Ultrasound transducer modeling-received voltage signals and the use of half-wavelength window layers with acoustic coupling layers

A general set of modeling equations for lossless one-dimensional multilayer ultrasound transducers is presented based on first principles. In particular, a direct relationship between ultrasound transducer results and the underlying physical principles of electroacoustics is given. As such, the model may provide better physical understanding for designers not fully versed in electrical circuits theory or in linear system analyses. The model is suitable for time-domain analysis and monofrequency design. Special attention is given to the determination of the time-dependent voltage across the receiver electrodes subject to a general voltage input, but information on any (dynamic) variable of interest is provided. The basic equations governing the dynamics of the multilayer structure acting as transmitter as well as receiver are solved by Fourier analysis and by imposing continuity of velocity and pressure between layers. Sound transmission between the two piezoelectric circuits is assumed to take place in a water bath such that the Rayleigh equation can be used to obtain the incoming pressure at the receiver aperture from the acceleration of the opposing transmitter aperture. Comparison with experimental results is possible by allowing coupling to external electric impedances. A numerical test case using a multilayered 1-MHz transducer for flow meter applications was considered and good agreement with experiments was obtained in terms of voltage signals. The transducer contains a half-wavelength stainless steel layer needed to resist corrosion, the ability to operate at temperatures in a wide range from 20 to 150 degrees Celsius, resistance to impact from flowing particles in the medium, high pressure or vacuum, and pH values up to 10 in some locations. The influence of epoxy glue and grease acoustic coupling layers-between the piezoceramics and the stainless steel layer-in the range from 1 to 70 mum was examined. It was shown that, for the same layer thickness, epoxy glue is preferred as compared with grease, both in terms of signal shapes and amplitudes. Finally, inclusion of appropriate electric impedances in the transmitter and receiver circuits is found to affect signal pulses strongly.     

Technique for nonuniform poling of piezoelectric element and fabrication of Gaussian transducers

A new technique has been developed to polarize piezoelectric ceramic elements with a nonuniform electric field. Used as an ultrasonic transducer, the piezoelectric element will produce a corresponding nonuniform sound field. Ultrasonic transducers for generating specific field profiles can therefore be made by having a predetermined spatial pattern of polarization strength poled into the piezoelectric element. One of the desirable beam profiles is a Gaussian; it has the advantages of being free from near-field fluctuations and far-field sidelobes, and it is much easier to model than the usual piston transducers. This method was used to fabricate Gaussian beam transducers, and their measured field profiles compared well with the Gaussian beam model. Such transducers containing the built-in Gaussian amplitude profile can be electroded and mounted in the same manner as conventional piston transducers.     

Efficiency of excitation of piezoceramic transducers at antiresonance frequency

The efficiency of piezoceramic transducers excited at both the resonance and antiresonance frequency was investigated. Losses in piezoceramics are phenomenologically considered to have three coupled mechanisms: dielectric, mechanical, and piezoelectric losses. Expressions for the resonance and antiresonance quality factors, which ultimately determine transducer efficiency, have been received on the basis of complex material constants for both stiffened and unstiffened vibration modes. Comparison of electric and mechanical fields, thermal and electrical losses of power supply, and their distribution in the transducer volume have been made. For a given constant mechanical displacement of the transducer top, the required electric voltage applied to the transducer at the antiresonance frequency is proportional to the resonance quality factor, but the changes in the intrinsic electric and mechanical field characteristics in the common case are not too essential. The requirements on the piezoceramic parameters, types of transducer vibration, and especially on the factor of piezoelectric losses in a range of physically valid values were established to provide maximal quality factors at the antiresonance frequency.     

SPICE model for lossy piezoceramic transducers

A transmission line equivalent circuit for piezoelectric transducers has been modified to provide modeling of lossy piezoceramic transducers. A lossy transmission line is used to model the mechanical losses. The equivalent circuit parameters are derived from analogies between electrical transmission lines and acoustic wave propagation. Implementation of the equivalent circuit model in SPICE is shown. Simulations and measurements in the time and frequency domain of a low-Q material and a multilayered ultrasonic sensor using a low-Q piezoceramic transducer are presented.     

