Embeddable wireless strain sensor based on RF resonant cavity for civil structural health monitoring applications

In this thesis we describe a new type of strain sensor which can be used to measure strain in civil structures. This strain sensor is a passive resonant cavity that can be embedded in a structure with an antenna attached. Changes in strain will by definition change dimensions of the sensor cavity, and hence change the sensor's resonant frequency. The resonant frequency of this device can be remotely interrogated using RF signals. Such a system has the advantage of requiring no permanent physical connection between the sensor and the data acquisition system. The design presented here is a coaxial cavity resonator that exhibits electromagnetic resonance for wavelengths two times its cavity length and operates at a frequency of approximately 2.4 GHz, and exhibits a shift in resonance of 2.4 kHz per microstrain. Methods of noise reduction are presented including time domain gating in the detector circuit and also using phase sensitive detection techniques. A power budget and error calculations are presented demonstrating that noise limits can still be improved upon. Experimental results have shown high linearity, low hysteresis, and a strain resolution of better than 1 microstrain with a bandwidth of 30 Hz on this sensor. Strain tests have also shown good agreement with measurements from metal foil strain sensors. This new class of embeddable sensor will have application in monitoring the health of and assessing damage in civil structures.