Underpowered robotics is an area of growing interest in robotics. While the loss of actuators offers many benefits in the application, it also poses significant challenges for the control of under-energized systems.
In many cases the indirectly controlled degrees of freedom are stable. More challenging and arguably more interesting systems arise when these degrees of freedom are unstable.
Nearly a decade ago, robot researchers led by ETH professor Raffaello D’Andrea impressed with Cubli, a cube-shaped robot that can jump up and balance on its fulcrum. As reaction wheels (mounted on three sides of the cube) spin at high speed, a brake can be applied to make the robot jump up and move around. Each wheel’s torque is adjusted to balance it or achieve a controlled drop in any direction. All of these skills can be combined to make the robot ‘walk’.
Now Cubli has been given a major simplification to create the single wheel Cubli, which requires only a single reaction wheel – as the name implies – for its balancing act.
Like its predecessor, One-Wheel Cubli has mounted the reaction wheel inside the housing. Instead of extra wheels, it is equipped with a balancing pole. When the wheel is accelerated by the electric motor, a reaction torque acts on the housing which is used to stabilize the system.
The system is carefully designed so that inertia is higher in one direction than the other by mounting two masses far from the center. As a result, the system moves faster in the lower inertia direction and slower in the higher inertia direction. This allows the One-Wheel Cubli to stabilize both directions at the same time with just one reaction wheel, researchers explain.
The single-wheeled Cubli has two main degrees of freedom, tilting fore-aft or side-to-side. What’s more, the engineers say that even if the One-Wheel Cubli is disrupted, it can recover and reliably balance on its fulcrum.
Now the team is working on further improving the system and is also investigating its possible application in satellite attitude control. In addition, the reverse shuttle system provides a simple yet challenging testing ground for control research and education.
Magazine reference:
- Matthias Hofer, Michael Muehlebach, Raffaello D’Andrea. The One-Wheel Cubli: a 3D inverted pendulum that can balance with a single reaction wheel. Mechatronics, 2023; DOI: 10.1016/j.mechatronics.2023.102965
