Wednesday, February 2, 2011

Snow Scooter: Why don't I have one?

That's what Cambridge looks like right now. We've had a blizzard a week for about the entire month of January, and February started with, oddly enough, more snow. So, there are now 2-3 foot high snow walls bounding every street and sidewalk with another 0-8" of fresh snow/ice/slush depending on the day of the week. The good news is that it probably won't melt until April now, but the bad news is that Quals have set me back about a month on completing the Snow Scooter.

In the last mechanical update from early January, I had just finished all four pulleys and the two drive-side end caps. The gearbox fit nicely in the drive side, but the motor terminals were going to interfere with the end caps on the idler side, so they got a bit of a re-design:

To accommodate the motor terminals and wiring, the idler side end caps are now dished out with a 2" ID. This allows plenty of room for 12-gauge wire soldered to the terminals and routed along the side of the motor to the wire exit slots in the center of the drive module. However, since the end cap wall thickness is now so small, my original plan of pressing a 1/2" steel shaft into the end cap to interface with the side plate bearing was no longer an option. Instead, the 1/2" shaft feature had to be integrated into the end cap:

Nothing too difficult here, since the idler side end cap does not need to transmit torque. (At least, not in any of the Plan A configurations of the Snow Scooter...) Since I had already bored out two disks for these end caps, I had to start from scratch with new stock. Minor setback, but I could think of no other way to clear the longer RS-550 motors. Here's what the rear drive module looks like with the newly-machined idler side end cap in place:

Totally Enclosed, Snow Cooled.

After the initial pulley-making blitz, I switched over to being an EE for a while and designed my own 4-quadrant 50A motor controllers so that I don't have to drop $300 on Victor 883s for this thing. As of the last update, I had only put one together. So, now that Quals are over, I assembled the second one. Since this would not be a Shane Colton project if there weren't some unforeseen motor controller issues, the high side drive on the second controller was dropping out. 

I know from a few years of experience that the HCPL-3120's have a pretty unforgiving UVLO of about 11V. So, I bumped the gate drive supply voltage up to about 14.75V (from 13.5V) and doubled the size of the bootstrap capacitors (to 2μF). This took care of the problem. Since the MITERS electronics bench was fully occupied, I decided to set up a Self-Sufficient Remote Electronics Outpost consisting of my nice soldering iron, 24V/32A power supply, and the immensely useful Sparkfun Pocket DSO.

Is it sad that I own better lab equipment than I have access to anywhere on campus?

The controllers are all set for now, so I turned my attention to assembling, or at least prepping for assembly, the pile of waterjet-cut panels that make up the deck and drive modules. Why, you ask, does one have to prepare waterjet-cut tab and slot panels for assembly? Well, I haven't yet mastered the art of tab/slot/t-nut assemblies yet, so my tabs and slots didn't quite fit together as-cut. Mainly, the tapered edges left by the waterjet make one side of the slot too narrow (or one side of the tab too wide, if you prefer). This can mostly be avoided by offsetting the tab or slot geometry enough to clear even at the narrowest (widest) points. Or, if you happen to own your own waterjet, just fiddling with the toolpath offset itself.

But, as I am but a novice, I just did ideal zero-gap tabs and slots. I also didn't bother so much with t-nuts, since I want some portions of the snow scooter to be waterproof anyway. The drive modules themselves fit together nicely, with just a bit of belt sanding to reduce the width of the tabs enough to fit the tapered slots.

The front drive module is fixed to the deck, while the rear is able to slide forwards and backwards. Two large compression springs will get stuffed in between the two drive modules to keep the belts tensioned. The motor controllers are mounted in the waterproof section of the front drive module, the part hat looks like (and is) a metal box.

Although the side panels fit together with minimal effort, the part that really screwed me over was the long swordfish-looking center panel. Slots with metal on all four sides are much harder to stuff tabs into than ones with only three borders. I knew this would be a problem, and decided that the best way to solve it was with a 20-ton hydraulic press... it turns out, this was not a very good idea. After pressing the center panel into the deck, I realized how stupid it was to do so, given the giant hole through which the motor fits in the front drive module. Pressing on this hole deformed the entire metabearing structure on which the front pulleys run. So, all the careful work I did ensuring a concentric bolt pattern for the mini bearings was pretty much undone. I found that no amount of force applied in any reasonable direction would push the bearings back enough such that the pulley would fit.

That doesn't explain the giant crack. Actually, I did that on purpose. It's a trick I've used before to fix circular bolt patterns that just don't quite line up no matter how hard I try. The idea is that by removing a small strip of material (hopefully somewhere that isn't structural) the remainder of the ring becomes flexible enough to force into place. How exactly this happens is a different story.

So, I think I can salvage the front metabearing. The next step will be a bunch of mill operations for boring out bearing holes and making reliefs for the belt to ensure that it clears the bottom of the deck. Then, final assembly! Now that classes and 2.007 have started up again, progress will be sporadic. But I guess sporadic is better than non-existent, which was the case for the last month. Even if it only lasts for one YouTube video, there will be a snow scooter in the near future.


  1. Well, I'll just post this here, since you'll be most likely to see it on your top post ...

    How do the RC speed controllers makes tones with brushless motors without spinning them? Do they energize two of the phases at once?

  2. I think so. As long as it doesn't try to step through phases, the motor won't spin. (It might still twitch.)