A Compact, Batteryless, and Chipless Intermodulation Sensor for Wireless Crack Detection

In this article, we introduce a wireless nondestructive evaluation (NDE) technique that exploits intermodulation interrogation from a compact, fully passive nonlinear antenna sensor, so as to suppress electromagnetic interreferences, such as clutters, echoes, and self-jamming. The antenna sensor consists of a local oscillator and a passive rectifier/mixer, connected to a microstrip patch antenna whose resonance frequency is sensitively tuned by the surface crack level (i.e., cavity perturbation). This nonlinear antenna sensor receives a frequency-hopped radio frequency (RF) monotone from the interrogator and reradiates an intermodulation signal produced by mixing the input RF signal (2.4 GHz) and the low-frequency signal (1.0–1.2 MHz) generated by the local oscillator. Through the frequency-hopped spread spectrum (FHSS) interrogation, the spectral signal strength pattern can identify the crack level encoded in the resonance frequency shift of the microstrip antenna. Compared with other nonlinear wireless sensors, such as harmonic/subharmonic sensors, the proposed intermodulation-based antenna sensor not only enables clutter-resistive passive wireless sensing, but also provides several advantages. First, it can significantly reduce the size and cost of the sensor and interrogator by removing high-harmonic antennas and reducing the design complexity of the interrogator. Second, it provides digital memory-free identification by assigning the oscillation frequency as the sensor’s identification (ID) key. Additionally, receiving intermodulation signals rather than harmonic tones can reduce the path loss and thus increase the wireless interrogation distance. The proposed fully passive intermodulation-based antenna sensor may open a new path to build portable, low-cost, low-noise, energy-efficient wireless pervasive NDE platforms.