Although robotic devices are used in everything from assembly lines to medicine, engineers have a hard time accounting for the friction that occurs when those robots grip objects — particularly in wet environments. Researchers have now discovered a new law of physics that accounts for this type of friction, which should advance a wide range of robotic technologies.
“Our work here opens the door to creating more reliable and functional haptic and robotic devices in applications such as telesurgery and manufacturing,” said Lilian Hsiao, an assistant professor of chemical and biomolecular engineering at NC State and corresponding author of a paper on the work.
At issue is something called elastohydrodynamic lubrication (EHL) friction, which is the friction that occurs when two solid surfaces come into contact with a thin layer of fluid between them. This would include the friction that occurs when you rub your fingertips together, with the fluid being the thin layer of naturally occurring oil on your skin. But it could also apply to a robotic claw lifting an object that has been coated with oil, or to a surgical device that is being used inside the human body.
Our work here opens the door to creating more reliable and functional haptic and robotic devices… ” — Lilian Hsiao
“Understanding friction is intuitive for humans — even when we’re handling soapy dishes,” Hsiao said. “But it is extremely difficult to account for EHL friction when developing materials that control grasping capabilities in robots.”
To develop materials that help control EHL friction, engineers would need a framework that can be applied uniformly to a wide variety of patterns, materials and dynamic operating conditions — which is what the researchers have discovered.
“This law can be used to account for EHL friction, and can be applied to many different soft systems — as long as the surfaces of the objects are patterned,” Hsiao said.
In this context, surface patterns could be anything from the slightly raised surfaces on the tips of our fingers to grooves in the surface of a robotic tool.
The new physical principle, developed jointly by Hsiao and her graduate student Yunhu Peng, makes use of four equations to account for all of the physical forces at play in understanding EHL friction. The research team demonstrated the law in three systems: human fingers; a bio-inspired robotic fingertip; and a tool called a tribo-rheometer, which is used to measure frictional forces. Peng is first author of the paper.
“These results are very useful in robotic hands that have more nuanced controls for reliably handling manufacturing processes,” Hsiao said. “And it has obvious applications in the realm of telesurgery, in which surgeons remotely control robotic devices to perform surgical procedures.”
Co-authors of the paper include Christopher Serfass, a Ph.D. student at NC State, and Catherine Hill, an undergraduate student at NC State.
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