Proportional control would work depending on how it’s being used.
Pellet smokers have more of a variable heat control than an electric stove. It’s not like the fire is either on or off. By controlling the amount of pellets in the pot, you can control how much fuel is burning, more like a gas grill.
In my case, I’m not using the P to control how long to run the auger, but how long to pause between runs. To maintain a temperature, you feed pellets for a known amount of time. Then you turn off the auger and let the current pellets burn. Then you feed pellets again. If you adjust how long you pause between feeding the next batch of pellets, then you can adjust how many pellets are currently burning. Keep adjusting that pause time until the amount of pellets burning is maintaining the temperature you want.
The shorter the pause, the hotter the temperature.
This is how the pit boss’s controller works during the ‘smoke’ setting. And adjusting the values on the smoke setting can adjust the temperature it runs at.
I do have an I and D value in my PID loop. I can tune those if necessary, but smokers are so under-damped, that I’m not sure it’s necessary. I’m assuming it’ll be more like trying to maintain a temperature in a fish tank.
Honestly, for my aquarium I always just ran a bang bang algorithm. Had it turn on at 1F below set point and turn off at .1F under set point. Residual heat in the heater would carry the temp up to set point. I only chose to try a PID because I haven’t done one in a very very long time. In fact. I don’t think I’ve ever used one on a personal project.
If you have zero error when you do the measurement, then you would feed pellets every 10 mins (or something). That 10 minutes is the feed forward. If you notice it is too hot (the error is positive) you would add some time proportionally to that error (your P might be 1 min / 10 degrees error or something). Too cold, and you will subtract a some time.
That might look like:
start with 10mins. Measure, it is too hot, add 1min, now it is 11mins.
start with 10mins. Measure, it is just right, keep 10mins.
start with 10mins. Measure, it is too low but just a little, subtract 30s. Now it is 9.5mins.
If you start from whatever delay you used last time, then that is just an integrator and not a proportional control. An example of an integrator would look like this:
measure the temperature, 10F too hot, add in a minute. Previous delay was 10mins, now it is 11mins.
measure the temperature, just right, keep the previous 11mins.
measure the temperature, too low but just a little, subtract 30s. Now it is 10.5mins.
The integrator has the danger of winding up and being totally wrong if you dramatically change the plant. If you put in a hunk of cold pork butt, it might take a while to catch up. And then the butt heats up, it may take a while to climb back down.
Yes, P only control can give you a nice flat line, either above or below the setpoint, and never reaching it. Adding the appropriate I factor then allows the temperature to reach setpoint. Having I wrong is what gives you the oscillation. What I’d start with is a proportional control with a small integral and a long time between PID calcs. As David said, the feed for a pellet grill is more analog than digital, even though the feed rate is likely to be stable when on. That means your actual control is more like PWM than true analog. If you actually control the auger rotational speed you could go full analog.
You may need logic to detect open lid so as not to overcompensate for heat loss.
The temp profile scheduler is a good idea. I could be wrong but I think heatermeter had this as well. I know it also has a feature to automatically bring the temp back down to a resting temp as the meat approaches a doneness temp.
Thanks for the reminder of how this all works. Like I said, it’s been a while since I’ve played with it… 18 years to be exact.
Right now, I have the controller set to check the temp every minute. For the initial startup I’m using the same on/off time that the pit boss controller uses. By default, the pit boss starts off in ‘P4’. I don’t remember if it saves previous changes, but I don’t think I’ve ever messed around with the settings either.
Part of the goal with pellet smokers is to have the ‘off’ time be as long as possible to maintain temp. The longer it’s off, the more the fire burns down, the more smoke it puts out.
This site is where I got the timings of the P settings
I’m not too worried about the door logic. I don’t typically open my pit once I put the meat on. I could put some simple logic in there that if the pit suddenly drops X degrees to ignore the changes. More than likely a sudden 10F drop wouldn’t happen during a normal cook.
The current PID loop does have an I value. I think it’s .1, so very minimal.
I would like to point out, as a Texan, I’ve never wrapped my brisket in foil. I know very few who do any more either. I do use butcher paper though because if it’s good enough for Franklin…
(That’s not a knock on anyone who does use it though! Texas has been so disappointing lately… just let us have our bbq!)
I don’t know what it is about the butcher paper, but it’s my favorite BBQ toy. You can take away my heatermeter and other gadgets, just leave me my butcher paper.
So, what’s the advantage to butcher paper over foil? It’s obviously not going to seal as well, so no braising action. I wouldn’t expect to get the temperatures up high enough to char the paper (Fahrenheit 451 remember) so it would just act as a way to continue heating without getting as much smoke into the surface.
Foil speeds up cooking by getting around the evaporative cooling stage but it can make the bark mushy and the meat taste more like a roast. Paper supposedly being a little more breathable keeps from messing up the bark as much but it still speeds up cooking over naked.
For competition cooks I get the impression that the correct time to wrap is when you’ve got the color you want and when the bark is set and passes the scratch test. I never get to the color I want in my egg so usually won’t wrap, but in my offset stick burner I usually wrapped after 3-4 hrs.
Franklin has a video on YT where he does a foil wrap, paper wrap, and naked briskets side-by-side…by-side and does a taste test and explains the differences. I’ve heard him remark a few times that he doesn’t always wrap, but in every video of his restaurant they’re all always wrapped in paper.
Paper helps hold in some of the heat like a blanket. Allowing the meat to cook faster, while still letting some of the moisture out so you’re not steaming the meat and making it mushy.
My favorite way if I have to wrap. The bark difference is the decision maker do I want it crunchy or soft. With pork it doesn’t seem to make as big a difference as with brisket.
Sorry to say it. But I just licked my screen. MAN that’s a pretty brisket. Smoke ring is on point too! Save all that juice and make au jus!!! I love to pour that stuff on a loaded baked potato, or a roast beef sammich.
I still have a little saved in the freezer, waiting for the right meal to bring it up a notch. I also did Salmon that week and I’m down to only one or two pieces left. Time to fire the smoker back up!
