Passive wireless displacement sensor based on RFID technology

This research demonstrates feasibility of using off-the-shelf radio frequency identification (RFID) tags to build a low cost passive wireless displacement sensor, suitable for applications such as crack detection in buildings and bridges. The sensor was constructed by splitting a RFID tag into two component parts consisting of the tag antenna and the chip-loop, which can be displaced by a distance represented by ‘d’. The displacement d could represent the width of a structural crack that may be found in a building or bridge. A loop antenna attached to a RFID reader can be placed at some distance ‘D’ in proximity to the sensor to measure the effect of the displacement (d) on the power transmitted by the RFID chip back to the reader. By incrementally increasing the power transmitted by the RFID reader to the sensor (Pt) when interrogating the RFID chip, calibration curves of the displacement d versus transmitted power Pt for different values of D were obtained. The use of the calibration curves along with known values of the transmitted power Pt allows for the determination of the displacement ‘d’.