Characterization Of Ultrasonic Transducers Research Articles

Lead‐free versus PZT: Acoustic characterization of ultrasonic immersion transducers fabricated with lead‐free NBT‐BT piezoceramic

AbstractAn evaluation of the performance of a recently developed formulation of (lead‐free) Sodium‐Bismuth titanate – Barium Titanate (NBT‐BT) piezoceramic is presented through the construction and testing of several immersion transducers based around a new material formulation “Pz12” from CTS Ferroperm (Kvistgaard, Denmark). Results are intended to yield insight into the applicability of this lead‐free material for use within industrial, NDT, medical or underwater sectors through the reporting of experimentally realised performance characteristics. Results of several performance tests are reported including: pulse‐echo amplitude, electrical insertion loss, transducer efficiency, power output and transmitting voltage response (TVR). Benchmarking is provided against comparable lead zirconate‐titanate (PZT) based transducers using an industry standard Navy Type 1 (Pz26, CTS Ferroperm). Performance of the Pz12 transducers was found to exceed expectations, although was lower than that of the comparable Pz26 transducers. Pulse‐echo amplitudes indicated a −4.3 dB relative amplitude between lead and lead‐free variants with electrical insertion loss and transmitting voltage response both showing an average −3.1 dB between the Pz12 and Pz26. Electroacoustic conversion efficiencies of up to 52% are reported for the Pz12 transducers compared with a 62% maximum efficiency for the Pz26 variant. Temperature stability is also investigated in the 10–70 ˚C range.

Acoustic and Thermal Characterization of Therapeutic Ultrasonic Langevin Transducers under Continuous- and Pulsed Wave Excitations.

We previously conducted an empirical study on Langevin type transducers in medical use by examining the heat effect on porcine tissue. For maximum acoustic output, the transducer was activated by a continuous sinusoidal wave. In this work, pulsed waves with various duty factors were applied to our transducer model in order to examine their effect on functionality. Acoustic power, electro-acoustic conversion efficiency, acoustic pressure, thermal effect on porcine tissue and bovine muscle, and heat generation in the transducer were investigated under various input conditions. For example, the results of applying a continuous wave of 200 VPP and a pulse wave of 70% duty factor with the same amplitude to the transducer were compared. It was found that continuous waves generated 9.79 W of acoustic power, 6.40% energy efficiency, and 24.84 kPa acoustic pressure. In pulsed excitation, the corresponding values were 9.04 W, 8.44%, and 24.7 kPa, respectively. The maximum temperature increases in bovine muscle are reported to be 83.0 °C and 89.5 °C for each waveform, whereas these values were 102.5 °C and 84.5 °C in fatty porcine tissue. Moreover, the heat generation around the transducer was monitored under continuous and pulsed modes and was found to be 51.3 °C and 50.4 °C. This shows that pulsed excitation gives rise to less thermal influence on the transducer. As a result, it is demonstrated that a transducer triggered by pulsed waves improves the energy efficiency and provides sufficient thermal impact on biological tissues by selecting proper electrical excitation types.

Fabrication and Characterization of Lead-Free BNT-6BT Ultrasonic Transducers Designed by an Intelligent Optimization Algorithm

Lead-free piezoelectric material-based ultrasonic transducers have been researched for several years, but the inefficient properties and design difficulties have troubled lead-free ultrasonic transducers for a long time. To improve the performance and design efficiency of lead-free ultrasonic transducers, in this work, an equivalent circuit model and intelligent optimization algorithm were combined for use in a transducer design. Firstly, 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3(BNT-6BT) lead-free piezoelectric ceramics were prepared and characterized. Then, BNT-6BT ceramics were used to fabricate the ultrasonic transducers. An equivalent circuit model-based software, PiezoCAD, and a genetic algorithm-based back-propagation neural network were used to optimize the design of the transducers. A 3.03 MHz center frequency and 60.3% −6 dB bandwidth of the optimized transducers were achieved, which were consistent with the neural networks optimization results. To verify the application potential of the lead-free transducers, tungsten rods phantom imaging and polystyrene spheres with 300 μm diameter manipulation were completed by the transducers, and the experiment results indicate that the BNT-6BT lead-free transducers have great potential in further biological and biomedical applications.

