Thursday, June 18, 2009

Both Wheels Now.

It's time for a new summer project.

In case you haven't been following along, I advise the Edgerton Center Summer Engineering Workshop, which is a relatively ad-hoc group of MIT and HS students bent on producing some of the most unarguably cool vehicles on campus / in the world. It's part research, part education, and part fun, and anyone who has a problem with that can go away. Our fleet currently consists of the DIY Segway, a homemade version of Dean Kamen's self-balancing scooter, and the Cap Kart, a pretty sophisticated electric go-kart with a 110F ultracapacitor regen/boost.

The DIY Segway (2007) and the Cap Kart (2008).

These vehicles, although certainly a lot of fun, are not really very practical. One is inherently unstable and the other weighs 350lbs and has a ground clearance of 3/4"; neither is a sort-of everyday ride. Inspired by this modified Razor scooter made by Charles Guan, we've decided to add a light vehicle to the fleet. I was originally thinking that a simple DC motor and belt drive with excessive amounts of torque would suffice, but apparently I'm boring and unimaginative. So instead, the team has shifted to a new design.

Artist's rendition.

This could stand for "Both Wheel Drive" or "Brushless Wheel Drive" depending on who you ask. The idea is that if you're gonna put in the effort to make a custom in-wheel hub motor, you might as well make two. (Also, they are somewhat limited in torque production, so having two might help there.) Anyway, it will be ultra-compact and ultra-light. Some preliminary specs from our first design meetings:

Base: Razor Spark. Really we are only using the front folding / steering thrust bearing mechanism. The wheels, handlebar, and deck will be custom.

Motors: Completely custom in-wheel hub motors (x2). They'll be a bit bigger than normal razor scooter wheels, but not much. The space savings elsewhere will more than make up for it. Basing our design very closely on the one built and tested by Charles Guan (more details). It's a 12-slot, 14-pole brushless DC motor, sometimes called an LRK motor, which provides relatively high torque and low ripple due to the strange slot/pole number.

Deck: Custom aluminum with battery tray underneath.

Batteries: LiFePO4 2.3Ah, 3.3V A123 cells. Pack size TBD. (Mostly limited by space under the deck.)

Control: Well, it'll probably start off with cheap model airplace ESCs, but the ultimate goal is to have a real torque-based controller that can do regenerative braking. That will take some effort.

Much more to come...


  1. Is there any way I could get the drawings or part numbers so I can get some lamination's from protolam? I don't have any real cad program to draw my own.

    Thanks JC

  2. The same question. Could you send me the drawing ? I'm fascinating about a scooter like this

  3. Sure thing. Here is a link to the drawing:

    The laminations we got for this project were 14mil M19 silicon steel. On a later project (Pneu Scooter) I used the same stator lamination but made some waterjet cut rotor plates out of 1/4" soft steel. The rotor Eddy currents are pretty low so this worked okay.