Transducer Excitation Research Articles

FoCUS: Fourier-Based Coded Ultrasound.

Modern imaging systems typically use single-carrier short pulses for transducer excitation. Coded signals together with pulse compression are successfully used in radar and communication to increase the amount of transmitted energy. Previous research verified significant improvement in signal-to-noise ratio (SNR) and imaging depth for ultrasound imaging with coded signals. Since pulse compression needs to be applied at each transducer element, the implementation of coded excitation (CE) in array imaging is computationally complex. Applying pulse compression on the beamformer output reduces the computational load but degrades both the axial and lateral point spread function, compromising image quality. In this paper, we present an approach for efficient implementation of pulse compression by integrating it into frequency domain beamforming. This method leads to significant reduction in the amount of computations without affecting axial resolution. The lateral resolution is dictated by the factor of savings in computational load. We verify the performance of our method on a Verasonics imaging system and compare the resulting images to time-domain processing. The computational savings are evaluated for a minimal sampling rate of four times the central frequency. We show that from 4- to 33-fold reduction is achieved as a function of the resulting lateral resolution, with no degradation of axial resolution. For an imaging system operating at a higher sampling rate, e.g., 10 times the central frequency, the savings can be as high as 77-fold. The efficient implementation makes CE a feasible approach in array imaging with the potential to enhance SNR as well as improve imaging depth and frame rate.

A Sizing Methodology for Rise-Time Minimization of Dickson Charge Pumps With Capacitive Loads

A novel sizing methodology for Dickson charge pumps with pure capacitive loads is presented. The methodology is based on dynamic analysis to minimize the rise time of the charge pump up to 25% under a given circuit area. The methodology is validated through the implementation of a six-stage charge pump-based driver in 180-nm standard low-voltage CMOS technology. The driver is used for the excitation of ultrasonic transducers with 34 V at a resonance frequency of 220 KHz. A rise time of only 512 nS is achieved. The driver consumes 10.6 mA drawn from a 5-V supply at a pumping frequency of 50 MHz and occupies an area of 0.2 mm2.

Acoustic field approximation with sparse arrays through optimization

In a previous work, the objective of approximating the acoustic field from an arbitrary continuous aperture with constrained sparse transducer arrays was established. Results from this showed limited accuracy due to the constraints and called for further consideration. The aim of this research is to extend the analysis and increase the accuracy of the field approximation through multi-variable optimization. System parameters such as transducer excitation level and location within the aperture space are varied. The acoustic coupling between transducers is also taken into account. Results from the optimizations will provide the system parameters that minimize error between compared radiation patterns.

Design and implementation of improved LsCpLp resonant circuit for power supply for high-power electromagnetic acoustic transducer excitation.

This paper investigates the design and implementation of an improved series-parallel inductor-capacitor-inductor (LsCpLp) resonant circuit power supply for excitation of electromagnetic acoustic transducers (EMATs). The main advantage of the proposed resonant circuit is the absence of a high-permeability dynamic transformer. A high-frequency pulsating voltage gain can be achieved through a double resonance phenomenon. Both resonant tailing behavior and higher harmonics are suppressed by the improved resonant circuit, which also contributes to the generation of ultrasonic waves. Additionally, the proposed circuit can realize impedance matching and can also optimize the transduction efficiency. The complete design and implementation procedure for the power supply is described and has been validated by implementation of the proposed power supply to drive a portable EMAT. The circuit simulation results show close agreement with the experimental results and thus confirm the validity of the proposed topology. The proposed circuit is suitable for use as a portable EMAT excitation power supply that is fed by a low-voltage source.

Ultrasonic tooling system design and development for single point diamond turning (SPDT) of ferrous metals

In ultra-precision diamond turning, work pieces with a surface roughness of R a < 5 nm and a form accuracy of <250 nm can be machined. Materials such as alumina, copper, electroless nickel and some plastics are mainly used in this process. In conventional ultra-precision diamond turning, it is not possible to machine ferrous metals such as hardened steel. A chemical reaction between carbon of the diamond and iron of the steel takes place and increases the tool wear significantly. The method of adding ultrasonic vibrations to the cutting process was developed to reduce the contact time between tool and work piece. This method leads to a significant reduction of the chemical process and thus enables the machining of ferrous materials with diamond tools. For the realization of this method, an ultrasonic tool holder was designed and a prototype of this tool holder was elaborated. Based on a longitudinal excited transducer, a longitudinal wave is transformed through a flexural sonotrode into a transversal wave. The aim of the new development was to reach a higher operating frequency of 100 kHz and minimize disadvantageous features related to weight and geometry of such devices.

