WEIRD temperature swings

[eriksw 0]

Correction: where I said it started spinning at 51%, this was actually at VALUE 51 (out of 255) on the controller, which is 20%.

And that tin foil was a feeble attempt, I admit. I did "earth" it by holding it, but let's forget about that. As said, I want it to work as it did before, so without shielded cables. It should not be necessary to change them...

Holding the fan was a good idea. Here's the test results:

I set the print temp to 200, and the fan speed at value 91.

While holding the fan, the temp readings were slightly longer apart, ranging between 198 and 202, with no steps larger than 2 degrees.

On releasing the fan, the readings were faster (slightly less than a second apart), ranging between 194 and 207, with steps averaging about 3 degrees, with some jumps of 8 degrees.

So definitely more interference when it is turning.

I have a gut feeling the PWM noise will normally be worse when you hold the fan ?

 

[anon4321 16]

Most fans of these type are DC brushless motors and as such are electronically commutated. The electronics usually have a stall detection mechanism that will detect when the fan is blocked and shut off power. Then it periodically pulses the motor to see if it spins.

So if you block the fan, it will momentarily shut off and try to restart in a second or two. If the tach signal isn't detected immediately (within 10s of milliseconds), they turn off the motor power and repeat.

 

[bbkkth 0]

Hello everybody,

We are having the same problem of unstable nozzle temperature. We bought our UM2 one month ago and it was printing very well until yesterday. Now without printing I try to heat up the nozzle and see if the temperature reaches the target temp or not, when set to 210 degrees it has a fluctuation of +-10 degrees. Please let me know how is the nozzle temperature control procedure in UM2, is the fan behind the nozzle used to control the temperature or the temperature is only controlled by the heating element?

Another question is that which fan are we setting in Cura, 2 side fans or the fan at the back of the nozzle?

Hope to hear good news about your printers!

 

[jonnybischof 60]

You should check the wiring with a multimeter! It sounds like you have a slack joint in a cable or a connector..

 

[gr5 2,167]

The 3rd fan (rear fan) is always on and keeps the UPPER part of the print head cool. If this doesn't work you can get filament jams because the filament is too soft.

Cura controls only the side 2 fans.

The temperature is only controlled through heating element and temperature sensor both are inside the print head. You probably have a bad connection on the temperature sensor. It's important to fix this because if the head is at 210C but the temp sensor says it is at 150C then it will keep heating and you can damage things.

Like Johnny says - probably a bad connection in the sensor wiring somewhere. With everything at room temperature I would look at the temperature reading and then poke on every possible part of the sensor wire hoping the temperature will jump/change suddenly. Remove larger bottom cover and check connection there also. But the most likely place for the problem is inside the nozzle or just outside so you might need to take the head apart.

 

[bbkkth 0]

Thanks Jonny and gr5 for your helpful replies.

Do you think that other than hardware (that's very likely as you said), any firmware bug can cause this kind of problems? The reason that I suspect the firmware can cause this is that we haven't used the printer for a long time but I upgraded the marlin firmware 3 times since we got the printer.

 

[gr5 2,167]

But noone else is having PID issues and lots of people have upgraded the firmware. Daid didn't mention any changes in the PID values or the PID code so I really doubt it's firmware. Plus this is a somewhat common problem.

Every time the print head moves back and forth the wires get stressed a little bit. If the wires short a bit that lowers the resistance which indicates a lower temperature.

 

[bbkkth 0]

Yes gr5, you are right. I tried to pull the sensor cable and deform it a little bit, then I could see changes in the magnitude of temperature swings. But if it is a common problem, then what do you guys do to fix it and prevent it the next time?

 

[eriksw 0]

Quite a while since last I posted on this thread, but here's an update.

I FINALLY found out that the temperature swing is caused by a firmware setting called "fast PWM". This setting causes much more PWM noise when you print with any fan speed other than 0 or 100. This noise interferes with the thermocouple signal.

I made a version of the firmware with the "fast PWM" switched off, and now I only get very moderate swings in the temperature reading (2 or 3 degrees max) and (more important) the actual nozzle temperature actually stays in the neighbourhood of the temperature reading, instead of dropping some 10-20 degrees below it. (You can check this by first printing with reduced fan speed, then set the fan speed to 100 on the controller, and then quickly go back to the info page to check the temperature reading.)

But HEY - I installed the new default firmware with Cura 15.2 and it looks like that also solves the problem !

I'll do some more checks later, but it seems like it is back to normal now.

So perhaps the "fast PWM" has been switched off in this version again, or perhaps something else was altered, but I'm not a firmware expert so I'll leave that to those who are.

 

[eriksw 0]

PS

I just noticed that gr5 already mentioned that this could be the cause, back in june. See page 1 of this thread, I quote:

"Another solution (maybe) is to use a copy of Marlin that uses "slow fan PWM". You can do that by unchecking the 4th checkbox from the bottom here. This won't eliminate the problem but it will cause less frequent spikes in the voltage so that hopefully you get less frequent "bad temperature" readings.

http://marlinbuilder.robotfuzz.com/

"

(It's the 5th box from below actually, if you want to try it out.)

Somehow I missed this first time around - too busy replacing all the parts I suspected, I guess....or perhaps I did read it but did not feel like messing with firmware (yet), which is tricky... and you can damage the printer if you don't know what you are doing, just so you know.

 

[randyinla 14]

I subscribe to the philosophy "Don't turn it on, take it apart!" (please read that quote aloud in an Australian accent).

 

Always good to find another Dave fan!