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.     

Quantitative Schlieren measurements of a high energy electromagnetic transducer acoustic shock field

Schlieren photography has been used to analyse quantitatively the acoustic field of an electromagnetic acoustic transducer (EMAT). By measuring the angle through which the rays are refracted it is possible to compute the refractive index gradient and thus determine both the absolute and complex pressure related structures of the images. Using this method, planar and focused shock transients generated by the EMAT have been evaluated and compared with transducer derived pressure measurements. The peak pressure in the unfocused shock was found to be 3.2 MPa and 4.6 MPa for the Schlieren and piezoelectric transducer measurements respectively. Corresponding values for the focused shock-wave agreed to within experimental error at about 19 MPa.     

Transmission circuits for thick-transducer ultrasound transmission test systems

Pulse techniques for the measurement of ultrasonic absorption over a wide frequency bandwidth are limited by the narrow band nature of conventional piezoelectric transducers. This problem can be overcome if the piezoelectric devices are operated as thick elements. In this paper the insertion characteristics of transducers operating in this mode are determined. The compromise between system sensitivity, bandwidth and noise generation in receiving circuits is examined.It is shown that high voltage generators with rise times of a few nanoseconds are required to drive thick transmission transducers. The design of suitable generators, based on avalanche operation of one or more bipolar transistors is described in detail. It is concluded that exciting signals of sufficient amplitude and short enough rise-time could be generated for absorption measurements to be made over at least two decades of frequency with a dynamic range of 100 dB without a change of transducer.     

Comparative study of wide-band ultrasonic transducers

The relative performance of several types of ultrasonic transducer are assessed using reproducible acoustic transients generated from Q-switched Nd:YAG laser pulses incident on an aluminium alloy sample. A laser interferometer, a capacitance transducer, two types of electromagnetic acoustic transducers (EMATs), and a broad-band piezoelectric transducer are examined as detectors. The comparison includes a study of their rise-times, and typical signal-to-noise ratios. In the case of the interferometer and capacitance transducer, displacement measurements are shown to be highly consistent with theory.     

Ultrasound pressure distributions generated by high frequency transducers in large reactors

The performance of an ultrasound reactor chamber relies on the sound pressure level achieved throughout the system. The active volume of a high frequency ultrasound chamber can be determined by the sound pressure penetration and distribution provided by the transducers. This work evaluated the sound pressure levels and uniformity achieved in water by selected commercial scale high frequency plate transducers without and with reflector plates. Sound pressure produced by ultrasonic plate transducers vertically operating at frequencies of 400 kHz (120 W) and 2 MHz (128 W) was characterized with hydrophones in a 2 m long chamber and their effective operating distance across the chamber’s vertical cross section was determined. The 2 MHz transducer produced the highest pressure amplitude near the transducer surface, with a sharp decline of approximately 40% of the sound pressure occurring in the range between 55 and 155 mm from the transducer. The placement of a reflector plate 500 mm from the surface of the transducer was shown to improve the sound pressure uniformity of 2 MHz ultrasound. Ultrasound at 400 kHz was found to penetrate the fluid up to 2 m without significant losses. Furthermore, 400 kHz ultrasound generated a more uniform sound pressure distribution regardless of the presence or absence of a reflector plate. The choice of the transducer distance to the opposite reactor wall therefore depends on the transducer plate frequency selected. Based on pressure measurements in water, large scale 400 kHz reactor designs can consider larger transducer distance to opposite wall and larger active cross-section, and therefore can reach higher volumes than when using 2 MHz transducer plates.     

Diffraction phenomena in the field of a circular piezoelectric transducer viewed in the time domain

Transient diffraction phenomena in the field of a piston transducer have been explained theoretically in terms of the direct and edge wave contributions to the piston response. Previous experimental studies by other workers have verified this concept for single cycles of 4 MHz radiation. Faster transients are difficult to study using thin piezoelectric transducers due to transducer resonance. Thick piezoelectric transducers of very low resonant frequency can be used to generate and receive step function acoustic transients with rise times of 20 ns or less and the problems of transducer resonance can be avoided. Experimental results are presented which show that when such a step function is propagated between two thick transducers the time course of the received wave can be explained in terms of the direct wave/edge wave model in conjunction with the theory of transient reverberations in piezoelectric elements.     

