Enhancing the Bit Density in Linear Electromagnetic Encoders for Chipless-RFID and Motion Sensing Applications

This paper presents a technique to increase the density of bits per unit length (DPL), a key figure of merit, in linear electromagnetic encoders. Such encoders are a low-cost alternative to optical encoders, and are useful for near-field chipless-RFID applications or for displacement and velocity sensing. The proposed encoders are made of two chains of metallic resonators (inclusions), transversally oriented with regard to the chains’ axis, etched (or printed) on top of a dielectric substrate, either rigid or flexible (for example, a polymer or paper). Encoding is achieved by the size of the resonators, which determines their frequencies. The reader is made of two transmission lines loaded with series gaps, axially aligned in the gap region, and oriented transversally to the axis of the encoder chains. By encoder motion over the reader, when a resonator lies on top of the line gap, it modifies the transmission coefficient of the corresponding line. Consequently, such resonator can be detected by the variation of the amplitude of a harmonic signal injected to the corresponding line, which should be tuned to the resonance frequency of the inclusion. Thus, as many harmonic signals as different resonator sizes must be injected to the lines, and the envelopes of the corresponding amplitude modulated (AM) signals at the output port of each line provide the ID code of the encoder in a time division multiplexing scheme (i.e., sequentially). As compared to other previous works where more than one chain of inclusions is used for encoding, thereby enhancing the DPL, the reader in the present paper is simply made of a pair of transmission lines, avoiding the need of power splitters. With only two chains, the proposed system provides 5.58 bits per chain position, and a density of DPL = 11.16 bit/cm. The preliminary results of the present paper allow us to envisage that it is possible to increase the number of chains (and hence the number of reader lines), with the potential to considerably further enhance the DPL.