Now that I have a PocketNC, the first thing I noticed was that I had a problem with noise volume.  The air compressor Pocket NC recommends is described as “quiet and durable”.  I can maybe believe the durable part, but quiet I have a harder time believing.

I’m running the machine in my home office and I measured the compressor at upwards of 85dB.  That’s about the same as a bulldozer.  Despite me adding some vibration damping padding, it also did a pretty good job shaking the whole house when in operation.

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:

The primary UI the Pocket NC presents is a web interface accessible over a virtual USB based ethernet port.  I wanted to be able to run mine not immediately near an ethernet jack, but also didn’t want to have to tote a laptop over every time to check on it.  I had plenty of raspberry pi’s lying around, so rigged one up as a wifi bridge.

First, I found a random case to print from thingiverse, the TurboPi:

With my efforts to build a gearbox transmission and subsequent plans for a quadruped, there are a lot of parts which just can’t be made effectively from 3d printed plastic.  To date, I’ve sent out a few parts of the gearbox to CNC shops, which while effective, has a relatively slow turn around.  The best you can get without paying an arm and a leg is something like a week turnaround.  One thing I’ve learned from having a 3d printer on site is how transformative it is to be able to have single day turnaround for parts.  Thus, I thought I would experiment with CNC machining on a small scale locally and recently acquired a PocketNC V2-50.

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:

As seen at Mech Warfare 2019, the existing gearbox motor shroud isn’t really up to the task of supporting the weight of a 20lb robot.  While I work on a more comprehensive redesign, I’ve got a short term fix in the form of another 3D print.  This is just a simple reinforcing ring, printed at 3mm thick, with the layer lines oriented so that layer separation will not be the primary failure mode.  It is attached to the outer housing via a thin layer of epoxy.

After a concerted push, I managed to get Super Mega Microbot “Junior” walking, for all of 15 minutes, then packed it up and went off to compete in Maker Faire.  Needless to say, with that much testing, I wasn’t expecting stellar results, and I wasn’t disappointed.  However, I did learn a lot.  Here’s some of the things that went wrong:

Gimbal and Turret EMI

For this new revision of SMMB, I updated the gimbal board to use RS485 and support the 5S system voltage.  I tested it some, but apparently not enough.  While I observed no problems during Thursday or Friday’s testing at the site, during the first Saturday match, after firing the gun a few times, the gimbal went into a fault state and stopped applying power.  The control scheme for SMMB relies on the turret being operational, so this not only made it impossible to aim, but also made it nearly impossible to drive.

Well, Mech Warfare at Maker Faire 2019 has come and gone.  Maker Faire was a really awe inspiring event, and RTeam did an excellent job organizing the Mech Warfare competition.  There were something like 13 teams with moderately functioning mechs who competed across the 3 days.

Super Mega Microbot “Junior”

My entry, Super Mega Microbot Junior, did manage to walk a bit in 3 matches, but had a previously unseen failure in the turret system that rendered it inoperable a short while into each match.  At the end of the 3rd match, one of the leg joints sheared off, and some other of the 3D printed parts were about to fail as well, so I declared it unrepairable at that point.

As mentioned last time, I needed to build a lot of gearboxes and new leg assemblies in a very short amount of time. So, I got to work.

Machining operations

I made a new fixture for holding stators to be extracted:

I turned down 8 more internal gears. To begin with, my mandrel had warped enough from the first gears that I had to add some heat set inserts to hold a cap to keep the gears on. Then on the last 2 gears, I got greedy, went too fast, and my lathe mandrel melted entirely.

Now that I could stand up and sit down, I needed to be able to walk reliably for the length of a match.  This wasn’t going to be easy because the direct drive motors were always a bit marginal in their power output to support the full robot, so I had my work cut out for me.

The short story is, I tried many things, spent about a day examining high speed video of walking, and made some improvements: