A Highly Sensitive Microwave Patch Sensor for Multidirectional Strain Sensing Based on Near Orthogonal Modes

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.