Ultrasonic transducers for wideband attenuation measurements

Includes bibliographical references.An application of ultrasound presently being investigated at the Council for Mineral Technology {MINTEK) is the measurement of the concentration of graphite particles in suspension in an agitated pulp. The principle of the measurement is based on the attenuation characteristics of the medium over a frequency range extending from 0.1 MHz to 10 MHz. This wide range and the highly attenuating medium (typically 3 dB/cm) require therefore that the ultrasonic transducers exhibit efficient wideband characteristics. Piezoceramic transducers vibrating in thickness-mode are the most suitable to generate ultrasound at these frequencies. However, their high acoustic impedances relative to their liquid loads make them inherently narrowband and inefficient. To overcome these undesirable features, impedance matching techniques are used whereby the impedance mismatch between a transducer and its load is reduced. In this thesis, various electrical and acoustic impedance matching schemes were implemented on a number of conunercial PZTSA transducers which were then evaluated in terms of bandwidth and efficiency. At 1 MHz, the 3-dB bandwidth of a PZTSA transducer operating in pulse-echo mode in water was improved from 0. 1 MHz for the unmat.ched case to 0. 83 MHz when 2 quarter-wave matching sections were bonded to the radiating surf ace and an electrical matching section was inserted at the electrical port. The resulting 2-way Insertion Loss over the pass band was 16 dB. Similarly, the 3-dB bandwidth of a 500 kHz PZTSA transducer improved from 50 kHz to 138 kHz with an accompanying Insertion Loss of 12 dB when both electrical and acoustic impedance matching were implemented. Certain radiation characteristics of thickness-mode transducers such as beam directivity and radiation resistance were investigated as functions of the "diameter to wavelength" ratio, D/)... , of the radiating transducer. Maximum radiation resistance (and therefore maximum acoustic coupling) was observed when D/ .A =13. The same ratio also produced the best performance in terms of directivity and sensitivity to parallelism between transmitter and reflector {or receiver). A comprehensive study of the resonance spectra of flexural, in-plane and thickness-mode vibrations of thin disks was also performed using various materials commonly used in transducer construction. The elastic constants of the materials were then determined by three different methods, each method making use of a different resonance spectrum

Investigation of a novel multiresonant beam energy harvester and a complex conjugate matching circuit

The aim of the work described in this thesis is firstly to improve the collection of vibration energy for piezoelectric cantilever harvesters, by a mechanical technique, so that the devices can harvest energy over a wider bandwidth. Secondly to investigate a new circuit topology for achieving complex conjugate load matching to the piezoelectric harvester. The thesis has been divided into two parts - the mechanical approach and the electrical approach. For the mechanical approach, a novel multiresonant beam, comprising piezoelectric fiber composites on a clamped-clamped beam and side mounted cantilevers, was proposed. The side cantilevers are tuned by tip masses to be resonant at different frequencies. A Rayleigh-Ritz model was developed to predict the vibration response of the proposed model multiresonant beam. This model showed that the bandwidth of the multiresonant beam was increased over that of a single cantilever harvester. A multiresonant beam for energy harvesting was experimentally tested and compared with a single cantilever energy harvester. The transmissibility and voltage responses were investigated, the beam showed a wide frequency response between 14.5Hz and 31Hz, whereas the single cantilever only showed one resonant frequency. Therefore the multiresonant beam system is feasible for wide band energy harvesting. For the electrical approach, the task was to investigate complex conjugate impedance matching for the piezoelectric energy harvesters, so that the output impedance from the piezoelectric harvester can be reduced, and maximum energy extracted from the device with a possibility of frequency tuning. A new amplified inductor circuit was proposed to enable the capacitive output impedance of the piezoelectric device to be cancelled. Experimental and software simulations are provided to verify the theoretical predictions. A prototype amplified inductor circuit was simulated and tested. The results showed that a variable effective inductance was achieved. However the circuit is lossy due to imperfections within the system, and needs further work to eliminate these imperfections.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Reconfigurable Bulk Acoustic Wave Resonators and Filters Employing Electric-field-induced Piezoelectricity and Negative Piezoelectricity for 5G

