New motion-sensing technology can detect a person’s presence and location anywhere inside a room

A closer look at the metamaterial that shapes radio waves to monitor movement within a room. Each cell can be individually tuned to interact with radio waves in a specified manner. Photo by Timothy Sleasman.

Relief may be on the horizon for anyone who has ever jumped around a room like a jack-in-the-box to get motion-sensing lights to turn back on. A new motion sensor based on metamaterials is sensitive enough to monitor a person’s breathing.

In a pair of new studies, researchers from Duke University and Institut Langevin, France, have shown that patterns made by radio waves can detect a person’s presence and location anywhere inside a room.

The findings appeared recently in Scientific Reports and Aug. 6 in the Physics Review Letters.

This new motion-sensing technology could lead to new smart home devices for energy savings, security, healthcare and gaming.

“Energy companies don’t love infrared motion detectors because they have lots of problems,” said David R. Smith, the James B. Duke Professor of Electrical and Computer Engineering at Duke. “The amount of space they can cover is limited, a person has to be within their line of sight to be detected, and probably everyone has had the experience where the lights have gone off because they’ve sat still for too long. Radio waves can get around all of these limitations.”

In their initial paper published earlier this year, the researchers took advantage of patterns created by radio waves bouncing around a room and interfering with themselves. These unique patterns change with the slightest perturbation of the room’s objects, allowing a sensitive antenna to detect when something moves in or enters the room. And by comparing how these patterns change over time, they can also be used to detect cyclical movements like a fan blade turning—or even a person breathing.

In the latest paper, the team shows that with a bit of training, the system can also extract information necessary to locate objects or people in a space. The demonstration system was taught the pattern of radio waves scattered by a triangular block placed in 23 different positions on a floor. That calibration is enough not only to distinguish between the learned 23 scenarios, but to also distinguish the positions of three identical blocks placed in any one of 1,771 possible configurations.

The technology works by taking advantage of the way radio waves behave in an enclosed room. Their ability to continuously reflect off multiple surfaces creates complex interference patterns throughout a room. In the past, this complexity has been an obstacle for systems trying to locate the origin of a signal. But Smith and his colleagues have now shown that this same complexity can be tapped to detect movement and locate objects within a room.

Learn more: Ricocheting Radio Waves Monitor the Tiniest Movements in a Room