In a physics laboratory in Amsterdam, there is a wheel that can spontaneously roll upwards while wiggling.
This “odd wheel” looks simple: just six small motors linked together by plastic arms and rubber bands to form a ring about 6 inches in diameter. When the motors are turned on, he begins to twist, performing complicated squishing and stretching motions and occasionally launching himself into the air, while slowly climbing up a bumpy foam ramp.
“I find it very playful,” said Ricard Alert, a biophysicist at the Max Planck Institute for Complex Systems Physics in Dresden, Germany, who was not involved in building the wheel. “I loved.”
The odd wheel’s unorthodox mode of motion illustrates a recent trend: physicists are finding ways to spontaneously bring out useful collective behavior in robots assembled from simple parts that obey simple rules. “I call it robophysics,” said Daniel Goldmanphysicist at the Georgia Institute of Technology.
The problem of locomotion, one of the most basic behaviors of living beings, has long preoccupied biologists and engineers. When animals encounter obstacles and rough terrain, we instinctively rise to those challenges, but how we do it isn’t so simple. Engineers have struggled to build robots that don’t collapse or tip forward when navigating real-world environments, and they can’t program a robot to anticipate all the challenges it faces. he might meet.
The odd wheel, developed by physicists Corentin Coulais from the University of Amsterdam and Vincenzo Vitelli of the University of Chicago and his collaborators and described in a recent preprint, embodies a very different approach to locomotion. The upward movement of the wheel results from a simple oscillatory movement of each of its components. Although these parts know nothing of the environment, the wheel as a whole automatically adjusts its waving motion to compensate for irregularities in the terrain.
The energy generated during each cyclical oscillation of the odd wheel allows it to push against the ground and roll up and over obstacles. (Another version of the wheel with only six motors was studied in a recent article.)Video: Corentin Coulais
Physicists have also created an “odd ball” that always bounces to one side and an “odd wall” that controls where it absorbs energy from an impact. The objects all come from the same equation describing an asymmetrical relationship between stretching and squeezing movements that the researchers identified two years ago.
“These are indeed behaviors that you would not expect,” said Auke Ijspeert, biorobotician at the Swiss Federal Institute of Technology in Lausanne. Coulais and Vitelli declined to comment as their latest article is under peer review.
In addition to guiding the design of more robust robots, the new research could provide insight into the physics of living systems and inspire the development of new materials.
strange matter
The odd wheel grew out of Coulais and Vitelli’s earlier work on the physics of “active matter” – a generic term for systems whose building blocks draw energy from the environment, such as swarms of bacteria , flocks of birds and some man-made materials. The supply of energy generates rich behaviors, but it also leads to instabilities which make the active matter difficult to control.
Vincenzo Vitelli from the University of Chicago.Courtesy of Kristen Norman
Physicists have historically focused on systems that conserve energy, which must obey the principles of reciprocity: if there is a way for such a system to gain energy as it moves from A to B, any process that brings the system from B to A must cost an equal price. amount of energy. But with a constant supply of energy from within, this constraint no longer applies.
In a 2020 paper in Natural Physics, Vitelli and several collaborators began to study active solids with non-reciprocal mechanical properties. They developed a theoretical framework in which non-reciprocity manifests itself in the relationships between different types of stretching and squeezing movements. “For me, it was just a beautiful mathematical framework,” said Nikta Fakhribiophysicist at the Massachusetts Institute of Technology.