Wireless Surface Strain Mapping by Passive Electromagnetic Resonators

A wireless multipoint strain sensing system is demonstrated via a compact and passive electromagnetic-coupling-based array of resonators. The system incorporates nested split-ring resonators (NSRRs) as the sensing element, since this geometry is known to enable higher sensitivity and miniaturization. The strain sensing system can be configured in two ways: 1) a single-antenna system where the near-field coupling of the antenna creates high field localization on the array and 2) a two-channel system (TX and RX), where the transmitter can be placed at a distant location to allow remote illumination of the array. Both configurations are validated by measurements, which demonstrate that the system exhibits at least 10-
$\mu \text{m}$
-level displacement resolution, corresponding to 1000 microstrains, and a sensitivity of 100 Hz/microstrain. Results of experiments with various antenna types, different numbers of elements, and array alignments are presented, along with simulations to understand the system characteristics. The results prove that frequency shifts of the sensors can be tracked without complex processing as long as the initial resonance frequency and allocated bandwidth of each sensor are selected so as to avoid overlaps. The system is particularly advantageous for structural health monitoring (SHM) applications, where measurement of 2-D strain is critical in assessment of the deformation in concrete, metal, or composite materials.