Yet another PID tuning query - UDS


 
B

BradCowain

New member
Hey everyone,

Long time lurker and heatermeter user on a UDS for the past year. Really love the kit and web interface and being able to tinker. For the past few months now I've been down the rabbit hole of PID tuning with mixed success so rather then continuing to stress about it, I thought I'd reach out to the experts.

My setup:
- Heatermeter with current software
- Adapt-a-damper with stock (kit) fan
- 3x 3/4" air intakes (2x closed & 1x fan)
- 2" exhaust with sheet magnet adjustable
- large charcoal basket usually using coconut briquettes
-

I don't have any issues bringing her up to temp and at times she holds temp fairly well. Other times it just wants to oscillate around the set point. You can see examples of both in my last cook.

I would really appreciate some input on my PID settings and how I can tune this baby in for set and forget.

Brad20210726_143137.jpgLastSession.png
 
Well, I am not a tuning expert, but will offer some suggestions anyhow. It would appear that you need more integral contribution (normally in steady state almost all of the output is from the integral term, so I would double that term and see what happens. Use the 'P' key on the home screen to display the pid internals to see what terms are driving the output. You may also want to go back to the default servo setup where it is open fully at 100% output until you establish some working settings and tweak from there. The theory behind using 50% makes sense ideally as it provides finer control, but can also create a very nonlinear airflow when the fan kicks in depending on the setup.

Good luck!!
 
Wow that is some of the most amazing oscillation I've ever seen, as in it looks like an untuned system and not just random stuff happening. It definitely looks like it needs more D term because that's the term that resists change. You'd want more resistance to change once the temperature starts changing so it doesn't keep going past the setpoint. I do think the P is too low as well so you might want to try boosting both of those, like P=5, D=8.

The PID debug info is really helpful with figuring out which value isn't doing its job. See what it says as the temperature is passing through the setpoint to see why it is going to keep going. Did the "I sum" get too big during the oscillation and there's not enough D to counteract it? Solution there would be (one or more of) A) Raise P so it reacts more quickly and doesn't give I time to accumulate B) Lower I so the value doesn't get so big C) Raise D to the point it cancels more I.

In general, higher gains work better, but the higher the gain the more random-looking the output gets so the balance is setting them high but not so high that output is jumping all over the place.
 
Thank you all for your your replies. I do recall from that cook that I was always higher in the PID debug. I'll give your suggestions a go Bryan.

For my damper and fan settings, do they also need tweaking? Ideally I'd love it to run completely on damper with no fan at lower temperatures. But could the damper being fully open at 50% and fan starting at 50% also be a factor of my results? For the time being I'll increase the P & D and back off the I and try to remind the scientist in me "one variable at a time" 🙄
 
I run my damper / fan with the same sequence (open at 50%, fan on at 50%) and it works just fine. There is a definite change in slope when the fan kicks in as the fully open damper doesn't pass nearly the same amount of air as the fan does when it kicks in at 1%.

You might try making sure you've got enough free airflow with the damper fully open such that you're not just effectively running at 0% output any time just the damper is open. You can do this by getting up to temperature then just setting the output to manual mode and see if just having the damper open causes the fire goes out or the temperature goes up. If the temp goes up over time (steadily increasing to like 250 or more from a 225 setpoint), then it should be good to go. If the fire struggles to stay running or goes out with just the damper, then you need more airflow by either loosening up the smoker or adding fan overlap.
 
Thanks Bryan,
Tomorrow is the day I'll try tuning my drum from scratch again. I have increased the air inlet where the adapt a damper is attached to 1in to help out breath. With all the changes I've made to PIDs I can't recall what the default settings are. Could you please let me know what the default PIDs are again?

Cheers
 
OK, so today I have been testing the system for airflow. After bringing the UDS up to temp at 250F I set the adapt-a-damper set at 50% (damper 100%/fan 0%) dragged myself away. The temp dropped and steadied at 200F - much too low indicating not enough air. I then tested opening the 3/4in ball valve on one of my other inlets at various settings until I found that a sweet spot where the temp rose slowly to 280F (and would have gone a bit higher). I then set the damper to 0% and the temp dropped down to around 220F over 30mins. So now I had my sweet spot, however not all the airflow will be controlled through the adapt-a-damper.

Moving on to PID calibration. With the UDS airflow loosened up I tried to set P to 10 and see if I could get a constant oscillation. No joy. With the air not being controlled completely by the damper the ramp down was super slow and on full fan I could feel warm air coming out of the inlet ball valve. Time for change of plans... I then put in the default PID settings and I'm getting a good trend as per below which I'm pretty happy with. Temp is being controlled by the manual air intake and the damper with no fan needed.

So she needed more air, and by "loosening" the intake I was able to provide this. My question: is there any way to draw better static airflow (with out fan/blower) through the adapt-a-damper, or is there another compatible damper/fan system that is able to draw more air? It would be great to be able to run it without the need for using a manual air intake.

Cheers
20210817_142833.png
 
@BradCowain on my UDS I have this for settings

P 3.5, I .0035, D 6
Fan output: 15% min 0%, Max 100%, startup max 65%
Servo pulse duration closed 1060, open 2140, fully open at 50%.
Click to expand...

I have barely a clue on it all, but it holds my UDS at 250 without issue. I have a single "chimney" that is wide open and my damper/fan hooked at the bottom inside a 3/4" opening - there is no restrictions between the damper/fan and the UDS except for the damper itself. no other inlets are open.
 
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