Radio waves reveal the presence of a person in a room, even beyond the sensor’s line of sight

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Relief may be on the horizon for anyone who has ever jumped into a room like a jack-in-the-box to turn motion-sensing lights back on, thanks to a new metamaterial-based motion sensor sensitive enough to monitor the a person’s breath.

In a pair of new studies, researchers from Duke University and the Langevin Institute in France have shown that patterns created by radio waves can detect a person’s presence and location anywhere. into a room.

The findings emerged recently in Scientific reports and August 6 in the Physical examination letters.

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

“Energy companies don’t like infrared motion detectors because they have a lot of problems,” said David R. Smith, 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 in their line of sight to be detected, and everyone has probably had the experience where the lights went out because they stayed stationary for too long. Radio waves can circumvent all these limitations.”

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

In the latest paper, the team shows that with a little training, the system can also extract the information needed to locate objects or people in a space. The demo system learned the pattern of radio waves broadcast by a triangular block placed in 23 different positions on a floor. This calibration is sufficient not only to distinguish the 23 learned scenarios, but also to distinguish the positions of three identical blocks placed in any one of the 1,771 possible configurations.

The technology works by taking advantage of how radio waves behave in an enclosed room. Their ability to continuously reflect off multiple surfaces creates complex interference patterns in 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 exploited to detect motion and locate objects in a room.

“The complexity of how radio waves bounce around a room and interfere with themselves creates a kind of fingerprint,” said Philipp del Hougne, a researcher visiting Smith’s lab at the Langevin Institute in Paris, France. “And every time an object in a room moves, even a little bit, that fingerprint changes.”

The challenge is to find the most efficient way to ink that fingerprint in the first place. It requires a lot of information, del Hougne explained, and there are several traditional ways to do it, but they all have drawbacks.

A large number of antennas could be installed in many places around a room to take several measurements, but this would be expensive and impractical. Another tactic would be to measure many different frequencies, as each bounces around a room in a unique way. This approach, however, would likely create interference with other radio wave signals like Wi-Fi and Bluetooth operating in a room.

The researchers’ solution is to dynamically control the shape of the waves using metamaterials – artificial materials that manipulate waves like light and sound through the properties of their structure. A flat panel metamaterial antenna can shape waves into arbitrary configurations and create many different wavefronts in rapid succession.

“It doesn’t matter what those particular waveforms are,” Smith said. “As long as they are diverse, the detector will detect enough different patterns to determine if there is something and where it is.”

“There are other technologies that could achieve similar wavefront shaping capabilities, but they are much more expensive both in terms of cost and power consumption,” said Mohammadreza Imani, a postdoctoral fellow in Smith’s lab who also worked on the papers. “Studies have shown that being able to adjust the temperature of a room when people leave and return can reduce energy use by around 30%. But if you’re trying to save energy by spending more energy by changing the antenna pattern, then you are not serving.”

And energy savings may just be the tip of the iceberg. The ability to count the number of people in a room, distinguish body positions, and monitor breathing patterns also has potential applications in security, healthcare, and gaming.

French project scientists created a related startup called Greenerwave.

“While we’ll definitely continue with the energy angle, we’ll also see where the research takes us,” Smith said.

This work was supported by the Air Force Office of Scientific Research (FA9550-12-1-0491).

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