Francisco J. Arnold

The resonance frequencies on mechanically pre-stressed ultrasonic piezotransducers

The piezotransducers employed in high power ultrasound are composed of piezoelectric ceramics and metallic pieces. These transducers are mechanically pre-stressed in order to avoid the ceramic fractures when high voltage is applied under resonance. The resonance and anti-resonance frequencies are shifted depending on the level of applied mechanical pre-stressing. This paper discusses some causes of this shifting on a experimental study. The discussion takes into account the variations on characteristic parameters of the ceramics and the acoustic coupling between parts of the transducer.

The mechanical pre-stressing in ultrasonic piezotransducers.

Composed piezotransducers submitted to mechanical pre-stressing present shifts on resonance and anti-resonance frequencies. Changes on characteristic parameters of the ceramic and in the coupling between the parts of the transducer can be the causes for this behavior. In applications where the level of pre-stressing is low (up to 50 MPa) the parameters of the ceramic are not altered, therefore, the shifting on frequencies are attributed to coupling between parts. This paper describes a mathematical model to explain this effect based on difference of effective cross-section between transducers parts under pre-stressing. The results show a proportional relation between pre-stressing and effective coupling of the parts.

The influence of the thickness of non-piezoelectric pieces on pre-stressed piezotransducers.

The mechanical pre-stress applied in piezotransducers used to generate high power ultrasound is needed to avoid ceramics fracture on traction cycle. Pre-stress levels inferior to 50 MPa can yield resonance shifting due to effectiveness of acoustic coupling between transducer pieces. Symmetrical transducers with different thickness of passive parts were submitted to axial mechanical pre-stress up to 50 MPa and their resonances were measured. The experimental results show the increasing of the resonances frequencies with the level of applied pre-stress. Similar effect is verified in simulations by using a model based on Masons equivalent electric circuit. Due to the similarity of these effects, a relation between applied pre-stress and pieces coupling was proposed for the transducer assembled. In addition, the dependence of the thickness of non-piezoelectric pieces on the coupling effectiveness between them is discussed. The results show that transducers with small thickness present more expressive shifting resonance ratio.

Estudo do amortecimento do pêndulo simples: uma proposta para aplicação em laboratório de ensino

The pendula are largely used in teaching activities of physical introductory courses. Often, in these activities an ideal model is used to study the behavior of the pendulum. In this work a model that takes into account the oscillation damping is suggested. An experimental methodology is purposed aiming to detect the falling down of oscillation amplitude and to fit these data to a mathematical model. The results show that the simple pendulum, set up by a small volume sphere and oscillating on low speed, can be studied with simplified models. The methodology and the mathematical model are appropriated for the knowledge level of the students from physic introductory courses and offer a more realistic insight about the pendulum behavior.

Resonance frequencies of the multilayered piezotransducers

Piezoelectric transducers used in high power ultrasonic applications are composed of piezoelectric ceramics, metallic blocks and a central bolt that pre‐stresses the assembly. The impedance characterization of these transducers has been done by using numerical methods but, in many cases, one‐dimensional simplified models are enough to a good physical interpretation of the problem. In this work was used the Masons equivalent electric circuit for one‐dimensional modeling of composed transducers. It was derived Thevenins equivalent circuit to simplify the problem and evaluate the effects of the central bolt on the resonances. Some transducers were mounted and the transmission line circuit was used to measure the resonance frequencies. The results show a good fitting between the experimental and simulated by electric circuit. By using the presented model it was identify other longitudinal vibration modes further that considered in previous models and, so that, it was increased the possibility of investigate...

The study of piezoelectric devices with Thévenin's Theorem

This work presents a study of the behavior of piezoelectric devices based on a modified Thévenins equivalent electric circuit. It aims to characterize piezoelectric devices using electric network theory and to complement the instruction of Thévenins Theorem in classes about electric circuits. The employed approach allows us to outline the most important concepts of operation of the piezoelectric transducers. The manner for obtaining the equivalent electric circuit and a simple experimental procedure are presented. The work shows that Thévenins Theorem, commonly taught in electric circuits classes, can be used for evaluating of piezoelectric devices. The methodology employed in this work facilitates the comprehension of the physical principles of the piezoelectric elements.

Simulação computacional de campos ultra-sônicos

When an electro-acoustical transducer is electrically supplied, it yields an acoustic field in the propagation medium. The acoustic field knowledge allows us to obtain significant help to transducers design and propagation medium interaction studies. In this paper, it is shown a proposal of classroom work involving the studied contents of physics, mathematic and computing in the introductory lessons of degree course of exact science. It is proposed the discussion about wave interference phenomena, the determination of acoustic field using cylindrical coordinates and the software of the mathematical method applying basic instructions of a computational language. The simulations of some acoustic fields are shown in bi and tri-dimensional graphics and the results are discussed.

Experimental evaluation of the performance of a wireless sensor network in agricultural environments

Um estudo experimental, visando a avaliar a distância de operacao adequada entre os nos de uma rede de sensores sem fio disponivel no mercado, foi realizado para diferentes cultivos agricolas (milho, pinhao-paraguaio e eucalipto), buscando oferecer aos agricultores e pesquisadores dados experimentais do desempenho da rede, que possa good yield are environmental factors, which include high temperatures and high humidity within the facilities, causing stress to the animal and consequent decreased production (SOUSA, 2005).am ser uteis para ser considerados no dimensionamento e no projeto de redes de sensores sem fio em situacoes semelhantes as estudadas. A avaliacao mostrou que a distância de separacao dos nos depende do tipo de cultura e e um fator critico para garantir a viabilidade do uso da RSSF. Na configuracao usada, com envio de pacotes a cada 2 segundos, a autonomia da bateria foi de cerca de quatro dias. Portanto, a autonomia pode ser aumentada com um intervalo maior de tempo entre o envio de pacotes.

Complex elastic coefficient and mechanical losses for piezoelectric resonators under transversal and thickness modes

ABSTRACT The performance of piezoelectric transducers depends upon a set of physical parameters. Among them, losses are one of the most important. Mechanical losses in piezoelectric devices are studied using complex numbers in the elastic coefficients of the models. Usually, numerical methods are employed to obtain the loss parameters through the minimization of an error function. We insert mechanical losses dependent on frequency in Masons model to evaluate how this dependency affects the electrical impedance curve. The results show that a better curve fitting is achieved when the mechanical loss is a first order function of frequency.

Prediction of the Performance of a Reconfigurable Resonator for RFID Chipeless Tags

A silicon optical switching has been applied to design a reconfigurable spiral resonator to be used in RFID chipless tags. The incidence of suitable wavelength light on the switching changes from conductive to dielectric its electrical properties. A multiresonator has an owner spectral signature defined by a set of bits 0 and 1 from each single resonator. In this work, the state of each resonator can be at resonance or non-resonance according to the optical switching allowing the reconfiguration of the whole system. Therefore, the reconfiguration based on optical switching can be performed many times allowing us to increase the number of access state of the tag. The approach is shown through a theoretical model for the characterization of silicon substrate behavior on light incidence. The validation of the system operation is performed using electromagnetic simulations in HFSS.