The ever-expanding wireless communications and sensing are influencing every aspect of human life. With the persistent demand for higher data capacity and recent advancements in wireless technologies, the design of current radio frequency front-end circuitry in communication devices calls for transformative changes. Frequency band proliferation is the biggest contributor to the added RF front-ends complexity in the design of future radios. To operate at various frequency bands, a complex combination of switches and filters is used in mobile devices, and the number of these frequency selective components in each device is expected to exceed 100 with the advent of 5th generation (5G) communication networks. Acoustic wave filters based on piezoelectric materials are the primary technologies employed in current communication systems, including mobile phones. Alternatively, the integration of multifunctional ferroelectric materials into reconfigurable frequency selective components promises reduced complexity, diminished size, and high performance for future radios, enabling them to support 5G wireless technologies and beyond. A promising reconfigurable bulk acoustic wave technology, employing electric-field-induced piezoelectricity and negative piezoelectricity in ferroelectrics, is presented in this dissertation. Successful implementation of ferroelectric filters would eliminate the need for external switcheplexers in the RF front-ends and reduce the number of required filters, leading to a significant reduction in size, cost, and complexity. Contributions of this work are categorized into three major parts. In the first part, an intrinsically switchable thin film bulk acoustic wave resonator (FBAR) based on ferroelectric BST with the highest figure of merit (i.e., Q_m×K_t^2) in the literature is presented. The BST FBARs are then employed to design intrinsically switchable filters with the lowest insertion loss to date. Such filters combine filtering and switching functionalities onto a single device, eliminating the need for external switches in RF front-ends. The second part of this work focuses on the development of frequency and bandwidth reconfigurable filters based on BST FBARs. The first switchless acoustic wave filter bank is presented in chapter 3, demonstrating the capability of BST FBARs in simplifying future agile radios. Next, a novel bandwidth reconfigurable filter based on BST FBARs is introduced in chapter 4, where the idea is experimentally validated with multiple design examples. Finally, through rigorous mathematical analysis and experimental validation, it has been demonstrated that a dynamic ‘non-uniform piezoelectric coefficient’ created within a composite structure made up of multi-layers of ferroelectrics allows the selective excitation of different mechanical Eigenmodes with a constant electromechanical coupling coefficient. Such technology overcomes the fundamental limitations associated with the electromechanical coupling coefficient of harmonic resonances in bulk acoustic wave resonators. To create ‘non-uniform piezoelectric coefficients’ in such structures, ferroelectrics’ electric-field-induced piezoelectricity and negative piezoelectricity has been exploited. This innovative technology provides a fundamentally new approach and a framework for synthesizing programmable frequency selective components, which leads to transformative advances in wireless systems’ front-end architecture. As part of the future direction, it is suggested that the multilayer structure presented in this section to be further studies as part of a new acoustic wave resonator design, which: (a) is capable of operation at a wide frequency range up to mm-wave frequencies designated for 5G (b). Such a structure has the potential to overcome the fundamental limitation of acoustic resonator’s ever-decreasing electromechanical coupling factors (Kt2) as their frequency of operation increases.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/163011/1/milad_1.pd

Wireless Sensors and Actuators for Structural Health Monitoring of Fiber Composite Materials

This work evaluates and investigates the wireless generation and detection of Lamb-waves on fiber-reinforced materials using surface applied or embedded piezo elements. The general target is to achieve wireless systems or sensor networks for Structural Health Monitoring (SHM), a type of Non-Destructive-Evaluation (NDE). In this sense, a fully wireless measurement system that achieves power transmission implementing inductive coils is reported. This system allows a reduction of total system weight as well as better integration in the structure. A great concern is the characteristics of the material, in which the system is integrated, because the properties can have a direct impact on the strength of the magnetic field. Carbon-Fiber-Reinforced-Polymer (CFRP) is known to behave as an electrical conductor, shielding radio waves with increasing worse effects at higher frequencies. Due to the need of high power and voltage, interest is raised to evaluate the operation of piezo as actuators at the lower frequency ranges. To this end, actuating occurs at the International Scientific and Medical (ISM) band of 125 kHz or low-frequency (LF) range. The feasibility of such system is evaluated extensively in this work. Direct excitation, is done by combining the actuator bonded to the surface or embedded in the material with an inductive LF coil and setting the circuit in resonance. A more controlled possibility, also explored, is the use of electronics to generate a Hanning-windowed-sine to excite the PWAS in a narrow spectrum. In this case, only wireless power is transmitted to the actuator node, and this lastly implements a Piezo-driver to independently excite Lamb-waves. Sensing and data transfer, on the other hand, is done using the high-frequency (HF) 13.56 MHz. The HF range covers the requirements of faster sampling rate and lower energy content. A re-tuning of the antenna coils is performed to obtain better transmission qualities when the system is implemented in CFRP. Several quasi-isotropic (QI) CFRP plates with sensor and actuator nodes were made to measure the quality of transmission and the necessary energy to stimulate the actuator-sensor system. In order to produce baselines, measurements are prepared from a healthy plate under specific temperature and humidity conditions. The signals are evaluated to verify the functionality in the presence of defects. The measurements demonstrate that it is possible to wirelessly generate Lamb-waves while early results show the feasibility to determine the presence of structural failure. For instance, progress has been achieved detecting the presence of a failure in the form of drilled holes introduced to the structure. This work shows a complete set of experimental results of different sensor/-actuator nodes

