To build a second demonstration quadruped and to generate some development kits, I’ve built up a set of 20 of the mk2 servo.  The production process is working out fairly well, in fact slightly better than I had predicted for overall cycle time.  The servos so far are coming out great, moving smoothly with full power.

I’m planning on building up a set of mk2 servos to test them on a quadruped and make some development kits.  As of now, I’ve got all the materials in house for the build and many things partially assembled!

To help get some better overhead camera shots, I made a simple bracket that I could bolt to my over-desk bookshelves.  It just has a 20mm tube on it that various camera attachments can be bolted to:

Here’s to more camera angles!

After casting the feet, the final step was to join the lower leg with the 3d printed foot bracket.  This I just did with some slow cure epoxy.

It seems strong enough for now, I was able to manually apply 10kg of load to a single leg while perfectly horizontal with no signs of stress, which should be good enough for a 4g 4 legged jump.

All the legs (and a spare) are now assembled with belts and a lower pulley ready to go on a robot!

Previously, I described the overall plan for my improved foot.  To make that work, I needed to cast a 3d printed part into the squash ball such that it would likely stay attached during operation, be suitable rigid and yet damped, and do so repeatably.

To start with, I used a random single yellow dot squash ball with a hole cut in one side using a pair of side cutters.  For the casting foam, I just used Smooth-On Flex Foam-IT 17, which is what Ben Katz originally used at least.  Initially I just mixed up a batch, poured it in to a random level, stuck my bracket in and hoped for the best.

As mentioned long ago in my post on failing more gracefully, it was obvious I wanted to strengthen the lower leg and foot mechanism to remove the point of failure observed there.  For now, I’m attempting to basically copy the original Mini-Cheetah foot principle, although with more 3d printing and less machining.

The basic idea is to print the entire lower leg in a single go laying on its side, so that delamination is unlikely.  The foot bracket will be cast into a squash ball, then epoxied onto the lower leg.

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.

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.

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:

Finally returning back to other pieces of my quad roadmap, I finished getting an updated power distribution board ready for the quad A0.  This board is one I had made many months ago and mostly brought up, but then didn’t quite finish.  The r1 was when I first discovered my unfortunate stm32g4 pinout problems that doomed 3 of my in flight boards.  The pictures here are of r2, which suffered from yet more pinout problems, resulting in more than my usual number of blue wires.  Fortunately, identifying those problems here let me fix them ahead of time for the fdcanusb and moteus r4 boards.

As mentioned previously, I’m releasing moteus controller development kits to a few lucky beta testers.  Building these wasn’t too hard, but was my first foray into low-volume production for someone who wasn’t myself.  Here are a few pictures of the build: