Transducer Bandwidth Research Articles

PMUT-Based System for Continuous Monitoring of Bolted Joints Preload.

In this paper, we present a bolt preload monitoring system, including the system architecture and algorithms. We show how Finite Element Method (FEM) simulations aided the design and how we processed signals to achieve experimental validation. The preload is measured using a Piezoelectric Micromachined Ultrasonic Transducer (PMUT) in pulse-echo mode, by detecting the Change in Time-of-Flight (CTOF) of the acoustic wave generated by the PMUT, between no-load and load conditions. We performed FEM simulations to analyze the wave propagation inside the bolt and understand the effect of different configurations and parameters, such as transducer bandwidth, transducer position (head/tip), presence or absence of threads, as well as the frequency of the acoustic waves. In order to couple the PMUT to the bolt, a novel assembly process involving the deposition of an elastomeric acoustic impedance matching layer was developed. We achieved, for the first time with PMUTs, an experimental measure of bolt preload from the CTOF, with a good signal-to-noise ratio. Due to its low cost and small size, this system has great potential for use in the field for continuous monitoring throughout the operative life of the bolt.

Parallel differential evolution paradigm for multilayer electromechanical device optimization

Design optimization of multilayer piezoelectric transducers is intended for efficient and practical usage of wideband transducers for fault diagnosis, biomedical, and underwater applications through adjusting layer thicknesses and volume fraction of piezoelectric material in each layer. In this context, we propose a parallel differential evolution (PDE) algorithm to mitigate the complexities of multivariate optimization as well as the computation time to achieve an optimized wideband transducer for the particular application. For lead magnesium niobate-lead titanate (PMN PT)- and PZT5h-based piezoelectric materials, the fitness function is formulated based on uniformity of mechanical pressure at the first three harmonics to achieve wide bandwidth in the required functional frequency range. It is carried out using a one-dimensional model (ODM), while input layer thicknesses and volume fractions of active material are evaluated using PDE. The simulation is performed on a parallel computing platform utilizing three different host machines to reduce computational time. Results of the proposed methodology for PDE are statistically represented in the form of minimum, maximum, mean, and standard deviation of fitness value, while graphically represented in terms of speedup and time. It can be observed that the execution time for parallel DE decreases with the increasing number of cores.

Influence of the liquid ionic strength on the resonance frequency and estimated shell parameters of lipid-coated microbubbles

Measurement of the resonance frequency and shell properties of coated microbubbles (MBs) is important in understanding and optimizing their response to ultrasound. In many applications MBs are in ionic liquids; however, the influence of the medium charges on the MB behavior is not well investigated. This study aims to measure the medium charge interactions with MBs by measuring the frequency-dependent attenuation of the same size MBs in mediums of varying charge density. In-house lipid-coated MBs were size isolated to a mean size of 2.35 μm using differential centrifugation. MBs were suspended in distilled water (DW), Phosphate-Buffered Saline solution (PBS1×) and PBS10×. The frequency-dependent attenuation of the MBs solutions was measured using a system of two aligned 100% bandwidth transducers with 10 MHz center frequency. The MB shell properties were estimated by fitting the linear equation to experiments. With increasing salinity, the frequency of the peak attenuation decreased (13, 7.5 and 6.25 MHz in DW, PBS1× and PBS-10×, respectively) and attenuation peak increased (∼140%). Estimated MBs shell elasticity decreased by 64% between DW and PBS-1× and 36% between PBS-1x and PBS-10x. The estimated shell viscosity reduced by ∼40% between DW and PBS-1× and 42% between PBS-1× and PBS-10×. The significant reduction in the fitted stiffness and viscosity is possibly due to the formation of a densely charged layer around the shell and requires proper inclusion in the related MB models.

