High-Density Microwave Encoders for Motion Control and Near-Field Chipless-RFID

A novel microwave system for measuring linear displacements and velocities with sub-millimeter resolution and for the implementation of near-field chipless radiofrequency identification (chipless-RFID) systems with very high data capacity is presented. The system is based on a reader, consisting of a half-wavelength straight resonator coupled (through capacitor gaps) to a pair of access lines, and a microwave encoder, in relative motion to the reader and consisting of a linear chain of strips orthogonally oriented to the chain axis. By displacing the encoder over the half-wavelength resonator of the reader, with the encoder strips parallel oriented to the reader axis, the relative velocity and position between the encoder and the reader can be inferred. For that purpose, the reader is fed by a harmonic signal tuned to the resonance frequency that results when an encoder strip is perfectly aligned with the reader. The encoder motion amplitude modulates the feeding signal at the output port, and both the position and the velocity are measured from the peaks, or dips, of the resulting envelope function. Moreover, by making certain strips inoperative, the system can be used for coding purposes. Due to the small period of the encoder (0.6 mm), a high per-unit-length data density in these near-field chipless-RFID tags (i.e., 16.66 bits/cm) is achieved. To illustrate the functionality and potential of the approach, 100-bit chipless-RFID tags with various ID codes are implemented and read.

I. Introduction

There are several approaches for the implementation of displacement and velocity sensors based on microwave technologies. In particular, a significant research effort has been dedicated in the last decade to the development of sensing systems where a transmission line is loaded with resonant elements in relative motion respect to it [1]–[10] (other sensors based on resonator-loaded lines, useful for dielectric characterization and related applications, have been also reported [11]–[25]). In several of the previous spatial sensors, the relative displacement is determined from the variation in the resonance frequency that results when the transmission line and the resonant element are in relative motion (either linear or angular) [2], [6]. In other displacement and velocity sensors, the working principle is coupling modulation [3]–[5], [7]. Although frequency variation and coupling modulation sensors are simple from a technological viewpoint, their resolution and dynamic range are typically limited. Moreover, frequency variation sensors may be subjected to cross sensitivities caused by environmental factors (e.g., frequency drifts caused by temperature variations or changes in ambient moisture), and coupling modulation sensors are very sensitive to noise.