Wednesday, February 23, 2011

Snow Scooter: 40 hours of work, 15 seconds of fame.

There's the 15 seconds of fame out of the way, now the 40 hours of work:

Last week, it was nearly 60ºF and the Cambridge permafrost and giant piles of snow all melted away. But they were pretty disgusting anyway, by this point. On Friday, I saw that there was a chance of a fresh coating of snow for Monday. And since I don't know what the rest of the spring will bring, I decided I might only have one chance to produce some marginally functional snow scooter. And as you can see from the video, it is exactly that: marginally functional. Or it was, for 15 seconds, until it sucked up a pebble and destroyed the left side motor. Here's how it all went down:

I finished the rear drive module a couple weeks ago, but because of me being an idiot issues with the front meta-bearing, the front drive module was left hanging. Also, the front drive module contains the Victor 883107-7 controller boxes, which needed to be water-resistant. I couldn't use t-nuts, so I had to drill and tap everything. To speed up this process, I marked all the holes in place through their matching clearance holes on other parts. Then, this:

It's actually not too bad when you have a depth setting and can just drill to a mark every time. Blind tapping sucks, but I recently invested in some nice new 4-40 plug and bottoming taps so I felt like I was putting them to good use. One problem is that, even without t-nuts, the tabs and slots still leave gaps, which are exaggerated by the taper of the waterjet. So despite everything, I still needed to seal the inside of the controller boxes with Goop to the best of my abilities:

To the controllers themselves, I added a 1/4" aluminum heat transfer plate to get heat off the bottom copper plane and into the scooter chassis, which is itself cooled by snow. In between the plate and the copper, a thin sheet of silicone thermal transfer pad provides electrical insulation while still conducting heat off the board. The board sits on 1/4" nylon standoffs (also for isolation). Here's the board in place:

I used 14AWG overcooked pasta wire from Hobby King, which turned out to be very necessary since I didn't leave much room at all for the power wiring to squeeze past the edge of the controller board. Only the absurd flexibility of this high-strand-count wire made it possible to get power to the bottom of the board. You can also see the motor wire exit strategy in that picture. Here's a closer look:

The wire runs down the body of the motor, between two of the metabearing bearings, and out of the pulley assembly through a pair of slots in the center rail. Again, only possible thanks to the magic of high-stand-count wire. Wiring the controllers was relatively easy. The controller boxes have Deans connector ports for interfacing to the outside world (battery, motors). Here's the overall layout:

A quick bench test at this point confirmed that both controllers were indeed still operational and that the front metabearing, which I violently bent back into something resembling a circle, was not totally destroyed. That's all the proof I needed to move on to the fun part, which was cutting and splicing the timing belt tank tracks. After calculating the desired track length, I cut the 2" wide timing belt (McMaster 7959K32) into two such lengths and prepared to join them into continuous belts.

One thing I learned accidentally when building my 2.007 robot way back when is that regular old cyanoacrylate (Super Glue) works disturbingly well for bonding rubber belts. I even tried commercial products advertised as rubber adhesives, but CA worked better than those, too. So I didn't bother with other adhesives this time. But in order to make a very strong bond, the CA joint needs to act in shear. This means a lap joint, and the best way I could think of to do this without increasing the maximum belt thickness was as such:

Speaking of my 2.007 robot, I found the missing tank tread and cut it up to make the rubber strips needed for the lap joint:

Poor robot.

One tread dies so that another may live:

These are both about 29.75" in unstretched length. Thanks to the giant compression springs, the pulleys should keep the belts in tension at around 25-50lbf each. I have enough room to remove one more tooth from each belt if necessary, and the tension in that case would be over 50lbf each.

Before final assembly, I made a Y-splitter for both the battery power and the throttle signal, so that one signal controls both motors. One of the goals that had to be dropped in order to meet the snow deadline was differential steering, where each controller gets a separate throttle signal and can drive the motors with different torques. The belts are so close together that I don't know how much net steering torque this would actually contribute, but it's an interesting idea that I want to try at some point. Here are the Y-cables passing through the open space between the two drive modules:

At this point, I was ready to put the belt on. Since the springs together push with upwards of 100lbf, I needed a way to hold them while slipping the belts on from the side:

Large clamp to the rescue.

The belt goes on first, then the front and rear side panels. I also made some quick bogie wheels out of 1" LDPE stock:

These take up much of the normal force on the treads, keeping them from rubbing on the bottom covers of the controller boxes. However, in the video you can see that the pulleys tend to slip a lot. Without the normal force of the tread against the drive pulley, it was hard to get enough torque transmitted. Possible solutions include more tension and sandpaper or other friction coating on the drive pulleys.

And here it is all together:

"Wait a minute. Did you just steal Pneu Scooter's front fork and attach it to snow scooter?!"

Yes. I'll give it back. But it was already snowing at this point so I had to get something on there. It's supposed to be a ski or a ski/wheel combo but for now I thought just a wheel would be fine.

I had some trouble with the belt getting sucked into one side's drive pulley. The problem seemed to be a small offset at the glue joint that was riding up on the rim of the pulley every so often and getting jammed between the pulley and the deck. The solution was to just turn the belt around, so that the small offset would move try to climb the more forgiving inboard pulley rim rather than the sharp outboard rim. If that made no sense...well just trust me.

After that, I borrowed a 19.8V FusionPack from Matthew and went outside...

And, well, you already saw what happened there. I guess the immediate cause of failure was a pebble...

...which got sucked into the thin gap between the tread and the deck, jamming the left side and burning out the (already heavily stressed) motor. The other obvious flaw was the lack of adequate friction between the pulley and the belt, which was not unforeseen and will need to be addressed. With all the weight on the bogie wheels, the pulleys prefer to just slip instead of driving the track. I think this can be easily solved. 

The more fundamental flaw is the debris protection and the fact that the motors are being run with no concern at all for overload conditions. I was sort-of imagining that the snow would cool them off, but the pulleys actually keep snow and water out decently enough that motor heating is now a problem. As for the debris, I just need some kind of brush to keep rocks and stuff out of that gap, I think.

Some parts of the design worked well, though. The metabearings did fine, even the one I mashed on the 20-ton press by accident. Overall, the 540-motor into Banebots gearbox seems to be a good solution, I just need to find the right 540 motor. Perhaps it's time to seriously consider going brushless...

1 comment:

  1. Cool man
    I was searching for rubber belts/treads for robots, saw your page good job