Hybrid Time/Phase Domain Synchronous Electromagnetic Encoders for Near-Field Chipless-RFID and Motion Control Applications

Synchronous electromagnetic encoder systems exploiting phase encoding to boost up the number of bits per encoder position are reported in this article. The reader is a one-port structure consisting of a transmission line terminated with a matched load and fed by a harmonic (single tone) signal. The encoders, or tags, are implemented by means of a single chain of strip resonators (inclusions) printed or etched on a dielectric substrate and oriented transversally to the chain axis. Phase encoding is achieved through the transverse position of the strips in the chain. Thus, in a reading operation, the encoder is displaced at a very short distance over the reader, in the direction orthogonal to the line axis. By this means, the resonant strips of the encoder cross the line in perfect alignment with it, significantly perturbing the reflection (and transmission) coefficient. By tuning the frequency of the feeding signal to the resonance frequency of the strips, total reflection is (roughly) expected each time a resonant strip is on top of the line. Moreover, under perfect alignment between the line and a resonant strip, the phase of the reflection coefficient depends on the distance between the inclusion (resonant strip) and the input port. Therefore, the identification (ID) associated with that inclusion can be retrieved by measuring the phase of the reflection coefficient. The number of states (and hence bits) per encoder position in the chain depends on the number of different transverse positions of the inclusions that can be resolved (16 in the prototype reported in this article). The achieved density of bits per unit length is DPL = 5.71 bit/cm and per unit surface is DPS = 0.57 bit/cm ^{2} . Nevertheless, the main relevant advantage of the proposed phase-modulated (PM) system is the fact that a single harmonic signal suffices for tag reading, contrary to other synchronous electromagnetic encoder systems based on frequency encoding, where multiple feeding harmonic signals are needed.