Abstract
A passive wireless strain-sensing system for strain monitoring of metallic structures is presented in this paper, wherein single-feed (SF) patch antenna with near orthogonal fundamental modes is initiated as the sensing element. A new approach is proposed by using input admittance of the orthogonal modes as the strain measurand. Strain sensing proof-of-concept of the SF patch-sensor was first established through electromagnetic simulation, then a cantilever beam strain measurement system was designed and employed to quantify the changes of input admittance while directional strain was arising. The simulated results reinforced with mathematical derivation and verified by experimental results to demonstrate that there is a linear relationship between normalized admittance and micro-strain with a high sensitivity factor of 97 ppm/με from simulation. Experimental results also verified that the single patch sensor can monitor the bidirectional (tensile and compressive) strain in multi-directions. An implementation example of the wireless passive sensing system is also presented in a proof-of-concept case. The change of minimum admittance that is linked with the applied strain was realized and interrogated using a monostatic radar system where the maximum radar cross section (RCS) and frequency differences were used as strain measurands. The interrogation method shows the RCS variations and the frequency shifts of the orthogonal modes in correspondence to the directional strains induced at the sensor in four uniaxial scenarios with a high sensitivity factor of 176 ppm/με.
Highlights
With the loading and environmental effect, the structural behavior will gradually degrade over time
A sensor of single-feed circularly polarized (SFCP) patch based on orthogonal mode was first initiated by the authors [33], where we presented a comparative study into the directional sensitivity factor of two coaxial probe-fed linearly polarized (LP)-patch sensors and CP patch
We further extended our studies from the probe-feed to microstrip-line feed patch sensor, where the microstrip-line is integrated with the strain sensor as a whole so that artificial hole is not required to accommodate the coaxial feed on the metallic structures in order to fulfill the integrity of structure
Summary
With the loading and environmental effect, the structural behavior will gradually degrade over time. A high sensitivity factor of sensing element is highly desired For those LP patch antenna-based strain sensors mentioned above, the sensing parameter was based on single (either TM10 or TM01) mode’s resonant frequency shifts with strain increasing. The practical novelty and contributions of this article can be summarized as follows: i) theoretical derivation is justified for the feasible use of patch-sensor with near orthogonal modes; ii) multidirectional strain is realized by using a single sensing element; iii) achieved ultra-high sensitivities under wire and wireless (RCS) scenarios are demonstrated; iv) relations on the input impedance, surface currents of CP-patch and the complex RCS under various strains are examined.
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