Symmetric Reflector Ultrasonic Transducer Modeling and Characterization: Role of the Matching Layer on Electroacoustic Performance.

Symmetric reflector ultrasonic transducers (SRUTs) are a new type of ultrasonic transducer that take advantage of the ultrasonic wave emitted on the rear face of the active element. In this work, the electroacoustic modeling and characterization of such a structure is reported. Using the well-known Krimholtz, Leedom, et Matthaei (KLM) model, the electroacoustic response of a SRUT based on a piezoelectric ceramic with one matching layer is calculated. Simulations show that the optimal acoustic impedance of the matching layer should be lower than for conventional transducers, leading to a relative bandwidth of approximately 50%. The characterization of such a transducer based on a piezoceramic plate has been carried out. Bandwidth and sensitivity are reported. They are found to be close to the simulation results and demonstrate that these new types of transducers need to be designed according to new rules compared to conventional ones.

Self-heating phenomenon of piezoelectric elements excited by a tone-burst electric field

Tone-burst excitation is often used for ultrasonic transducers of specific operation modes or for overcoming transducer overheating problems associated with continuous wave (CW) excitation. In this study, a theoretical model for the self-heating phenomenon of a piezoelectric disc element is established to estimate the temperature rise induced by a tone-burst electric field. An analytical solution for the temperature rise of the piezoelectric element is obtained by using Laplace transform method. Numerical simulations and experimental measurements are performed to investigate the influence of different excitation parameters on the temperature rise. By comparing the experimental results with the simulation results, the temperature-rise difference between tone-burst and CW excitations is quantified, and the validity of the theoretical model is verified. Furthermore, a multiparameter estimation method is proposed for the heat convection coefficient and dielectric properties under high-field operating conditions. These results are useful in both optimization of heat dissipation performance and characterization of high-power ultrasonic transducers.

Novel application of wavelet synchrosqueezed transform (WSST) in laser-vibrometer measurements for condition assessment of cementitious materials

Early damage detection is of interest in the non-destructive testing of civil infrastructure. Traditional wave velocity-based methods are not sufficient as they use only one data point of information, neglecting the frequency content of ultrasonic signals. This paper demonstrates the advantages of applying the wavelet synchrosqueezed transform (WSST) to include the wave attenuation approach for condition assessment of cementitious materials.The application of the WSST is presented in three steps. First, synthetic models are studied to demonstrate the capabilities and limitations of the WSST technique, focusing on the ultrasonic frequency range. Next, the WSST is used to extract the main vibration modes to characterize ultrasonic transducers using a state-of-the-art laser vibrometer. Finally, a new WSST-based attenuation analysis technique is used for the evaluation of localized damage in a cemented sand specimen.Our results show that the WSST technique can be used to detect damage using ultrasonic measurements as long as the main frequency components are well separated. The characterization of ultrasonic transducers using WSST gives a better understanding of the frequency content of transmitted signals, which are generated not only by the resonance of piezoelectric crystal but also by the resonance of the surface-wearing plate of the transducer itself. The application sections of the paper show that the proposed approach can be successfully used for early damage detection in the case of localized damage. The WSST-based technique can improve the sensitivity of attenuation analysis in the detection of localized damage up-to 36%.

Network Optimisation and Performance Analysis of a Multistatic Acoustic Navigation Sensor

This paper addresses some of the existing research gaps in the practical use of acoustic waves for navigation of autonomous air and surface vehicles. After providing a characterisation of ultrasonic transducers, a multistatic sensor arrangement is discussed, with multiple transmitters broadcasting their respective signals in a round-robin fashion, following a time division multiple access (TDMA) scheme. In particular, an optimisation methodology for the placement of transmitters in a given test volume is presented with the objective of minimizing the position dilution of precision (PDOP) and maximizing the sensor availability. Additionally, the contribution of platform dynamics to positioning error is also analysed in order to support future ground and flight vehicle test activities. Results are presented of both theoretical and experimental data analysis performed to determine the positioning accuracy attainable from the proposed multistatic acoustic navigation sensor. In particular, the ranging errors due to signal delays and attenuation of sound waves in air are analytically derived, and static indoor positioning tests are performed to determine the positioning accuracy attainable with different transmitter–receiver-relative geometries. Additionally, it is shown that the proposed transmitter placement optimisation methodology leads to increased accuracy and better coverage in an indoor environment, where the required position, velocity, and time (PVT) data cannot be delivered by satellite-based navigation systems.

