Wireless antenna sensors for strain and crack monitoring
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In order to accurately evaluate the safety condition of various engineering structures, numerous electronic sensors have been developed over the past few decades to measure both structural performance and operating environment. This research investigates low-cost wireless antenna sensors for monitoring stress concentration and crack propagation in structures. Operating like a strain gage, the planar-shaped antenna sensor is bonded to the structural surface. When experiencing deformation with the structure, the antenna shape changes, causing shift in electromagnetic resonance frequency of the antenna. Upon interrogation by a wireless reader, the antenna sensor gathers operating energy from the interrogation signal and radiates back to the reader. The radiation parameters of the antenna sensor, such as resonance frequency and power level, are thus wirelessly interrogated by the reader. Owing to the wireless power delivery, the sensor operation is completely wireless and battery-free. This dissertation first proposes a multi-physics modeling and simulation framework for designing the antenna sensors. The simulation couples mechanics and electromagnetics, in order to characterize electromagnetic behavior change of the antenna sensor under mechanical deformation. To further improve the simulation efficiency for multiple strain steps, the dissertation investigates eigenvalue perturbation techniques for determining electromagnetic eigenfrequency shift. For distinguishing antenna sensor response from background reflections of reader interrogation signal, radiofrequency (RFID) and frequency doubling technologies are adopted. The former uses an off-the-shelf RFID chip for signal modulation, while the latter uses a Schottky diode to double the sensor response frequency for differentiating from background reflections. A number of RFID antenna sensors and frequency doubling antenna sensors have been developed for strain and crack sensing. Extensive experiments are conducted to validate the sensor performance, including strain sensing resolution, strain sensing range, wireless interrogation distance, sensor array performance, and crack sensing performance.