Subharmonic response of lipid-coated monodisperse microbubbles

Subharmonic emissions from lipid-coated microbubbles, which is not radiated by tissues, can be leveraged to improve contrast-enhanced diagnostic ultrasound. In our work, we investigated subharmonic emissions from monodisperse lipid-coated microbubbles under different acoustic pressure and frequencies. First, the resonance frequency of microbubble monodispersion was determined from measured attenuation spectrum. Next, acoustic emissions from the microbubbles were detected by a transducer positioned orthogonal to the excitation transducer. Our study showed that subharmonic emissions were maximized when bubbles were driven at nearly twice the pressure-dependent resonance frequency rather than the linear resonance frequency. We also observed subharmonic emission at low excitation acoustic pressure (< = 30 kPa) for microbubbles coated with densely packed lipid shells, which suggests that minimizing the initial surface tension can enable subharmonic emissions at very low excitation pressures. Further studies were conducted with shells composed of varying lipids to test the influence of shell composition on the initial surface tension and subharmonic emissions. Theoretical simulations were carried out and agreed with the experimental trends. Implications of these results on the use of monodisperse lipid-coated microbubbles for subharmonic imaging will be discussed.

An acousto-electric effect logging detector in boreholes

An acousto-electric effect logging detector in boreholes is introduced. The detector can measure the first and second type of acousto-electric effect in the downhole. The acoustic transducer of the detector is composed of a transmitting transducer T and a receiving transducer array R1, R2, and R3, and the electrode is composed of power supply electrodes A and B, and measuring electrodes E1, E2, E3, and E4. The minimum spacing between the transmitter transducer T and the receiving transducer R1 is 2500 mm, and the spacing between the receiving transducer is 300 mm. The spacing of the power supply electrode is 1500 mm, and the spacing of the measuring electrode is 300 mm. The detector circuits include the acoustic transducer excitation circuit, power supply electrode excitation circuit, acoustic signal-processing and acquisition circuit, electrical signal-processing and acquisition circuit, system main control circuit, and telemetry interface circuit. Laboratory and field tests were conducted. The sound pressure of the acoustic transmitting transducer at 1 m is up to 70 kPa. The typical acousto-electric logging signal has been observed from the field test in the downhole formation.

Power Efficiency Calculation of Acoustic Energy Transmission by a Stepped-Plate Radiator

One performance measure of in-air ultrasonic radiators, such as wireless power transmission, is the power efficiency of the transducers. The efficiency of most in-air acoustic radiators is low, even at ultrasonic frequencies; however, a large radiating plate with steps introduced by Gallego-Juarez et al., can provide efficient radiation. Their in-air acoustic radiator consists of a Langevin transducer for wave excitation, a mechanical amplifier, and a stepped plate with a large radiating area. This study describes a design processing technique for a stepped-plate radiator developed for optimum energy transmission at the target point in air. The total efficiency required to transfer the acoustic energy was divided into three categories, and the design parameters of each category were calculated to maximize the efficiency. This design technique allows optimum acoustic radiation efficiency and maximum acoustic energy transmission depending on various acoustic energy transfer conditions.

Excitation of a circular-aperture transducer with an echo pulse reflected from a spherical surface

A theoretical model is suggested that allows one to explain peculiarities of the frequency response of echo signals at the output of a receiving transducer versus geometrical parameters of reflecting surfaces. It is shown that the way the instantaneous frequency of signals at the output of the receiving transducer changes depends on the curvature radius of reflected wavefront at the receivingtransducer aperture. It has been established that the frequency modulation of the pulsed echo signal is due to nonuniformity of the distribution of the phase of acoustic signal at the receiving-transducer aperture; this can be characterized by the radii of Fresnel zones that correspond to the reflected wavefront in relationship to the aperture radius of the circular transducer. Data on the frequency modulation of pulsed echo signals expressed in terms of dimensionless parameters (normalized frequency deviations) that were obtained experimentally and calculated by the suggested theoretical model demonstrate fair qualitative and quantitative agreement.

Electrical method for depressing the transmitting response fluctuation of the broadband doubly excited transducer

For the problems of the high transmitting response fluctuation of the broadband transducer, a method is proposed to depress the fluctuation and expand the work bandwidth of the transducer in this paper. By connecting two resistances in series with the upper piezoelectric ceramic stack and the lower stack of the doubly excited transducer respectively, the fluctuation of the transmitting response of the doubly excited transducer can be depressed. The equivalent circuit of the doubly excited transducer is derived and the relationship between transmitting response and frequency is obtained when different resistances are connected in series with the upper stack and the lower stack, respectively. The results are validated by finite element numerical simulation. Based on theoretical calculation, the doubly excited transducer which works at the frequency range from 14 to 34 kHz is produced and measured. The dependence of transmitting voltage response on the frequency is measured when different resistances are connected in series with the upper stack and the lower stack. The experimental results show that the proposed method can depress the transmitting response fluctuation. The transmitting response fluctuation of doubly excited transducer decreased from 12 to 7.3 dB in the bandwidth, which ranges from 14 to 34 kHz. The method proposed in this paper will have certain significance for the design of doubly excited broadband transducer.

