Load cell for z probe and hot end pressure messuring

  • hi guys how can i connect a load cell as z probe on my duet2 ?

  • Moderator

    Can you give some more information?

  • i have a 4 wired load cell mount on my nozzle as bridge to the extruder ( direct drive ) and now i want to messure the pressure from the filament to the nozzle ! maybe as gauge at the display ! and i think i can use this also for bed leveling and auto 0 point ( hot nozzle ) before the print starts ! like the renkforce rf2000 ? my printer is an cartesian printer with double z motors and 2 endstops and 1 Motor on x y axes !

  • Moderator

    Thanks. What kind of signal do you get from the load cell?

  • i have a board with the connectors ( E+ E- A- A+ B- B+) for the load cell and GND DT SCK VCC to the board !

  • ore should i pin the load cell directly into the board ?

  • I also have the header extansion for external stepper drivers connected !

  • Moderator

    I'm not sure if what you want is possible, but now you've provided enough information that hopefully someone else will be able to answer.

    Also, do you have a link to the load cell in question?

  • thanks for your attempt !

  • The HX711 board is an instrumentation amplifier which reads the tiny deviations due to the load cell changing with pressure and converts it into a digital readable signal. Unfortunately these are not yet directly supported with the duet firmware. You could however use a small Arduino to convert the signal into a trigger signal understood by the duet board.

    Just one word of caution: the analog signals (E+, E-, A+, A-) are very sensitive to noise, if you want to continue this approach you need to install the hx711 board as close as possible to the load cell and as far away as possible from heater wires and steppers.

  • OK ! Thank you very much for your fast support ! mayby dc 42 will work for this thinks in future !

  • I’ve worked on several custom designs for this already, including the electronics. A couple of big draw backs are:
    1: you need to limit heat on the sensor - the heat will cause sensor errors.
    2: you need to pay special attention to the mechanical load path. In engineering, it’s known as a free body diagram. If the load transfers through anything other than the cell, it’s essentially useless. You need something between the motor / Bowden tube and the hot end.
    3: combine 1 and 2. Generally to sense the load path, you’re going to run into either 1 or 2. If you’re far enough away where you don’t get heat problems, you’re probably no longer in the load path.

    I ultimately came up with a plausible design and manufactured most of the system, but then I had another breakthrough and measuring the filament force was no longer necessary.

  • administrators

    I really don't advise using the HX711. It's a low-cost device intended for cheap weighing scales, the sort that set themselves to zero when you turn them on, then when it reads zero to step on it (or weigh whatever it is that you want to weigh). The datasheet doesn't even specify a maximum input offset voltage, just an "input offset voltage drift" whatever that means (it doesn't say). Users report that it drifts quite a lot.

    For accurate results, I suggest you use either a good instrumentation amplifier such as the INA333 or an all-in-one load cell interface IC such as ADS1242.

  • Thank you for your support

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