This Monowheel Robot Never Topples

A monowheel is a type of vehicle that rides on a single wheel. You may think that sounds a lot like a unicycle or a Onewheel-style electric skateboard, but the distinguishing feature of a monowheel is its massive diameter. A traditional monowheel vehicle carries its rider within the middle, where the hub would be. In the case of smaller robots, the motor and all of the electronic components sit within the circle of the rim. But like a unicycle, it is hard to keep a monowheel upright while moving slowly or standing still. To skirt that issue, James Bruton built a mechanically stable monowheel robot.

Calling this a "monowheel" is a bit of a cheat, because it technically has two wheels — they're just very close together. The use of two wheels comes with advantages. First, it lets the robot balance in place without tipping to either side. Second, it makes steering simple. Like a tank, this robot has differential drive: spin one wheel faster than the other and it will turn. It can even rotate in place if the wheels spin in opposite directions.

But the big innovation here is the type of wheels that Bruton chose. Each of those is a pedrail wheel, which is a strange contraption with a whole bunch of sliding legs in a radial arrangement with pivoting feet. It rotates like a wheel, but keeps at least a few feet in contact with the ground at any given time. Not only does that help it traverse certain obstacles, like steps, but it also means the pedrail wheel can remain stationary without rolling forwards or backwards.

Bruton 3D-printed almost all of the mechanical parts for this robot, including the legs and feet. The only parts that weren't 3D-printed were the fasteners and bearings. Because of the unique drive system, this robot's electronics are quite simple — no IMU (inertial measurement unit), gyroscope, or accelerometer necessary. There are two DC motors that spin the wheels and a Teensy 3.2 development board controls those. It receives commands from Bruton's universal remote via OrangeRX DSM transceivers.

Testing proved that this concept works fairly well. The robot moves around easily and can remain still without any kind of active balancing. It can also climb over small obstacles, though the top-heavy nature of the design makes it somewhat unstable if uneven terrain causes it to tip to either side. Spacing the wheels further apart would solve that issue, but then it would be even harder to classify this robot as a monowheel.