Range Extension in Coupling-Based Wireless Passive Displacement Sensors for Remote Structural Health Monitoring

Wireless monitoring of displacement between two points is demonstrated with an interrogation range (
${D}_{\boldsymbol {m}}$
) which is dramatically larger compared with previous configurations based on near-field coupling. The sensing system comprises a transmitting antenna, a nested split-ring resonator (NSRR) sensor functioning as a tag, and a probe, called a “sniffer coil.” In classical configuration, the NSRR and the antenna constitute an electromagnetically coupled system which exhibits high sensitivity and resolution but is only functional within the near-field of the antenna, which in practice is much smaller than one wavelength, which limits the capabilities of the sensor. In this work, the system is separated into the TX and RX channels, which enables the transmitter antenna to be placed at a location which is several wavelengths away from the sensor. To remove the effect of clutter that disrupted the sensing signal at higher
${D}_{\boldsymbol {m}}$
in previous works, the variation with frequency of the power level coupled to the sniffer coil is tracked instead of the variation in the scattering parameters with frequency. While extending
${D}_{\boldsymbol {m}}$
, the high sensitivity and resolution are retained by the coupling between the NSRR and the sniffer coil. Wireless sensing of displacement is essential for several application areas, most specifically, structural health monitoring (SHM). The fact that the sensing system can work in far-field and that the NSRR is passive allows for remote and nondestructive testing (NDT) of damaged structures and buildings.