Thursday, October 30, 2008

Prototype That

In addition to Time Warp, the Discovery Channel has another new show called Prototype This. Although you definitely get the impression that Discovery is trying to ride the Mythbusters wave a bit with these new shows, this one features an interesting balance of mechanical and electrical/computer geeky hosts.

This week's prototype, a truck that could navigate above a traffic jam and slide sideways into an occupied parking space, featured a bit of technology that I've seen in action before: Mecanum wheels. Rather than try to explain how they work, a short video:

(Check out the new TechTV player!)

These pretty cool sideways-action wheels have been gaining popularity in the FIRST competition for some time. So, if you don't have the budget of a Discovery Channel prime-time show, you can obtain a set of these wheels at reasonable cost from FIRST supplier AndyMark. They only have small robot versions for now, but are working on a 21" version which I think would suffice for making your car slide into a parking space.

Most of my projects fall squarely in the "prototype" category - the first take at something or other. After that is the "beta" phase, which is an interesting in-between ground for working out the finishing touches. Then, the final mass-manufactured product release. I've seen a bit of the whole sequence, but most of the time I enjoy working in the early "whiteboard" stages.

Monday, October 20, 2008

On Simplicity

I love simple systems. I think somewhere in the process of building a 300-amp motor controller from scratch, we forgot how remarkably simple the electric go-kart really is. It took a while to get there, but in what I think is a good example of engineering system design, the individually complex pieces (most of which I would have preferred to buy off-the-shelf if they actually existed) together make something so easy to understand that it must work.

When the kart engages its regenerative braking circuit, there are only really two places for energy to be stored: in the moving mass, or in the capacitor. The batteries are out of the equation, since the main controller, in an act of almost absurd simplicity, cuts them off and shorts the motor terminals across the capacitor. Of course there is friction, but let's say we idealize by using a 54lb steel flywheel instead. ("That is one of the scariest things you've ever built." -Matt R.)

The only way to move energy from the mass to the capacitor is by energizing the motor's magnetic field, forcing current to flow as it acts like a generator. The flywheel slows down, the capacitor fills up, and along the way a sizable portion of the energy is dissipated in the resistance of the motor. Since we can measure current the whole time, the energy dissipated can be calculated. And guess what? It almost exactly matches the energy difference between the final value of the capacitor and the initial value of the flywheel, every time. The data is up on the site.

I'm not sure why I was surprised by this. There's a huge spinning disk, a huge capacitor, and a huge copper motor winding. There's no where else for 10kJ to go in a matter of seconds without destroy something, so that must be all there is to it. I'm pretty sure I thought this all through and came to a similar conclusion before we even started building. But after months of fiddling with power electronics nuances, it was wonderful to see the conceptual simplicity come to life and to see the data confirm that physics does indeed work.

Take-away: If I draw a resistor and a capacitor and ask people if that's a simple thing, they say yes. If I draw a go-kart with regenerative braking, they say no. Hopefully I've taught a few people to say they're the same.

Monday, October 13, 2008

Time Warp

Time Warp, the new Discovery Channel show that was partially filmed in the Edgerton Center (where I work often) at MIT, premiers this week. Their cameras are very nice, but a while back I wondered what kind of high speed work you could do on a budget. Turns out there are now a few consumer-level cameras that can shoot at relatively high speeds. One is the Casio EX-F1 (and its newer, cheaper cousin the EX-FH20). Here is a compilation of random things shot with a borrowed EX-F1 and finger in one day:

Thursday, October 9, 2008

Large Spinning Disks of...Aluminum?!

I was wondering how UPS had managed to alter the material properties of steel to make it so light. A minor setback, steel disks are now on their way. I hope somebody out there is designing something really cool that can be cut out of the inside material of left-over 14" aluminum disks.

Monday, October 6, 2008

Large Spinning Disks of Doom (or Steel)

An annoyingly blog-like title to start with. Aside from just testing out how this blog thing works, I will attempt to catch my readers (hah!) up on one of my current fun projects, an electric go-kart with a 110F ultracapacitor boost. (Think railgun-you-can-ride.) Working with a crazy - but very competent - group of high school students from the MIT Edgerton Center Summer Engineering Workshop, we built it over the summer and have had some fun driving it around campus:

By the way, I do this kind of stuff because I sometimes get bored with business-as-usual in my life as an engineering student. But this particular project is interesting because I think, as fun as it is, it also has significant serious research potential. Since we've already proven it is a fun ride, I'm thinking it might be time to satisfy academia with some more controlled experimentation. Hence, the spinning disks of doom:

These 11lb, 14" steel disks will provide rotational inertia directly to the electric motor on the kart, enough to simulate the effect 500lbs of kart + driver. And all without leaving the table in our shop. Not as much fun, but with the wireless data acquisition system it should rake in results fast enough to meet some paper deadlines. And even if the papers get rejected, guess what, now we have a sweet electric go-kart and five massive steel plates.

If you are wondering what magic MIT technology we use to produce custom-cut steel disks, it is called an abrasive waterjet. But in fact you can use one yourself. I sent the order out at 2:45PM and they were cut and shipped by 5:00PM. Can't beat that.