A spiral wave front beacon for underwater navigation: basic concept and modeling

A spiral wave front source produces an acoustic field that has a phase that is proportional to the azimuthal angle about the source. The concept of a spiral wave front beacon is developed by combining this source with a reference source that has a phase that is constant with the angle. The phase difference between these sources contains information about the receiver's azimuthal angle relative to the beacon and can be used for underwater navigation. To produce the spiral wave front, two sources are considered: a "physical-spiral" source, which produces the appropriate phase by physically deforming the active element of the source into a spiral, and a "phased-spiral" source, which uses an array of active elements, each driven with the appropriate phase, to produce the spiral wave front. Using finite element techniques, the fields produced by these sources are examined in the context of the spiral wave front beacon, and the advantages of each source are discussed.     

PVDF梳状换能器接收非线性兰姆波的实验研究*

该文基于PVDF压电薄膜对梳状换能器进行设计制作,并将该换能器应用于接收非线性兰姆波信号实验研究。首先通过输出矢量和方法分析PVDF梳状换能器的工作特性,然后将设计制作的PVDF梳状换能器应用于铝板中非线性兰姆波信号的接收,并与传统压电陶瓷换能器经斜劈接收的非线性兰姆波信号进行比较分析。实验结果表明,两种换能器所接收到的信号幅值随传播距离的变化趋势相近,线性增长的积累效应表征结果相似,且PVDF梳状换能器可以对基波和二次谐波信号同时响应。除此之外,PVDF梳状换能器用于接收非线性兰姆波信号更加稳定。因此,PVDF梳状换能器有望应用于复杂构件的在线检测与监测研究。     

Nonlinear characterization with burst excitation of 1-3 piezocomposite transducers

Ultrasonic transducers made with 1-3 connectivity piezocomposites are frequently used in Medical applications and nondestructive testing. When the transducer is used for special applications as, for instance air-coupled transmission, it is necessary to compensate for the high difference of acoustic impedance between transducer and medium using high amplitude pulses to generate high acoustic signal. Thus, the nonlinear behavior of the transducer must be taken into account in similar application conditions. The newly developed method, which performs the nonlinear characterization with burst signal excitation near the thickness resonance frequency, is based on the measure of the current as well as the vibration velocity of the piezocomposite transducer. The current of the stationary response is measured before the end of the burst signal excitation. Burst excitation enables us to measure the nonlinear characterization without producing overheating in the transducers. The amplitude level dependence of mechanical losses tandelta(m) and the stiffness increases |Deltac/c(0)| have been studied, as well as the velocity dependence of a point of the transducer, measured with a laser vibrometer. In this method, the power level applied to the transducers can be higher than other nonlinear measurement methods, providing measurements of high accuracy.     

一种非均匀厚度压电复合材料片宽带换能器的实验研究

本文利用１－３压电复合材料作为换能元件，通过特殊工艺制备成一面是平面，一面是球面的非均匀厚度压电振子，这种压电晶片加上激励电压后，不同厚度处以不同的共振频率振动，从而使压电晶片的振动频带加宽，本文介绍了这种压电晶片的机电特性，以及用这种压电晶片制成的宽带聚焦超声换能器的脉冲回波特性，此外，文中还对这种换能器与均匀厚度压电晶片换能器的脉冲回波特性进行了比较。     

对2-2型压电复合材料横波换能器产生的纵波特性的研究

在固体测量中，横波换能器所产生的波列中常伴随有纵波，认识纵波产生的原因可以解释横波测试中出现的现象，以及抑制纵波提供有效的方法，本文利用实验研究的方法，用一对纵波偶极子换能器的对比实验，分析了２－２型压电复合材料厚度切变换能器产生纵波的机理，发现这是一种伴随着晶片厚度切变振动而产生的“偶极子”纵波，并由此解释了这种纵波的伴随性，频率特性及其首波与横波首波位相相反等问题。     

Assessing the relationship between the inter-rod coupling and the efficiency of piezocomposite high-intensity focused ultrasound transducers

