Alternative Power Transfer for Passive RFID Systems in Challenging Applications
This dissertation presents a case study which attempts to implement a passive Ultra High Frequency Radio Frequency Identification (UHF RFID) system on aircraft landing gear (LG) to permit component configuration management. It is shown that a monostatic RFID system with two reader antennas, one on the LG main fitting and one in the wing bay allows up to 64 kbits of data to be associated with each LG component. A 7 dB system margin allows data on each LG component to be updated wirelessly and will also enable a passive UHF RFID-based LG health and usage monitoring system when tags with required sensors become available. Results from an electromagnetic simulation show that when a metal is illuminated by a nearby antenna the E-field distribution close to its surface is stronger than in free space. To explore if the stronger E-field can be used to enhance the performance of a conventional passive tag, a 5 cm × 6 mm × 3.02 m aluminium bar has been selected as the tagging object and connected to the reader via an RF feed. It is shown that a conventional metal tag which has a maximum free space range of 1.3 m when mounted on a metal plate can be detected up to 30 m along the aluminium bar from the RF feed. When orientated with the long axis normal to the metal surface a conventional passive tag with a dipole antenna can efficiently harvest the E- field and can be read at least 50 m away from the antenna feed. The proposed use of metal objects as a nearfield antenna is well suited to some applications, but in others a significant wireless path is still required. In such a case, a semi-passive tag can be used. It is demonstrated that a semi-passive tag only requires 14.4 𝜇𝑊 to be read over 42 m in a bistatic RFID system. Such a power consumption can be easily achieved by most energy harvesting techniques. It is demonstrated that a solar-powered semi-passive tag can be read at a range of 22 m, but its performance is still limited by multipath effects. A distributed antenna system (DAS) can be used to overcome these effects by using frequency and phase hopping techniques. It is demonstrated that 50 solar-powered semi-passive tags can be read with no missed detections over a 10 m × 20 m office area with 4 dB system margin.