Testing of this seems to confirm that lower intergrator values are the way to go. We had excellent locking with <1.2 kHz/min for a period of about 12 hours. This is almost good enought to move down to 500 Hz/V VCO slope with some averaging.
Settings were P=0.0015, I=0.000050, D=0.0. The heater diff value hovers around 0.68 W for a setpoint of 69.2 MHz.
Bad news is we had a glitch that spiked the pre-cavity BN detector frequency at about 5:07 am this morning. This spiked the intergrator value and drove the heater value up for about 30 min before the BN crossed the 0 Hz point. There wasn't enough time for the intergrator to wind back down the sign change of the BN frequency ment that the loop then just drove away from the setpoint eventually railing at 1.1 W diff heating.
This will be more a problem down at the 27 MHz point that we want to lock to. As the set point is so close to the BN flip across 0 Hz any disturbance can drive our loop into this runaway state. Its not clear how we reject these glitches or reliably sense the flip across 0 Hz. Filtering pre-cav BN for glitches seems bad as it will introduce poles that then affect the loop. Trying to sense a change in BN slope (I think anchal implemented this?) will be hard if the slope is very shallow as it crosses, this will give confusion about the sign if the noise dominates over the derivative value.
Machine learning would be helpful here maybe. If you include information about the laser slow frequency controls (for both lasers) its really obvious when the direction of the BN evolution is inconsistent with frequency direction of the laser evolution for both cavities.
For now I am going to narrow the hard stops on the PID script. This should prevent the loop from railing hard and taking hours to recover. It seems that without a drastic change in the environment temperature we will never need more than 0.1 W of movement either side for the actuator center point. I'll set the limits to [0.65, 0.75] W for now, this should be enough to gently hold the BN in the 0- 100 MHz range and will prevent hard railing way out of range.