Wiper Pad Alternatives

A forum dedicated to the stock LulzBot Mini 3D Printer
IDEngineer
Posts: 42
Joined: Sun Feb 05, 2017 2:36 pm

Re: Wiper Pad Alternatives

Post by IDEngineer » Tue Nov 20, 2018 5:07 pm

Thank you for this response, see my question below.
Galadriel wrote:
Tue Nov 20, 2018 3:30 pm
Enter that information into your Start Gcode in Slic3r using the following format...
I only use Cura. Are the GCode commands the same in Cura?

IDEngineer
Posts: 42
Joined: Sun Feb 05, 2017 2:36 pm

Re: Wiper Pad Alternatives

Post by IDEngineer » Tue Nov 20, 2018 5:12 pm

Galadriel wrote:
Tue Nov 20, 2018 3:30 pm
Something that might help would be completing a PID tuning on this toolhead.
And: I understand what PID control loops are, and understand how tuning the PID parameters might help the nozzle temperature be held more consistent during a print job, but I don't think they will be more accurate. As long as the only reference the firmware has for measuring the nozzle temperature is this thermistor that is off by ~25C, there's no way to make the temperature more accurate. There needs to be some sort of external reference against which this thermistor can be calibrated. After that, the firmware can adjust for the specifics of this (or any) thermistor.

In this Mini's case, we know based on an external, calibrated lab temperature meter that this thermistor is off by -25C in the range used by the printhead. Sure would be nice and helpful if there were some way to inform the firmware (or even Cura) of this known error.

Galadriel
Aleph Objects | LulzBot
Posts: 271
Joined: Mon Sep 25, 2017 10:23 am

Re: Wiper Pad Alternatives

Post by Galadriel » Wed Nov 21, 2018 9:51 am

IDEngineer wrote:
Tue Nov 20, 2018 5:07 pm
Thank you for this response, see my question below.
Galadriel wrote:
Tue Nov 20, 2018 3:30 pm
Enter that information into your Start Gcode in Slic3r using the following format...
I only use Cura. Are the GCode commands the same in Cura?
Sorry about that! Those GCode commands will be the same in Cura as they are in Slic3r.
IDEngineer wrote:
Tue Nov 20, 2018 5:12 pm
Galadriel wrote:
Tue Nov 20, 2018 3:30 pm
Something that might help would be completing a PID tuning on this toolhead.
And: I understand what PID control loops are, and understand how tuning the PID parameters might help the nozzle temperature be held more consistent during a print job, but I don't think they will be more accurate. As long as the only reference the firmware has for measuring the nozzle temperature is this thermistor that is off by ~25C, there's no way to make the temperature more accurate. There needs to be some sort of external reference against which this thermistor can be calibrated. After that, the firmware can adjust for the specifics of this (or any) thermistor.

In this Mini's case, we know based on an external, calibrated lab temperature meter that this thermistor is off by -25C in the range used by the printhead. Sure would be nice and helpful if there were some way to inform the firmware (or even Cura) of this known error.
I have never personally encountered a thermistor behaving as you have described so the PID tuning was mostly a suggestion of something that it sounded like may apply to the situation and be helpful.

Aside from trying to fine tune temperature control on your toolhead if you are seeing that drastic of a difference between your thermistor read out and the actual temperature I would suggest checking your thermistor itself to see if it might be damaged. If you have access to a digital multimeter, you can check the resistance measurements on the tool head to see if the thermistor or heater cartridge is bad. With the printer turned off and unplugged, disconnect the hot end from the wiring harness and test hot end for the proper resistance. The smaller black and red wires coming from the hot end are for the thermistor and should read ~100k ohms at room temperature. The larger red wires are connected to the heater cartridge and should read ~20 ohms.
Here is a link to a photo of the pinout diagram for your toolhead that may also be helpful: https://ohai.lulzbot.com/media/uploads/ ... x0_q85.jpg
Your heater wires are in positions 5&6
Your thermistor wires are positions 15 and 16

IDEngineer
Posts: 42
Joined: Sun Feb 05, 2017 2:36 pm

Re: Wiper Pad Alternatives

Post by IDEngineer » Wed Nov 21, 2018 2:43 pm

Galadriel wrote:
Wed Nov 21, 2018 9:51 am
Aside from trying to fine tune temperature control on your toolhead if you are seeing that drastic of a difference between your thermistor read out and the actual temperature I would suggest checking your thermistor itself to see if it might be damaged.
It would have to be defective, as it's done this since Day One.
If you have access to a digital multimeter, you can check the resistance measurements on the tool head to see if the thermistor or heater cartridge is bad... thermistor should read ~100k ohms at room temperature.
Yep, did this when I was debugging with your tech support a while back - when they sold me the new print bed, that I then couldn't install until I ALSO spent more money and bought the new heater for it too. I don't recall the value but I read it to your tech support guy in real time, on the phone, and he was happy with it.

I can do it again, if you like, but nothing has changed since so I suspect we don't discover anything helpful.

IDEngineer
Posts: 42
Joined: Sun Feb 05, 2017 2:36 pm

Re: Wiper Pad Alternatives

Post by IDEngineer » Thu Nov 22, 2018 12:46 pm

Galadriel wrote:
Wed Nov 21, 2018 9:51 am
Your thermistor wires are positions 15 and 16
To be accurate, on this Mini the thermistor wires are in positions 17 (red) and 18 (black). See Figure 6 here:

https://ohai.lulzbot.com/project/86797b ... 0547/mini/

My Fluke meter shows 145.5K with the nozzle at 19.7C. Quick linear interpolation based on the 104GT's spec sheet suggests the ideal value at 19.7C would be ~135K, so it's "off" by ~7%. If our target nozzle temperature is 220C and the Mini actually yields (220 - 25 =) 195C, that's an error of ~12%. So the thermistor isn't the (only?) problem in this Mini.

What we need is some ability to inform the Merlin firmware (or at least Cura, which could then offset it) of each particular thermistor's curve offset. The spec sheet itself quotes a tolerance of +/-3%, so even if you don't want to calibrate them at the factory there should be a way for more precise users to calibrate to the specifics of their equipment.

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