SOLVED Interference between extruder motors and PT100

  • Hello.

    I'm in the process of repairing an old BigRep ONE v1.2 printer that had a fatal failure on the mainboard. Im replacing the old electronics, manual PIDs and small stepper drivers with the Duet 3 6HC, but keeping stuff that would be expensive or hard to change, like the original beefy drivers for the cartesian movement or the temperature sensors PT100. Right now the extruder motors are connected directly to the 6HC, the rest of the motors are connected to external drivers though 1XD boards and the temperature sensors (two for the double hotend and one for the bed) are connected through two sensor daughter boards. Everything else is more or less standard. When I test every component of the machine individually everything seems to work fine, machine moves, hotends heat, extruders move, temperature readings are fine, etc. But the problem arises when I try to extrude material, every time when I try to extrude o retract material the temperature reading of both extruders goes up to 2000°C. I've realized that the temperature is misread every time the extruder motors are enabled and they go back to working properly when I disable the motors. There must be some kind of interference between the motors and the sensors, and their cables goes through the same tubing for several meters, but I have not been able to find any continuity between any of the motor coils and the sensors or the machine ground and I can read correct sensible values on the PT100 when the motors are enabled using a normal multimeter. I come to ask for any recommendations, help or advice that could help me troubleshoot this problem, or maybe information regarding how the sensor daughter board reads the PT100 and what can be interfering with this reading.

    Thank you for your attention

  • pt100 are susceptible to interference

    you could try twisting the cables and see if that reduces the interference
    or shielding the cables

  • @Idir
    On one of my printers I could reduce that by grounding the hotend-heatsink.
    What I always do, is twisting the stepper cables.

  • @o_lampe said in Interference between extruder motors and PT100:

    What I always do, is twisting the stepper cables.

    or braiding them 4 way braid

  • The factory sensor comes connected on a 3 wire configuration and sadly I can't access them because they are heavily encapsulated all the way though, but I can try grounding the extruder-hotend setup as suggested by @o_lampe. If that doesn't work I guess I'll try switching them for PT1000 since they seem more resilient. I'll update with the working solution if I get there for future reference.

    Thank for the quick response!

  • Moderator

    3 wire should be pretty resilient to interference. 4-wire is best, but 3 should be ok. Hopefully the grounding helps.

  • @Idir
    there's still a chance, you have a broken sensor or bad solder joint. The connections between mainboard and daughterboard can also cause issues. Maybe the extruder draws too much current, it causes a brownout or huge voltagedrop on the daughterboards?

  • administrators

    Unfortunately, stepper motor cables generate interference in other wires that they run close to.

    Please post a photo of the PT100 daughter board so that we can see what revision it is. The newer boards have additional capacitors to help reject interference. It's possible to add capacitors to the older boards.

  • @dc42

    Here's the picture of the boards. Sorry for the quality, but it's what I managed to get from mi client.

    PT100 Board 1 PT100 Board 2

    I have not been able to try yet grounding the head yet.

  • administrators

    As the PT100 wires run close to any stepper motor wires for a substantial distance, then using shielded cable for the PT100 wires should solve it.

  • The PT100 wires and the extruders wires go through different cables (although I suspect they are not insulated) and are relatively separated, but the cables go through the same ducts for about 4-5 meters, so I guess that might be a problem. I don't have detailed photos of the cable arrangement, but here's an overall look of the machine in case is helpful.


  • @Idir said in Interference between extruder motors and PT100:

    The PT100 wires and the extruders wires go through different cables (although I suspect they are not insulated)

    Sorry, I intended to say that they are probably not shielded. They are of course insulated and I've checked for any derivation.

  • 2000 deg sounds more like a connection issue then interference .

  • After fiddling everything for several hours I didn't manage to find a bad connection or to solve the problem. I ended up changing the sensors for thermocouple cartridges and after connecting them directly to the board they just work, so I guess that's the end the problem for now.

    Thanks to everyone for the quick response and the help.

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