PID Tuning - big difference?


 

MilesB

New member
Did a test run with the HM4.2 two days ago and got the chart below, which I was pretty happy with:



This was with my UDS running empty.

I fired it up today and put a small lump of gammon on as a test for christmas ham, and have ended up with this:



The spread is quite frustrating and i've not changed anything else.

Current settings as follows:

UDS
Adapt-A-Damper with servo and standard fan
P: 3.5
I: 0.0035
D: 6

Fan Output: Pulse
On above: 45%
min: 0%
max 45%
startup max: 50%

Most annoyingly these settings kept triggering the "lid open" - I changed this to 20% below setpoint until I can refine the PID settings but don't know where to start - at least it's constant...!
 
I do not use the 4.2 controller by I do use the 4.3. I see that you use a UDS. If it is air tight and well insulated, then what is what I am noticing is the "P" setting set to high. Controls break into oscillation and now you get frustrated chasing the PID dragon. Here are that settings I use on my small Primo Jr.oval and they work well for me. Make sure you pit probe id positioned such that the heat/smoke flow passes the probe tip smoothly. Careful you do not get it too close to the coals. IMO this can cause control issues.
P 2.8
I 0.0035
D 6
Fan output voltage
On above 0%
Min 10%
Max 30%
Startup Max 70%
 
Thanks for responding Gary.

Would having a "heat shrink" in the form of a slab of ham in there make a difference? My gut feeling is not.

The only other thing that was different is I re-cycled the charcoal and there wasn't much left when I finished. I'm wondering if it had problems catching because there wasn't enough of it?
 
I think that you should do you fine tuning with product in grill. But if your going to make big changes I would load something in there that you can cook low and slow to give you time to tune. Make sure you let the grill adjust at least 15 minutes between tweaks. I always have a mix of fresh and used lump charcoal. One other thing is I also use ceramic plates which separate the coals from the grill grate. This really helps dampen spikes in the control so keep that in mind as well. Just enjoy tuning the grill. take your time, goods results will come.
Good luck and enjoy
 
........... snip..........Would having a "heat shrink" in the form of a slab of ham in there make a difference? My gut feeling is not.
............. snip.............

I think the word you're looking for is "heat sink" which is simply something that absorbs heat. That is frequently something like a piece of aluminum clamped to a component which gets hot to help with dissipating potentially damaging heat. In other words, a heat sink is simply a set of cooling fins in many cases. Chances are the processor inside your computer has one. Heat sinks are generally not made from ham. The chilled piece of steel (called an enswell) a cutman holds against the swollen eye of a boxer between rounds is also a heat sink designed to slow the local blood flow and reduce the swelling.

However, I think what you really want to discuss is the effects of a "thermal mass". This is something (like a ham) which will absorb and hold heat. A feather pillow has a low thermal mass while a cast iron engine block has high thermal mass.

The idea of a PID controller is simply to have a way to adjust the power to a heat source which is more sophisticated than a simple on/off relay. A simple relay triggered by a particular set point will always overshoot the target temperature. But a PID controller has a "brain" which can anticipate when you approach a set point and reduce or turn off the power ahead of time to reduce or eliminate overshoots.

As you might imagine, if your BBQ were made of thin steel without insulation and weighed around 5 pounds, then it would have a very low thermal mass. Heating it up and cooling it off would happen very quickly. On the other hand, a heavy, thick-walled ceramic cooker would take a while to heat up and cool down. The PID settings for the two units would be quite different. So we experiment with our BBQ rigs and our PID controllers in order to fine tune the PID settings to get stable holding and reasonable overshoots. A well tuned PID system will normally overshoot the set point a little bit because that lets the system reach the set point more quickly. If you tune for no overshoot at all, the system will take forever to "creep up" on the desired temperature, so we normally accept some reasonable amount of overshoot.

Now let's consider the ham. Since it adds thermal mass to your system, it WILL make a difference in your PID settings, so ideally you would fine tune your cooker with a representative chunk of food in there along with a proper sized fire. For example, when I was breaking in a new ceramic cooker a few weeks ago, I burned a small fire at low temperatures for quite a long time. I tried tuning the PID settings and had some success, but the fire was so small and the temperature low enough that the results were not ideal. Later on I had to adjust them when I was cooking chicken at higher temperatures.
 
