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...

Saturday, June 6, 2009

Design Table...err...Table Design

Alex Slocum teaches about how to make design tables in SolidWorks. I never really got it, so I decided to make a table design in SolidWorks instead. I've been wanting to design and build a cool-looking glass table for a while now. Sources of inspiration:

2.009 Green Team Final Project, "Elika"

80/20 Conference Table

I wasn't really looking for something fancy. Just a small kitchen table for my "efficiency apartment" in Cambridge. A place to eat breakfast in the morning, or to work when my desk gets too boring and a need a new place to think. The idea probably would have stayed in my head for a while, if not for a generous donation of a slightly-used glass tabletop. With the heavy lifting done, all the way left was to design a cool aluminum frame. Here's the build, from start to finish:

Like any good engineering project, I started in CAD. I drew up some cool-looking corner brackets that would also hold the glass, which sits on rubber bumpers, in place.

Here's the "before" shot. The corners were cut on a water jet.

Sanding jig.

Oooooh, pretty.

Just before final assembly.

All together, complete with $20 folding chairs. Now I have a place to eat my cereal in the morning!