The electroacoustic conversion efficiency of the ultrasonic transducer is a critical performance index for high-power applications. The material properties, volume fraction (VF) and aspect ratio (AR) are typically regarded as the design parameters of the piezocomposite transducer. We hypothesized that the spacing between piezoelectric rods was also a dominant factor. Therefore, the inter-rod coupling effects on the efficiency of 1–3 piezocomposite ultrasonic transducers were investigated in this study. The efficiencies of six flat and three curved 1.0 MHz PZT4 epoxy composite transducers with different geometric parameters were measured. Finite element transient analyses of the inter-rod electrical-mechanical coupling in the composites were carried out to explain the measured results. The experimental results showed that for 0.47 AR, the 79% VF transducers had lower efficiency than the 64% VF and 53% VF transducers. For 0.19 AR, the efficiency of the 59% VF transducer was not greater than the efficiency of the 39% VF transducer. Numerical analyses demonstrated that the positive peak voltage induced by the coupling of the side rods was more than twice the level induced by the coupling of the diagonal rods for any spacing. The diagonal coupling voltage peak did not change for spacings larger than 0.2 mm. Moreover, for spacings of 0.05 and 0.1 mm, the inter-rod coupling caused 24% and 20% waveform shifts of the driving voltage, respectively, while the 0.2 mm spacing coupling caused a 14% reduction in the amplitude of the driving voltage. As a result, the asymmetry of the driving voltage degraded the efficiency of the composite transducers and became more severe when the spacing was decreased. We concluded that the efficiency loss induced by inter-rod coupling as a function of spacing should be considered when designing piezocomposite transducers.     

同时发射和接收纵波和横波的压电复合材料换能器

通过控制压电复合材料中压电相的分布形式，本文具有在百度模振动的１－３型压电复合材料和具有扭转模振动的拼接２－２型压电复合材料组合在一起，构成一种混俣模式压电材料，并测试了这种压电材料晶片的性能参数，使用这种晶片作的换能器可以根据使用者的要求，单独产生纵波或横波，也可同时产生这两种波。     

Experimental and finite element modelling studies on single-layer and multi-layer 1-3 piezocomposite transducers

Finite element modelling (FEM) using ATILA code and experimental studies have been carried out on 1-3 piezocomposite transducers. FEM study was initially carried out on a piezocomposite infinite plate and then extended to transducers of finite size. The infinite-plate model results agree well with that of a simple analytical model and experiments. The acoustic performance of multi-layer finite-size piezocomposite transducers was also studied. Transducer stacks were fabricated with different number of layers. The transducer characteristics such as the electrical impedance, the transmitting voltage response (TVR) and the receiving sensitivity (RS) of the 1-3 piezocomposite transducers were evaluated as functions of frequency, ceramic volume fractions and the number of layers. TVR increases and RS decreases with increase in ceramic volume fractions. The model results are found to agree with the experimental data, especially when the number of layers is less.     

1-3 connectivity piezoelectric ceramic-polymer composite transducers made with viscous polymer processing for high frequency ultrasound

Potential applications of high frequency ultrasound exist because of the high spatial resolution consequent upon short wavelength. The frequencies of interest, typically from 25 MHz upwards, are easily supported by modern instrumentation but the capabilities of ultrasonic transducers have not kept pace and the transducers in high frequency commercial ultrasonic systems are still made with single-phase crystal, ceramic or piezopolymer materials. Despite potential performance advantages, the 1-3 connectivity piezoelectric ceramic-polymer composite materials now widely used at lower ultrasonic frequencies have not been adopted because of manufacturing difficulties. These difficulties are centred on fabrication of the 1-3 piezoceramic bristle-block comprising tall, thin pillars upstanding from a supporting stock. Fabrication techniques which have been explored already include injection moulding, mechanical dicing, and laser machining. Here, we describe an alternative technique based on viscous polymer processing (VPP) to produce net shape ceramic bristle-blocks. VPP produces green-state ceramic with rheological properties suitable for embossing. We outline how this can be created then report on our work to fabricate PZT bristle-blocks with lateral pillar dimensions of the order of 50 microm and height-to-width ratios of the order of 10. These have been backfilled with low pre-cure viscosity polymer and made into complete 1-3 piezocomposite transducer elements. We outline the performance of the transducers in terms of electrical impedance and pulse-echo behaviour and show that it corresponds well with computer modelling. We conclude that VPP is a promising technique to allow the established advantages of piezocomposite material to be exploited at higher frequencies than have been possible so far.     

