To switch to the full rotation gear design, I needed to get all my gearbox motors, some bearings, and a lot of other bits and pieces disassembled and ready for re-use.
The remaining 3 non-broken legs from Maker Faire
Taking everything apart took a surprising amount of time, nearly a full day. Each leg resulted in quite a collection of fasteners. Seeing them all in one place made me realize how complex this has become!
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:
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:
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.
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.
Alert! I’m at Maker Faire Bay Area all weekend in the Mech Warfare area in Zone 2 (May 17-19, 2019 for you time travelers from the future). Drop by and say hi!
If you were left in suspense last time, yes, the robot can walk! Getting it to do so in a minimal way was relatively painless. What I found, which hadn’t happened in earlier iterations, is that many types of dynamic motions would cause the lower leg belts to jump a tooth. Needless to say, this was nearly universally fatal, as there is no direct position sensing of the lower leg. This robot is heavy enough that my simulacrum 3d-printed timing belt pulleys just don’t cut it.
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.
I made a new fixture for holding stators to be extracted:
Stock in the vise
Countersinks milled
Stator mounted and fractionally machined
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:
Before SMMB could function in the Mech Warfare event it needed to be able to start and stop unattended. That meant standing up and sitting down on its own. Being that hack that this was, I went for a two pronged approach.
The direct direct servos I have for the upper and lower legs are somewhat underpowered for this size of robot. Especially so when the machine is fully squatting down. Also, the servos aren’t really encapsulated at all, and there are plenty of leg configurations that can self-intersect resulting in robot harm.