Structural Health Monitoring of Laminate Structures Using Shear-Mode Piezoelectric Sensors

Structural health monitoring (SHM) employing embedded piezoelectric transducers has shown potential as a promising solution for inspection of different engineering structures such as aircraft, bridges, and renewable energy structures. Despite advancements in the field of ultrasonic SHM, inspection of laminate structures is still a major challenge due to their susceptibility to various joint defects. This thesis presents a novel approach to tackle the challenge of inspecting laminate structures using shear-mode (d35) piezoelectric transducers that are made of lead zirconate titanate (PZT). This study begins with the characterization of d35 piezoelectric transducers using analytical, numerical, and experimental approaches. The results were found to match well. A finite element (FE) simulation of a laminate structure was developed based on multiphysics analysis to identify the propagating waves generated by d35 PZT actuators embedded within the bondline of the laminate structure. The group velocities of voltage signals as well as the distributions of normal displacements and stresses induced by the propagating waves showed that the elastic waves generated by the d35 PZT actuator exhibit the characteristics of antisymmetric (flexural) waves coupled with strong transverse shear stress across the thickness of the adhesive layer. The FE results were validated by testing laminate specimens with bondline-embedded d35 PZTs in a pitch-catch arrangement. A parametric study was performed to provide design guidelines for d35 PZT sensors and actuators. The thickness and length of d35 PZT transducers were varied while monitoring the actuation strength and the sensed voltage signal. It was found that thicker and shorter d35 PZT sensors can produce stronger signals compared to thinner and longer d35 PZT sensors. On the contrary, d35 PZT actuators were noticed to exhibit the opposite response to d35 PZT sensors. The selectivity of d35 PZT sensors was also investigated in multiphysics simulations by comparing voltage signals obtained from a bondline-embedded d35 PZT sensor and a surface-mounted conventional (d31) PZT sensor. It was found that d35 PZTs offer a selective hardware filter that primarily captures antisymmetric wave modes in the laminate structure while suppressing symmetric wave modes. Filtering symmetric modes significantly reduced the complexity of signal processing and this could potentially enhance the process of SHM as well. Various joint defects including disbonds, cracks, and voids were introduced in the bondline of laminate structures to investigate the feasibility of embedding d35 PZT transducers in the bondline of laminate structures for detection of joint defects. It was observed that antisymmetric waves generated by d35 PZT actuators exhibited strong interaction with joint defects especially nonlinear defects such as cracks and disbonds. By placing the transducers within the bondline and at the neutral axis of the laminate structure, it provided a direct strong coupling between the bondline and the d35 PZT transducers resulting in high transmission and sensitivity of flexural waves to joint defects. Several specimens were prepared and tested. The results obtained from experiments and simulations were found in good agreement. The proposed approach was also evaluated experimentally for health monitoring of bondline integrity. A laminate specimen with bondline-embedded d35 PZT and surface-mounted d31 PZT piezoelectric transducers was subjected to a three-point bending test to create joint defects. Damage indices were implemented to detect the presence of damage and its severity. The experimental results demonstrate the ability of bondline-embedded d35 PZTs to be used as sensors and actuators for ultrasonic SHM of bondline integrity. The proposed approach successfully produced promising results for detection of joint defects that often impose a significant challenge to detect using conventional nondestructive evaluation techniques. The results presented in this thesis provided fundamental work towards creating embedded, automated damage detection systems for laminate structures using bondline-embedded d35 piezoelectric transducers

Development and characterization of sensors fabricated from polymer based magnetoelectric nanocomposites