扭转压电复合材料及其应用

本文描述了轴向－切向连通的圆柱２－２型压电复合材料。用ＰＺＴ／环氧树脂制作的普通２－２型压电复合材料可拼接成一种具有扭转振动的压电复合材料。这种压电复合材料和为圆柱２－２型压电复合材料的近似。本文还给出两组这种拼接型压电材料晶片的参数，用这种压电材料制作了产生扭转振动的换能器，测试了换能器的波形，频谱特性和偏振特性，指出了使用适当的高通滤波器可以抑制横波中的径向模，分析了接收波列中纵波抑制较强的原     

水声工程中基于均匀圆阵列的涡旋声波性能分析*

声波作为信息和能量的载体,一直以来在水下通信中被广泛采用,但尚未解决带宽窄、速率低的问题。在光学领域和电磁波领域,轨道角动量都表征了螺旋相位结构的自然属性;通过引入轨道角动量到声学领域中,水声通信系统的传输能力以及频谱效率都得到扩展。基于换能器圆阵列产生涡旋声波进行分析和检测,研究涡旋声波波束的阵列产生方法,给出涡旋声波波束在水下传播的特性。在主轴方向,采用均匀圆阵列产生不同拓扑模式的涡旋声波波束,确定轨道角动量拓扑模式与换能器阵列之间的对应关系;为生成不同拓扑模式下的涡旋声波,研究阵列单元数目、阵列半径、传输频率等对涡旋声波的影响。通过研究发现模式数越高,涡旋声波主瓣波束角越大,主瓣峰值越小。阵列半径越大,主瓣波束角越大,而主瓣峰值则随着阵列半径的增大而减小;频率越高,主瓣波束角越小,主瓣峰值变化不大;阵列单元数对主瓣波束角无影响,但与主瓣峰值成正比关系,阵列单元数越多,主瓣峰值越大。     

Selecting passive and active materials for 1.3 composite power transducers

1.3 PZT-polymer composites were fabricated using the dice and fill method with various PZT types and volume fractions. These composites were evaluated for power underwater transducer applications with an air backed and no matching layer configuration. Electrical input and acoustical output powers were monitored as a function of the drive level. Total acoustic power densities of 30 W/cm2 were obtained with a P189/epoxy piezocomposite vibrating at 350 kHz with a low duty cycle (1-5%) and with a 90% efficiency. Power densities up to 20 W/cm2 were measured with a 50% duty cycle. Evolution and destruction of the transducers were monitored versus increasing averaged power. It was observed that better efficiencies were obtained with low volume fraction configurations allowing natural acoustic impedance matching to water. It was found that hard PZT type (Navy III) are optimal compositions even for piezocomposite transducers. It is shown that, unlike a common belief, the polymer mechanical losses are comparable to those of the active ceramic justifying that 1.3 piezocomposites are suited for low-cost power applications. In fact, the main limitation induced by the polymer phase is a strong thermal breakdown when the temperature of the transducer approaches the glass transition region of the polymer. Measurements of the polymer losses as a function of the temperature were obtained confirming this point and offering interesting new alternatives for future composite power transducers.     

