RFID-based sensing technology with microstrip lines

We have developed a sheet-type sensing technology using radio frequency identification (RFID) tags with microstrip lines that can detect the presence of objects on a sheet. The sensing sheet is composed of microstrip lines, a spacing material on the lines, and RFID tags attached to the spacing material. The RFID tags are electromagnetically coupled with the microstrip lines. The thickness of the spacing material and the horizontal distance between the RFID tag and the microstrip line influence the coupling coefficient. The microstrip lines are connected to an RFID reader that collects the tag IDs with their received signal strength indexes (RSSIs). When an object composed of metal or a dielectric is on an RFID tag, the coupling coefficient is influenced and the RSSI changes, thus enabling the presence of the object to be detected by the difference of the RSSI. We describe the characteristics of this sensor and demonstrate detection using plastic drink bottles and metal cans.