I'm having issues tuning a PID controller - specifically the I-term. First - here's a little info about my system:
I'm heating a coil (R ~= 1.8 Ohms) to 750°F. My power delivery system is working fine. I'm applying a maximum of 12V to the coil, so there's about 7A going through the heater. I'm monitoring temperature with a k-type thermocouple. I've double-checked with another thermocouple and DMM to make sure my microcontroller is reading the right temperature data. It is. If you have clarifying questions about the system hardware I'd be happy to answer but I'm pretty sure my issue is coming from my tuning method.
I set my I-term and D-term gain to zero and slowly increased my P-term gain until I got stable oscillations:
After achieving stable oscillations and choosing a P-term gain of 8.0, I calculated my new P-term and I-term values per the equations in the tutorials (P Gain = 3.63, I Gain = 0.42):
As you can see, although the oscillations are much more centered, they have not diminished as they do in the first tutorial. When I calculate and implement the D-term gain, I still get unsatisfactory behavior:
My Question is this: where do I go from here? Should I start over with a new P-term? Change I-term and keep everything else the same? Change D-term and keep everything else the same? Any input is welcome.
From 130°F with error term: