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. Credit: Timothy Sleasman
An international team of scientist have discovered a way to produce more accurate motion sensors using radio waves.
Researchers from both Duke University and the Institut Langevin in France have found patterns made by radio waves can detect where a person is inside of a room, which could yield new motion-sensing technology for smart home devices for energy savings, security, healthcare and gaming.
“Energy companies don’t love infrared motion detectors because they have lots of problems,” David Smith, the James B. Duke Professor of Electrical and Computer Engineering at Duke, said in a statement. “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.”
Initially, the researchers looked to take advantage of patterns created by radio waves bounding around a room and interfering with themselves that change with the slightest perturbation of the room’s objects.
This allows a sensitive antenna to detect when something moves in or enters the room and by comparing how the patterns change over time, they can be used to detect cyclical movements like a fan blade turning or a person breathing.
For the current study, the team found that they could train a system to also extract information necessary to locate objects or people in a space. The scientists taught the demonstration system the pattern of radio waves scattered by a triangular block placed in 23 different positions on a floor.
That calibration can distinguish between the learned 23 scenarios, as well as the positions of three identical blocks placed in any one of 1,771 possible configurations.
The new system takes advantage of the way radio waves continuously reflect off multiple surfaces to create complex interference patterns throughout a room.
“The complexity of the way radio waves bounce around a room and interfere with themselves creates a sort of fingerprint,” Philipp del Hougne, a researcher visiting Smith’s laboratory from Institut Langevin in Paris, said in a statement. “And each time an object within a room moves, even a little bit, that fingerprint changes.”
However, the researchers found it challenging to efficiently ink the fingerprint in the first place. One method is to install several antennas around the room to take multiple measurements, which would be both expensive and inconvenient.
Another method would be to measure several different frequencies, as each bounces around a room in a unique way. This method would likely create interference with other radio wave signals like Wi-Fi or Bluetooth operating within the room.
The researchers were able to dynamically control the shape of the waves using a flat-panel metamaterial antenna that can shape waves into arbitrary configurations and create several different wave fronts in rapid succession.
“There are other technologies that could achieve similar wave front shaping capabilities, but they are much more expensive both in cost and energy usage,” Mohammadreza Imani, a postdoctoral fellow in Smith’s lab who also worked on the papers, said in a statement. “Studies have shown that the ability to adjust a room’s temperature when people leave and come back can reduce power consumption by around 30 percent. But if you’re trying to save energy by spending more energy changing the antenna pattern, then you’re not helping.”