Wide-bandwidth immersion ultrasound transducer made of Mn:PIMNT single crystal/epoxy 1–3 composite

The sensitivity and bandwidth of immersion ultrasound transducer directly determine the echo signal intensity and axial resolution of the immersion ultrasonic testing (IUT) equipment. The active piezoelectric material, which is the core part of the transducer, usually operates in a thickness-stretching vibrational mode. Hence, it is necessary for piezoelectric materials to possess high piezoelectric performance. In this work, a relaxor-based ferroelectric Mn:PIMNT single crystal with excellent electromechanical performance (d33=1220 pC/N, g33=37.87×10−3 Vm/N, and k33=90%) was used to fabricate a crystal/epoxy 1–3 composite transducer. The crystal-based transducer has a center frequency of 3 MHz. The −6 dB bandwidth is 111.4%, and the insertion loss is −25.2 dB, which are much better than PZT-5 H 1–3 composite transducer (74.5% and −32.9 dB). Additionally, the imaging test reveals that the 1–3 composite transducer made of Mn:PIMNT single crystal exhibits better axial resolution, which shows a good application prospect in the field of ultrasonic nondestructive testing.

On the Importance of Low-Frequency Signals in Functional and Molecular Photoacoustic Computed Tomography.

In photoacoustic computed tomography (PACT) with short-pulsed laser excitation, wideband acoustic signals are generated in biological tissues with frequencies related to the effective shapes and sizes of the optically absorbing targets. Low-frequency photoacoustic signal components correspond to slowly varying spatial features and are often omitted during imaging due to the limited detection bandwidth of the ultrasound transducer, or during image reconstruction as undesired background that degrades image contrast. Here we demonstrate that low-frequency photoacoustic signals, in fact, contain functional and molecular information, and can be used to enhance structural visibility, improve quantitative accuracy, and reduce spare-sampling artifacts. We provide an in-depth theoretical analysis of low-frequency signals in PACT, and experimentally evaluate their impact on several representative PACT applications, such as mapping temperature in photothermal treatment, measuring blood oxygenation in a hypoxia challenge, and detecting photoswitchable molecular probes in deep organs. Our results strongly suggest that low-frequency signals are important for functional and molecular PACT.

Low-frequency directional wideband transducer based on lead-free piezoelectric ceramics

In recent years, lead-free piezoelectric ceramic devices have been extensively researched and applied in the high-frequency range, specifically above 20 kHz, for airborne acoustics applications. However, few applications have been reported on low-frequency underwater acoustic transducers, which are crucial in the fields of underwater acoustic communication, ocean resource development, etc., because this imposes more stringent requirements for lead-free piezoelectric ceramics in terms of size, mechanical coupling coefficient, piezoelectric coefficients, and temperature stability. This paper explores the application of high-performance potassium sodium niobate based lead-free piezoelectric ceramics in a low-frequency directional wideband transducer. It covers design, fabrication, and performance testing of the transducer. The transducer measures Φ236 × 83 mm2 and weighs 4.45 kg, which achieves wideband emission (3500–8200 Hz) and directional sound emission with a front-to-back sound pressure ratio exceeding 6 dB across a significant frequency range. Both the theoretical analysis and the experimental results confirm the competitiveness application of the lead-free piezoelectric ceramics in the field of low-frequency underwater acoustic transducers applications.

Synergistic Enhancement of Bandwidth and Sensitivity of Phased Array Ultrasonic Transducer With Novel Acoustic Mismatch Structure

Synergistic Enhancement of Bandwidth and Sensitivity of Phased Array Ultrasonic Transducer With Novel Acoustic Mismatch Structure

Review of Deep Learning Approaches for Interleaved Photoacoustic and Ultrasound (PAUS) Imaging.

Photoacoustic (PA) imaging provides optical contrast at relatively large depths within the human body, compared to other optical methods, at ultrasound (US) spatial resolution. By integrating real-time PA and US (PAUS) modalities, PAUS imaging has the potential to become a routine clinical modality bringing the molecular sensitivity of optics to medical US imaging. For applications where the full capabilities of clinical US scanners must be maintained in PAUS, conventional limited view and bandwidth transducers must be used. This approach, however, cannot provide high-quality maps of PA sources, especially vascular structures. Deep learning (DL) using data-driven modeling with minimal human design has been very effective in medical imaging, medical data analysis, and disease diagnosis, and has the potential to overcome many of the technical limitations of current PAUS imaging systems. The primary purpose of this article is to summarize the background and current status of DL applications in PAUS imaging. It also looks beyond current approaches to identify remaining challenges and opportunities for robust translation of PAUS technologies to the clinic.

