[pt] Os sensores de deformação convencionais, por exemplo, os strain gages resistivos e ópticos, consagrados pelo mercado, são empregados em diferentes ambientes e estruturas, oferecendo um nível de flexibilidade que permite sua integração a vários tipos de sistemas de medição. No entanto, requerem uma fonte de energia elétrica local ou não podem funcionar sem cabos, limitando o seu uso em alguns cenários, como em partes móveis de máquinas. Os dispositivos SAW (Surface Acoustic Wave) podem ser usados como sensores de deformação piezoelétricos, pois possuem sensibilidade à deformação e podem operar de forma passiva por meio de antenas, podendo ser integrados a sistemas de monitoramento sem fio. Seu funcionamento é baseado em ondas de superfície geradas em um meio piezoelétrico. O estado de tensão na superfície do meio altera as características dessas ondas e induz mudanças na frequência de operação. O presente trabalho analisou a sensibilidade à deformação de dispositivos SAW ressonadores feitos de quartzo ST-X com frequência central de 433,92 MHz colados com diferentes adesivos, e testou sua operação como sensores passivos sem fio. A metodologia incluiu testes de tração não destrutivos em diferentes temperaturas e também simulações com elementos finitos. Os resultados experimentais mostraram relação linear entre a variação de frequência dos ressonadores e as deformações aplicadas, concordando com a literatura. A interrogação sem fio foi bem sucedida, confirmando o grande potencial dessa tecnologia. Os resultados numéricos, combinados a um modelo teórico, foram próximos aos experimentais, validando o modelo numérico. / [en] The conventional strain sensors, e.g., resistive and optical strain gages, established in the market, are deployed in different environments and structures, providing the flexibility of integration with different measurement systems. However, they require a local energy source to work or cables, limiting their use in some scenarios such as moving parts of machines. The SAW (Surface Acoustic Wave) devices can be used as piezoelectric strain sensors since they have sensitivity to strain, can operate passively by antennas and can be integrated to wireless monitoring systems. Its working principle is based on surface acoustic waves generated on piezolectric medium. The stress state of the medium changes the characteristics of these waves and induces changes in the operating frequency. The present work analyzed the strain sensitivity of SAW resonators made of ST-X cut quartz operating at a central frequency of 433,92 MHz, bonded with different adhesives, and tested their operation as passive wirelesss sensors. The methodology included non-destructive tensile testing at different temperature, and also finite elements simulations. The experimental results showed linear relation between the frequency change and the applied strain, agreeing with the literature. The wireless interrogations was successful, confirming the great potential of this technology. The numerical results, combined to a theoretical model, matched well the experiments, validating the model.

A passive wireless surface acoustic wave sensor of a delay-line type is composed of an antenna, a transducer that converts the EM signal into a surface acoustic wave, and a set of acoustic reflectors that reflect the incoming signal back out through the antenna. A cavity forms between the transducer and the reflectors, trapping energy and causing multiple unwanted echoes. The work in this dissertation aims to reduce the unwanted echoes so that only the main transit signal is left--the signal of interest with sensor information. The contributions of this dissertation include reflective delay-line device response in the form of an infinite impulse response (IIR) filter. This may be used in the future to subtract out unwanted echoes via post-processing. However, this dissertation will use a physical approach to echo suppression by using a unidirectional transducer. Thus a unidirectional transducer is used and also optimized for 3rd harmonic operation. Both the directionality and the coupling of the 3rd harmonic optimized SPUDT are improved over a standard electrode width controlled (EWC) SPUDT. New type of reflectors for the reflective delay-line device are also presented. These use BPSK type coding, similar to that of the Walsh-Hadamard codes. Two types are presented, variable reflectivity and variable chip-lengths. The COM model is used to simulate devices and compare the predicted echo suppression level to that of fabricated devices. Finally, a device is mounted on a tunable antenna and the echo is suppressed on a wireless operating device.