Standard Installation for the Transmission of Units of Sound Pressure in the Ultrasonic Frequency Range

We present information about a standard installation used for the calibration of hydrophones at frequencies of 0.1–3.0 MHz. The installation is created with an aim to extend the frequency range of the GET 55-2017 State Primary Standard to the region of high-frequency ultrasound. The necessity of development of this installation is justified. We briefly consider basic principles of its operation and the possibilities of measurements. The requirements to the characteristics of ultrasonic transducers are discussed. In the choice of standard transducers, we give priority to their time stability because, in the course of transmission of the unit with the help of a tone signal, the uniformity of the frequency characteristic of sensitivity of the transducer is not of primary importance. To reduce the influence of directivity, we use a precise four-coordinate system of positioning of the transducers, which includes the possibility of control of their mutual positions with the help of a laser beam. We also consider the problems connected with the possibility of passing to higher frequencies.

The standard facility for the transmission the unit of sound pressure at ultrasonic frequencies

Information is provided on a standard facility for calibrating hydrophones at frequencies from 100 kHz to 3 MHz, which created to expand the frequency range of the State primary standard GET 55-2017 to the area of high-frequency ultrasound. The actuality of the standard facility creation is justified. The possibilities of measurements and operation principles of the facility are briefly reviewed. The requirements to the characteristics of ultrasonic transducers are discussed. When selecting standard transducers, priority was given to their temporal stability, since the unevenness of the transducer frequency response is not of fundamental importance when calibrating using a tone signal. To reduce the influence of directivity, a precision four-coordinate positioning system of transducers applies, which including the control of transducers mutual position using laser beam. Attention is paid to the problems related with the mastering of higher frequencies.

Fabrication and Characterization of High-Sensitivity Ultrasonic Transducers With Functionally Graded Design

In this paper, a PZT/epoxy piezo-composite transducer with functionally graded design was proposed, which facilitated high sensitivity and broad bandwidth. The functionally graded piezocomposite was fabricated using a modified dice and fill method. Finite-element simulation was performed to study the vibration modes of the piezocomposite and to predict the transducer performances. To evaluate the performances experimentally, the proposed and conventional transducer based on monolithic PZT ceramic with similar dimension was fabricated and characterized systematically. We determined that the functionally graded transducer has a slightly higher center frequency (5.37 MHz), broader −6-dB bandwidth (22.5%), and a much lower insertion loss (−13.57 dB) compared to the conventional transducer (5.26 MHz, 18.3%, and −17.72 dB, respectively). The experimental results are in good agreement with the simulation ones. This preliminary investigation suggests that the functionally graded design is a promising approach to enhance the performance of ultrasonic transducers in high-sensitivity applications.

Investigation of the repeatability and reproducibility of hydrophone measurements of medical ultrasound fields

Accurate measurements of acoustic pressure are required for characterisation of ultrasonic transducers and for experimental validation of models of ultrasound propagation. Errors in measured pressure can arise from a variety of sources, including variations in the properties of the source and measurement equipment, calibration uncertainty, and processing of measured data. In this study, the repeatability of measurements made with four probe and membrane hydrophones was examined. The pressures measured by these hydrophones in three different ultrasound fields, with both linear and nonlinear, pulsed and steady state driving conditions, were compared to assess the reproducibility of measurements. The coefficient of variation of the focal peak positive pressure was less than 2% for all hydrophones across five repeated measurements. When comparing hydrophones, pressures measured in a spherically focused 1.1 MHz field were within 7% for all except 1 case, and within 10% for a broadband 5 MHz pulse from a diagnostic linear array. Larger differences of up to 55% were observed between measurements of a tightly focused 3.3 MHz field, which were reduced for some hydrophones by the application of spatial averaging corrections. Overall, the major source of these differences was spatial averaging and uncertainty in the complex frequency response of the hydrophones.

