3 motor bed leveling

The round heated bed for my printer design will be mounted to a hexagon shaped 2020 extrusion frame. That support frame will mount to three motor driven carriages (like a delta), each connected to one vertical leg of a delta type chassis. The print head is a fixed polar rotating assembly mounted at the top of the chassis and is non-adjustable for leveling, other than base leveling the frame itself. The bed and bed support frame will move along the Z axis.

So I'm thinking to place a horizontal pivot connection at each of the three carriages, where carriage position relative to each of the other two, can be determined in programming via 3 position sensors (mounted flush with the print bed surface) triggered by the print head tip. So much like end stops, the bed will rise via the three motor drives at the beginning of each print, to trigger each of the 3 position sensors (one at each 120° carriage position in the bed) . Programming will 3 point auto level the bed depending on print heat tip position at each sensor location across the bed surface.

So the design would require 2 motors for the print head assembly, and 3 motors for (1 each) for the three vertical risers that position the bed assembly. Each of those 3 bed motors would need its own stepper driver acting independently to position the bed at the beginning of a print calibration routine. Ideally, one would like then for the three bed motors to be driven in parallel by a single stepper controller for the duration of the print cycle.

Does anyone see how to configure the bed motor drivers for such a configuration. The step drivers could all be external units with sufficient load capacity to handle one or all of the 3 bed motors independently or simultaneously.

I proposed using three Z motors to auto level a large print bed several months ago, but nobody seemed to like the idea. You don't need 3 position sensors because you can use a single height sensor on the print head, such as my differential IR sensor described at [miscsolutions.wordpress.com].

A Duet plus a DuetX4 expansion board would provide more than enough stepper motor drivers for this arrangement. In RepRapFirmware (which runs on the Duet), all the homing operations are defined in gcode macro files. There are also special gcode commands to drive motors independently that are normally driven together. So there is already most of the parts of a mechanism that would let you do bed levelling at the same time as Z homing. Or you could do it as a separate operation, because bed levelling is also defined by a macro file.

There is still some work that would need to be done in RepRapFirmware, for example to drive all 3 motors together in normal use. I would be willing to do this, because I would like to see this idea tried out.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Hi Dc42...I do recall reading something about that but can't find it now in your past posts. Please share the link again here so I can go back and study what you proposed then. I appreciate your offer, and I'm very interested in discussing this and more with you relating to my Polar Hex design. Thanks...TP

I finally found your post on the 3 motor bed leveling idea David.... I was thinking of a different one involving micromotors that you also commented on. Having now read your post, I see and agree that we are talking about the same concept. I'd like to talk to you more on that, and generally the idea of using a Duet controller and DuetX expansion board with your configuration of RepRapFirmware for the Polar Hex printer design. Please PM me when you have time to discuss putting a package like that together. Thanks...TP.