Study on the multifrequency Langevin ultrasonic transducer

Langevin ultrasonic transducers are widely used in high-power ultrasonics and underwater sound. In ultrasonic cleaning, a matching metal horn rather than a metal cylinder is used as the radiator in order to enhance the radiating surface and improve the acoustic matching between the transducer and the processed medium. To raise the effect of ultrasonic cleaning, the standing wave in the cleaning tank should be eliminated. One method to eliminate the standing wave in the tank is to use the multifrequency ultrasonic transducer. In this paper, the Langevin ultrasonic horn transducer, with two resonance frequencies, is studied. The transducer consists of two groups of piezoelectric ceramic elements: the back metal cylinder, the middle metal cylinder and the front matching metal horn. The vibrational modes of the transducer are analysed, and resonance frequency equations of the transducer in the half-wave and the all-wave vibrational modes are derived. According to the resonance frequency equations, transducers with two resonance frequencies are designed and made. The resonance frequencies, the effective electromechanical coupling coefficients and the equivalent electric impedances of the transducers are measured. It is shown that the measured resonance frequencies are in good agreement with the computed results, and the transducer can be excited to vibrate at two resonance frequencies, which correspond to the half-wave and the all-wave vibrational modes of the transducer.     

Optimization of ultrasonic transducers

The so-called KLM-model for ultrasonic transducers is employed to optimize transducer design. Some new performance characteristics are defined which change monotonically with design parameters. These characteristics are based on the area of the envelope of the echo waveform produced by the transducer and of the corresponding amplitude spectrum. The efficiency of the transducer is defined by the round trip energy factor. The performance characteristics are used in a composite performance measure, which is then employed as a criterion in the optimization procedure. Two transducers are investigated: for medical imaging purposes and for spectral analysis of clinical echograms. The influence of electrical matching, backing impedance, matching layer impedance, bond line thickness and series induction on the optimized transducers is investigated.     

Automatic system for dynamic control of resonance in high power and high Q ultrasonic transducers

This paper describes the fundamentals, design criteria and electronic structure of a new frequency control system to keep permanently at resonance high Q ultrasonic transducers whose mechanical resonance band may vary within wide limits under normal operating conditions. The procedure developed is based on keeping constant at its zero value the phase of the motional admittance of the transducer by automatically and instantaneously adjusting the frequency of the driving signal provided by a voltage controlled oscillator. The characteristics of this system, especially the fact that the transducer is not an integral part of the feed-back loop of the oscillatory circuit and the frequency tracking mechanism does not depend directly on the magnitude of the motional variables of the transducer, offer some advantages in construction and performance with respect to the conventional motional positive feed-back systems.     

正交偶极声波测井换能器有限元分析*

三叠片型换能器是正交偶极声波测井仪器的重要组成部分。换能器的性能直接影响声波测井数据的质量。本文在前人工作基础上,针对三叠片型声波测井发射换能器长度方向的一阶弯曲振动工作模式,利用有限元方法,模拟了换能器结构尺寸和边界条件变化对换能器性能的影响。结果显示,结构尺寸和边界条件对换能器的谐振频率和电导值均有不同程度的影响。参考数值模拟结果,选取合适的结构尺寸参数和边界固定方式,可以调整和选择正交偶极声波测井仪器发射换能器工作频率和最大发射效率。本文研究结果对三叠片型换能器的优化设计有较好的指导意义。     

The performance of thick piezoelectric transducers as wide-band ultrasonic detectors

A Q-switched Nd:YAG laser has been used to generate reproducible acoustic displacements in metallic samples. The acoustic waveforms were initially detected by an absolute displacement-sensitive, capacitance transducer. The capacitance transducer was then replaced in turn by two piezoelectric ceramic transducers of different thickness and diameter. Comparison of waveforms from the two types of detector indicate that thick piezoelectric transducers are useful in the detection of fast rise-time displacement waveforms at metal surfaces. Their performance is optimized by making them as thick as possible, with a small area of contact with the surface.     

