Microwave Encoders for Chipless RFID and Angular Velocity Sensors Based on S-Shaped Split Ring Resonators

In this paper, it is demonstrated that a chain of S-shaped split ring resonators (S-SRRs) etched on a dielectric substrate can modulate the amplitude of a carrier signal injected to a transmission line (a coplanar waveguide (CPW). To this end, the S-SRR chain must be transversally displaced above the CPW, in close proximity to it. By this means, the transmission coefficient of the line is modulated by the time-varying electromagnetic (inductive) coupling between the line and the S-SRRs of the chain, related to their relative motion. Based on this principle, two different applications can be envisaged: 1) angular velocity sensors and 2) near-field chipless radiofrequency identification (chipless-RFID) tags. In the former application, the S-SRR chain is circularly shaped and the S-SRRs are distributed uniformly along the perimeter of the rotor, at equidistant positions. By this means, the amplitude-modulated signal generated by rotor motion exhibits envelope peaks, whose distance is related to the angular velocity of the rotor. In the use of S-SRRs as microwave encoders for chipless RFID tags, not all the S-SRRs of the chain are present. Their presence or absence at the predefined (equidistant) positions is related to the logic state “1” or “0.” Tag reading is sequential, and it is achieved through tag motion (at constant velocity) above the reader, a CPW transmission line fed by a carrier signal. The ID code is contained in the envelope function of the resulting amplitude modulated signal, which can be obtained by means of an envelope detector. With the proposed approach, a high number of pulses in angular velocity sensors can be achieved (with direct impact on angle resolution and sensitivity to changes in instantaneous rotation speed). Moreover, chipless-RFID tags with unprecedented number of bits can be obtained. The proposed angular velocity sensors can be useful in space environments, whereas the chipless-RFID systems based on the proposed tags are useful in applications where reading range can be sacrificed in favor of high data capacity (large number of bits), e.g., security and authentication.