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Message ID: 2254     Entry time: Tue Nov 20 17:31:13 2018
Author: anchal 
Type: DailyProgress 
Category: Other 
Subject: Precav Beatnote Freq counter and PID optimization 

I learned today from Andrew on how to setup the pre-cavity beat note frequency counter and use it for cavity heater PID. Now, I need to figure out good PID parameters to start heating up cavities in a controlled fashion. For now, I have kept the differential heating off to keep the cavities stable.

But then, I realized that the Slow PIDs in FSS aren't functioning. Whenever I switched them on, it would take the SLOWOUT so far away that the frequency lock will break. So to fix this, I read more about PID tuning and figured that the pre-defined PID constants for Slow PID might be wrong. I ran the PIDAutoTuner (which I have changed a little bit, see git commit) which gave very different values for these constants. Further, the newly found values were even worse. So I started looking into the PID code itself and when I was checking the error signal, the obvious truth revealed. The FASTMON channels which are used as the process variable in the SLOW PID were not being updated. Then I realised a whole batch of channels are stuck to 0.

So I restarted the channels by restarting the AcromagBoot.conf service on the acromag computer. This suddenly made everything SUPER UNSTABLE. Even with locks and autolockers switched off, the laser frequency was just dancing and giving a light show out of the PMC! While trying to fix this, I also realized that the variable gains in the PMC controls have been reset to 0 in some cycle of restarting services. So I found the right values again by monitoring the error signal.

Finally, I had to restart the acromag computer as it was running 2 different instances of EPICS channel causing the above mashup.

After everything got settled back, I ran the PIDAutotuner code and it updated the PID constants values in the SLOW PID. The setpoint for both south and north SLOW PIDs is 1V. I have updated default values in .db files.
 


PMC Variable Gain value:

South: 30 dB (No oscillations happen upto here, so no harm in keeping it here unless we find any wacky behavior)
North: 22 dB (Oscillations start at 24 dB, I'll increase the margin if need be)


SLOW PID parameters:

  K_p K_i K_d
South -0.00040 -0.00017 -0.00062
North -0.00041 -0.00015 -0.00076

My take:

I personally feel the SLOW PIDs are just fine with a long range of values unless we have a better measure of what is bad and what is good, particularly, along with the cavity heater PID. Because SLOW PIDs for frequency are important only when we are bringing the cavities to 26.5 MHz apart. Once they are there, it doesn't matter much. I'll think more about this.


Near Future Steps:

Once I set the cavities on course to reach 26.5 MHz, I'll start finishing the code to take weekly noise budgets. Then I'll work on the weekdays to improve something in the experiment and check with noise budget taken during the weekends. I'll start with optimizing lens positions and then installing ISS.
 

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