BrewPi AVR starts oscillating loop

Hello everybody,

I hope you can help me, i have some problems with my brewpi setup.
It starts working corectly but after some time it starts oscillating the beertemps.

I use a thermowell, fridge and 300w heating pad. PID settings are default.

Thank you for any help.


I just found this thread and my graph looks similar.

So i halved my PID settings to Kp = 2, Ki = 0.1, Kd = -1

The fridge temperature follows the fridge setting very nicely, so it is really the PID settings that need to be changed.
The problem arises from the fact that the beer reacts to the fridge temperature very slowly.

I see you already halved your PID constants, which is good. If you continue to get overshoot in the beer temperature, decrease Ki (integral) and increase Kd (derivative). You can see the effect in the control algorithm tab and see which one has most effect on the fridge setting.

Thank you elco.

The last settings were still oscillating.

I am now at:
Kp = 2, Ki = 0.05, Kd = -1

Can you share a new chart? Also look at the control algorithm tab at a moment you think the fridge setpoint is wrong to see how it is calculated.

Still oscillating.

Settings are @ Kp = 2, Ki = 0.05, Kd = -1

I am going to try Kp = 2, Ki = 0.025, Kd = -1 next.

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Maybe I have some sort of the same thing happening. I’m using an old freezer (put this on on the lowest setting possible), and I’m currently using a heating pad from an old waterbed, on the middle setting (30°C). I tought it was working perfectly, and then this started happening.

Had my settings on 2 / 0.1 / -1 …just changed them to 2 / 0.05 / -0.5 to see what will happen.

What I don’t understand is, why does the algorithm doesn’t use shorter heating times, instead of these larger heatings, followed by agressive cooling. Seems to me it keeps itself in a perpetual up and down loop.

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@doomy86: it looks like your fridge temperature is going up and down even when the heater/cooler are doing nothing… Could one of the temperature sensors be unreliable?

@PlazzmiK: It looks like your beer responds very fast to cooling. I don’t think the heating is the problem: the beer temperature barely goes over the setpoint. The problems started when the cooler got in on the action.

You can try doing 2 things:

  • don’t let your beer make direct contact with the freezer wall, let heat transfer go via the air for a gentle nudge in the right direction.
  • Move your fridge air temperature sensor closer to the cooler
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Thanks Elco, I admire your quick and usefull responses!

UPDATE: Adjusting the fridge temp sensor seems to have fixed it!

The screenshots were too small, the ups and downs are actualy heating cycles:

Would it make sense to reduce Kd for less heating cycles?

@doomy86: in your last screenshot, the fridge setting rises because the beer temperature does not respond to the rising fridge temperature for quite a while. The integrator accumulates the error and increases to correct this ‘steady state’ error. Decreasing the integrator action by reducing Ki might be better for you, because of how slow the beer responds. How much beer do you have in there?

If you reduce Kp, it will result in a lower difference between fridge and beer temperature. I think a too low Kp could actually cause your problem, because then it will have to rely on the integrator alone to correct the temperature.

Thank you elco i really appreciate your help…

Its a 60l batch, the thermowell seems to work really well.

Ok, so i try to lower Ki to 0.01 and post results as soon as the next beer is in the fridge…

Thanks again, you are the best!

I would increase Kp too (back to 5 or even 10), the reason the integrator builds up is because the proportional part is not doing it’s job.