In this paper we will be describing noise reduction techniques for new type of wireless sensor for use in monitoring strain in civil structures. This strain sensor is a passive sensor that can be embedded and then interrogated through an attached antenna and hence has the advantage that is requires no permanent electrical or optical connection. The sensor is a metal coaxial cylindrical cavity embedded or attached to the object in which strain is to be measured. As the structure changes dimension in response to applied forces the electromagnetic cavity also changes dimension and hence its resonant frequency also changes. The sensor can then be interrogated via the antenna and the resonant frequency of the electromagnetic cavity determined. Once the resonance frequency is determined it can be used to calculate the strain in the structure. We will present results on the use of time domain gating to reduce environmental and instrumental noise. We will also present results using peak fitting techniques that make optimum use of signals to locate the resonance. These noise reduction techniques make the use of this type of sensor applicable in a wider range of environments. We have demonstrated a strain resolution of 8 microstrain in a noisy environment by using peak fitting techniques. These techniques were much less sensitive to environmental sources of noise than FM modulation and phase sensitive detection.
In this paper we describe a new type of strain sensor which can be embedded in civil structures for structural health monitoring applications. This strain sensor is a passive device that can be embedded in a structure and remotely interrogated using RF signals via an attached antenna. Such a sensor has the advantage of requiring no permanent physical connection between the sensor and the data acquisition system. The sensor is a metal coaxial cavity that can be embedded or bonded to the structure in which strain is to be measured. The design presented here exhibits a dominant mode of electromagnetic resonance for wavelengths two times its cavity length. When the material in which the sensor is embedded is strained, the strain will be reflected in changes in the sensor dimensions, and hence will cause a shift in the resonant frequency of the cavity. The resonant frequency, or shift therein, can be easily obtained by various methods. The acquired resonant frequency is then used to calculate the strain on the structure. The sensor presented in this paper operates at a frequency of approximately 2.4 GHz, and exhibits a shift in resonance of 2.4 kHz per microstrain. The sensor has been embedded in concrete test cylinders and interrogated using external antenna. Experimental results show a strain resolution of better than 1 microstrain with a bandwidth of 30 Hz on this sensor. We will also present results of different interrogation systems including a simple switched detector circuit, a gated detector circuit and also using lock in techniques. This new class of embeddable sensor will have application in monitoring the health of and assessing damage in civil structures.