Design and characterization of immersion ultrasonic transducers for pulsed regime applications

Ultrasonic transducer design is focused to maximize performance in specific applications, usually leading to complex design and expensive construction and assembly. With the aim to overcome this drawback, a general-purpose immersion ultrasonic transducer for pulsed regime applications has been developed. The design of each element of the transducer is described in this paper, wherein materials and geometries for each part have been recommended. A simple theoretical model has been proposed in order to predict the form of the received electric signal in the target transducer. The model is based on the assumption that the piezoelectric element acts as an underdamped oscillator, forced by the acoustic field coming from the propagation medium. Excellent agreement between the experimental measurements and the analytical model is achieved. Electrical impedance measurements reveal negligible differences between the resonance frequency of the active element and that of the assembled transducer. The designed devices have been characterized in water using two identical transducers placed face to face with changeable orientation. The experimental results show a highly linear response and the generation of a collimated acoustic field. The effects of the thickness of the matching layer on the transmission coefficient have been also studied, resulting in a smooth decrease in the received amplitude, which may significantly lower large-scale production costs.

Reliability of Interpretation of the Acoustic Characteristics of Ultrasonic Transducers in Estimation of the Geometric Parameters of a Subject of Inspection

A relationship between the interpretation of results obtained in ultrasonic inspection and model representations of the subject of inspection and the precision characteristics of instruments with phased antenna arrays is demonstrated. Experimental data that confirm that the interpretative model exerts an effect on the inspection reliability are presented.

An automatic compact Schlieren imaging system for ultrasound transducer testing

The current standard used for the characterization of ultrasonic transducers is the hydrophonic technique, which is able to measure the acoustic pressure profile. This technique allows a quantitative analysis, but is marred by several problems. A scan of the region of interest is a time-consuming operation, especially when measuring a long acoustic beam. Furthermore, a hydrophone placed near the radiating surface is certainly an impediment to the free propagation of the field. Off-axis measurements can be inaccurate because of the angular response of the hydrophone. These problems, together with the costs, have encouraged the search for an alternative quick and inexpensive test system. The well-known Schlieren technique allows a real-time visualization of the whole pressure range of the transducers, but to display the entire beam emitted by the transducers it is necessary to use very large-diameter lenses, with focal lengths of several meters. Such systems are very cumbersome and difficult to use. The system developed in this paper allows the imaging of an acoustic beam up to 200 mm in length, but the system is compact, being only about 1 m long and 0.30 m wide. A similar system based on the classic Schlieren effect would be several meters long, with lenses of 200 mm in diameter. Finally, the system can reconstruct the section of the beam at any height, using an acoustic tomography technique, and can also implement a quantitative analysis. Because it uses only commercial components, the developed and fabricated ultrasonic beam analyzer is a very low-cost imaging system. The goal of this work is to create a compact, low-cost system based on this technique to test a wide range of ultrasonic transducers up to 40 MHz and above.

High-intensity therapeutic ultrasound: metrological requirements versus clinical usage

High-intensity therapeutic ultrasound (HITU) is an appealing non-invasive, non-ionizing therapeutic modality with a wide range of tissue interactions ranging from transient permeabilization of cell membranes to thermal ablation. The ability to guide and monitor the treatment with an associated ultrasonic or magnetic resonance imaging device has resulted in a dramatic rise in the clinical use of therapeutic ultrasound in the past two decades. Nevertheless, the range of clinical applications and the number of patients treated has grown at a much higher pace than the definition of standards. In this paper the metrological requirements of the therapeutic beams are reviewed and are compared with the current clinical use of image-guided HITU mostly based on a practical approach. Liver therapy, a particularly challenging clinical application, is discussed to highlight the differences between some complex clinical situations and the experimental conditions of the metrological characterization of ultrasonic transducers.

