Low-Cost, Passive UHF RFID Tag Antenna-Based Sensors for Pervasive Sensing Applications

Abstract

In the future, large-scale sensor deployment would enable many areas such as infrastructure condition monitoring and supply chain management. However, many of today's wireless sensor technologies are still too expensive to meet this need. Radio Frequency IDentification (RFID) offers good potential for the development of pervasive sensors: RFID tags have a proven track record of large-scale, highly integrated deployment for object identification in the retail and consumer goods industry. Furthermore, the last decade has seen much progress in making RFID a reliable, standardized wireless communication medium with the ability to mass produce low-cost RFID tags. My thesis introduces the concept of RFID Tag Antenna-Based Sensing (RFID TABS). In this approach, a change in the sensed parameter of interest induces a controlled change in the geometry or boundary conditions of an RFID tag's antenna. The resultant change in the tag's response signal can then be detected by an RFID reader. My approach builds upon current developments in RFID technology. For instance, the manufacturing techniques for the mass production of low-cost RFID tags can be used for pervasive tag-sensor development. My thesis examines TABS in a two-pronged approach: First, I demonstrate how three fundamental tag and reader signal properties can be used for sensing and propose three classes of TABS: -- Amplitude Modifying (AM) TABS use RFID reader transmitted power and tag response power for sensing. I illustrate proof of concept using a displacement sensor. I demonstrate that both these power metrics can be used to reliably measure structural displacement to a precision of 2.5 mm using commercial RFID tags. -- Frequency Modifying (FM) TABS relate changes in the sensed parameter to a shift in the tag's optimal operating frequency - the carrier frequency for which the tag is best tuned to respond to the reader. I demonstrate proof of concept using a temperature threshold sensor - the crossing of a design temperature threshold results in a shift in the sensor's optimal operating frequency. I demonstrate that the sensor works reliably over a 3 m read range and in different environmental conditions. -- Phase Modifying (PM) TABS use tag backscatter phase for sensing. I provide a brief summary of the factors influencing RF phase and outline the design for a PM TABS fluid level sensor that uses RFID tag response phase to detect the presence or absence of fluid in a beverage glass. I highlight the challenges in the practical implementation of this approach by demonstrating the sensitivity of RFID tag phase to three extraneous factors. Second, I introduce the concept of Non-Electric Memory to record short timescale threshold crossovers in the sensed parameter that may occur when the tag-sensor is unpowered. When information about, rather than the exact time of, the threshold occurrence is sufficient, non-electric memory provides a solution. I demonstrate how non-electric memory can be integrated into sensor design at minimal added cost. In the proof of concept of a temperature threshold sensor, I design a thermally actuated shape memory polymer switch to permanently change the electrical properties of an RFID tag when the temperature threshold is crossed. I demonstrate that the design works reliably over a read range of 3 m and is independent of the material on which the sensor is deployed. In summary, this thesis demonstrates how an RFID tag can be adapted for low cost, pervasive sensing. Sensor prototypes illustrate proof of concept in three application areas. Extensions to two other applications are also discussed.