Sub-terahertz photoacoustic effect enabling broadband ultrasound generation for underwater communication

Conventional wireless communication faces limitations in underwater environments due to the high electromagnetic wave attenuation. Ultrasound provides an alternative, enabling communication for underwater drones and intrabody communication for implantable devices. However, the limited bandwidth of resonance-based piezoelectric transducers remains a challenge. In addition, the significant acoustic impedance mismatch between water and air hinders cross-boundary communication and necessitates direct contact between the medium and the transducers. To overcome these challenges, we propose to use a sub-terahertz photoacoustic effect for the generation of broadband ultrasound in a non-contact manner. The photoacoustic effect generates acoustic signals in a medium through thermal expansion caused by optical irradiation. We demonstrate that the use of periodically modulated continuous-wave sub-terahertz waves around 0.1 THz enables broadband signal generation efficiently owing to their high attenuation in water. We characterize the frequency response of the generated acoustic signals and demonstrate a proof of concept for underwater communication.

Design, fabrication and testing of a high frequency broadband hydroacoustic transducer for sonar systems

A high frequency broadband hydroacoustic transducer was fabricated for applications such as small target detection and fine imaging in sonar systems. The utilization of the structure design of multimode coupling theory is employed to expand the operational bandwidth of the transducer by leveraging the characteristics of 1–3 type piezoelectric composites that possess a broad frequency range. Piezoelectric composite sensitive elements of varying thicknesses were developed through optimization of dimensional parameters using theoretical and finite element simulation analyses. These composite sensitive elements were then bonded to a spiral stepped backing to enable lateral stacking, ultimately resulting in the fabrication of a high frequency broadband hydroacoustic transducer. The outcomes of the underwater performance evaluation indicate that the transducer's central frequency is 310 kHz, with a −3 dB bandwidth capable of reaching 100 kHz. The maximum transmit voltage response is 159 dB, while the maximum receive sensitivity is −182 dB. Additionally, the directional beam opening angle of −3 dB at the center frequency is roughly 9°, which effectively broadens the transducer's bandwidth.

Design of acoustic impedance gradient matching layers

This article proposes a new acoustic impedance gradient matching layer based on phononic crystal theory to further expand the bandwidth of piezoelectric ultrasonic transducers and improve their working efficiency. The matching layer is made of a composite of two different materials and its transverse structure can be regarded as a one-dimensional phononic crystal. This structure can effectively reduce the transverse vibration of the transducer and enhance the longitudinal vibration. In addition, the gradient alteration of acoustic impedance in the longitudinal direction of the matching layer improves its acoustic impedance matching performance with the medium. Through theoretical calculations and finite element simulations, we obtained the equivalent acoustic parameters of the gradient matching layer and investigated the suppression of transverse vibration by its structural properties based on the band gap theory. Finally, a sample of the transducer equipped with the gradient matching layer was prepared and its working performance was measured by pulse echo experiments, which is a significant improvement compared to the conventional λ/4 matching layer transducer.

A liquid column resonance transducer driven by Class IV flextensional transducer.

A liquid column resonance (LCR) transducer, also referred to as an organ pipe transducer, is a type of transducer that utilizes the liquid column resonance mode to produce acoustic energy underwater. Traditional transducers, such as piezoelectric rings or Janus transducers, are commonly used as the driving source in LCR transducers. A flextensional transducer (FT) is introduced into the LCR transducer as the driving source because of the relatively larger volume velocity at low frequencies. Moreover, the eigen-mode of the Class IV FT is easier to couple with the LCR mode to broaden the bandwidth of a LCR transducer. To overcome the problems associated with the low stiffness of elliptical metal pipes, an improved aluminum pipe, which has a cross-beam to increase the stiffness, was proposed and utilized in a LCR transducer driven by a Class IV FT. The fabricated LCR transducer prototype driven by the Class IV FT has two resonance peaks from 700-2000 Hz, and the transmitting voltage response values of these peaks are 132.1 and 137.8 dB (re 1 μPa/V @1 m). Comparing with an LCR transducer driven by a 33-mode ring, the results show that the LCR transducer driven by a Class IV FT provides good efficiency and broadband characteristics.

