Multi-parametric and distributed RFID sensing

The radio-frequency identification (RFID) sensors are considered one of the enabling technologies for the Internet of Thing (IoT) paradigm, capable to connect every physical object to the cloud for exchanging information. Starting from the its intuitive working principle, based on the backscattering modulation, the strengths of the RFID technology rely on its stealthy nature, low-complex electronics and low-power demanding. These features all together made this technology competitive not only for logistic purposes, as natural replacement of the barcode, but also in all those contexts and applications where embedded, remote or even passive sensing capabilities are required. Moreover, the RFID sensing technology can support large scale implementation thanks to its low-cost manufacturing process. Despite of the above mentioned advantages w.r.t. other active technologies (such as Bluetooth and WiFi), they are not as pervasive as expected in industry and daily life yet. In the context of the digital -passive and battery assisted passive (BAP)- Ultra-High-Frequency (UHF) RFID sensors, this thesis focuses on four main challenges that the UHF-RFID technology should face up with to boost its rise outside the logistic scenario: detuning, complexity, density and reliability. The natural dependence on the surrounding materials makes the design of the antenna a delicate task. Although the recent progress in the development of IC modules with self-tuning capabilities, the impedance mismatch between antenna and IC can still seriously limiting the tag performance when it operates in an environment not considered during the antenna design stage. Considering the high electromagnetic variability of the human body, this thesis investigated a compact, time-saving and bidirectional tuning mechanism to overcome the detuning effect, hence performance loss, when the RFID transponder is applied on different body parts. Exploiting one of the most attractive feature of the digital RFID sensors, i.e. their II being low-complex electronic platforms, this thesis shown the development of an epidermal flexible RFID sensor board suitable for multi-sensing purposes. The potentiality of the developed RFID board was demonstrated by the simultaneous acquisition of three parameters, such as skin temperature, pH of the sweat and sodium concentration in a buffer solution. Compared to other active technologies, the proposed UHF-RFID multisensing board is a valid candidate for the massive and automatic monitoring of patients/user or athletes. Another challenge concerns the application of sensors on large surfaces to monitor their status, such as the presence of structural damages. This issue includes two aspect, namely the deployment of discrete sensors and the development of distributed ones. By focusing on the distributed sensors, this thesis introduced the novel idea of using pre-fractal curves as distributed crack detectors. By exploiting the features of some Space-Filling Curve (SFC) families, the leading idea is to wrap the SFC-based electrode to the target surface like a tattoo; the connection of that distribute electrodes to an UHF-RFID IC with anti-tamper capabilities allows the real-time crack detection and digitalization. Unlike the common crack detection techniques based on the shift of the resonance frequency of the antenna due to the crack occurrence, the proposed method keeps the sensor separated by the RFID transponder. As shown in this thesis through several experiments, this feature extends the applicability of this approach to different kind of materials, from plastic to metals, passing through stretchable objects. A further step towards the stable adoption of the UHF-RFID sensing-oriented ICs into industrial processes might be done by improving the reliability of such technology in extreme conditions. This thesis carried out a thermal analysis on two different UHF-RFID transponders, since their behavior in medium/high temperature is mostly unknown. The study revealed the need for a correction factor to overcome acquisition errors due to high temperature exposition. The compensation in the thermal drift facilitated the adoption of those RFID sensors in a real industrial scenario, driving the manufacturing process towards the Industry 4.0 framework. Overall, all the insights provided by this thesis can be considered as a further milestone for the advancement in the exploiting the sensing capabilities of the RFID technology in some strategic fields, such as manufacturing, healthcare and structural health monitoring.