Passive RFID for Automotive Sensor Applications - Gregor Lasser - Ph.D. Thesis

Future vehicular sensors will further increase active and passive safety systems of cars and trucks. One particular promising objective is to gather tyre data as temperature, vibration, acceleration and tyre pressure by mounting sensors in the tyres. Since they are the actual elements which transfer the occurring acceleration and — more important — deceleration forces, any additional information about these essential load transmission components aids safety systems. Whereas the tyre pressure is already monitored in many vehicles using sensors mounted at the rim, the other mentioned parameters are only retrievable for a tyre-mounted sensor. The particular benefit about the knowledge of the acceleration forces lays in the possibility to compute the tyre’s contact area, which is directly proportional to the maximum force that is transferable without wheelspin. In this thesis, I evaluate the applicability of Ultra High Frequency (UHF) or microwave passive Radio Frequency IDentification (RFID) technology to communicate with tyre mounted sensor nodes to form an Advanced Tyre Monitoring System (ATMS). First, I focus on Onboard Unit (OU) antennas for this application. Vertically polarized dual-band antennas, which exploit the body floor pan beneficially, are presented and characterized. Further, a horizontally polarized Switched Beam Antenna (SBA) is developed, which is capable of steering a beam to one of the four tyres of a car. To shield this antenna from the deleterious metallic body floor pan, I propose, construct, and characterize two dual-band Frequency Selective Surface (FSS) structures based on the Artificial Magnetic Conductor (AMC) principle. Antenna measurements of the overall system are discussed. Next, my thesis addresses broadband leakage cancellation. This topic is the key to RFID systems that are range-limited only by the tag sensitivity. Theoretical bounds for the isolation bandwidth are derived and confirmed in experiments. I present a comparison of leakage canceller adjustment algorithms, and derive the estimation noise and bias of a novel, fast algorithm. Again, this analysis is experimentally confirmed. Finally, I report on dual-band channel measurements for RFID-based ATMS using previously characterized antennas. The analysis of these measurements demonstrates, that communication with UHF RFID-based sensors is feasible in 75% of all possible rotational angles, using state-of-the-art dual-antenna RFID tags and power combining.