基于激光全息法的聚焦换能器近场声特性分析

研究了基于激光全息法的低频聚焦换能器近场测量方法,分析了利用激光全息法测量近声场特性时的基本原理,构建了一套实验测量系统。为验证该方法的可行性与准确性,利用激光测振仪对1个由25个压电小柱按5×5排布构成,频率为80kHz且表面带有自聚焦声透镜的聚焦换能器的声场进行实验测试和推算,获得了距离该换能器辐射凹面中心50mm处振动膜片上的声压幅度和相位分布,推算出此聚焦换能器声轴线上的声压分布与焦点处声压分布,同时使用COMSOL模型仿真与水听器测试,对比验证了激光全息近场测量后远场外推结果的高准确性。     

Elastic, dielectric, and piezoelectric losses in piezoceramics: how it works all together

The quality factor along with electromechanical coupling coefficient (CEMC) is commonly used as a measure of the energy efficiency of a piezoelectric transducer (PT) working as an energy converter. Losses in piezoceramics are phenomenologically considered to have three coupled mechanisms: dielectric, elastic, and piezoelectric. Their cumulative performance first of all determines the PT quality factor characterizing the efficiency of vibrational energy accumulation, and related to it dissipative effects. The extended definition of the PT electromechanical quality factor (EMQ) with permanent energy exchange between electrical source of excitation and PT was proposed. The EMQ analysis has been conducted on the basis of complex material constants for both stiffened and unstiffened canonical vibrational modes. The efficiency of mechanically free and electrically excited piezoceramic transducers in a wide frequency range of PT harmonics, especially between the fundamental resonance and antiresonance frequencies, was investigated, and the effect of piezoelectric loss anomaly with extremely low total losses was predicted. Particularly, optimization of PT excitation with connected reactive (capacitive) element was conducted to provide higher PT mechanical vibrational characteristics with less total losses. The requirements to the piezoceramic material parameters, types of transducer vibrations, and especially to the piezoelectric loss factor in the range of physically valid values were established to provide maximal EMQ.     

Preparation and Properties of 1-3 Piezoelectric Composite Transducers

This study demonstrated the preparation of 1-3 piezoelectric composite transducers by using 1-3 piezoelectric composites as the active phases, air as the backing layer and polyurethane as the matching layer. The composite electrode was produced by electroless nickel coating and the effect of the matching layer on transducer performance was investigated in detail. The results showed that the matching layer could improve the receiving sensitivity of the transducer significantly. The receiving response value of the transducer at first increased and then decreased with the increase of the matching layer thickness. This receiving response had the largest head-wave amplitude value when the thickness of the matching layer was about 2.2 mm, which was in good agreement with the theoretical value. The resonance frequency of the transducer in water showed a tendency of low frequencies, and the bandwidth of the transducer in water became much wider than that in air. The reasonable matching layer could improve performance of the transducer effectively.     

Double frequency piezoelectric transducer design for harmonic imaging purposes in NDT

Harmonic imaging (HI) has emerged as a very promising tool for medical imaging, although there has been little published work using this technique in ultrasonic nondestructive testing (NDT). The core of the technique, which uses nonlinear propagation effects arising in the medium due to the microstructure or the existence of defects, is the ability to design transducers capable of emitting at one frequency and receiving at twice this frequency. The transducers that have been used so far are usually double crystal configurations with coaxial geometry, and commonly using a disc surrounded by a ring. Such a geometry permits the design of broadband transducers if each transducer element is adapted to the medium with its corresponding matching layers. Nevertheless, the different geometry of the emission and reception apertures creates difficulties when resolving the images. In this work, a new transducer design with different emission and reception apertures is resented. It makes use of the traditional construction procedures used to make piezocomposite transducers and the well-known theory of the mode coupling in piezoelectric resonators when the lateral dimensions are comparable with the thickness of the piezoceramic. In this work the design, construction, and characterization of a prototype to be used in NDT of metallic materials is presented. The acoustic field is calculated using water as a propagation medium, and these theoretical predictions then are compared with the experimental measurements. The predicted acoustic performances for the case of propagation in stainless steel are shown.     

Design considerations for piezoelectric polymer ultrasound transducers

Much work has been published on the design of ultrasound transducers using piezoelectric ceramics, but a great deal of this work does not apply when using the piezoelectric polymers because of their unique electrical and mechanical properties. The purpose of this paper is to review and present new insight into seven important considerations for the design of active piezoelectric polymer ultrasound transducers: piezoelectric polymer materials selection, transducer construction and packaging requirements, materials characterization and modeling, film thickness and active area design, electroding selection, backing material design, and front protection/matching layer design. Besides reviewing these design considerations, this paper also presents new insight into the design of active piezoelectric polymer ultrasonic transducers. The design and fabrication of an immersible ultrasonic transducer, which has no adhesive layer between the active element and backing layer, is included. The transducer features direct deposition of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer onto an insulated aluminum backing substrate. Pulse-echo tests indicated a minimum insertion loss of 37 dB and -6 dB bandwidth of 9.8 to 22 MHz (71%). The use of polymer wear-protection/quarter-wave matching layers is also discussed. Test results on a P(VDF-TrFE) transducer showed that a Mylar/sup TM/ front layer provided a slight increase in pulse-echo amplitude of 15% (or 1.2 dB) and an increase in -6 dB pulse-echo fractional bandwidth from 86 to 95%. Theoretical derivations are reported for optimizing the active area of the piezoelectric polymer element for maximum power transfer at resonance. These derivations are extended to the special case for a low profile (i.e., thin) shielded transducer. A method for modeling the non-linear loading effects of a commercial pulser-receiver is also included.     

