Absolute encoder for Z axis



  • Hi, I'm new to the forum but I've been using Duet Ethernet for some time now.

    I would like to know if there is a way to use an absolute encoder for the Z axis.

    I own a machine center and it's really fast to just turn on the machine and continue working knowing that your origins are in the exact same position as they were the day before.

    Now for the 3d printer I find it really frustrating to deal with Z homing sensors. To the point I've actually remove it from my printer and set it manually every time I need to use it. That way I always have a perfect first layer.
    But if the Z axis would have an absolute encoder I would (most of the times) only need to set Z0 once. And no matter if I turn of the printer, the Z0 would be retained.

    It only need to function at homing, not while printing, so perhaps it´s something doable...


  • administrators

    What type of encoder would you propose?



  • Well, I actually didn’t find one, nor did I looked for too much. But this got my attention https://www.kickstarter.com/projects/301805155/ustepper

    An expansion board with only the encoder would be feasible? I know it’s something I would buy.

    It would only need to function when homing Z for the first time and then only when power up the printer.
    That way, unless something goes wrong with your bed like a collission, you would never need to home Z.

    Sorry if my English is not very good, it’s been a long time since I wrote something on an English forum.



  • Seems pretty similar to a Mechaduino?

    http://tropical-labs.com/index.php/mechaduino



  • It seems the same, only one uses 12bit encoder and the other 14bit (I don't know the difference).
    But, I ordered this:

    https://www.rls.si/en/rmk4-evaluation-board#ow_related

    Wich is what I have in mind.

    I don't know if I'll be able to use it though.



  • Excuse me, but these products have nothing to do with what you were describing in the opening post. They do have an absolute encoder for the position of the shaft, but it will not tell you the absolut position of your Z axis after a power cycle.
    You have to look into absolute linear encoders. Do a google search, you will find plenty of information about them. Incorporating such encoders into Duet is another story...



  • The ustepper claim it can maintain encoder value after a power cycle. But I think you are right, it maintain angular position.
    On the machine center the absolute encoders and the PLC have batteries to keep that information, it doesn't have linear encoders.


  • administrators

    It would be possible in theory to save the machine position to SD card when the power is lost, as long as you are not in the middle of a file upload or some other operation that increased the SD card latency. But it's hard to be sure that the stepper motors will hold position when not powered.



  • Ok, so I read a bit more, and what I assume were absolute encoders are in fact an absolute encoder and a multiturn encoder. Together they give the position information (wich is what machine centers have). They have a battery backup for power loss. What I still don't know is if they continue reading with the battery or only holds the last position known.
    If that 's the case, would it be possible to park the motor at a full step so it will keep that position at power loss?

    So to sumarise, we have angle and turn count, knowing the pitch of the screw we can calculate the position at start. After that it keeps working open loop as always.

    I found this: https://www.ichaus.de/PV1M1

    What do you think?


 

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