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.
I did manage to connect to the turret manually after the match to diagnose the problem, and discovered that the IMU had stopped communicating over I2C. I had some half-baked logic to try and recover from that, but it was broken, and the only effective way to recover was to power cycle the whole unit.
Unfortunately, my matches on Saturday were all close together, so I didn’t have enough time to prepare a fix in between. Thus, each match I got one or two shots off, and then the machine as a whole became effectively inoperable.
Likely, something in the new board, either in the layout or the decoupling capacitors, results in worse electrical noise than the old one when the AEG is fired. This shouldn’t be too hard to resolve, either through tweaking the layout, or perhaps moving the AEG control to an entirely separate board.
Walking and Leg Robustness
When I got the gearbox system walking for the first time, I quickly noticed that one or more of the timing belts connecting the lower legs to their motor had a propensity to skip a tooth. Since there is no position sensing directly on the lower leg, when that occurs the gait logic just has the incorrect position, causing the robot to fall over pretty soon afterwards. I had never observed any tooth skipping in my previous direct drive leg, even when jumping for over an hour. The first difference I thought which might be causing the problem was the lower pulley print, which I had initially done at 0.15mm but in the gearbox revision it was at 0.2mm. So I printed a full set at 0.15mm, and swapped them in. However, that didn’t fix it and I didn’t have any more time for mechanical solutions, so I tried to work around it by tuning the gait to be as gentle as possible.
Unfortunately, I wasn’t really able to come up with a gait that both could effectively move on the foam mat in the arena, and not occasionally result in belt skips. Also, as I went along, the skips got worse and worse. I tried upping the tension on the belt, lowering the tension on the belt, walking with a straighter leg and more bent leg, nothing much made a difference.
Finally, before my third match, I did more examining and realized that the shoulder joint was deforming significantly under the tension of the belt, resulting in the timing belt only contacting maybe half the pulley or less, and the rest dangling off. Also, the pulley was out of alignment, so the belt was probably only effectively making contact in an even smaller patch. Unfortunately, there was very little I could do about that aside from hope for the best. As it turns out, that problem, while limiting the gaits I could use significantly, didn’t result in ending my run.
Gearbox Outer Housing Strength
The entire gearbox effort was undertaken somewhat at the last minute, and with little thought to analysis or design for structural integrity. At best, I made a gut check of “that’ll probably work”, and at worst, I gave it no thought at all.
It was an instance of the latter that caused the final and fatal failure in SMMBJ at Maker Faire. In the gearbox chassis design, the lateral servos themselves support the entire weight of the robot. Those gearbox servos transmit the entire load from the front plate of the servo, through the outer housing, then to the back plate, and finally to the chassis itself. The problem in this case is that the outer housing is a 1.5mm thick (or rather thin) PETG shroud printed with layer lines perpendicular to the primary load.
On reflection then it was not too surprising that a 20lb robot walking around was enough to cause a motor’s shroud to separate at the layer lines, which is what ended SMMBJs run. I had a spare motor and could have replaced it, however, it would likely have failed shortly afterwards too, and the shoulder was about to rip itself apart due to the leg tension problems mentioned above. Thus I turned it into a “static display” and switched to a “show and tell” mode for the rest of the event.
Despite those problems, the kind organizers at RTeam awarded me the “Most Innovative” award for trying to push the limits!
Fixing the problems
Clearly, all of these issues can be fixed in a variety of ways, both easy and hard. Keep coming to see my attempts!