Tese de Doutoramento em Engenharia Electrónica e de ComputadoresSensors are increasingly used in many applications areas, integrated in structures, industrial machinery, or in the environment, contributing to improve the society level of well-being. It is expected that sensorization will play on of the most relevant roles in the fourth industrial revolution, and allow, together with mechanization and informatization, a full automation. Particularly, magnetic sensors allow measurements, without physical contact, of parameters such as direction, presence, rotation, angle, or current, in addition to magnetic field. In this way, for most applications, such sensors offer a safe, noninvasive and non-destructive measurement, as well as provide a reliable and almost maintenance-free technology. Industry demands for smaller, cheaper and low-powered magnetic sensors, motivating the exploration of new materials and different technologies, such as polymerbased magnetoelectric (ME) composites. These composites are flexible, versatile, lightweight, low cost, easy to model in complicated shapes, and typically involve a lowtemperature fabrication process, being in this way, a solution for innovative magnetic sensor device applications. Therefore, the main objective of this thesis is the development of polymer-based ME sensors to be incorporated into technological devices. Thus, the ME effect is increasingly being considered an attractive alternative for magnetic field and current sensing, being able to sense static and dynamic magnetic fields. In order to obtain a wide-range ME response, a nanocomposite of Tb0.3Dy0.7Fe1.92 (Terfenol-D)/CoFe2O4/poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) was produced and their morphological, piezoelectric, magnetic and magnetoelectric properties investigated. The obtained composites reveals a high piezoelectric response (≈-18 pC∙N- 1) that is independent of the weight ratio between the fillers. In turn, the magnetic properties of the composites are influenced by the composite composition. It was found that the magnetization saturation values decrease with increasing CoFe2O4 content (from 18.5 to 13.3 emu∙g-1) while the magnetization and coercive field values increase (from 3.7 to 5.5 emu∙g-1 and from 355.7 to 1225.2 Oe, respectively) with increasing CoFe2O4 content. Additionally, the films show a wide-range dual-peak ME response at room temperature with the ME coefficient increasing with increasing weight content of Terfenol-D, from 18.6 mV∙cm-1∙Oe-1 to 42.3 mV∙cm-1∙Oe-1. The anisotropic ME effect on a Fe61.6Co16.4Si10.8B11.2 (FCSB)/poly(vinylidene fluoride) (PVDF)/FCSB laminate composite has been used for the development of a magnetic field sensor able to detect both magnitude and direction of ac and dc magnetic fields. The accuracy (99% for both ac and dc sensors), linearity (92% for the dc sensor and 99% for the ac sensor), sensitivity (15 and 1400 mV∙Oe-1 for the dc and ac fields, respectively), and reproducibility (99% for both sensors) indicate the suitability of the sensor for applications. A dc magnetic field sensor based on a PVDF/Metglas composite and the corresponding readout electronic circuits for processing the output ME voltage were developed. The ME sensing composite presents an electromechanical resonance frequency close to 25.4 kHz, a linear response (r2=0.997) in the 0–2 Oe dc magnetic field range, and a maximum output voltage of 112 mV (ME voltage coefficient α33 of ≈30 V∙cm-1∙Oe-1). By incorporating a charge amplifier, an ac–rms converter and a microcontroller with an on chip analog-to-digital converter (ADC), the ME voltage response is not distorted, the linearity is maintained, and the ME output voltage increases to 3.3 V (α33effective=1000 V∙cm-1∙Oe-1). The sensing device, including the readout electronics, has a maximum drift of 0.12 Oe with an average total drift of 0.04 Oe, a sensitivity of 1.5 V∙Oe-1 (15 kV∙T-1), and a 70 nT resolution. Such properties allied to the accurate measurement of the dc magnetic field in the 0–2 Oe range makes this polymerbased device very attractive for applications, such as Earth magnetic field sensing, digital compasses, navigation, and magnetic field anomaly detectors. A dc current sensor device based on a ME PVDF/Metglas composite, a solenoid, and the corresponding electronic instrumentation were developed. The ME sample exhibits a maximum α33 of 34.48V∙cm-1∙Oe-1, a linear response (r2=0.998) and a sensitivity of 6.7 mV∙A-1. With the incorporation of a charge amplifier, a precision ac/dc converter and a microcontroller, the linearity is maintained (r2=0.997), the ME output voltage increases to a maximum of 2320 mV and the sensitivity is increased to 476.5 mV∙A-1. Such features indicate that the fabricated ME sensing device is suitable to be used in non-contact electric current measurement, motor operational status checking, and condition monitoring of rechargeable batteries, among others. In this way, polymer-based ME composites proved to be suitable