Passive inductively coupled wireless sensor for dielectric constant sensing
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In order to address the challenges of capacitive sensing in harsh environment, self resonant passive wireless sensors are studied. The capacitive sensing elements based on interdigitated capacitor (IDC) sensor are used. A semi-empirical model providing accurate capacitance calculation for IDCs over a wide range of dimensions and dielectric constants is developed. An equivalent circuit model based on electric field distribution is proposed, leading to a closed form approximation for IDC capacitance calculation. The conductivity of the material under test is also considered and a model is proposed to calculate effective capacitance as a function of conductivity and measurement frequency. The model is used to study the design optimization of IDC sensor and suggested design procedure is proposed. To wirelessly interrogate the capacitive sensor, it is connected to an inductive element to form a resonant circuit, while the measurement is made at remote reader coil. Advantages and disadvantages of different type of resonant structure design are analyzed. In order to assist the design process, a SPICE circuit model is developed to estimate the resonant frequency of the self resonant sensor. Miniaturized sensors with different dimensions are designed, fabricated and tested. The sensor is integrated with silicon nanowire fabric coated with polymer. Measurements are made to illustrate the enhancement in sensing capability by integrating chemical selective material.