Infrared motion detectors can cover only a limited amount of space, a person has to be within the detector's line of sight to be detected, and lights can go off when a person remains still for too long. Work has demonstrated that patterns made by radio waves can detect a person's presence and location anywhere inside a room. A new motion sensor based on metamaterials is sensitive enough to monitor a person's breathing.
Radio waves bounce around a room and interfere 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.
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 it has been shown that this same complexity can be tapped to detect movement and locate objects within a room. The complexity of the way radio waves bounce around a room and interfere with themselves creates a sort of fingerprint. Each time an object within a room moves, even a little bit, that fingerprint changes. The challenge lies with finding the most efficient way to ink that fingerprint in the first place. It requires a lot of information, and there are multiple traditional ways it can be done, but they all have drawbacks.
A large number of antennas could be installed in many places around a room to take multiple measurements, but this would be costly and inconvenient. Another tactic would be to measure many different frequencies since 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 within a room. The solution was to dynamically control the shape of the waves using metamaterials — artificial materials that manipulate waves like light and sound through properties of their structure. A flat-panel metamaterial antenna can shape waves into arbitrary configurations and create many different wave fronts in rapid succession.
It has been shown that the ability to adjust a room's temperature when people leave and come back can reduce power consumption by around 30 percent. And energy savings may be just 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.