Improved integral formulae for supersonic reconstruction of the acoustic field

In this work we derive surface integral formulae for the frequency dependent power operator in arbitrary geometries. This operator is combined with layer representations and measured pressure (or normal velocity) near-field holograms to construct a non-negative intensity distribution quantifying the areas of the vibrating structure that produce radiation to the far-field. It is demonstrated that the sound producing regions of the structure are clearly revealed using the derived formulae and that the spatial resolution is limited to a half-wavelength. We validate our methodology using numerically generated data that corresponds to the excitation of an elastic spherical shell by a point force.     

Ultrasonic density sensor for liquids

This paper presents an ultrasonic density sensor for liquids that unifies high accuracy with high durability and is suitable for on-line measurements in a wide range of tube diameters. The sensor consists of a transducer with a piezoceramic disk mounted between two reference rods of quartz glass. Additionally, a second transducer is used as a sound receiver. The density is obtained from the reflection coefficient of ultrasound at the interface between the quartz glass rod and the liquid and the transit time of sound between this interface and the second transducer. Parameters, such as high long-term stability and accuracy of +/-0.1% of full scale, were obtained by an internal acoustic reference measurement. The reference signal is generated using the sound radiated from the rear side of the piezoceramic disk. Design aspects such as sensor materials and signal-to-noise ratio are discussed, and experimental results are given in this paper. Applications of the sensor include concentration measurement, and ultrasonic mass flow measurement.     

大功率夹心式压电换能器结构参数计算分析及设计

基于一维纵振理论和机电等效原理,得到了单匹配层夹心式压电换能器辐射声压级的计算公式,通过该公式对换能器结构参数对其谐振频率和辐射声压级(sound pressure level,SPL)的影响进行了计算分析。计算结果表明:换能器径向尺寸对谐振频率的影响不大,但前盖板径向尺寸的增大会提高辐射声压级;纵向尺寸的增大会使换能器系统谐振频率降低,但增加前盖板厚度,或增大前后盖板厚度比,有利于辐射声压级的提高;增加压电晶片的片数、厚度和面积都可使换能器辐射声压级提高;换能器发射端增加匹配层能有效提高辐射声压级,匹配层的厚度和密度对声压级幅值和带宽有影响。依据计算结果,进行了给定指标要求的换能器的设计计算。     

Triple-resonant transducers

A detailed analysis is presented of two novel multiple-resonant transducers which produce a wider transmit response than that of a conventional Tonpilz-type transducer. These multi-resonant transducers are Tonpilz-type longitudinal vibrators that produce three coupled resonances and are referred to as triple-resonant transducers (TRTs). One of these designs is a mechanical series arrangement of a tail mass, piezoelectric ceramic stack, central mass, compliant spring, second central mass, second compliant spring, and a piston-radiating head mass. The other TRT design is a mechanical series arrangement of a tail mass, piezoelectric ceramic stack, central mass, compliant spring, and head mass with a quarter-wave matching layer of poly(methyl methacrylate) on the head mass. Several prototype transducer element designs were fabricated that demonstrated proof-of-concept.     

Piezoceramics for high-frequency (20 to 100 MHz) single-elementimaging transducers

The performance of transducers operating at high frequencies is greatly influenced by the properties of the piezoelectric materials used in their fabrication. Selection of an appropriate material for a transducer is based on many factors, including material properties, transducer area, and operating frequency. The properties of a number of piezoceramic materials have been experimentally determined by measuring the electrical impedance of air-loaded resonators whose thickness corresponds to resonance frequencies from 10 to 100 MHz. Materials measured include commercially available compositions of lead zirconate titanate (PZT) with relatively high dielectric constants and a modified lead titanate (PT) composition with a much lower dielectric constant. In addition, materials which have been designed or modified to result in improved properties at high frequencies are studied. Conclusions concerning the influence of the microstructure and composition on the frequency dependence of the material properties are made from the calculated properties and microstructural analysis of each material. Issues which affect transducer performance are discussed in relation to the properties. For transducers larger than about 1 mm in diameter, the use of a lower dielectric constant material is shown to result in a better electrical match between the transducer and a standard 50 Ω termination. For transducers whose impedance is close to that of the connecting cables and electrical termination, equivalent circuit model simulations show improved performance without the need for electrical matching networks. Measurements of fabricated transducers show close agreement with the simulations, validating the measurements and showing the performance benefits of electrically matched transducers     

