Tag Archives: leg

mk2 leg knee stud

robots, , , ,

One of the parts on the original quad A0’s leg that was prone to failure was the “knee stud”, a little cylinder that acted as the mating interface between the upper leg and the lower leg.  It directly attaches to the upper leg, and has bearings that ride between it and the lower leg.  The entire tension of the leg belt is born in shear by this part.

20200206-knee-stud

In the mk1 leg, this part was 3d printed with heat set inserts used to form the threaded holes.  This mostly worked, although occasionally the stud could shear along the 3d printed lamination lines.  Thus, for the mk2 leg, I’m making this part out of 6061.

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The first op takes a 0.875 inch cylinder, and does all the work on one of the sides.  That includes roughing it down to length, getting the outer diameter that the bearing rests on accurate, and drilling and threading the holes.

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5x knee studs with the first op done

At that point, the part is turned over and bolted into a 3d printed fixture.

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Then, all the tool paths are repeated on the other side, as well as the middle being cut away.  I didn’t really worry about surface finish on the middle section, since it will never be seen.  This of course would be much easier on a CNC lathe with live tooling, but hey, you use what you’ve got!

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All 5 that I made

Updated leg design for mk2 servo

robots, ,

Since the mk2 moteus servo has slightly different dimensions and a different mounting pattern than my original, I needed up update the full rotation leg design to handle it.  The basic concept is the same, except for some in-progress work on the foot design which I’ll write up later.  The only significant changes were that because of the mk2 design, access to the power and data connectors is much easier.

Here’s a brief CAD snapshot:

20200205-mk2-leg

First assembled full rotation leg

robots, ,

As I described earlier, the first draft brushless quadruped leg design was insufficiently robust for the gearbox driven motors and I am updating it to a geometry that allows full rotation.  I’ve made at least some progress on that front, so here is an intermediate report.

First, after doing some analysis, it appeared that the 3mm pitch 6mm wide belt was unlikely to be able to carry the full torque from the motors.  So I’ve switched to a 5mm pitch 15mm wide belt, which while still unable to carry the full torque indefinitely is only a factor of 2 or 3 off instead of a factor of 20 off.  Secondly, I added a bearing opposite the upper pulley so that it is supported from both sides.  The recommended belt tension for this belt works out to something like 120lb, which is a fair amount of cantilevering, even over the 16mm wide pulley.  The updated CAD looks like:

20190531-full-rotation-cad.png

And the newly added bearing can be seen in this section view:

20190531-upper-leg-bearing.png

I did a first test print of all these parts and put them together.  While there were a few tweaks necessary for the second revision, it looks like this leg is probably usable.

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The shoulder and upper leg adapter installed, all the leg pieces splayed out
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Lower leg all assembled with belt and upper pulley installed
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All assembled!

Next up is building 3 more of them!

 

Full rotation leg design

robots, ,

Another of the failure modes observed during the 2019 Maker Faire was in my quickly slapped together leg design.  The shoulder joint was required to squeeze two motors together against a strongly tensioned belt, using nothing but a relatively thin section of printed plastic.  This caused it to deform, leading to belt tooth skipping, and then eventually to fail, leading to delamination of the shoulder joint.

My plan to resolve this is to switch to a leg design where the upper and lower leg are in series rather than opposing one another.  This is more like the Mini-Cheetah design from Ben Katz.  This has the benefit of getting the leg out to the side, so the upper leg is free to rotate 360 degrees, only limited by cable harnessing.  As seems to be my pattern, I’ll try making something out of 3d printed PETG first, optimize it some, and if I fail there, switch to metal.  Here’s a render of the current CAD:

full_rotation_leg

 

Eric from CireRobotics helpfully pointed out that I’m way over the design limit for the 6mm Gates belt I was using, so I’ll also be trying to bump up to a beefier belt in this iteration.