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With conventional discrete point sensors, there are several tradeoffs and concerns in structural measurement and structural control. In post-processing, spatial aliasing is one of the possibilities for inaccurately reconstructing structural responses; in structural control strategies, the spillover effect has been one of the most significant challenges. Theoretically, these concerns can be eliminated by utilizing spatially distributed sensors. In addition, distributed sensors can be implemented for in-situ measurement, which can provide improved and thorough monitoring of aerospace structures in flight. The distributed sensors made of smart materials such as piezoelectric materials have the capacity to improve aerospace structural safety. One of the most widely studied smart materials for distributed sensors is polyvinylidene fluoride (PVDF). Due to its low stiffness and high flexibility to be combined with various structures without affecting the properties of host structures, PVDF has the potential to be economically implemented in structural sensing. Since the 1990s, after Lee and Moon’s innovative design of piezoelectric modal sensors, the interest in PVDF modal sensor has increased. Lee and Moon’s design is based on shaping the sensor’s width to be proportional to the “curvature eigenfunction.” [12]. By utilizing the orthogonality between eigenfunctions, the sensors can be only sensitive to the selected modes. In this work, the characteristics and applications of the PVDF bonded to the Euler-Bernoulli beam are first discussed. The sensitivity analysis of Lee and Moon’s design is performed numerically, and a concept of the piezoelectric film sensors based on rectangular segments is proposed. The experiment to validate the performance of rectangular-shaped piezoelectric modal sensors is also conducted

Innovative sensing modalities for wearable devices continue to emerge with the advancement of microelectromechanical systems and material science. Compared to conventional sensing modalities, these are either more accurate or more energy efficient. In keeping with this trend, this thesis introduces innovative sensing modalities, aiming to quantitatively improve on accuracy or energy consumption and finally lead to qualitatively difference in working scenarios. The contributions of this thesis are three-folded. Firstly, via implementing a new type of highly sensitive, stretchable, optical transparent, and low-cost strain sensor, we addressed the problem of human posture detection with casual loose-fitting smart garments. We developed a smart garment system—E-Jacket—by attaching novel sensors to an off-the-shelf casual jacket. In addition to the novel highly sensitive strain sensor, we implemented a state-of-the-art human activity recognition tool, the combination of a convolutional neural network and a long short-term memory recurrent network. Our evaluation showed that the E-Jacket can achieve up to 91.7, 84.4, and 88.4 per cent detection accuracy, respectively, for these three case studies. Second, we developed H2B, which is a system for securely pairing wearable devices by generating a shared secret key from interpulse interval between heartbeats. In the H2B system, we implemented cost- and energy-efficient piezoelectric sensors to replacing conventional electrocardiogram or photoplethysmogram sensors. To handle the high rate of bit mismatch, we also proposed a novel compressive sensing-based reconciliation method that can be adopted on all kinds of symmetric key generation systems. Our results showed that H2B has a pairing success rate of 95.6 per cent. Third, we explored the feasibility of using KEHs to collect gait signals for device authentication and symmetric secret keys generation continuously. Based on preliminary analysis, we developed KEHKey, a gait-based authentication and key generation system that uses KEH to replace accelerometers. Our results showed that KEHKey is able to generate the same key for two KEH-embedded devices at a speed of 12.57 bps, while reducing energy consumption by 59 per cent, compared to accelerometer-based methods, which makes it suitable for continuous operation.

Abstract: The health monitoring of critical structures plays a crucial role in locating damage positions timely and preventing catastrophic failures. Much attention has been devoted to exploiting piezoelectric sensors/actuators to develop techniques of recording elastic wave signals to realize structural health monitoring (SHM). This thesis is to conduct a systematic investigation of the dynamic behaviour of piezoelectric sensors and their application in quantitative crack identification in SHM systems. A typical SHM system contains piezoelectric sensors bonded to a host structure to be monitored. This structure is subjected to a dynamic excitation which will induce elastic wave propagation in it. When the wave encounters cracks, it will be scattered and the scattered wave will be recorded by the piezoelectric sensors. The recorded signals contain the information of the cracks thus can be used to identify the parameters of the cracks. In this study, theoretical modelling and simulation are conducted to investigate the load transfer between the sensor and the host structure, the dispersion relation of wave propagation, and the multiple scattering of elastic waves. In addition, a crack identification technique is investigated using the voltage signals based on optimization method. Four aspects of the work were accordingly studied and examined. Firstly, a new model is developed for surface-bonded piezoelectric thin-sheets with bending effect. The coupled electromechanical behaviour and the effect of bending upon load transfer and local stress field are studied. Secondly, a new analytical treatment is provided for wave propagation in layered piezoelectric structures, including dispersion characteristics and harmonic wave propagation. The two lowest wave modes of the guided wave in such structures are analyzed. Thirdly, a new semi-analytical solution is determined for the complicated dynamic interaction between piezoelectric sensors and cracks using pseudo incident wave method and superposition. This method has the advantages of the reliability of analytical solutions and the flexibility of typical numerical methods, and finds explicit relations between the voltage output of the piezoelectric sensor and the crack parameters. Inversely, by integrating this relation and known voltage data into an optimization process, a novel crack identification technique is established. This technique quantitatively identifies the position, the length and the orientation of typically embedded cracks effectively. The methods proposed in this thesis can be used to understand the dynamic behaviour of piezoelectric based SHM systems, multiple scattering of elastic waves and provide insights into developing new methods for quantitative crack identification.

