# Chipless Radio Frequency Identification (RFID) Sensor for Angular Rotation Monitoring
## 1 Introduction  
The identification paradigm offered by Radio Frequency Identification (RFID) technology has been increasingly exploited in more and more applications, such as inventory management, access control, assembly process monitoring and supply chain integration [1; 2]. Together with this employment based on the encoding of a univocal identification (ID) number in the electromagnetic footprint of the RFID tag, (which has reached a good point of maturity), an additional declination of the RFID technology is represented by the realization of sensors. Several directions have been explored in this sense, spanning from something as simple as measuring temperature [3; 4] and gas [5; 6], to more complex usage in healthcare [7; 8], structural monitoring [9; 10], localization [11; 12; 13], as well as food chain control [14; 15]. However, although the presence of an integrated circuit (IC) in a RFID tag guarantees interesting features (in terms of large data storage and reprogrammable capability), it also poses some constraints on the item price and operating environmental conditions. These shortcomings have fostered the research toward chipless RFID tags, which encode the information in the electromagnetic (EM) footprint of the tag without resorting to an IC. This task obviously presents different challenges that have been faced in the last years by original approaches to the exploitation of different EM signature strategies for encoding the information. For example, the ID has been stored in the frequency response of the tag [16; 17], as well as in the timing of the response [18; 19]and also by exploiting the phase [20; 21] or the polarization [22; 23] of the scattered field. Moreover, a suitably designed chirpless RFID can be also employed as a sensor for monitoring quantities, such as humidity [24; 25; 26], temperature [27], and gas [28], even in harsh environments [29]. An interesting quantity to be monitored is represented by the rotation angle of an object revolving on its axis. Recently, some solutions aimed at estimating the rotation angle, or measuring the angular velocity, have been proposed. In Reference [23], the co-polar response of a chipless RFID tag to a linearly polarized incident field has been exploited in order to detect the tag orientation. In particular, the provided accuracy for the estimated rotation angle is equal to 20 degrees, whereas the coding capacity is 6 bits if three resonant elements are used. In Reference [30], a coplanar waveguide (CPW) loaded with a horn-shaped split ring resonator (SRR) is employed for realizing a rotation sensor operating at a single frequency with a dynamic range of up to seven degrees. A rotation sensor comprised of two meandered U-shaped resonators, coupled to a microstrip line, is described in Reference [31]. This metamaterial-inspired sensor exhibits a dynamic range of up to 180 degrees and requires a bandwidth of 350 MHz around the center frequency of 1.0 GHz, since the angular encoding is based on the downshift of the minimum transmission coefficient of the device. A procedure for recovering the angular velocity by using two circularly polarized antennas with a demodulator for Frequency Modulated (FM) carriers, is shown in Reference [32]. A solution relying on ultra-high frequency (UHF) RFID tags has been presented in Reference [33], where the rotation speed is inferred by exploiting the level of polarization mismatch in the time domain between the reader and the tag. A preliminary study on the possible use of a cross polarization scattering (CPS) system has been presented in Reference [34; 35], using chipless RFID tags to design angular rotation sensors, where different resonator geometries (loop, SRR, stubbed loop and dipole) were investigated. A coplanar waveguide loaded with SRRs has been proposed in Reference [36] for measuring angular displacement and velocity.