Balancing act (Harrison Shull/Aurora/Getty)

AS ANY unicyclist knows, balancing on one wheel while staying in one spot is pretty tricky. But a robot that can do just that could be used to squeeze into tight spaces during search and rescue missions, or possibly even as a form of transport.

Most wheeled robots have more than one wheel, but this means they need a chassis – something that gets in the way when trying to squeeze into gaps. Mono-wheeled robots would avoid this problem. But while the same gyroscopic effect that lets a spinning top balance as it spins also helps keep a moving wheel upright as it turns, once the wheel stops, balance and control become much harder.

Previous efforts to create a mono-wheel that is stable when stationary, like the Gyrover, developed by Ben Brown at Carnegie Mellon University in Pittsburgh, Pennsylvania, have a second wheel that spins within the first to act as a gyroscope. But this uses a lot of energy.

So Patryk Cieslak, a PhD student at AGH University of Science and Technology in Cracow, Poland, and colleagues decided to take a different approach. Their robot is packaged within a single wheel: its motor, battery and controls remain stationary in the centre of the wheel while a rubber tyre rotates around the outside.


To keep it upright, the team have placed a weighted lever that can tilt to either side within the body of the wheel (see diagram). If the wheel starts to fall to the right, for example, three sensors will detect the movement: an inclinometer to detect tilt, plus an accelerometer and a gyroscope to detect changes in direction. These then send a signal to a control circuit to move the lever to the left to provide counter-balance. By making constant adjustments, the lever keeps the wheel upright (Robotics and Autonomous Systems, DOI: 10.1016/j.robot.2011.05.002).

“There are many potential uses for this kind of robot because of its thin body, simple construction, good mobility and traction, and the efficiency of using only one wheel,” says Cieslak. He says that the next step is to set the robot rolling, possibly using the lever to steer the robot as it moves, before equipping it with vision and extra sensors that will let it guide itself.

Eventually the concept could be used for transportation. There are already mono-wheel motorcycles in which the rider sits inside a giant rotating wheel and steers by leaning, says Cieslak, but they can be hard to control. A scaled up version of the mono-wheeled robot could be safer, as it would actively balance itself.

Using a lever to balance a wheel is a valid approach, says Brown. “It simplifies things in some ways. Of course it makes the control a lot more complicated.” The robot will be harder to steer the faster it goes, Brown adds, as the lever has to overcome an increased gyroscopic effect.