PID tuning to the wrong temp!

Hi all,

I am having an odd issue with my hotend that comes and goes.

Printing ABS at 225, so i ran the PID autotune (M303 S225) and got....
#define DEFAULT_Kp 15.84
#define DEFAULT_Ki 0.51
#define DEFAULT_Kd 123.51

I entered these into my firmware and uploaded it.

BUT, when i set the temp via pronterface to 225, the current to my hotend switches off (@0) at 215.
Admittedly, it is very good at keeping at 215, but i need it hotter.

I had a look through the firmware and it seemed an odd coincidence that my PID functional range was set to 10
#define PID_FUNCTIONAL_RANGE 10

So i changed it to 8 - #define PID_FUNCTIONAL_RANGE 8 - and guess what happened.

My hotend peaked at 217!

I thought the functional range was the band inside which PID operated, outside it was PID max.

So shouldn't it max up to 215, then continue to climb up to 225?

I could set it to zero, but that would just be bang-bang wouldn't it?

To further complicate things i have had limited success using M104 S225 to fix the temp, but not always.

Does anyone have any ideas what is going on?

Thanks CT

Are you sending enough power to the hot end? Maybe check the wiring and resistor values. It could be that you're not capable with your current setup to supply enough power to get above that temp.

Thanks for your help Veen, but I dont think its an electical issue.
If it was an eletrical issue the temp would stall even though the PWM setting for hotend was on full.

The software is actually turning the hotend power down before the temp is reached (@128 below 215C, to @0 above 215C).

I will do another experiment tonight and see what happens if i set the temp to 235C. If it gets to 225C and then switches off it is definetly a software issue.

Thanks CT

Edited 1 time(s). Last edit at 05/16/2013 08:23PM by ChaseT.

EDIT: I posted wrongly.

Edited 2 time(s). Last edit at 05/22/2013 01:40PM by changfrancis.

Ok - I am now very confident it is not an electrical issue, but rather a software one.

I have tried with a variety of temperatures and it always comes up 8 degreed short.

233=225
225=217
185=178

Given i have set #define PID_FUNCTIONAL_RANGE 8 this seems to be the culprit, but i have no idea how to fix it.

Does anyone have any suggestions?

Thanks CT

It's probably that D is way too high.
Try increasing P slightly and decrease D by about 1/2 to start with.

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For me PID autotune has -never- worked. Tried it many times, and with the standard settings in Marlin the hotend heats up quickest and never loses temp. No idea why it doesn't work. I put in the values in the firmware correctly, but the hotend just underperforms compared to the standard...

I hope you manage to get it working.