPD is a little bit too low by a few mm.
[NOT GOOD] IPANG : maybe hitting a post or something because the spot is vertically split into two. The spot is too low.
[FINE] POX/POY/POP : they all are coming out. POP is visible with an IR viewer.
[FINE] REFL : no clipping but the beam looks a little bit too low relative to the CCD camera.
[FINE] AS : no clipping and the spot position on the AS camera looks fine.
[FINE] Green beams : both X and Y beams are successfully landing onto the PST table without no clipping.
[FINE] Suspensions : all of them are reasonably quiet without the oplevs, which is good.
The output matrix in the C1ASS servo were coarsely readjusted and the servos seemed working.
However it is difficult to say the servo is very good or so-so,
because the ETMY suspension moves a lot and hence the cavity eigen axis moves a lot too.
+ Modification of C1ASS (Kiwamu)
The servo for aligning the Y arm is working fine with the coarse gain coefficients.
However then I found the ASS_Xarm servo was not healthy.
So the next step is to refine the X arm servo in C1ASS.
+ With the ETMY oplev the Y arm became quieter after we recovered the oplev whitening filter (#5523)
+ The Y arm alignment scripts can be run from the usual C1IFO_CONFIGURE screen.
It will servo the spot positions on ITMY and ETMY, and align the input beam pointing. It brings the Y arm power to about 1.
+ The X arm servo is doing something funny. It doesn't bring the arm power up to 1.
I thought the X arm didn't need any modifications because the X arm servo doesn't include PZT1 and PZT2.
So it maybe a simple bug (for example, some switches are disable and so on)
The channels for IPPOS had been assigned in a wrong way.
Because of this, C1:ASC-IP_POS_X_Calc corresponds to the actual vertical motion and C1:ASC-IP_POS_Y_Calc is for the horizontal motion.
We should fix the database file to get the correct vertical/horizontal corrdinate.
Now the C1ASS servos are working fine.
However at the end of the scripts sometimes it changes the DC force (e.g. C1:SUS-ITMX_PIT_COMM and so on) by a wrong amount.
So for this bug, it misaligns the suspensions a lot. I will take a look at the script tomorrow.
The input matrix of IPPOS were fixed so that the horizaontal motion correctly shows up in X and the vertial is Y.
(what I did)
+ The data base file, QPD.db, were edited.
QPD.db is a part of the c1isxaux slow machine and it determines the input matrix for deriving the X/Y signals from each quadrant element.
+ The previous input matrix was :
X = (SEG1 + SEG4) - (SEG2 + SEG3)
Y = (SEG1 + SEG2) - (SEG3 + SEG4)
+ The new matrix which I set is :
X = (SEG1 + SEG2) - (SEG3 + SEG4)
Y = (SEG1 + SEG4) - (SEG2 + SEG3)
The new matrix is a just swap of the previous X and Y.
+ Then c1isxaux was rebooted by :
+ The I did the burt restore it to this morning.
Did somebody delete all the scripts in /opt/rtcds/caltech/c1/scripts/ASS ?
I have moved all the MC_ASS scripts to a directory called MC under ASS
In any case, the daily backup of the scripts are found in /cvs/cds/caltech/scripts_archive.
I have moved all the MC_ASS scripts to a directory called MC under ASS
Initial pointing or IP-ANG is a pointing monitor of the MC. This beam is launched after the second pzt steering mirror.
IP-ANG is missing the pick up mirror by a few inches at ETMYchamber
1000 days plot show last appearance in Feb 2010
Looks good. Any way that you can tell in an unambiguous way, where the beam is, is very good. Ideally we want to have1-2 mm accuracy.
I wanted to center beams on the XARM cavity mirrors using c1ass model. I've run XARM setup script and then turned dithering on. Cavity went out of lock because calculated offsets were incorrect.
I was using TRX only and calculated rotation phases for ITM and ETM pitch and yaw. For this I've added a low pass filter into Q-quadrature bank and made DC value at the output to be zero by adjusting the phase. I've put gains (+1 or -1) in the I quadrature such that output was positive.
Then I've set the sensing matrix to identity as I decided to deal with separate loops. Of coarse, they are mixed by the cavity, but at least in the control system they are distinguished. Old matrix summed error signals in one degree of freedom from both mirrors. This makes more sense but still not precise because coils are not ideally diagonalized.
Then I've adjusted gains for control loop for every degree of freedom. I've ended up with (0.1; 0.1; 0.1; -0.1). I did not use large gains as I wanted slow convergence because of the demodulation low-pass filter time response constant of 20 sec. Coupling (I quadrature) was reduced from (0.9, 0.3, 2.4, 1.2) to zeros (0-0.1) in ~5 minutes, TRX increased from 0.73 to 0.90.
There is one thing that I do not understand yet. I think controllers should minimize angle -> length coupling that is proportional to I-quadrature if phase is correct. But phase depends on alignment and when the feedback loops are on, phase drifts. I could see it during my measurement. But I did not find any script that smoothly tunes phase such that coupling is all in I-quadrature. I guess this is not hard to set a gradient descent algorithm that minimizes DC value of Q-quadrature. Or how this is usually done?
Gouy not Guoy:
pronounced Goo-eee, with the emphasis on the second syllable.
It's OK; even Siegman got it wrong---48 times.
RA: NO, stil not OK.
I modified our existing c1ass model to include alignment of input steering TT1 and TT2 for YARM and BS for XARM. Corresponding medm screens are also created.
ETM_PIT: frequency = 6 Hz, amplitude = 100 cnts
ETM_YAW: 8 Hz, 400 cnts
ITM_PIT: 11 Hz, 800 cnts
ITM_YAW: 14 Hz, 1200 cnts
These values were chosen by looking at cavity transmission and length signals - excitation peaks should be high enough but do not shake the optics too much.
LO for each degree of freedom is mixed with cavity length and transmission signals that are first bandpassed at LO frequency. After mixing low-pass filter is applied. Phase rotation is chosen to minimize Q component
8 * 8 matrix was measured by providing excitation at 0.03 Hz to optics and measuring the response in the demodulated signals. Excitation amplitude was different for each optics to create cavity transmission fluctuations of 25%
Though coherence was > 0.95 during the measurement for each element (except for TT -> Length signals), after inverting and putting it to control servo, loops started to fight each other. So I decided to try a simple diagonal matrix:
TT1_PIT -> ETM_PIT_TRANS, TT1_YAW -> ETM_YAW_TRANS, TT2_PIT -> ITM_PIT_TRANS, TT2_YAW -> ITM_YAW_TRANS,
ITM_PIT -> ETM_PIT_LENGTH, ITM_YAW -> ETM_YAW_LENGTH, ETM_PIT -> ITM_PIT_LENGTH, ETM_YAW -> ITM_YAW_LENGTH
And this matrix worked much better.
8 loops are running at the same time. UGF for input steering loops is 20 mHz, for cavity axis loops - 80 mHz. Slower loop is stronger at low frequencies so that cavity axis servo follows input steering alignment.
When I started experiment the cavity was misaligned, transmission was ~0.4. Servo was able to align the cavity in ~30 seconds. This time depends on mirrors misalignment as well as input optics and cavity axis misalignment relative to each other.
When servo converged I disturbed ETMY, ITMY, TT1 and TT2. Servo was able to compensate for this.
Excitation lines seen by transmission and length of the cavity are suppressed as shown on the attached as pdf figures.
Though the servo is able to align the cavity during my tests, this does not mean it will work perfectly any time. So please, if you lock, try to use the servo for alignment. If something goes wrong we'll fix it. This is better then to align IFO by hands every time.