Ultrasonic transducers characterisation for evaluation of stiff geomaterials

The pulse velocity test (PVT) is an ASTM standard for the dynamic characterisation of geomaterials such as stiff clays, cemented sands, rooks and concrete. The PVT is based on the first arrival of the compressional wave. The analysis of full waveforms in ultrasonic testing is important in evaluating the variation with frequency of the dynamic properties of materials (phase velocity and material damping). Reliable full waveforms are also required to calibrate numerical simulations for the interpretation of ultrasonic wave propagation in viscoelastic materials. However, the analysis of full waveforms is rarely performed in practice because of the difficulties involved in the characterisation of ultrasonic transducers and their coupling with the medium. This paper presents a new methodology for measurement of the frequency response and impulse response functions of ultrasonic transducers. The proposed methodology uses the complex exponential method to extract dynamic properties from impulse response functions. Ultrasonic tests and numerical simulations are conducted on a calibration aluminium bar and a cemented sand specimen to demonstrate the applicability of the new methodology. The results show that this methodology can be successfully used for dynamic characterisation of ultrasonic transducers for evaluation of the wave velocity and damping ratio of stiff geomaterials.

Characterization of cylindrical ultrasonic transducers using acoustic holography

We present the results of studying the vibrational velocity distribution over the surface of cylindrical ultrasound transducers by acoustic holography. We describe two approaches for acoustic holography: the spatial spectrum method and the Rayleigh integral method. In the case of cylindrical sources the spectral method has a specific feature in comparison to the case of quasi-plane sources: small-scale spectrum components having the form of evanescent (nonpropagating) waves near the source, turn into propagating waves at a certain distance from the source. The use of such a mixed type of waves makes it possible to increase the holographic resolution. To conduct holography of cylindrical sources by the Rayleigh integral method, a modification consisting in the superimposing of boundaries on the integration region is proposed. We present the results of numerical simulation and physical experiments on holography of small cylindrical piezoelectric transducers. We demonstrate that the proposed methods of holography make it possible to recover the vibration structure of source surfaces up to order of the wavelength scales.

Modeling, design and characterization of limited diffraction ultrasonic transducers

Ultrasound piezoelectric transducers are used in medical applications and for non-destructive testing of materials. Among them, a special type, the so named limited diffraction ultrasonic transducers present a great depth of acoustic field, without important effects of diffraction, besides other qualities, which make them optimal in applications of medical images. In this paper, the design of a class of these transducers is explained through a simple technology that uses a method of direct polarization. The application of the Finite Elements Method to simulations in the design process of not uniformly poled transducers, as well as the experimental characterization of their dynamic behaviour, is described. The comparison of the experimental results with those simulated, using Finite Element Analysis, guarantees with an excellent agreement the use of this powerful method of simulation, during the construction and characterization of limited diffraction transducers.

Measurement of the spectral directivity of optoacoustic and ultrasonic transducers with a laser ultrasonic source

Comprehensive characterization of wideband ultrasonic transducers and specifically optoacoustic detectors is achieved through the analysis of their frequency response as a function of the incident angle. The tests are performed under well-defined, repeatable operating conditions. Backillumination of a blackened, acoustically matched planar surface with a short laser pulse creates an acoustic impulse which is used as a wideband ultrasonic source. Upon illumination with a short laser pulse, the bandwidth of our source shows a -6 dB point of 12 MHz and a low-frequency roll-off around 300 kHz. Using proprietary software, we examine thoroughly the planarity of the emitted wave front within a specified amplitude cutoff and phase incoherence. Analysis of the angular dependence of the frequency response yields invaluable directivity information about the detector under study: a necessary component toward accurate optoacoustic image reconstruction and quantitative tomography. The laser ultrasonic source we developed is the main feature of our directivity measurement setup. Due to its simplicity, it can easily be adapted to various calibration devices. This paper focuses on the development and characterization of the flatness and the bandwidth of our wideband ultrasonic source.

Design, Manufacture and Characterization of Ultrasonic Transducers

Piezoelectric transducers are key elements in ultrasound systems used in applications such imaging and flaw detection in non-destructive testing (NDT). NDT intend the analysis of materials and components without affecting their properties, leaving them suitable for their later use, which constitutes actually a valuable tool of engineering. In the present work it was successfully manufactured and characterized a piezoelectric transducer for NDT. The active element of the transducer was a PZT ceramic prepared by the conventional method of solid state reaction. Dielectric, piezoelectric, mechanical and acoustic properties were determined. The transducer was characterized by both electroacoustic and optoacoustic methods.