Bed PID Tuning: How long should it take?

moth4017

@DonStauffer the bed PID tune does take a long time i have a 340mm x 340mm x 8mm thick Keenovo 700W /220Vac , if you have a heated or insulated chamber even longer .
here is my PID tune if it helps

My sensor is in a hole drilled in the edge of the aluminum bed . not using the one on the silicone heater

M307 H0 R0.838 K0.331:0.000 D3.08 E1.35 S0.90 B0

if you send M307 H0 in the console you will get a report
Heater 0: heating rate 0.838, cooling rate 0.331, dead time 3.08, max PWM 0.90, mode PID
Predicted max temperature rise 198°C
PID parameters: heating P69.2 I2.156 D149.1, steady P69.2 I6.103 D149.1

moth4017

from the settings above i ran an auto tune to see how long it would take , ambient air 18C
M303 H0 P1 S100
the PID tune took 37 minutes and i got back this response

Warning: heater behaviour was not consistent during tuning
Auto tuning heater 0 completed after 4 idle and 25 tuning cycles in 1620 seconds. This heater needs the following M307 command:
M307 H0 R0.812 K0.268:0.000 D3.35 E1.35 S1.00 B0
Edit the M307 H0 command in config.g to match this. Omit the V parameter if the heater is not powered from VIN.

My first auto tune took much longer

so my bed has been working fine with those settings above and the results were not much different .

previous settings working ok "M307 H0 R0.838 K0.331:0.000 D3.08 E1.35 S0.90 B0"
settings after the PID tune "M307 H0 R0.812 K0.268:0.000 D3.35 E1.35 S1.00 B0"

DonStauffer

@moth4017 Mine took 2 hours. I'm a bit disappointed that the bed doesn't heat any faster with the insulation. I've done this before almost the same way and did get a massive decrease in heat time (it was a Marlin machine). Maybe this tuning algo doesn't handle insulated beds very well. If so, maybe tuning it manually might help.

The bed is around 300x300mm. No big deal, everything works. Once the tuning finished I put the line in config.g.

Phaedrux

What M307 did it give you and how did it compare to what you had before?

engikeneer

@DonStauffer the PID tuning cycle measures both heating and cooling rates. Its always going to take longer to cool down, especially with the additional insulation, hence the tuning cycle took longer over all despite the heating being slightly faster.

As the bed goes through the tuning cycle it kicks off heat and warms up the air around it, more so if the printer is partially enclosed. This means the cooling rate changes slightly over time. This may happen more with the insulation applied as less heat comes out the first few measurement cycles.

Your tuning took the full default 25 cycles to complete. I'm guessing this is because the cooling rate was changing over time (as explained above), meaning it wanted to use more cycles to collect more data, and hence the warning message you got.
I'm guessing previously it did not need to use as many cycles, so also took less time.

The important thing is how it performs in a print. Does it heat up any faster/slower? Is it stable over a print? Those are all that really matter

DonStauffer

@engikeneer It does not heat up faster or slower. I'm disappointed and I wonder if I can manually tune it better.

droftarts

@DonStauffer said in Bed PID Tuning: How long should it take?:

I'm disappointed and I wonder if I can manually tune it better.

Insulation won't make it heat up faster, but may/should make it cool down slower. Tuning just means it should hold it's temperature more consistently. If you want to do it manually, see https://docs.duet3d.com/en/User_manual/Connecting_hardware/Heaters_tuning#setting-the-model-parameters-manually

Ian

DonStauffer

@droftarts My experience is different, and it makes sense.

If you're not losing much heat at all, even while you're heating the bed, it will heat up faster. My experience with aerogel insulation is that it loses very little heat. My other printer will heat up in half the time it did before the insulation.

droftarts

@DonStauffer you said the heater was a Keenovo heated pad. Those have silicone on the back, which provides pretty effective insulation. Was the heater on your other printer the same? Same size and/or power density (ie watts per square cm)? It may have suffered more heat losses if it had a lower power density, that extra insulation improved.

PID doesn’t kick in until the target temperature is approached, so the majority of heating from cold will be at full current. The firmware/Duet isn’t limiting this.

Ian

DonStauffer

The other was also Keenovo, but 400W @24V rather than 750W @120V. This isn't subtle: the insulation on the other one doubled the heat up speed and also stabilized running temperature. The insulation on the Railcore has no effect on either, it seems.

It's not a huge big deal. Everything works. I just expected different behavior, and wondering if I really got a good PID tuning or not.

Phaedrux

@DonStauffer said in Bed PID Tuning: How long should it take?:

400W @24V rather than 750W @120V

The effect would be more noticeable at 24v than 120v.

DonStauffer

@Phaedrux Would it? I would think the wattage might affect it, but I don't see why voltage would.

Phaedrux

Wattage is the result of voltage and current. The lower voltage heating rate would be more affected by the cooling rate of heat dissapating into the air, which is more easily overcome by the higher voltage.