PID Tuning help needed

Reefwarrior

@jay_s_uk said in PID Tuning help needed:

the oscillations are normal and it can take up to an hour or more to PID tune something. just let it run its course

Thanx, but is it normal to shoot 30+ degrees Pas target, and the oscillations do not get smaller!?

jay_s_uk

@Reefwarrior yes. its measuring cooling and heating times

Reefwarrior

@jay_s_uk Thank you - I will try again and let you know. Weird is that I started the PID process with a temperature of 240C, but on initial heating now it went past 320C. I suspect that the glass thingy (I forgot the name now!) that measures the temperature may not be calibrated/defined correctly.

I know in Marlin you can choose the different options, but I haven't come across it in the duet firmware yet. Or am I missing something?

Tinus

jay_s_uk

@Reefwarrior yea, it shouldn't go as high as 320 on a 240 degree tune.
The M308 line in config sets the thermistor. best finding out what you had in marlin so you can replicate it here

deckingman

@Reefwarrior It sounds to me like you have a very over powered heater. Might it be 12V heater running in a 24V system? Are you able to measure the resistance of the heater cartridge? If so, post the value here and one of us will work out the wattage.

Reefwarrior

@deckingman Hi, I get 4.1ohm. I don't know if this can be correct, as it 'feels' too little?

jay_s_uk

@Reefwarrior typical resistances

P\V --------------------- 12V---- 24V
40w (Red Leads) 3.6Ω 14.4Ω
25w (Blue Leads) 5.76Ω 23.04Ω

deckingman

@Reefwarrior said in PID Tuning help needed:

@deckingman Hi, I get 4.1ohm. I don't know if this can be correct, as it 'feels' too little?

So, if my maths serves me correctly, V=IR, so I=V/R = 24/4.1 = 5.8536Amps. Wattage = VI = 24 x 5.853 = 140.48 Watts !!

If the voltage was 12, then the current becomes 2.9 Amps so the wattage would then be 12 x 2.9 = 35 Watts.

So it looks you have a nominal 40 watt \ 12V cartridge so when you put 24V through it, you get roughly 4 x the heating power.

Note. A 40 Watt/24V cartridge would have a resistance of about 14.5 Ohms.

Edit. A 40W/ 24v heater cartridge would draw about 1.65 Amps which is perfectly safe to use with the onboard MOSFETs - no need to use an external one.

Reefwarrior

Thank you all for the feedback. Yes seems I am running a 12V then at 24V. I take it I can't just put in a power factor of P0.25 to fix this? Or will it be better to rather just swap it out for a 24V cartridge?

deckingman

@Reefwarrior said in PID Tuning help needed:

Thank you all for the feedback. Yes seems I am running a 12V then at 24V. I take it I can't just put in a power factor of P0.25 to fix this? Or will it be better to rather just swap it out for a 24V cartridge?

Running 25% PWM is bad idea IMO. The problem is that, if a MOSFET fails, then you could end up with the full ("permanent" as in non PWM switched) 24V which could be a fire risk.

jay_s_uk

@Reefwarrior swap it. it will be unsafe until you do so

Reefwarrior

Cool thank you. I will swap it out this afternoon then, and see if it is better.

With regards to the temperature measurement. Is it normal for it to jump up and down a few degrees? The graphs show spikes, instead of a normal flat line....

deckingman

@Reefwarrior said in PID Tuning help needed:

Cool thank you. I will swap it out this afternoon then, and see if it is better.

With regards to the temperature measurement. Is it normal for it to jump up and down a few degrees? The graphs show spikes, instead of a normal flat line....

I think you'll find it's a lot more stable with a cartridge that is a better match to the hot end. The PID control can only do so much and when you have a heater that is over powered by a factor of 4 then you are likely to get overshoots when it turns on.

droftarts

@Reefwarrior said in PID Tuning help needed:

Is it normal for it to jump up and down a few degrees?

No, check your wiring. Either it's picking up interference (usually from stepper motor or endstop wiring), or there's a poor joint. It will also cause the PID tuning to fail, as it can't get a consistent reading, and it's probably why it's taking so long to tune.

Are you using a thermistor PT1000, PT100 or thermocouple? There are ways to make the last two more reliable, by using four wires rather than just two.

Ian

Reefwarrior

@droftarts Hi Ian,

It looks like one of those little glass bead thingies that measures the temperature....

droftarts

@Reefwarrior Sounds like a thermistor. Check your wiring, and make sure the leads of the thermistor aren't grounding on the heater block, and that the thermistor bead is making good contact with the heater block (ie not flapping about in a big hole!). Cartridge thermistors, or PT1000, are generally easier to use and get good contact with. What kind of hot end is this?

Ian

Reefwarrior

@droftarts E3d v6 clone. I will take a look at the hotend too....

deckingman

@droftarts Did you check the earlier posts? It transpires the OP has a 12V cartridge and a 24V system. So around 140 Watt instead of 35. Methinks, that's more likely to cause the oscillations.

droftarts

@deckingman Yes, read the whole thread. @Reefwarrior said "The graphs show spikes". While I'm sure the heater can create a spike up in temperature, it probably can't create a drop as quickly! But a dodgy thermistor can, though I guess it could be a power drop across the board when the heater switches on, if the PSU isn't up to it.

@Reefwarrior A screenshot of the temperature graph misbehaving would go some way to disambiguate the cause of the "spikes". Try wiggling the thermistor wires, see if that causes the spikes, too.

Ian

deckingman

@droftarts I guess it depends on the definition of "spike". I read this (quote)

Other strange thing is, there are quite a lot of oscillations with regards to the temperature being sensed. (Up to 3 to 4 degrees up and down)

As you say, we need a picture of the graph......