高频宽带换能器研究

主要研究高频换能器的匹配层技术,通过匹配层技术拓宽换能器的频带,首先利用等效电路法分析高频匹配层换能器,其次通过Matlab仿真分析匹配层材料的密度、声速、厚度变化对换能器电声参数性能的影响,进而对其电声性能进行优化设计,最终制作出一高频宽带换能器。通过实验测得结果与仿真结果基本一致,实验测得换能器的最大发送电压响应为178dB,工作频带为260～370kHz,带内发送电压响应起伏为-3dB,300kHz时换能器指向性-3dB开角为6．5°。     

High frequency piezoelectric MEMS ultrasound transducers

High-frequency ultrasound array transducers using piezoelectric thin films on larger structures are being developed for high-resolution imaging systems. The increase in resolution is achieved by a simultaneous increase in operating frequency (30 MHz to about 1 GHz) and close coupling of the electronic circuitry. Two different processing methods were explored to fabricate array transducers. In one implementation, a xylophone bar transducer was prototyped, using thin film PbZr(0.52)Ti(0.48)O(3) (PZT) as the active piezoelectric layer. In the other, the piezoelectric transducer was prepared by mist deposition of PZT films over electroplated Ni posts. Because the PZT films are excited through the film thickness, the drive voltages of these transducers are low, and close coupling of the electronic circuitry is possible. A complementary metal-oxidesemiconductor (CMOS) transceiver chip for a 16-element array was fabricated in 0.35-microm process technology. The ultrasound front-end chip contains beam-forming electronics, receiver circuitry, and analog-to-digital converters with 3-Kbyte on-chip buffer memory.     

Controlled-source analogous circuits and SPICE models forpiezoelectric transducers

Transmission line analogous circuits for piezoelectric transducers are developed which employ controlled sources rather than the traditional transformer to model the coupling between the electrical and the mechanical systems. A novel method is used to derive each model that consists of adding a term that is equal to zero to one of the device electromechanical equations. When this is done, it is shown that the equations can be cast into the form of the familiar telegraphist's equations for the voltage and current on an electrical transmission line. The circuits are derived for both the thickness-mode piezoelectric transducer and the side-electrode bar piezoelectric transducer. SPICE models of the analogous circuits are presented and an example simulation is given     

Modeling and optimization of high-frequency ultrasound transducers

Obtaining an accurate transducer model for a high-frequency transducer can be troublesome using traditional models, such as the KLM model, since it is often difficult to measure precisely the piezoelectric, dielectric, and mechanical properties of the transducer. This paper describes an alternative method of modeling transducers using network theory. The network theory model for a transducer is determined from a measurement of the transducer impedance in water and the pulse-echo response of the system for a given electrical source and load. A discussion of how this model can be used to optimize the design of an electrical matching circuit is given. This method is illustrated by designing a two-element transmission line matching circuit for a miniature 53 MHz transducer. Excellent agreement between the network model prediction and the experimental response is obtained     

Ultrasonic Phased Array Device for Acoustic Imaging in Air

An ultrasonic phased array device is developed to provide mobility aid for visually impaired people. To perform acoustic imaging, two different linear transducer arrays are constructed using commercially available transducers. The transmitter and receiver arrays are formed with six and four transducer elements, respectively. Individual transducer elements are discrete components with a radius of 1.9 wavelengths and a half-power beamwidth of 43$^{circ}$ at 40.8 kHz center frequency. The transmitter array is formed by aligning the transducers with minimum spacing between the elements. Even this placement leads to the occurrence of unwanted grating lobes in the array response and decreases the field-of-view to 30$^{circ}$ . To eliminate these grating lobes, the elements of the receiver array are placed with a different spacing. Forming the receiver and transmitter arrays with nonidentical element spacing makes the grating lobes to appear at different places. Since the response of the overall system is the product of the directivity patterns of receiver and transmitter arrays, the grating lobes diminish for the overall system and the field-of-view increases.      

Radially composite piezoelectric ceramic tubular transducer in radial vibration

The radially composite piezoelectric tubular transducer is studied. It is composed of radially poled piezoelectric and a long metal tube. The electro-mechanical equivalent circuit of the radially poled piezoelectric and metal tube in radial vibration is obtained. Based on the force and velocity boundary conditions, the six-port electro-mechanical equivalent circuit for the composite tubular transducer is given and the resonance/anti-resonance frequency equations are obtained. The relationship between the resonance frequency and the dimensions is analyzed. Numerically simulated results obtained by the finite element method are compared with those from the analytical method. Composite piezoelectric tubular transducers are designed and manufactured. The resonance/anti-resonance frequencies are measured, and it is shown that the theoretical results are in good agreement with the simulated and experimental results. It is expected that radially composite piezoelectric tubular transducers can be used as high-power ultrasonic radiators in ultrasonic applications, such as ultrasonic liquid processing.