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Hotend on 24V, Kp Ki Kd trouble...

Posted by HugoW 
Hotend on 24V, Kp Ki Kd trouble...
September 07, 2017 02:12PM
Hi,

tonight I am trying to finalise my hotend settings and melt the first PLA. I heated the nozzle to 200C and overshot by about 70 degrees. Not good. I have a cheap Mk8 extruder from Ali Express which is made for 12V and I run it on 24V, so there is probably the issue; wrong p, i and d settings. So I tried Marlin's pid autotune: m303s200c10 and GO! It quite quickly shot to 270 degrees C and autotune called it quits, temp to high. I understand. I need some pid settings to start, more carefull than my current ones, for autotune to work. I currently have:
Kp 22.2
Ki 1.08
Kd 114
But I have no clue to what I should change it. Googling the terms has got me even more confused. Please help me on some better figures to start with so I can use autotune to do the rest.

Cheers,

Hugo

Edited 2 time(s). Last edit at 09/08/2017 02:30AM by HugoW.
Re: Please advise on Kp, Ki and Kd settings to start
September 07, 2017 03:15PM
Check to see if your firmware has a maximum PWM setting that can be changed, you might want to set it to 50% of its present value.
Re: Please advise on Kp, Ki and Kd settings to start
September 07, 2017 04:38PM
OK, good tip. Didn't work at 50%, but made me get closer. I finally lowered the value from 255 (full blast) to 32 (1/8th blast...) and now autotune is running. I just got new settings and I will try them I will gradually up the pwm again and let autotune run again and again.

Cheers,

Hugo

Edited 1 time(s). Last edit at 09/08/2017 02:41AM by HugoW.
Re: Hotend on 24V, Kp Ki Kd trouble...
September 08, 2017 02:40AM
I tried a bit more but above 1/4 power, PWM 64, it runs out of control, overheats to the point the autotune gives up. Is this normal? I read about other people running a full set-up on 24V, do they all tune back the hotend PWM?

Besides that, running at PWM 32 (very low power) gives +/- 2 degree fluctuation, 64 gives a +/- 5 degree variation. Is +/- 5 degrees acceptable? I think it is a bit much but I don't like reducing the power as I want to melt a lot of PLA printing at 100mm/s.

And I think it would overshoot a lot less running autotune (or in use) while running the extruder. I think the PLA melting will take a lot of energy hence reduce the overshoot considerably. But how would I test that? Is there an autotune setting which makes the extruder run?

All input is appreciated,
Cheers,

Hugo
Re: Hotend on 24V, Kp Ki Kd trouble...
September 08, 2017 03:47AM
Quote
HugoW
......, do they all tune back the hotend PWM?........

Hi Hugo,

maybe most of them using 24V Heaters smiling smiley

Another way can be to decrease the voltage of the PSU by turning down the Potentiometer.


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Siggi


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Re: Hotend on 24V, Kp Ki Kd trouble...
September 08, 2017 04:00AM
Hmmmm, now you're making too mucht sense... That might be a solution, although I wonder why I wouldn't be able to run my hotend at 256 PWM (half power) which should be the same, power-wise. But I'll give it a shot, for the price it doesn't hurt to try.

Cheers,

Hugo
Re: Hotend on 24V, Kp Ki Kd trouble...
September 11, 2017 06:06PM
The reason for your issue is that at 24v a 12v heater produces 4 times the power not 2 times. So PWM 64 will give you about the right overall amount of output. But you are still outputting 4 times the rated power per pulse, you're just sending 1/4 of the pulses.

The other issue is that should your mosfet fail closed you will melt your heater block 4 times faster.

Better to get a 24v to 12v converters to run your 12v heater on your 24v system, or spend £3 and get a 24v heater, which will have higher resistance.

Edited 1 time(s). Last edit at 09/11/2017 06:07PM by DjDemonD.


Simon.

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Re: Hotend on 24V, Kp Ki Kd trouble...
September 11, 2017 06:11PM
As DJDemon says, you want to reduce your PWM to 25% of full power.

However your P factor is probably set too high aswell. Drop this to 5 and then see how heating goes with PWM at 25%. If it is slower and doesnt overshoot, then you can dial up the P value a little bit, or just run the PID with the lower P value.

You can look at PID in a different way winking smiley


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