| 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.
Building your own
Feel the need 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 & 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 Extra
Large Adults" from Unicycle.com
and spent a couple months learning to ride it before I built the
electric version.
Components
All together the components, in single unit retail quantities,
cost
about $1500. They are:
- A microcontroller board from BDMicro featuring the Atmel
AVR Mega 128. $125.
- A gyroscope and acceleromoter by Rotomotion. $149.
- The OSMC motor controller by Robot Power. $199.
- 40 NIMH cells, made into nice packs by Robot Marketplace. $218.
- A Magmotor S28-150.
$299
- The Whyachi TWM3M gearbox. $345.
- A 16-inch bicycle rim with spokes. $35 from your local bike
shop. I use a 100 psi tire, which makes turning slightly easier.
- A custom-machined hub.
- A GT2 5 mm pitch belt, and 28 and 72-tooth pulleys. $21, $26 and $14.
- Some 1" diameter tubing, TIG-welded together. $40
- A guard for the upper pulley, machined from black Acetal
plastic.
- Some spacer bushings to center the wheel.
- A combination dead man's switch and key. $12.
- A unicycle seat. $24.
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.
Downloads
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.) |