This work presents an investigation of a piezoelectric sensor based energy harvesting system, which collects energy from the surrounding environment. Increasing costs and scarcity of fossil fuels is a great concern today for supplying power to electronic devices. Furthermore, generating electricity by ordinary methods is a complicated process. Disposal of chemical batteries and cables is polluting the nature every day. Due to these reasons, research on energy harvesting from renewable resources has become mandatory in order to achieve improved methods and strategies of generating and storing electricity. Many low power devices being used in everyday life can be powered by harvesting energy from natural energy resources. Power overhead and power energy efficiency is of prime concern in electronic circuits. In this work, an energy harvester is modeled and simulated in Simscape™ for the functional analysis and comparison of achieved outcomes with previous work. Results demonstrate that the harvester produces power in the 0 μW to 100 μW range, which is an adequate amount to provide supply to low power devices. Power efficiency calculations also demonstrate that the implemented harvester is capable of generating and storing power for low power pervasive applications.

Generalizations are made for three types of well-known and widely used twodimensional scalar differential equations in the literature describing transversely varying thickness modes in piezoelectric plate resonators. They are for singly-rotated quartz plates, doubly-rotated quartz plates, and plates of crystals of class 6mm with the c-axis along the plate thickness, respectively. The purpose of the generalizations is to include the effects of surface mechanical loads such as mass layers or fluids for resonator -based acoustic wave sensor applications. Surface acoustic impedance is introduced to take into account various surface loads in a general manner for time-harmonic motions. Both unelectroded and electorded plates are treated. For electroded plates, both free and electrically-forced vibrations under a time-harmonic driving voltage are considered. Simple two-dimensional scalar differential equations are constructed from the asymptotic dispersion relations quadratic in the small wave numbers of transversely varying thickness waves at long wavelengths. The equations obtained can be reduced to the equations in the literature in the special case when the surface acoustic impedance is set to zero. As illustrations of the usefulness and effectiveness of the equations obtained, simple examples of pure thickness vibrations of unbounded plates with surface loads, propagation of long thickness waves in unbounded plates with surface loads, and vibrations of finite plates with surface loads are presented. It is expected that a lot of theoretical results can be obtained in the future using the equations derived in this dissertation for piezoelectric plate acoustic wave sensors.

The brain, like many parts of the human body, can experience swelling, also known as cerebral edema. Cerebral edema may occur because of an injury, health related issues, tumors, or even high altitudes[1]. When cerebral edema occurs, a rise in intracranial pressure (ICP) becomes prevalent and may cause a serious threat. Without immediate treatment, increased intracranial pressure can prevent blood from flowing to the brain and depriving it of necessary oxygen it needs to function. A normal ICP is usually between 5-15 mmHg (666 Pa - 1333Pa). Any ICP observed to be above 20 mmHg (2666Pa) can be associated with brain ischemia and is usually treated[2, 3]. If prolonged, high intracranial pressures can be fatal. Current methods of measuring increased ICP are invasive and may involve drilling into the skull. Extreme invasive measures are not always suitable for certain situations. This thesis presents a study of a non-invasive sensor using piezoelectric PVDF wire to measure the ICP. The PVDF wire sensor is wrapped around the outer portion of the human head to measure the integrated hoop strain. Using this hoop strain, the pressure is then calculated from a known coupling factor of strain to pressure outputted from finite element modeling simulations. The coupling factor is then incorporated into a final calibration factor to calibrate the piezoelectric PVDF wire sensor from charge (Picocoulomb) to pressure (Pascal). These calibration factors are proven to be primarily dependent on the circumference of the human skull. Furthermore, part of this study analyzed the effectiveness and validity of the sensor due to asymmetries in the human skull. A comparison of analytical analysis results versus computational results using finite element modeling simulations show that the PVDF wire sensor neglects any asymmetries presented within the test subject. The results of this study show that this sensor will output correct ICP measurements of different subjects using appropriate calibration factors and is a viable option for measuring ICP non-invasively.

