Wednesday, January 5, 2011

MEMS / Snow Scooter / Panzerkitten / Thing

Cambridge got its first dumping of snow, about two feet of it. And even though it's already mostly melted or trucked away, the pressure's on to create what I call the Micro Electrical-Maniacal Snowmachine, or MEMS for short. (Just so I bait and switch more common search hits like "MIT MEMS"?) Or maybe, as I was thinking yesterday, I should call it Panzerkitten. After all, it is a small, cute, harmless tank tread vehicle.

Thank you, Charles, for the illustration... You can go back to building unnecessarily dangerous and impractical competing vehicles now.

But then again, the seed for this name, "Panzerketten," seems to refer to jewelery that resembles tank tracks, not tank tracks themselves as the literal translation offered by Google would suggest? So it might not actually make any sense. I'll probably just keep calling it "snow scooter" or "the thing" anyway, so it doesn't really matter. If you have a strong opinion, let me know...

Anyway, I got the first batch of waterjet-cut parts for the thing from Big Blue Saw. (I've grown out of the phase where I try to negotiate with the on-campus shop staff about waterjet fees. Plus this way, anyone can make a snow scooter!) For budgetary reasons, I started with just the center plates, which are where all the action happens:

The front (swordfish) plate is like the spine of the scooter, giving the deck some rigidity as well as supporting the front drive module. The drive modules are probably the most interesting part of this build, and I'm trying some new tricks with them to mount the gearmotors inside the track pulleys. To facilitate this, I start with four stainless steel bearings on each side of the center plate:

Pictured: about $40 of stainless hardware. :(

These four bearings form a metabearing on which the pulley itself rides. The pulley is just an aluminum tube with a lip and a metabearing race machined in it on one side. The pulleys are hollow so that the motor and gearbox fit inside them.

Metabearing race.

Copy-paste three more times.

You can really see how the metabearing works in this last picture. The four small bearings ride around in the race cut out on the inside of the pulley, and the result is a lot more space efficient than one giant ring bearing. It only works because the loads are low: the treads get maybe 2/3 of the gross vehicle weight, so maybe 100lbs, and that's spread out across four metabearings on the center plate, four regular bearings on the side plates, and the bogie wheels. The last picture also shows the exit strategy for the two motor leads. The only way to get them out is through slots in the center plate, since there will be spinning pulley and tank tread everywhere else.

The pulleys also get end caps. So far, I've only managed to work out the details of the drive end caps, the ones on the gearbox side. The idler end caps will need to be redesigned to accommodate the longer 550 motors I have planned. As it is, there is virtually no room for wiring. In the mean time, I made two drive caps, complete with 1/2" keyed bore:

They're pressed into the pulleys with Loctite 609, since they transmit torque to the tracks. The small raised edge around the bore is for spacing the pulley out a bit from the side plate's bearing. Both the pulley and the end cap are tapered a little to help keep the track from snagging or trying to jump off. Speaking of tracks, here's where it starts to look real:

What do you normally use 2" wide timing belt for?

I made all these parts while waiting for my Beanbots gearmotors. Beanbots gearboxes occupy a very special place in my mind. On the one hand, they used to make some pretty crappy gearboxes. They lacked proper alignment features, had miserable axial tolerances, tore the shit out of their brass gearteeth, and stripped out the low-carbon output carriers. But this was like...back in 2007. I spent so much time reverse-engineering these gearboxes that I have a spare carrier plate I made out of tool steel on my key chain.

My bad luck charm.

Now, though, they are very well-made. Steel gears, hardened carrier plates, axial spacers, and alignment features other than through-bolts are all standard. Everything is tight, the backlash is minimal, and they run smooth, all for a fraction of what you'd pay for precision planetary gearboxes. Totally legit. It's almost like a totally different company...


After I geased my grearboxes, I popped one into the rear center plate to check the fit:

I spec'ed 8-32 bolts to attach the gearbox to the center plate, when in fact the regular gearbox screws are 6-32. But since I had to drill out the tapped holes in the gearbox anyway, that was no big deal. The 8-32s clear everything just fine. With the two pulleys in place, it disappears:

Except for the motor length, there were no clearance issues and it runs smoothly. With the stock RS550 motor, 12V gives 1,000rpm no load at the pulley. That's 9mph at the ground. At 18V, 13mph. The next test will probably be to see how much torque this drive puts out. If it's more than adequate, I can trade back for a little more speed by going to 16:1 gearboxes, changing to a faster 550 motor, or both. If the torque is merely adequate, I'll probably have to live with the 10mph top speed. If the torque is inadequate, well then I'm screwed.

Before I make that call, though, there are more non-trivial tasks to accomplish. The idler pulley end caps need to be redesigned to handle the longer motor. I need a way to join the belt and to make sure I can get it on and off easily. I need the side plates, the ski, and another Razor A3 to sacrifice. And I need controllers. Speaking of which......well this post is too long already so I'll hold off on that.

No comments:

Post a Comment