Topic: The Omni-wheel - basis of self balancing mechanisms.
Have a look at https://www.youtube.com/watch?v=byUo4EagXu0
There is a device called an omni-wheel that has to be seen to believed. The author demonstrates things like unicycles, and robots balancing on one wheel. The omni-wheel is a larger wheel with several small wheels at its rim, each rotating on a peripheral axis. There is a general gyroscopic theme to the mechanics.