Default settings, standard fridge build

Standard fridge build.
I did a test with warm water and beer constant and it looked good so here’s my first actual fermentation so far.
It was a little warm to start with but within my normal pitching range.

The beer temp is much better than my normal ambient - you can see the temp swings in the room when the sun comes out !
I had two concerns -
1 Is my compressor on for periods that are too short - should I be protecting it more
2. Does that initial curve zeroing in look good.

Obviously this is way better than before, I was just trying to make sure my parameters were sensible. They are the defaults of:
Beer to Fridge Kp 2, Ti 7200, Td 1200, max diff 10, input filter 18 secs, derivative delay 159 secs
Cooler Kp 10, Ti 1800, Td 200, PWM period 1200, min off 180, min on 120, input filter 18, derivative delay 159
Heater 1 Kp 10, Ti 600, Td 60, PWM period 4, input filter 18, derivative delay 159
dead time between heat / cool 1800

I know PID tuning is an art in itself and the control is way better than I’m used to, I just wanted any useful comments in case I can improve things.

  1. If you want to run with longer cycles for the compressor, you can increase the cooler PWM cycle.
  2. The overshoot at the start is because of the integrator winding up and holding the fridge setpoint low. The time constant for the integrator (2 hours, 7200 seconds) is too fast for your system. I would increase it to 14400 at least and also increase Td to take more overshoot into account. Set Td to 3600.
    You can also argu that Kp is too low, which causes the integrator to make up for it. I’d set Kp to 5.

You can also increase the filtering in the beer-to-fridge PID to get a smoother setpoint.

Thanks Elco, I’ll make those changes in the Beer to Fridge settings and see how they work out for the next batch. I’m not too worried about the fuzzy setpoint because it all gets smoothed out in the end. It was also interesting to see how the chamber temp was influenced by room temp, particularly on the test (not posted here). Then I found out my kids had been peaking and hadn’t closed the fridge door properly!

So changed params as suggested, but not filter

I know this isn’t zoomed in but the thing I noticed is the fact everything is much fuzzierafter the change.
It hasn’t affected my beer temp but the whole system seems to be trying harder. It just looks like it needs to do less!
If I zoom in I can see a kind of oscillation.

Perhaps I changed too many parameters at once.
Beer still rock solid at 18.5 :slight_smile:

Do increase the filtering settings. Your fridge setpoint is fuzzy because it is reacting to bit flips of the beer sensor (0.06 degrees). On a bit flip, the derivative is high. Filtering smooths this out. Set beer-to-fridge filtering to max and you’ll have a less grassy line.