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This paper was concerned with the current level of progress towards the development of chipless radio frequency identification (RFID) sensors that are capable of sensing strain and temperature. More specifically, it was interested in the possibility that the resulting devices could be used as a passive wireless structural health monitoring (SHM) sensor technology that could be printed in situ. This work contains the development and performance characterization results for both novel strain and novel temperature sensor designs with resulting sensitivities of 9.77 MHz/%ε and 0.88 MHz/°C, respectively. Furthermore, a detailed discussion on the interrogation system required to meet the relevant aerospace sensing requirements was also discussed, and several methods were explored to enhance the multi-sensor support capabilities of this technology.

This paper contains two main areas of research: First, this work outlines a novel, highly sensitive strain sensor design that should support various levels of deformation, depending on the substrate type used. Physical implementations in this work have focused on proving its large deformation capabilities, and simulations have been used to assess its more general electromagnetic response. The other part of this paper focusses on exploring other effects that will impact the sensing of strain of resolutions below 10 με, which is a capability achieved by other aerospace-grade strain sensor technologies. These effects are limited to mechanical swelling and sensor orientation in the azimuth and elevation planes, as these appear to be unexplored and highly relevant issues to the topic of chipless RFID-based strain sensing. From this exploration, it is apparent that the effects of mechanical swelling and sensor orientation (amongst others) will need to be addressed in any real-life implementation of the sensor, requiring a strain resolution below 10 με.

To date, no printable chipless Radio Frequency Identification (RFID) sensor-related publications in the current literature discuss the possibility of thermocouple integration, particularly for the use in extreme environments. Furthermore, the effects of a time-dependent stimulus on the scattering parameters of a chipless RFID have never been discussed in the known literature. This work includes a review of possible methods to achieve this goal and the design and characterization of a Barium Strontium Titanate (BST) based VHF/UHF voltage sensing circuit. Proof-of-concept thermocouple integration was attempted, and subsequent testing was performed using a signal generator. These subsequent tests involved applying ramp and sinusoid voltage waveforms to the circuit and the characteristics of these signals are largely extracted from the scattering response. Overall conclusions of this paper are that thermocouple integration into chipless RFID technology is still a significant challenge and further work is needed to identify methods of thermocouple integration. With that being said, the developed circuit shows promise as being capable of being configured into a conventional chipless RFID DC voltage sensor.