moteus servo mk2

As described in my roadmap, making a new revision of the moteus servo is up there on my list of things to do.  The initial servos were a work of art, yes, but also pretty fragile, very labor intensive, and still not all that robust.  My goals this time around are:

  • Manufacturability: The servo mk1 took about 2 or 3 man-days of manufacturing time per servo once all the steps were factored in.  I’d like to get that down to an hour or two at most per servo.
  • Robustness: The planet input, outer housing, back housing, and controller cover of the mk1 servo were 3d printed, mostly to save cost and time.  This necessitated adding reinforcing rings on the outer housing, as it is nearly impossible to 3d print something with the required material properties in a single print.  At this point, all of these components should just be made of aluminum like the others.
  • Repairability: Once the mk1 was assembled, there was no way to disassemble it, as installing the stator interfered with the ability to remove the outer housing, and the outer housing in place interfered with the ability to remove the stator.
  • Convenience: The mk1 servo used the r3.1 moteus controller, which had RS485 connectors sticking straight out the back, and bare power wires coming out the back.  That orientation for connecting things was not terribly convenient in the full rotation leg design, and required making extension cables.  The newer moteus controller has the connectors sticking out the bottom, so the servo needs to accommodate that.

CAD explosion

moteus_mk2_side_view
CAD rendering

Changes in moteus servo mk2

The current design for the moteus servo mk2, as the exploded video above shows, has a number of changes.

First, the outer housing has been changed to be purely cylindrical.  This allows it to be machined out of round tube stock, and also assembled and removed in any order.  Thus the front housing now has a slightly larger outer diameter, and has threaded holes around the perimeter and 8 primary mounting holes instead of 6.

The rotor is custom machined, so that the sun gear holder assembly is no longer required.  A not shown in the explosing mini 3d printed adapter will hold the magnet and fit inside the rotor bearing on the back.

The planet input now has a small indexing slot to eventually register a magnet holding assembly that can be used to sense the position of the output stage, and a position sense board is installed in the front housing to sense it.

All of the gears are now custom manufactured (at shop.mjbots.com – Internal Gear, Planet Gear, Sun Gear).  This reduces the cost considerably, improves tolerances, and requires no post-machining.

The back housing has been updated to mount a newer moteus controller, provide heatsinking to it, and also be slightly slimmer due to being manufactured from aluminum.

The overall dimensions are approximately the same as the mk1, with the depth increasing by a few millimeters (largely because of the connectors on the new moteus controller), and the outer diameter decreasing by a few millimeters.  I believe I should be able to get the weight to be about the same as the mk1, around 430-450 grams.

Next steps

First, I’ll make a functional prototype to verify that all the parts fit together and work.  Then I’ll work to get the weight back down to closer to the mk1, after which I’ll start producing enough of these to make more robots.

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