RFID tag arrays can be used to track a person's movement

Cheap, washable, and battery-free RFID tags could form the basis for a new type of wearable sensor.

Radio-frequency identification (RFID) tags have become a key component of global commerce, enabling stakeholders to track physical assets quickly and reliably. Deployed properly, the tags could be used in a new class of wearable designed to track physical movement and shape change.

Researchers at Carnegie Mellon University have figured out how RFID tags could be used do things like control virtual avatars or tell slouching users to sit up straight.

RFID tags are cheap, battery-free and washable, which makes them appealing to developers.

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"By attaching these paper-like RFID tags to clothing, we were able to demonstrate millimeter accuracy in skeletal tracking," says Haojian Jin, a Ph.D. student in CMU's Human-Computer Interaction Institute (HCII).

The innovation is the strategy Jin and his collaborators used to track the tags. RFID tags work by bouncing back certain frequencies. If you wanted to devise a method to track movement using multiple RFID tags, the simplest way would be to track backscatter using multiple antennae to triangulate location. But that's not practical in real-world applications.

Instead, the CMU researchers used a single mobile antennae to essentially rig a time-of-flight sensor. As backscatter comes back from multiple tags embedded in clothing, for example, subtle differences in signal timing from one sensor to another can be used to calculate the motion of and changes of shape to an object. 

RFID-embedded clothes could be an alternative to motion tracking devices like the Fitbit. RFID could also surmount camera-based systems like Kinect for controlling avatars in virtual environments.

Backscatter from RFID arrays can also be used to interpret the changing shapes of objects, which is the subject of research from Jin and Jingxian Wang, a Ph.D. student in CMU's Electrical and Computer Engineering (ECE) Department.

"We can turn any soft surface in the environment into a touch screen," Wang said. A CMU spokesperson told me that smart carpets, for instance, could detect the presence and locations of people or be used to control games or devices. 

"Soft toys could respond to or otherwise register squeezes and bends," according the CMU spokesperson. "Smart pillows might help track sleep quality."

RFID could also monitor vital infrastructure. The CMU researchers, for instance, measured the curvature of a bridge in Pittsburgh by dragging a string of RFID tags across.

"We're really changing the way people are thinking about RF sensing," Jin says.

The tags, on average, cost less than a dollar. Most smartphones can't currently read 900 MHz RFID applications, but including that capability in future handsets could unlock a cheap alternative to pricey wearable sensors.