Riding a unicycle is the most useless thing you can do.
-- Ricky Gervais.
Some time ago I built a self-balancing two-wheeled scooter. Since then I realized that two wheels are redundant, and only a single wheel is needed to make a ridable vehicle. A vehicle with a single wheel is much smaller and lighter. It weighs under 30 lbs and is easily carried with one hand when going up stairs or on public transportation.
The Electric Unicycle's only control is the on-off switch. The rider controls everything else by shifting his weight. You lean forward to accelerate, lean backwards to brake, and gyrate your arms wildly to turn. With a little practice you can get more graceful and keep your arms mostly by your side.
The unicycle balances itself using a simple feedback loop between a solid-state gyroscope and the wheel motor. When it detects itself tilting forward, it runs the wheel forward to keep it vertical. When it detects itself tilting backwards it runs the wheel backwards. It does this so rapidly (200 updates per second) that it feels perfectly smooth. This is really the same thing that a conventional unicycle rider does with his legs.
Although I could have reused the electronics for my balancing scooter, I designed a better system, partly to lower cost and partly to improve performance. Version 1 of the scooter used a RoboteQ motor controller, which takes tens of milliseconds to respond to motor speed commands. The unicycle uses an OSMC motor controller which takes a PWM drive signal directly from the microcontroller, reducing delay to less than a millisecond. The result is a smoother, stiffer feel with less wobble. It's also smaller, lighter, and cheaper.
Update:: This instructables project is an easier place to start.
Want to look more eccentric as you cruise the neighborhood? Build your own electric unicycle!
You need the parts listed below, some welding, fabrication, electronics, programming, and control skills, some physical skill, and some bravery. My code is downloadable below, but you'll probably need to tweak it a bit for your exact electronics and hardware configuration. Or better yet, build something else and tell me about it! There are lots of possibilities for balancing vehicles that haven't been explored.
What I said about safety for the two wheel scooter applies, but there is a major difference. While the 2-wheeled scooter is easy to ride (I've let maybe 100 people ride it without a single fatality) the Electric Unicycle takes a good deal of practice. You don't want to be learning how to control such a vehicle at the same time as debugging it, so you really need to learn to ride a regular unicycle first. I got a "United 24-inch Trainer for Adults" from Unicycle.com and spent a couple months learning to ride it before I built the electric version.
All together the components, in single unit retail quantities, cost about $1500. They are:
The component I would most like to replace is the Whyachi gearbox. It makes a very loud gear whirring sound, which is fine for battlebots but undermines the graceful gliding look.
You can download the complete software here. The mechanical fabrication drawings are available in Postscript (use the Ghostscript viewer) and as an eDrawing (use the eDrawing viewer.)