The One-Wheel Cubli Balances Perfectly with a Single Reaction Wheel — and Could Inspire Satellites
A cubic robot designed to move using three reaction wheels has been simplified, and can now balance itself using just one.
Cubli, a three-reaction-wheel cube-shaped robot capable of jumping and "walking," has received a major simplification to create the One-Wheel Cubli — which, as per its name, uses only a single reaction wheel yet manages to balance perfectly on its pivot.
The One-Wheel Cubli is based on the Cubli, a 15×15×15cm (around 5.9×5.9×5.9") cubic robot unveiled by Gajamohan Mohanarajah and Raffaello D'Andrea in 2012 and that was originally designed to feature reaction wheels on three of its faces. As the reaction wheels spin at high speeds, a brake can be applied to make the robot jump — or the torque of each wheel adjusted in order to make it balance or fall in a particular direction, all of which can be combined to make the robot "walk."
The One-Wheel Cubli, though, has only one wheel — as the name implies. Normally, a Cubli with a single wheel wouldn't be able to do much — but D'Andrea, working with Matthias Hofer, Michael Muehlebach, Michael Egli, and Matthias Müller, came up with a way to do more with less. "The key is to design the system such that the inertia in one direction is higher than in the other direction by attaching two masses far away from the center," the researchers explain. "As a consequence, the system moves faster in the direction with the lower inertia and slower in the direction with the higher inertia. The controller can leverage this property and stabilize both directions simultaneously."
"Seeing the OWC [One-Wheel Cubli] balance for the first time was counter-intuitive as the working principle is not obvious," Hofer recalls in an interview with IEEE Spectrum, which brought our attention to the project. "It was very satisfying for us, as it meant that every puzzle piece of the project that Michael Muehlebach, Raffaello D’Andrea, and I, along with our technical staff (Michael Egli and Matthias Müller), contributed to finally worked — including the theoretical analysis, the prototype development, the modeling, the state estimation, and the control design."
The team is investigating extending the system to provide three degrees of freedom, and also its potential use in satellites — where reaction wheels are already used to keep them pointed towards ground stations or other areas of interest.
The team's work, which was presented earlier this year as a Master's thesis, had not yet been publicly released at the time of writing.