Building a cube that can jump up and balance.
The Cubli takes it’s name from a combination of the English word ‘cube’ and the Swiss-German diminutive ‘li’ indicating something of small size. With the side length of 15 centimeters it mainly consists of three reacton wheels each attached to a brushless DC-motor, motor controllers, inertial sensors and embedded processor and batteries.
Inertial sensor measurements are fused to get an estimate of the tilt and angular velocity of the Cubeli. And the whole sensors provide the angular velocity of the reaction wheels. Based on these estimates a non-linear controller is used to balance the Cubeli on it’s corner while the angular velocity of the reaction wheels is kept close to zero. Although this method of using reaction torques is similar to the one that is used to stabilize satellites in space due to gravity the algorithms used here are very different. With this set up the Cubely can robustly balance around its corner while rejecting disturbances, rotate while balancing and balance on its edge.
In addition the momentum wheels can rotate at very high angular velocities and build up sufficient angular momentum. A rapid braking of the wheel allows it to transfer the angular momentum to the Cubeli’s body. This allows the Cubeli to jump up from a resting position to a balancing position without external support.
Balancing on a corner is achieved through two subsequent jump up maneuvers. The first maneuver raises the Cubeli from one of its faces so that a balance is along one of its edges. On the second raises it from its edge to one of its corners.
In addition to balancing the motor torques can be used to achieve a controlled fall such that the Cubeli can be commanded to fall in a specific direction.
Combining these three abilities: jumping up, balancing and controlled falling allows the Cubeli to walk across a surface. Some researchers hope to use this method of locomotion for planetary exploration while others are using it for self assembling robots. But for us it is just a cool little cube that can jump up, balance and walk.