Automatic bed levelling using 2 or 3 Z motors



  • Hi Bart,

    No problem. I think the idea that David is putting forward is this. Probe the bed in the centre. i.e. enable all 3 motors then home the bed to the trigger height - for the sake of argument, lets say it's 2mm. Then move close to one of the screws, disable the other 2 Z motors then using just the one motor, drop the bed by 1mm (to ensure that it is below the trigger height, then probe the bed at this point - i.e using just one motor adjust the bed back to the trigger height of 2mm. If the bed was level to start with, this would be the same position, otherwise it will be adjusted. Repeat this process for the other two screws. A second iteration might be necessary but after that, in theory the bed should be level as long as the motors remain in sync (and nobody turns one of the screws when the machine is switched off).

    So there aren't any offsets to remember and a macro could be created which would run this process automatically.

    HTH

    Ian



  • That could indeed work. It will end up with a leveled bed.

    Only negative I see is that on an allready leveled bed this is moving 1 z motor while others are not moving, so producing tear on the axis which is unneeded.

    I do agree that most systems will not see a lot of problems in this. It is just something that bothers me a bit, but its not the end of the world.

    I plan on leveling my 326x326x8mm aluplate (pei coated) with 3 T8x1 leadscrews (like you use too on your corexy) and then mount the plate RIGID onto the frame that connects to the 3 nuts. I don't plan on implementing leveling screws between the z stage and the buildplate itself since technically speaking this solution should be able to level the plate to 0,01mm anyway and thereafter you can do a mesh bed leveling to factor out al unevenness.

    I still think engineeringwise the best option is to probe the 3 points with all motors on, then calculate offsets and reposition the motors. An already leveled bed will need no adjustments and leveling procedure will be very fast and even on a unleveled bed (which we make this calibration for) the out of sync movements which are more or less harmful for the guide system will only move the least possible (calculated) offset.

    PS. While I have your attention. On your printer what do you use for guiding your Z table so it can't have horizontal wiggle room? I see wheels. Are these special types ? I plan un using 16mm precision shaft and LMF16UU bearings.


  • administrators

    @educa:

    I still think engineeringwise the best option is to probe the 3 points with all motors on, then calculate offsets and reposition the motors. An already leveled bed will need no adjustments and leveling procedure will be very fast and even on a unleveled bed (which we make this calibration for) the out of sync movements which are more or less harmful for the guide system will only move the least possible (calculated) offset.

    I already agreed with you, see my previous post.



  • Yes I understood that David. It was more an answer to Ian actually.

    Is something like this planned to be implemented in firmware anytime soon ?

    I would make a difference on which boards I have to buy for my build. Normally I would buy a duet wifi, connect X, Y, Extruder1 and Extruder 2 to the duet wifi and then map the z drive on expansion header to do my own stuff with it.

    If it is working in firmware, then I will need to buy at least an extra duex-2 so I can hook the X and Y + the 3 Z motors to die duet wifi and then the 2 extruder motors to the expansion board. At east that is what I think I have read that it is best to have 1 axis on the same board, so for 3 z motors that would possibly be my best bet?


  • administrators

    It doesn't make any difference whether you have the 3 Z motors on one of the boards or split between them.



  • Ah ok. I thought I read that somewhere. So if I need X, Y, 3 times Z and 2 extruders, I just need to make sure that I have a DUET + a Duex2 and it doesn't matter how I map them. Nice 😉



  • @educa:

    T….............................

    PS. While I have your attention. On your printer what do you use for guiding your Z table so it can't have horizontal wiggle room? I see wheels. Are these special types ? I plan un using 16mm precision shaft and LMF16UU bearings.

    Hi Bart,

    The best way to answer that is to give you this shareable link to my google drive folder that has pics of the lead screw and Z axis mounting arrangement. https://drive.google.com/drive/folders/0B_MwtHtQR_ZvdHY1dXdxb2RQVm8?usp=sharing

    As with the rest of the printer, the guides are open builds V slot (2020 in this case). However, I started with 2 only guides but when I built it, I found it was possible to "rotate" the bed slightly by hand. Closer inspection revealed that the guides were able to twist because, being 2020 it is only possible to use 1 bolt where they fix to the horizontal rails. The easiest fix at that stage was to install a 3rd guide but if I was starting from scratch, I'd have used 2040 with 2 bolts at each vertical to horizontal intersection.

    I used open builds mini gantry plates and twin bearing Delrin wheels. The coupling between the guide and the plate mount is via a rod end rose joint which allows the vertical bolt itself to swivel to allow for any misalignment whilst still constraining the bed from any "wobble". Then I fitted "O" rings between the brass lead screw nut and the bed mounting plate. The bolts are tightened to just slightly compress the "O" ring and nyloc nuts prevent them from coming loose. This arrangement allows the bed to be tilted slightly without putting any twisting forces on the guides (which I know you are concerned about).

    As I mentioned earlier, I originally planned to use 3 motors for the automatic levelling feature when it became available - that's why I designed the printer this way. I don't have any form of levelling other than by slackening the pulleys at the bottom of the lead screw and rotating the screw. Now that it's built and running, I've found that I adjusted the bed initially, then again after the initial settling in period of a few hours, but since then I've never had to adjust the bed level. So, I've abandoned all plans to change to 3 motors.

    In fact, I don't use any form of compensation either, levelling, flatness or orthogonality (is that a word?). I have "home all" as part of my slicer start code along with bed and tool heating, so when I want to print something, I just turn on the printer, select the file, hit print, and walk away. Now that I've got it all dialled in, it's as easy as printing a document on my PC's printer (takes s bit longer though 🙂 )

    In case you haven't seen it, here is a link to a video that I have shameless bragging rights to. It's printing a 300mm x 300mm square without using any form of bed compensation.https://www.youtube.com/watch?v=U733PMTou7M

    HTH

    Ian



  • That looks great Ian. It's great when a machine is well built, sturdy and reliably level, but these aren't really the machines that auto levelling is aimed at. Once you have a machine like yours it all seems a bit pointless. I can't really see it appealing to people with a z axis with multiple leadscrew with multiple guides, these things are sturdy and once set by manually turning screws relative to one another, it's set for good long while. It's really the cheap more mass produced machines that need it. The bed has to be level/orthogonal, and the easiest way to do this on a cheapo printer would be to have a bed with 2 tiny motors driving small screws to adjust 2 points with a third fixed point and a sensor. DC motors would work if the screws were quite fine pitch.

    I started with ABL on my corexy, then improved the mecahnics, then got rid of ABL as it never went out of level.



  • I'm running a duet 0.6. Upgrade to DuetWifi|Ethernet would require me to get the new board and expansion card. I can't see the Due4 for the Duet 0.6 for sale anywhere.

    Is this card suitable for this feature?

    Can't see why not, but rather ask a question and get a seemingly obvious answer as opposed to getting a surprise after I try it out!


  • administrators

    Yes that should work if 6 drivers is enough. I can't remember what driver numbers the 2 extra drivers are, possibly 5 and 6 instead of 4 and 5.



  • Thanks. I'll bare the numbering in mind!


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