Study for imaging of inside bone using FM-chirp pulse compression system

We measured the attenuation of ultrasound in animal bone in vitro, and determined successfully the correct dynamic range for the detection of echo signals from inside the bone. It was possible to display a B-mode image of a 6-mm thick porcine rib bone using a high frequency (5 MHz) transducer. However, it was difficult to obtain the image of a large bone (20-mm thick femur). This paper describes a possibility of the improvement for the detection of echoes from inside the bone using the FM-chirp pulse. An ultrasonic wave generator that generates the FM-chirp pulse for a frequency range of 100 KHz to 5 MHz was fabricated. And the pulse was transmitted from a transducer and received by a hydrophone in the water. The received waveform and the frequency spectrum were displayed on an oscilloscope. We measured the amplitude spectrum of the received waves before and after inserting a sample bone between the transducer and the hydrophone. In the experiment, three types of focused transducers (0.5, 1 and 2.25 MHz) and three kinds of sample bones (spine, femur and rib of a pig) were used. From the results of the measurement, we could find the useful information for imaging of inside the bone.     

多匹配层空气耦合压电超声换能器*

该文针对超声无损检测与成像功能空气耦合换能器开展了分析计算和研制。为解决压电材料与空气间巨大的阻抗失配问题,进行了多匹配层设计,并基于有限元技术仿真设计了1-3压电复合材料参数。借助复数压电方程,导出考虑损耗的多匹配层压电复合材料换能器厚度振动等效电路,获得其等效导纳,以此计算电导谱,同时基于有限元技术数值计算相应电导谱,二者有较好的一致性。在此基础上分别设计制作复合压电材料,多匹配层材料以及由此构成的空气耦合超声换能器。换能器的实测电导谱与数值仿真结果一致。进一步的换能器回波信号测试及其谱分析结果表明,所研制的160 k Hz中心频率空气耦合换能器样品有较好灵敏度和带宽。这些结果说明,该文研制的空气耦合超声换能器的初样是成功的。     

A spatial impulse response based method for determining effective geometrical parameters for spherically focused transducers

This paper proposes a novel method for determining effective geometrical parameters (GPs) of spherically focused transducer. Theoretical basis of the method is the spatial impulse response method (SIRM) that is a powerful tool to calculate transient fields from a piston-like transducer. Experimentally, the method is implemented by measuring arrival times of the pulse signals (either transmission or pulse echo signals) and using them to determine the time intervals between the direct and edge wave pulses. Since the time intervals for a given measurement position depend on the transducer's GPs, then the effective GPs can be obtained. The measurements are carried out in the near-field so that the time intervals can be resolved. The proposed method has been applied to a spherically focused transducer operating in pulse echo mode. The results have shown that the effective GPs used in the SIRM give a closer agreement between the theory and the measurements than the nominal GPs. The method is best suited for characterizing broadband transducers with short pulse excitations. With certain modification it can be also applied to narrowband transducers. The method can be also applied to transducers of other shapes, e.g., planar circle, and planar or curved rectangle provided the piston approximation is appropriate.     

Acoustic field imaging by light reflection at the free surface of a liquid

In this paper we present a novel technique for acoustic field imaging. This technique is based on reflection of a collimated laser beam at the free surface of a liquid. The reflected beam becomes phase modulated by the acoustic wave as in acoustical holographic systems. We do not use a reference acoustical beam for holographic reconstruction but we observe this phase modulation using dark-field techniques. It gives a measurement of the acoustic field power as a function of the position. The authors have built an optical imaging system and carried out experiments with piezocomposite transducers. The technique presented in this work is able to give fast quantitative information about the performance of individual ceramic rods of the piezocomposite.     

Development of a compact self-focusing piezoelectric generator using electrical pre-strain piezocomposite material