Alternating current polarization to enhance piezoelectric performance of single crystal composites

Pb-based relaxor piezoelectric single crystal composites (PSCCs) have attracted widespread attention for improving the bandwidth and sensitivity performance of acoustic transducers due to their excellent electromechanical and piezoelectric properties. Alternating current polarization (ACP) has been proven as an effective method for enhancing the dielectric and piezoelectric properties of relaxor-PbTiO3 single crystals. Herein, we investigated the effects of the amplitude, frequency, and cycle number of the poling electric field on the piezoelectric and dielectric performances of PSCCs and obtained the optimum poling condition of ACP. Compared with the traditional direct current poling method, both the dielectric permittivity (ε33T/ε0) and piezoelectric coefficient (d33) of the AC-poled PSCCs were both increased by up to 20%. Notably, the ACP PSCC with a volume fraction of 60% exhibited a high d33 of 1610 pC/N, which is superior to values previously reported for PSCCs. This work provides an alternative strategy for enhancing the properties of PSCCs and may contribute to the further development of piezoelectric applications.

Improving the quality of ultrasound images acquired using a therapeutic transducer

To enhance the effectiveness and safety of focused ultrasound (FUS) therapy, ultrasound image-based guidance and treatment monitoring are crucial. However, the use of FUS transducers for both therapy and imaging is impractical due to their low spatial resolution, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR). To address this issue, we propose a new method that significantly improve the quality of images obtained by a FUS transducer. The proposed method employs coded excitation to enhance SNR and Wiener deconvolution to solve the problem of low axial resolution resulting from the narrow spectral bandwidth of FUS transducers. Specifically, the method eliminates the impulse response of a FUS transducer from received ultrasound signals using Wiener deconvolution, and pulse compression is performed using a mismatched filter. Simulation and commercial phantom experiments confirmed that the proposed method significantly improves the quality of images acquired by the FUS transducer. The −6 dB axial resolution was improved 1.27 mm to 0.37 mm that was similar to the resolution achieved by the imaging transducer, i.e., 0.33 mm. SNR and CNR also increased from 16.5 dB and 0.69 to 29.1 dB and 3.03, respectively, that were also similar to those by the imaging transducer (27.8 dB and 3.16). Based on the results, we believe that the proposed method has great potential to enhance the clinical utility of FUS transducers in ultrasound image-guided therapy.

Harmonic-Reduced Bias Circuit for Ultrasound Transducers.

The gain of class-C power amplifiers is generally lower than that of class-A power amplifiers. Thus, higher-amplitude input voltage signals for class-C power amplifiers are required. However, high-amplitude input signals generate unwanted harmonic signals. Therefore, a novel bias circuit was proposed to suppress the harmonic signals generated by class-C power amplifiers, which improves the output voltage amplitudes. To verify the proposed idea, the input harmonic signals when using a harmonic-reduced bias circuit (-61.31 dB, -89.092 dB, -90.53 dB, and -90.32 dB) were measured and were found to be much lower than those when using the voltage divider bias circuit (-57.19 dB, -73.49 dB, -70.97 dB, and -73.61 dB) at 25 MHz, 50 MHz, 75 MHz, and 100 MHz, respectively. To further validate the proposed idea, the pulse-echo measurements were compared using the bias circuits. The peak-to-peak echo amplitude and bandwidth of the piezoelectric transducer, measured when using a harmonic-reduced bias circuit (27.07 mV and 37.19%), were higher than those achieved with a voltage divider circuit (18.55 mV and 22.71%). Therefore, the proposed scheme may be useful for ultrasound instruments with low sensitivity.

A Martian sonic anemometer

On Mars, we have yet to fully quantify atmospheric turbulent transport [Banfield, JASA (2016)]. The key required instrument is a wind sensor that can resolve horizontal and vertical winds, and the perturbations associated with turbulent eddies. On the surface, 10–20Hz sampling and a sensitivity of 5 cm/s are required (McBean, 1972). Viking, Pathfinder, Curiosity, and Insight used hot film or hot wire anemometers that can be confused by radiative heating and sample at approximately 1 Hz. Sonic anemometry is an alternative high speed, high accuracy approach which uses differences in the acoustic time of flight to determine flow speed. However, operation on Mars can be challenging due to the low acoustic source strength and high attenuation in the thin (6––11 mbar) CO2 atmosphere, as well as large temperature variations (−100°C to 20°C). In this paper, we describe an instrument that addresses these challenges: a 3 axis ultrasonic anemometer using custom low impedance, wide band transducers, low noise electronics, and an optimized mechanical structure. With an earlier prototype in wind tunnel tests in 6 mbar CO2, we demonstrated resolution better than 10 cm/s, and an accuracy of 3% at flow speeds up to 12 m/s. Data from an instrument flight to 128 000 feet on a stratospheric balloon will be presented, and progress towards a flight qualifiable instrument will be described.

