One wheeled robots are extremely difficult to keep balanced due to the single point of contact with the ground.

Multi-wheel statically-stable mobile robots tall enough to interact meaningfully with people must have low centers of gravity, wide bases of support, and low accelerations to avoid tipping over. These conditions resent a number of performance limitations.

Accordingly, we are developing an inverse of this type of mobile robot that is the height, width, and weight of a person, having a high center of gravity, that balances dynamically on a single spherical wheel. Unlike balancing 2-wheel platforms which must turn before driving in some direction, the single-wheel robot can move directly in any direction. We present the overall design, actuator mechanism based on an inverse mouse-ball drive, control system, and initial results including dynamic balancing, station keeping, and point-to-point motion.

A significant, but frequently overlooked problem is that statically-stable one wheeled mobile robots can easily become dynamically unstable.
-If the center of gravity is too high,
-or the robot accelerates/decelerates too rapidly,
-or is on a sloping surface,
-the machine can tip over.

A robot must be tall enough to be able to interact with people and the human environment at a reasonable height. On the other hand, it must be skinny enough to easily make its way around without bumping into things or getting into peoples’ way.