New clinical concepts in lithotripsy demand small shock heads. Reducing the size of piezoelectric shock heads will only be possible if the pressure generated at the surface of each transducer can be increased so that, the total pressure at the focus remains very high. So, we propose a new method allowing the generation of large surface pressures. It is well known that the piezoelectric rods in piezocomposite material are more fragile in the extension mode than in the compression mode. For this reason, actuators are mechanically pre-stressed between two flasks. This method cannot be used for transducers working at high frequencies, about 0.5 MHz. For this reason, we proposed to electrically pre-strain the piezoelectric material by applying high electric field in the opposite direction of the polarisation. In a first mode we proposed to pre-strain in continuous mode the transducer. Unfortunately we noticed a rapid de-poling and re-poling in the inverse direction. In a second mode to reduce depolarisation, this field was applied only during a short time just before the generation of the pulse which generate the compressive wave and in a third mode, the transducer was re-poled between two successive electrical pulses. Using this last method, it was possible to increase the maximum pressure at the surface of a 20 mm diameter plane piston to 20% and reach 4 MPa. According to this idea a very compact shock wave generator was designed. The generator made of a 1-3 piezocomposite material has a diameter of 120 mm and focused at 120 mm. The maximum pressure and the width of the compressive wave at the focus were, respectively, 60 MPa and 1.5 micros. The focal zone measured at -3 dB is an ellipsoid 6 mm high in the propagating axis and 3 mm width in the perpendicular direction. The efficacy of this generator was measured as the number of shocks necessary to totally disintegrate plaster balls 15 mm in diameter mimicking the kidney stones. At full power the number of shocks was only 150 which is rather the same number as the one obtained using electrohydraulic machine generally considered as the gold standard. This results show that piezoelectric material may be advantageously used for the manufacturing of shock wave generators.     

Interlaboratory evaluation of hydrophone sensitivity calibration from 0.1 to 2 MHz via time delay spectrometry

Knowing the low-frequency response of hydrophones, down to 100 kHz at least, is important for accurate biomedical ultrasound measurements. However, current international standards do not extend below 500 kHz. Furthermore, commercial hydrophone sources typically do not supply sensitivity data below 1-2 MHz. Therefore, to help identify and validate practical calibration methods below 2 MHz, the authors have extended their previous individual efforts in an interlaboratory evaluation of sensitivity calibration using the swept-frequency technique, time delay spectrometry (TDS). Calibrations were performed for needle and membrane PVDF hydrophones using each laboratory's TDS system. Each site employed the same purpose-built broadband source transducers, comprising both plano-concave and biconcave 1-3 piezocomposite elements 4 cm in diameter, with maximum and minimum thicknesses of approximately 1.5 and 0.1 cm. Agreement between laboratories was within the estimated measurement precision of +/-0.6 dB. The results demonstrated that a TDS system employing such transducers constitutes a viable method for hydrophone calibrations in this frequency range.     

Generation of very high pressure pulses at the surface of a sandwiched piezoelectric material

New clinical concepts in lithotripsy demand small shock heads. Reducing the size of piezoelectric shock heads will only be possible if the pressure generated at the surface of each transducer can be increased so that the total pressure at the focus remains very high. We propose for the first time to increase the pressure without increasing the transducer voltage by using sandwiched transducers, which are a combination of several stacked transducers. When excited at appropriate time intervals, the pressure waves generated by each one reinforce when they reach the load. This new technique has been successfully tested. A pressure of 2.5 MPa was generated with two stacked, 5 mm-thick 1-3 piezocomposite transducers operating at an excitation voltage of 8 kV. No transducer damage was detected after 10(6) shocks, which corresponds approximately to the treatment of 500 patients.     

Finite element modelling of dense and porous piezoceramic disc hydrophones

The acoustic characteristics of dense and porous piezoceramic disc hydrophones have been studied by finite element modelling (FEM). The FEM results are validated initially by an analytical model for a simple disc of dense piezoceramic material and then it is extended to a porous piezoceramic disc replicating a foam-reticulated sample. Axisymmetric model was used for dense piezoceramic hydrophone due its regular geometric shape. 3-dimensional model was used for the porous piezoceramics, since the unit cell model is inadequate to fully represent transducers of finite lateral dimensions. The porous PZT discs have been synthesised by foam-reticulation technique. The electrical impedance and the receiving sensitivity of the hydrophones in water are evaluated in the frequency range 10-100 kHz. The model results are compared with the experimental data. The receiving sensitivity of piezocomposite hydrophones is found to be reasonably constant over the frequency range studied. The sharp resonance peaks observed for the dense piezoceramic hydrophone has broadened to a large extent for porous piezoceramic hydrophones, indicating higher losses. The flat frequency response suggests that the 3-3 piezocomposites are useful for wide-band hydrophone applications.