In an effort to improve the CTN experiment's temperature control I've reconfigured the cavity heaters to operate with differential and common mode heating. By making the actuation symmetric and elevating both cavities well above the vaccan temperature hopefully this will improve the linearity of the actuator.
The heating wire on the north path is 156.8 Ω and south is 85.6 Ω (don't know why they are different). I've configured an additional channels for the south heater (previously not hooked up). Units of the new channel are in watts. Two additional calcout channels were made that set the common heating value (in watts) for both channels and a difference between the heaters (in watts) that ranges from -1.1 W to +1.1 W. After rebooting the IOC and setting the common and diff heating to produce 0 W on south and 0.7734 W on north I get roughly the same beat note frequency as before.
After the change PID feedback to the differential heater channel, it was possible to use PI feedback to drive the beat note to a set point of 100 MHz but tweaking gain values to kill oscillations was takes a very long time (still not there yet). I reattempted to implement the relay auto tuning method from before (see PSL:2142). However, I had the same converging cycle limit to zero after a few hours. I think that the large lag on the actuator/sensor/plant in these temperature tuning situations may rule out finding objective Kp, Ki, Kd values using relay tuning methods: the lag puts us away from the -pi first real axis crossing the nyquist curve and estimates of the plant's critical gain and frequency are bogus.
Short of making some really smart controls that can anticipate the trajectory of the temperature and make more optimal estimates for the feedback I may need to vent the can and get some low noise platinum RTD sensors on the shields.