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PID Tuning Fails - "bad curve fit"

Posted by Tryptamine 
PID Tuning Fails - "bad curve fit"
April 26, 2021 08:56AM
Hey guys,

My printer is using a 12V heating patron on a 24V PSU.
Suboptimal, i know, but hard to change due to the design of my printer.
According to ohms Law, the heat output is 4 times higher as if used with 12V, which i could compensate for with a PWM Duty cycle of 0.25.

I used RepRap 2.x before. Back then i was able to autotune the Heater and it was completely stable.
In RRF 3.x this changed.

I updated firmware and PID tuned, which oddly worked back then and lead to stable temps. Settings were:
M307 H1 R10.72 C150.0 D3.34 S0.60 V24.0

Today the temp overshot by 20ish Degrees. So i tried to autotune again and again, using
M303 T0 P0.5 S220 A24

and i always get

Warning: Auto tune of heater 1 failed due to bad curve fit (R=10.395, 1/C=0.0067:0.0127, D=3.3)

Also for P of 0.3, 0.6, etc.
Using the report I calculated C to be 113.996. With the settings

M307 H1 R10.395 C113.996 D3.3 S0.50 V24.0

I get a perfectly flat curve at 220°C, if heated from like 200. If i heat from ambient, i overshoot by 20 degrees and the printer shuts down.

Why so?
Are the M307 settings of config.g used when i run M303?
Can i somehow fit a model by myself?
I can't find a proper explaination for R,C,D and how they relate to P,I,D so i can maybe tune them manually.

Any help is greatly appreciated.

Edited 2 time(s). Last edit at 04/26/2021 09:15AM by Tryptamine.
Re: PID Tuning Fails - "bad curve fit"
April 27, 2021 03:34AM
I have replied to your similar question on the Duet3D forum. Basically, RRF has measured your heater parameters, estimated that the hot end would reach nearly 1600C if run at full power, and treated that as not reasonable.

I think you will find that your manually-constructed M307 command is being rejected and RRF is using default hot end parameters for that heater, hence the severe overshoot.

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
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