Piezoelectric transducer vibrations in a one-dimensional approximation

Summary The theory of piezoelectric transducer vibrations, which may be treated as onedimensional, is developed in detail for thin discs vibrating in a pure thickness extensional mode. An effort has been made to obtain relations of general validity, which include losses, and which are in a simple explicit form convenient for practical calculations. The behaviour of transducers is discussed with special attention to their characteristics at the two fundamental frequencies, the so-called parallel and series resonances. Several peculiarities occur when transducers are coupled to media with considerably different acoustic impedances. These peculiarities are discussed and illustrated by numerical results for quartz and PZT 4 piezoelectric discs radiating into water, air and liquid hydrogen. The application of the theory to different types of vibrations is briefly illustrated for thin bars vibrating longitudinally. Short discussions are included on compound transducer systems, and on the properties of thin discs as receivers.     

A novel method for estimating the focal size of two confocal high-intensity focused ultrasound transducers

Estimating the focal size and position of a high-intensity focused ultrasound (HIFU) transducer remains a challenge since traditional methods, such as hydrophone scanning or schlieren imaging, cannot tolerate high pressures, are directional, or provide low resolution. The difficulties increase when dealing with the complex beam pattern of a multielement HIFU transducer array, e.g., two transducers facing each other. In the present study we show a novel approach to the visualization of the HIFU focus by using shockwave-generated bubbles and a diagnostic B-mode scanner. Bubbles were generated and pushed by shock waves toward the HIFU beam, and were trapped in its pressure valleys. These trapped bubbles moved along the pressure valleys and thereby delineated the shape and size of the HIFU beam. The main and sidelobes of 1.1- and 3.5 MHz HIFU beams were clearly visible, and could be measured with a millimeter resolution. The combined foci could also be visualized by observing the generation of sustained inertial cavitation and enhanced scattering. The results of this study further demonstrate the possibility of reducing the inertial cavitation threshold by the local introduction of shock wave-generated bubbles, which might be useful when bubble generation and cavitation-related bioeffects are intended within a small region in vivo.     

Acoustic impedance matching of medical ultrasound transducers

Ultrasonic transducers for pulse-echo systems are studied both theoretically and experimentally. For the theoretical calculations the Mason model for thickness-mode disc transducers with and without backing and matching layers is used. By building several of the theoretically investigated transducer configurations it is shown that theory and experiment agree well. Thus the properties of a transducer can be predicted to a good approximation before its experimental realization. To find transducers with good sensitivity and short pulses, the pulse shape and frequency response for the following classes of transducers were studied both theoretically and experimentally: transducers with backing only, transducers with heavy backing and front matching layers, and air-backed transducers with front matching layers.     

Progress in stacked piezocomposite ultrasonic transducers for low frequency applications

A stacked ultrasonic transducer comprises multiple individual layers connected mechanically in series and electrically in parallel to reduce the fundamental thickness mode resonance to a frequency corresponding to the transit time of the complete stack and the electrical impedance to a value which corresponds to that of the layers of the stack in parallel. In turn, this allows lower frequency resonant operation than would be possible with a single layer, and facilitates electrical impedance matching to typical transmission circuitry. On transmission, an ideal stack of uniform layers will have an output amplitude larger than that of the equivalent single layer by a factor equal to the . However, using conventional signal amplification circuitry on reception, the output voltage amplitude will be smaller than that of the equivalent single layer by a similar factor. In the past, stacks have commonly been assembled from layers of conventional piezoceramic material but more recently there have been reports of stacks of 1–3 piezocomposites and it is this type that is considered here. The work described in this paper is motivated by the need to operate at frequencies lower than are possible using conventional piezocomposite fabrication technology. Progress in comparison of experimental and simulated results is outlined and the highlights of a theoretical design study are presented. These show that although the general behaviour of a stacked structure is easily predicted, a rigorous theoretical analysis is essential to understand the detail of even a limited range of possible designs.     

