Wireless sensing systems based on passive UHF RFID technology for physical parameters measurement

Abstract

Sensing and monitoring using the conventional microwave, optical, and waveguide methods have been the focus of many researchers for the past two decades. However, the limitations imposed by these techniques such as power inefficiency and the use of complex readout circuitries are the important challenges for their successful implementation in the internet of things (IoT) based applications, when widespread deployment of sensors is essential. This dissertation focuses on the design of a new type of sensing systems utilizing a battery-less UHF RFID technology with the capability of power efficient remote sensing, reduced sensing node complexity, and lower maintenance over time. Various capacitive interdigitated electrode (IDC) sensing elements were designed and evaluated for different sensing applications. The working principle is based on the capacitance variation of the sensing element due to the changes in the permittivity profile of the test medium that alter phase of the backscattered RFID signal. In order to use the high sensitivity advantages of the capacitive sensing element and the passive nature of the RFID technology; the designed IDC structures were integrated with a three-port RFID sensing architecture to develop a reliable non-contact passive remote sensing systems. In the next step the proposed passive wireless sensing system was practically tested for remote liquid level detection in a real-world scenario. The presented solution is a simple three-port sensing architecture without any internal power source at the sensing node; that transduces liquid level variations into corresponding capacitance change, that adds an additional phase delay to the input RFID signal. The phase variation of the backscattered signal is used to estimate the liquid level in real-time. Furthermore, this concept is expanded to the volume fraction analysis of a binary as well as multivariable mixtures by modifying the sensing element structure and utilizing the limited RFID frequency band while taking advantages of the multitemperature measurements. The proposed method was evaluated by exposing the sensor to various mixtures containing different ratios of oil, alcohol and water. The difference in the phase values of the backscattered signal observed as a result of measurements at a single frequency (927 MHz) and multi-temperatures is used to calculate the unknown volume concentrations of the three components in a mixture based on the unique permittivity profile of the test samples. The presented sensing techniques provide a new solution for powerless real-time remote measurement in a densely populated sensing environment.