Measuring sound velocity based on acoustic resonance using multiple narrow band transducers

The sound velocity in a medium is closely related to its material properties, including its composition, structure, density, pressure, and temperature. Various methods have been developed to determine the sound velocity through materials. Among them, a strategy based on ultrasound resonance frequency has been most widely used due to the simplicity. However, it requires a transducer with a wide bandwidth to cover enough resonance frequencies to perform the consequent calculations. In this paper, we develop a resonance method for measuring sound velocity, using multi-frequency narrow-band transducers breaking through the limitation of transducer bandwidth on the utilization of the resonance method. We use different transducers at different center frequencies and with different bandwidth to measure the sound velocity in 100-μm and 400-μm thick steel pieces. The measurement results of different combinations are in good agreement, verifying that the use of multi-frequency narrow-band transducer combinations. Given that most therapeutic transducers have a narrow bandwidth, this method can be used during intracranial ultrasound stimulation to optimize targeting by non-invasively measuring the sound velocity in the skull, especially at thinner locations.

Multi-frequency broadband piezoelectric micromachined ultrasonic transducer utilizing helmholtz resonance

Piezoelectric micromachined ultrasonic transducers (PMUTs) with multi-frequency are appealing for ultrasonic applications, which can meet the requirements of large detection depth and high resolution simultaneously. Hence, a novel structure with series Helmholtz resonators under the PMUT membrane term as dual-cavity serial PMUT is presented, which utilizes both the piezoelectric effect and Helmholtz resonant effect to achieve multi-frequency and further widens the bandwidth. Based on the mechanism of vibration coupling between the membrane and Helmholtz resonator, a lumped model and an equivalent circuit model are formulated. Meanwhile, the finite element method (FEM) is developed to investigate the multi-resonance and make comprehensive validations. The results demonstrate that the multi-frequency is contributed by the induced acoustic impedance of the Helmholtz resonator, causing a simulated average sound pressure of up to 260 Pa/V at the resonance frequency and a 77% -6 dB frequency bandwidth in air by theoretical analysis and parameter optimization. The proposed dual-cavity serial PMUT provides a new approach for the application of high transmission power and broad bandwidth transducers.

Double-armed cone-shaped flexoelectric transducer

Compared with a single energy harvester, this tuning fork double-armed cone-shaped flexure transducer can fundamentally solve the problems of high resonance frequency, low output efficiency, and narrow bandwidth of the flexure transducer. Since the transducer limits the clamping losses of the cantilever beam, it provides sufficient stiffness-mass balance to lower the resonance frequency of the system and extend the bandwidth. The tapered structure provides sufficient strain gradient for the flexoelectric effect, improving the output efficiency of the whole system. The vibration frequency of the tuning fork structure and the derivation of the bandwidth formula are analyzed theoretically, and then the electrode width and thickness of the cone structure are studied through simulation to analyze the transducer output response, resonant frequency, bandwidth, and output impedance.

3-D Protoacoustic Imaging Through a Planar Ultrasound Array: A Simulation Workflow.

Bragg peak range uncertainties are a persistent constraint in proton therapy. Pulsed proton beams generate protoacoustic emissions proportional to absorbed proton energy, thereby encoding dosimetry information in a detectable acoustic wave. Here, we seek to derive and model 3D protoacoustic imaging with an ultrasound array and examine the frequency characteristics of protoacoustic emissions. A formalism is presented through which protoacoustic signals can be characterized considering transducer bandwidth as well as pulse duration of the incident beam. We have also collected an experimental proton beam intensity signal from a Mevion S250 clinical machine to analyze our formalism. We also show that proton-acoustic image reconstruction is possible even when the noise amplitude is larger than the signal amplitude on individual transducers. We find that a 4μ s Gaussian proton pulse can generate a signal in the range of MHz as long as the spatial heating function has sufficiently high temperature gradients.