Dependence of sonochemical luminescence on various sound fields

To understand the effect of the sound field on sonochemical luminescence, the exact sound pressure must be determined in each field. In this study it was determined by the Shlieren method, which measures the sound pressure without mixing the sound fields. We compared the efficiency of the sonochemical luminescence in three different ways: changing the diameter of the transducer, combining two transducers to obtain crossed propagating directions and surrounding the sound field by a glass cylinder. In the last case cylinders with various sizes were studied. We found that (i) at the same sound pressure, the larger transducer induces stronger luminescence per unit volume, (ii) driving two transducers produces stronger luminescence than the sum of each transducer and (iii) a glass cylinder surrounding the sound field induces stronger luminescence.     

Acoustic power calibration of high-intensity focused ultrasound transducers using a radiation force technique

To address the challenges associated with measuring the ultrasonic power from high-intensity focused ultrasound transducers via radiation force, a technique based on pulsed measurements was developed and analyzed. Two focused ultrasound transducers were characterized in terms of an effective duty factor, which was then used to calculate the power during the pulse at high applied power levels. Two absorbing target designs were used, and both gave comparable results and displayed no damage and minimal temperature rise if placed near the transducer and away from the focus. The method yielded reproducible results up to the maximum pulse power generated of approximately 230 W, thus allowing the radiated power to be calibrated in terms of the peak-to-peak voltage applied to the transducer.     

A systems feedback representation of piezoelectric transducer operational impedance

An analysis of thickness-mode piezoelectric transducer impedance under a variety of mechanical load conditions is presented. A wideband, systems model is used to describe this impedance, which is representative of the electrical input impedance in the transmitting mode and the electrical output impedance for the receiving mode.For a fixed input voltage, the total current flowing through the transducer is considered as the vector sum of two current quantities. Firstly, an input current arising directly from the applied voltage source and secondly, a feedback current, which is generated by secondary piezoelectric action. By treating the problem in this manner, an extremely valuable insight into the nature of the electrical characteristics is provided. Complex variations in the impedance behaviour are readily explained and the physical processes that contribute to such deviations are clearly isolated.     

High precision, short range ultrasonic sensing by means of resonance mode-locking

This report describes a novel method of continuous-wave operation of ultrasonic transducers, which enables short-range measurement with a resolution much finer than the sound wavelength. The transducer is locked into a particular resonant mode of the transducer-gap-reflector system by a resonance-locking phase-locked loop. The resonant frequency depends on the relative coherence of the transmitted and reflected waves, so that a change in the position of the reflecting surface will require a corresponding change in the resonance frequency if the resonant mode is to be retained. A simple system of this type has been implemented using a cheap, commercially available transducer with far-from-optimum characteristics. The system has been found to operate successfully in a range well below the typical ring-down range of a corresponding pulse-echo system, and has displayed a resolution much finer than the sound wavelength for the transducer.     

Detection limits for single small flaws and groups of flaws when using focussed ultrasonic transducers

The performance of ultrasonic imaging systems, using line-focussed transducers, was evaluated theoretically by employing a simple technique based on convolution. The analysis considered the response of single reflectors, which are mainly voids, and also multiple reflectors located both on a single plane and on multiple planes, which represent a porous layer or bond-line. The limits of detection for each of the above cases, in the presence of different levels of acoustic noise in the object under test were also calculated. The results presented are expressed as a function of a dimensionless flaw size, incorporating the ultrasonic frequency and beam F-number. The results can be used to identify the best match between ultrasonic equipment and a specific imaging problem. They can also serve to guide decisions regarding the most suitable detection method for use with the chosen equipment. If the inspection is properly designed, surprisingly small flaws could be reliably detected — even in objects where significant levels of incoherent acoustic noise are found. The method and results presented in this paper for line-focussed transducers fit qualitatively the case of spherically focussed transducers. The method is highly versatile and it can be extended to cover a range of problems, outside the scope of this paper, including, quantitatively, that of the spherically focussed transducer by employing a formulation which uses more computer time and memory.