Flexible skin lets robots, prosthetics sense shear force

A flexible sensor skin that can be stretched over any part of a robot’s body or prosthetic can accurately convey information about shear forces and vibration.

Developed by engineers from the University of Washington (UW) and the University of California, Los Angeles (UCLA), the bio-inspired robot sensor skin mimics how a human finger experiences tension and compression as it slides along a surface or distinguishes among different textures, measuring tactile information with precision and sensitivity similar to human skin.

“Robotic and prosthetic hands are really based on visual cues right now – such as, ‘Can I see my hand wrapped around this object?’ or ‘Is it touching this wire?’ But that’s obviously incomplete information,” says senior author Jonathan Posner, a UW professor of mechanical engineering and chemical engineering. “To hold on to a medical instrument, it needs to know if the object is slipping. This all requires the ability to sense shear force, which no other sensor skin has been able to do well.”

Some robots use fully instrumented fingers, but sense of touch is limited to that appendage and users can’t change its shape or size to accommodate different tasks. Wrapping a robot appendage in a sensor skin provides better design flexibility, but these skins don’t yet provide a full range of tactile information.

“Traditionally, tactile sensor designs have focused on sensing individual modalities: normal forces, shear forces, or vibration exclusively. However, dexterous manipulation is a dynamic process that requires a multimodal approach. The fact that our latest skin prototype incorporates all three modalities creates many new possibilities for machine learning-based approaches for advancing robot capabilities,” says co-author and robotics collaborator Veronica Santos, a UCLA associate professor of mechanical and aerospace engineering.

Manufactured at the UW’s Washington Nanofabrication Facility, the stretchable electronic skin made from silicone rubber is embedded with tiny serpentine channels – roughly half the width of a human hair – and filled with electrically conductive liquid metal that won’t crack or fatigue when the skin is stretched.

When the skin is placed around a robot finger or end effector, these microfluidic channels are strategically placed on either side of where a human fingernail would be.

As a human finger slides across a surface, one side of the nailbed bulges out while the other side becomes taut under tension. The same thing happens with the robot or prosthetic finger – the microfluidic channels on one side of the nailbed compress while the ones on the other side stretch out.

When channel geometry changes, so does the amount of electricity that can flow through them. The research team can measure differences in electrical resistance and correlate them with the shear forces and vibrations of the robot finger.

In experiments, the skin has detected tiny vibrations at 800 times per second, better than human fingers.

University of California Los Angeles (UCLA)
www.ucla.edu

University of Washington
www.washington.edu

The research was funded by the National Science Foundation; Grant numbers - NSF: CBET - 1264046 and CBET - 1461630.