Logging While Drilling (LWD) instruments used in oil and gas exploration are subjected to extreme environmental conditions that make reliable operation a major challenge. The sensors directly exposed to this environment experience accelerated aging and may suffer physical damage leading to failure. The cost of drilling and rig operations is very high magnifying any failures or issues with LWD tools. The goal of this report is to present a built-in self-test for an instrument sensor that provides a means to evaluate sensor functionality. The sensor is a piezoelectric ultrasonic transducer. A brief review of the sensor physics will be given. I will review some methods for characterizing piezoelectric ceramic materials and transducers. The application of sensor test methods is applied in an ultrasound pulse-echo application. A brief review of the application circuit will be covered including state of the art in commercial ultrasound integrated circuit design. A prototype of the BIST method is evaluated using test transducers to verify the circuit provides indication of a transducers ability to function correctly. The prototype is achieved through the AD5933 demo board and MATLAB is used for data processing.

Abstract: The performance of smart structures depends on the dynamic electromechanical behavior of piezoelectric sensors/actuators and the bonding condition along the interface. This thesis contents a theoretical study of the coupled electromechanical characteristics of a surface-bonded piezoelectric sensor with interfacial debonding, which is subjected to high frequency mechanical loads. A one dimensional sensor model is proposed. Analytical solutions based on the integral equation method are provided. Numerical simulation is conducted to evaluate the effects of different parameters upon the dynamic load transfer between the sensor and the host medium. The results indicate that, the material combination, the sensor geometry, and the loading frequency, affect the load transfer significantly. The analytical solution of the elastic wave field in the host medium is obtained and used to evaluate the effects of different parameters upon the resulting wave field. The theoretical solution demonstrates the basic properties of wave propagation under current loading conditions.

The development of micro-electro-mechanical systems (MEMS) is constrained by the range of materials that can be processed lithographically. Bulk ceramics (including piezoceramics) are of particular interest due to unique properties for application to micromachined transducers and packages. This effort has two goals: the development of lithography-compatible batch-mode micro ultrasonic machining (µUSM) of bulk ceramics; and the demonstration of this technology by its application to piezoelectric sensors for guiding fine needle aspiration (FNA) biopsy. The new micromachining process, LEEDUS, uses lithography, electroplating and batch micro-electro-discharge machining (µEDM) to define stainless-steel microtools, which are then used in batch µUSM of ceramic substrates. This die-scale pattern transfer provides high throughput and resolution. A related process (SEDUS) uses serial µEDM without lithography for rapid prototyping of simple patterns. A computer-controlled µUSM apparatus with force feedback is developed as part of this effort. Die-scale patterns with 25µm feature sizes can be transferred onto MacorTM ceramics at a machining rate of >18µm/min. Other process characteristics are also described. Spiral in-plane actuators are machined from bulk lead zirconate titanate (PZT) for demonstration purposes. The process is applied to piezoelectric sensors integrated on biopsy needles to aid in real-time tissue differentiation during FNA biopsy, which is challenging because of the precision required to obtain samples from small target tissue volumes. The 200µm-diameter and 50µm-thick sensors are batch-fabricated from bulk PZT and located on a steel diaphragm formed at the needle tip by µEDM. Tissue contrast detection is demonstrated, showing resonance-frequency shifts (≈13MHz) in sensor impedance when the needle is moved between tissue layers (porcine fat/muscle). In vitro characterization shows proportional relationship between the frequency shift and sample acoustic impedance, demonstrating its potential utility during FNA biopsy. For tissue depth >15mm, differential sensor configurations are designed with oscillating interface circuits, which include an analog CMOS circuit fabricated in the UM 3µm process. The circuit functionality is experimentally verified: an inverter amplifier provides a voltage gain of >390V/V; oscillating signals of ≤19.67MHz are generated with quartz crystals. For the piezoelectric sensor to be used with this interface a higher Q is necessary and various options have been defined.