Wednesday, June 29, 2011

ECSEW V: tinyKart

Believe it or not (I don't), this is the fifth year of the Edgerton Center Summer Engineering Workshop, an ad-hoc team of like-minded engineering students interested in things you can ride. Of all my past projects, I think the ones that I've enjoyed the most have been the Workshop ones. Since 2007, we've made:
  • The DIY Segway, probably definitely our most popular vehicle, which helped spawn a new era of smaller, lighter, cheaper, simpler DIY self-balancing...things.
  • The Cap Kart, our largest, most ambitious vehicle, which underwent a revision in 2010. Its name comes from the 16V/110F capacitor that gives it a speed boost from a bit of regenerated energy.
  • BWD Scooter, which led me in the direction of brushless motor design and control that occupies so much of my time these days.
But for 2011, we are returning to four wheels to create an ultralight go-kart. (Okay, maybe not this ultralight.) One of the drawbacks of Cap Kart, even the newer, lighter version, is that it takes three people and a truck to move around. With tinyKart, the target weight is something like "less than one of Cap Kart's old batteries."

Cap Kart's original 53lb deep-cycle marine batteries.
So obviously we're gonna need lighter batteries. Cap Kart v2.0 uses 42lbs of LiFePO4 batteries from Thundersky, now Winston Battery Limited. For that weight, you get 1.58kWh of energy storage and a peak power of 7.5kW. The only way to do better than that, in terms of power and energy density, is with lithium polymer.

tinyKart will use between two and four 10S, 4500mAh LiPos, making it only marginally less of a fire hazard than a gas go-kart. At a maximum, this gives 0.67kWh of energy storage. But since they specialize in absurdly high power density, there is the possibility of using just two batteries, a weight of just over 6lbs, to power the whole kart for a few minutes at a time.

Next comes the question of propulsion. The ultralight kart as I've always imagined it was powered by two short Magmotors, but they're just not cost-effective anymore. For less than the cost of one Magmotor, we got a set of four Turnigy SK6374-170 brushless outrunners. They're not the biggest brushless motors, but two will certainly propel a go-kart and four would make it a competitor for Cap Kart, in terms of acceleration. (One of the emerging themes of this project seems to be modularity; we can change the number of motors and batteries quickly to go between the lightest configuration and the most powerful.)

The drivetrain is really simple: RWD with independent belt drives on each wheel. This is enabled by the cheap and light aluminum rims from electricscooterparts, which have a 72-tooth belt pulley integrated into the rim casting. The rim and tire together weigh less than 2lbs. The motors are light enough to face-mount to a 1/4" aluminum plate and ball bearings on an eccentric standoff provide adjustable belt tension. As-pictured, the gear ratio is 4.5:1, yielding a top speed of about 30mph with the 8" wheel.

These wheels are designed for electric scooter use, where they would fit on 10mm shafts supported on both sides by the rear fork of a scooter. One of the easily-overlooked little details that I think makes this all possible is 6903 bearings. These are the same O.D. (30mm) as the 6200 bearings normally used in these rims, but they have a 17mm I.D. instead of 10mm. Thus tinyKart's four wheels are supported by cantilevered 17mm aluminum shafts.

Ask Max why the plate is at a weird angle like that...
One of the things I've always had in mind for an independent RWD electric kart is torque vectoring, a neat trick where you force more current into the motor driving the outside wheel to help shove the vehicle into the turn. This of course requires measurement of the steering wheel angle, but that's not hard. It might make up for the natural understeer of a kart that has 70+% of its weight on the rear wheels.

Between all of us, we have a ton of experience building electrically-propelled drivetrains, so I'm not very worried about the propulsion side of things. What makes this project interesting is that, unlike Cap Kart, we're building the entire chassis from scratch. And as you can probably tell from the CAD, it is not a typical go-kart build. We briefly debated the idea of making a welded tube frame, but then decided that 80/20 is the only structural material we know how to utilize ultimate expression of lightweight structural modularity.

The kart chassis is split into two halves: the front, which has a lot of platework for aligning steering linkages, and the back, which contains drivetrain components and the seat. The overall width is about 34" and the length is 48", though it can easily be made an few inches longer or shorter to accommodate different drivers. The front and back overlap by an adjustable amount to provide rigidity.

The front wheels are also electricscooterparts rims, but with brake disks instead of drive pulleys. We'll machine off the drive pulleys on those wheels and face the brake mount inwards. This will cause a bit of an issue on the left side, since the brake disk hubs are threaded and the braking force will tend to unscrew that side's hub. But I suspect this problem is easy to solve with enough Loctite.

I went with a relatively low-tech method for working out the geometry of the brake caliper mounts:

The other tricky bit of geometry to solve in the front of the kart was the steering linkage.

From above, it's a pretty straightforward Ackermann steering geometry, with a drive link off the steering column very similar to Cap Kart. But I tried my hardest to contain the entire thing in the 1" vertical space between the plates, which meant that I couldn't use ball joints on the steering pivots. Instead, the two degrees of freedom are split into a horizontal and a vertical pin joint:

The horizontal pin joint is just a shoulder screw with some brass washers. The vertical pin joint, which is far enough inside the kart not to interfere with the plates, attaches to threaded rod that goes to the drive plate on the steering column. Since I know nothing about proper steering geometry, everything is adjustable. The only thing we've sacrificed from Cap Kart's steering is the caster angle, which is by necessity zero in this design. To make up for it (or do nothing at all, idk), the center of the front wheels is just slightly behind the kingpin pivots.

Since the 1/8" aluminum plates don't really provide much rigidity, the gap between the 80/20 rails framing the front of the kart are spanned above and below by 1/4" steering reinforcement plates, which also hold the radial kingpin bearings. The thrust bearings are between the 1/8" plates. It might be overkill, but flimsy steering is one failure that we don't want to deal with.

There are plenty of other little details that went into the design so far, but I'm sure we'll be highlighting them as they get built. For now, we're waiting on parts and I'm on my first real vacation in a long time. The build begins next week and it will drive by mid-August.

1 comment:

  1. why build a cart when you could by winston battery limited car?