The Cubli is a 15 × 15 × 15 cm cube that can jump up and balance on its corner. Reaction wheels mounted on three faces of the cube rotate at high angular velocities and then brake suddenly, causing the Cubli to jump up.
Once the Cubli has almost reached the corner stand up position, controlled motor torques are applied to make it balance on its corner. In addition to balancing, the motor torques can also be used to achieve a controlled fall such that the Cubli can be commanded to fall in any arbitrary direction.
Combining these three abilities — jumping up, balancing, and controlled falling — the Cubli is able to ‘walk’.Balancing is not necessarily difficult to achieve (although it looks amazing).
The trickiest part was in getting the cube to jump up from a resting position to a balancing position, since it releases a burst of energy to do so, and needed to be kept stable. The solution was to use momentum wheels, which are the same kind of flywheel used for altitude control in spacecraft.