Fan Output: Pulse
On above: 45%
min: 0%
max 45%
startup max: 50%
With these settings, your damper is going to be 45% open before the fan turns on, then the fan will only get up to 45% max speed, and it is running in the less controllable "pulse" mode. I think what you're seeing is that at <50% PID output, there's just not even close to enough air getting in. In your second graph, note how the temps only start going up when the output is like 90%. 90% would be a 90% open damper plus just ~40% blower speed.

I'd say #1 thing is definitely set the Fan Output from Pulse -> Voltage and never consider changing it back. Pulse is just for people who don't have the hardware to support Voltage mode, or want to run sous vide relays.

Then try setting your servo output to be "fully open: 45%". That will make your damper open completely before the fan turns on, because I think it being half closed when the fan is barely running is limiting the airflow too much. Think, if the fan is on 25% and the damper is only 75% open, you're only getting 75% of 25%. It just makes the control band too small, since in that case, you're at 75% output but only getting 18% forced air output.
 
I started to confuse myself a fair bit with the PID settings for my UDS. A member on here, Darren C. kindly offered to share with me a write-up he did on PID tuning which helped me a lot, I still occasionally refer to it. I prefer not to share it without his permission, but suggest to contact Darren if you would like to use his explanations. He named his document: "PID Tuning Makes Me Cuss"

( I did a short thread on my initial UDS/HM experience, and sadly have been too slack to add to it, but i am still very happy with how it performs. https://tvwbb.com/showthread.php?70981-My-HM-build-for-UDS)
 
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One of the things that makes the heater meter so good, is that you can adjust the tuning and control parameters.

What works at 350 on chicken with the full load of hot coals, will not work at 150 on bacon with a small coal amount.

what works great at the start of a cook when you've got a lot of hot coals does not work well as you approach the end of a 14 hr cook and you have very few coals left and they're covered with ash.

You can cycle by plus or minus 10 F, or 1F, with just a small change in the gain sometimes. You have to learn how do you control your cooker best with the ATC. It's a tool, not a cure-all. When it starts to cycle during a cook, you make whatever adjustments are needed to calm that down.

. With other controllers that don't graph and allow you to see what's happening.. and change tuning..and clamping, You're lost when that's occurs. Rapid temperature swings can produce dirty smoke which definitely can end up affecting the quality of your cooks in the end.
 
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One of the things that makes the heater meter so good, is that you can adjust the tuning and control parameters.

What works at 350 on chicken with the full load of hot coals, will not work at 150 on bacon with a small coal amount.

what works great at the start of a cook when you've got a lot of hot coals does not work well as you approach the end of a 14 hr cook and you have very few coals left and they're covered with ash.

You can cycle by plus or minus 10 F, or 1F, with just a small change in the gain sometimes. You have to learn how do you control your cooker best with the ATC. It's a tool, not a cure-all. When it starts to cycle during a cook, you make whatever adjustments are needed to calm that down.

. With other controllers that don't graph and allow you to see what's happening.. and change tuning..and clamping, You're lost when that's occurs. Rapid temperature swings can produce dirty smoke which definitely can end up affecting the quality of your cooks in the end.

Thanks for all the feedback guys. I made a couple of changes and currently have this, which is much better (12k lump of ham in there at the minute). I also used fresh briquettes...



Which is much more like it (I have to admit I did wonder if it was broken, I don't think my oven can get that close. Lid Open mode didn't trigger, I've changed this setting back to 10%/120sec and expect to go back to default shortly.
 
Thanks for all the feedback guys. I made a couple of changes and currently have this, which is much better (12k lump of ham in there at the minute). I also used fresh briquettes...



Which is much more like it (I have to admit I did wonder if it was broken, I don't think my oven can get that close. Lid Open mode didn't trigger, I've changed this setting back to 10%/120sec and expect to go back to default shortly.

Settings to get this:

P: 3
I: 0.0035
D:6

Fan Output: Voltage
On above: 45%
Startup Max: 50%
Servo fully open at 45%
 
Nice! Yeah my HeaterMeter maintains a tighter temperature control than my oven does as well, by a long shot. HeaterMeter has the advantage of a continuous heat source compared to the oven which only pulses power on heating elements that take a long time to heat up and cool back down.
 

 

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