WearID: A Wrist-worn Backscatter Reader to Measure Interactions with the Physical World
In this work, we explore the challenges in designing a practical wearable backscatter reader that operates within the form-factor and power constraints of a smartwatch-class device, and the opportunities that such a device presents for monitoring interactions. Our work is inspired by early work on the design of glove and bracelet-sized NFC readers for activity recognition iBracelet. However, an NFC-based approach has two key limitations: a) NFC operates at much lower frequency than UHF RFID and therefore does not provide useful phase information that is crucial for a range of interactive RFID-based applications, and b) NFC operates at short ranges of a few centimeters with smartwatch form-factor loop antennas, and is therefore more suited for intentional interactions (e.g. credit card payment). In contrast, UHF RFID readers operate at a wavelength of roughly 30cm and does not have the inherent range limitations of NFC, which makes it is more suitable for measuring tactile interactions with tagged objects. We refer to our design as WearID.
The receiver is mainly passive components, which reduce sensitivity of the system and as a resut reduce the reading range of the system. We also reduced the RF output power to further reduce power consumption, while losing more range, but all in all, the range loss comes in handy in detection of objects in close proximity of user's hand.
Hardware design files can be found here.
System is able to distinguihs between grabbing a tagged object, releasing a tagged object, touching a tag, and passing by a tag, with 86% accuracy.