Battery-Less Sensing of Body Movements Through Differential Backscattered RFID Signals

RFID tags not only can be used as identification devices, with antenna sensing techniques, they can also be used as low-cost and battery-less sensors. However, RFID antenna sensing relies on the impedance and/or gain modulation of the tag’s antenna, and extracts the sensing information through the RSSI variation of the backscattered signals. The sensing information is prone to the interferences and noises in the reader-tag communication channel, as well as the distance variations if the sensors are in constant motion. In this paper, we propose a sensor structure that is constructed from adjacently placed double tags to address these issues. The EM properties of the antennas of the double tags behave differently (or even oppositely) under the same sensing variation; therefore, the differential backscattered signals of the dual-tag sensor can eliminate the common-mode interferences while preserving the sensing information. We have constructed one curvature sensor and one deformation sensor with embroidered antennas (using conductive yarns) and all-fabric materials, and sewn the sensors on apparels to demonstrate their use in smart clothing applications. The experiments show that even when the human body is in constant motion, the curvature sensor sewn on the knee area can effectively monitor human’s body gesture, while the deformation sensor sewn on the chest area can precisely detect the respiration activities.