for magnetic field and current sensor applications.Os sensores estão a ser cada vez mais utilizados em diversas áreas, integrados em estruturas, máquinas industriais ou projetos ambientais, contribuindo para melhorar o nível de bem-estar e eficiência da nossa sociedade. Espera-se que a “sensorização” contribua decisivamente para a quarta revolução industrial, e que permita, em conjunto com a mecanização e a informatização, uma completa automação. Em particular, os sensores magnéticos permitem medir parâmetros como a direção, presença, rotação, ângulo ou corrente, para além do campo magnético, tudo isto sem qualquer contacto físico. Assim, para a maioria das aplicações, estes sensores oferecem uma medição segura, não invasiva e não destrutiva, para além de garantirem uma tecnologia confiável e de escassa manutenção. A indústria procura e exige sensores magnéticos mais pequenos, mais baratos e de baixo consumo, daí a motivação para explorar novos materiais e diferentes tecnologias, tais como os compósitos magnetoelétricos (ME) baseados em polímeros. Estes compósitos são flexíveis, versáteis, leves, de baixo custo, fáceis de se modelar em formas complexas e tipicamente envolvem um processo de fabricação a baixa temperatura, constituindo uma solução fiável e de qualidade para os sensores magnéticos. É da constatação deste potencial que surge este estudo e o objetivo desta tese: o desenvolvimento de sensores ME de base polimérica. O efeito ME é cada vez mais considerado como uma alternativa credível para a medição de campo magnético e da intensidade da corrente elétrica, podendo detetar campos magnéticos estáticos e dinâmicos. De modo a obter uma gama mais alargada de resposta ME, produziram-se nanocompósitos de Tb0.3Dy0.7Fe1.92 (Terfenol-D)/CoFe2O4/poli(fluoreto de vinilideno trifluor-etileno) (P(VDF-TrFE) e as suas propriedades morfológicas, piezoelétricas, magnéticas e magnetoelétricas foram investigadas. Os compósitos obtidos revelam uma elevada resposta piezoelétrica (≈-18 pC∙N-1) que é independente da percentagem de cada material magnetoestrictivo. Por sua vez, as propriedades magnéticas são influenciadas pela composição dos compósitos. Verificou-se que a magnetização de saturação diminuí com o aumento da percentagem de CoFe2O4 (de 18.5 para 13.3 emu∙g-1) enquanto que a magnetização e o campo coercivo aumentam (de 3.7 para 5.5 emu∙g-1 e de 355.7 para 1225.2 Oe, respetivamente) com o aumento da percentagem em massa de CoFe2O4. O efeito ME anisotrópico num compósito Fe61.6Co16.4Si10.8B11.2 (FCSB)/ poli(fluoreto de vinilideno) (PVDF)/FCSB laminado foi utilizado para desenvolver um sensor de campo magnético capaz de detetar tanto a magnitude como a direção de campos magnéticos ac e dc. A exatidão (99% para ambos os sensores ac e dc), linearidade (92% para o sensor dc e 99% para o ac), sensibilidade (15 e 1400 mV∙Oe-1 para o sensor dc e ac, respetivamente) e reprodutibilidade (99% para ambos os sensores) indicam a aptidão destes sensores para aplicações avançadas. Desenvolveu-se ainda um sensor de campo magnético dc baseado num compósito ME de PVDF/Metglas, bem como a correspondente eletrónica de leitura para processar a tensão de saída ME. O compósito ME apresenta uma ressonância eletromecânica de aproximadamente 25.4 kHz, uma resposta linear (r2=0.997) para uma gama de campos magnéticos dc entre 0–2 Oe e uma tensão de saída máxima de 112 mV (coeficiente ME α33≈30 V∙cm-1∙Oe-1). Ao incorporar um amplificador de carga, um conversor ac–rms e um microcontrolador com um conversor analógico-digital (ADC), a tensão ME não é distorcida, a linearidade manteve-se e a tensão ME aumentou para 3.3 V (α33efectivo=1000 V∙cm-1∙Oe-1). O sensor, incluindo a eletrónica de leitura, obteve um desvio máximo de 0.12 Oe com um desvio total médio de 0.04 Oe, uma sensibilidade de 1.5 V∙Oe-1 (15 kV∙T-1) e 70 nT de resolução. Tais propriedades aliadas à medida exata do campo magnético dc entre 0–2 Oe tornam este dispositivo indicado para aplicações como sensores de campo magnético terrestre, compassos digitais, navegação e detetores de anomalia no campo magnético. Foi ainda possível desenvolver e otimizar um sensor de corrente baseado num compósito ME de PVDF/Metglas, num solenoide e na correspondente eletrónica de instrumentação. A amostra ME exibe um α33 máximo de 34.48V∙cm-1∙Oe-1, uma resposta linear (r2=0.998) e uma sensibilidade de 6.7 mV∙A-1. Com a incorporação de um amplificador de carga, um conversor ac/dc de precisão e um microcontrolador, a linearidade manteve-se, a tensão ME aumentou para um máximo de 2320 mV e a sensibilidade subiu para 476.5 mV∙A-1. Estas propriedades tornam este sensor ME apropriado para a medição de corrente elétrica sem contato, para a verificação do estado de funcionamento de motores e para monitorização da condição de baterias recarregáveis, entre outros. Concluindo-se deste modo que os compósitos de ME com base em polímeros provaram ser adequados para aplicações na medição de campos magnéticos e intensidade de corrente elétrica