Development of an apparent face-shear mode (d36) piezoelectric transducer for excitation and reception of shear horizontal waves via two-dimensional antiparallel poling

The non-dispersive fundamental shear horizontal (SH0) wave is extremely useful in guided-wave-based inspection techniques. However, the generation or reception of the SH0 wave by using a piezoelectric transducer is always a challenge. In this work, first, we realized the apparent face-shear (d36) mode in PbZr1-xTixO3 (PZT) ceramics via two-dimensional antiparallel poling. Then, we demonstrated via finite element simulations that the apparent d36 mode PZT wafer can behave as both a SH0 wave actuator and a SH0 wave sensor. Next, by using the apparent d36 PZT wafer as an actuator and a face-shear d36 0.72[Pb(Mg1/3Nb2/3)O3]-0.28[PbTiO3] crystal as the sensor, almost a pure SH0 wave with a high signal-to-noise ratio was successfully excited in an aluminum plate from 180 kHz to 200 kHz. Later, experiments showed that the proposed apparent d36 PZT wafer can also serve as a sensor to detect the SH0 wave over a wide frequency range (160 kHz to 230 kHz). Finally, the amplitude directivity of the SH0 wave generated by the apparent d36 PZT wafer was also investigated. The wave amplitude reaches its maxima at the main direction (0° and 90°) and then decreases monotonically when the propagation direction deviates from the main directions, with the symmetric axis along the 45° direction. The proposed apparent d36 PZT wafer is very suitable for severing as SH0 wave actuators and sensors in structural health monitoring systems.

The Use of Binary Quantization for the Acquisition of Low SNR Ultrasonic Signals: A Study of the Input Dynamic Range.

Low-power excitation and/or low sensitivity transducers, such as electromagnetic acoustic transducers, piezoelectric paints, air-coupled transducers, and small elements of dense arrays, may produce signals below the noise threshold at the receiver. The information from those noisy signals can be recovered after averaging or pulse compression using binary (1-b) quantization only without experiencing significant losses. Hence, no analog-to-digital converter is required, which reduces the data throughput and makes the electronics faster, more compact, and energy efficient. All these are especially attractive for applications that require arrays with many channels and high sampling rates, where the sampling rate can be as high as the system clock. In this paper, the theory of binary quantization is reviewed, mainly from previous work on wireless sensor networks, and the signal-to-noise ratio (SNR) of the input signals under which binary quantization is of practical interest for ultrasound applications is investigated. The main findings are that in most practical cases binary quantization can be used with small errors when the input SNR is on the order of 8 dB or less. Moreover, the maximum SNR after binary quantization and averaging can be estimated as 10log10N-2 dB , where N is the number of averages.

Improved and Automated Primary Ultrasonic Power Measurement Setup at CSIR-NPL, India

Ultrasonic power generated by an ultrasonic transducer must be measured and declared for the ultimate safety of patients. According to IEC-61161 radiation force balance is internationally recommended primary method for measurement of total, time averaged ultrasonic power radiated by transducer. In a manual RFB system, the output of microbalance is recorded manually at no radiation force and after transducer excitation. Manual technique suffers from various errors, such as overshoot due to momentum of target, disturbance due to small tilt in target and buoyancy change of target. The developed automated system provides scope to visualize these effects and enables us to considerably reduce effects of above sources and hence improves uncertainty. In this article developmental details and the functionality of an improved automated system is described.

Controlled Suppression of Wear on the Nanoscale by Ultrasonic Vibrations.

Wear on the nanoscale, as evidenced by the formation of periodic ripples on a model polystyrene thin film while a sharp tip is sliding on it with a normal force in the μN range, is shown to be suppressed by the application of ultrasonic vibrations of amplitude Aexc. An accurate calibration of the transducer excitation amplitude is achieved by a home-built setup based on a laser Doppler vibrometer. The corrugation of the typical ripple pattern that is formed in the absence of vibrations is reduced when the excitation frequency matches the contact resonance of the system and Aexc progressively increases. Above a critical value of Aexc, the ripples completely disappear, while the friction levels off at a finite value determined by the normal force and the vibration amplitude. This value can be significantly smaller than the value of the macroscopic friction coefficient. In addition to the control of wear in general, this opens up the possibility of controlled nanolithography with improved accuracy.

WiSeREmulator: An emulation framework for wireless structural health monitoring

Many competing approaches exist in evaluating sensor network solutions differing by levels of ease of use, cost, control, and realism. Existing work concentrates on simulating network protocols or emulating processing units at the machine cycle level. However, little has been done to emulate the sensors and the physical environments that they monitor. The main contribution of this work is the design of WiserEmulator, an emulation framework for structural health monitoring, which gracefully balances the trade-offs between realism, controllability, and cost. WiserEmulator consists of two main components - a testbed of wireless sensor nodes and a software emulation environment. To emulate the excitation and response of piezo-electric transducers, as well as the wave propagation inside concrete structures, the COMSOL Multi-Physics software was utilized. Digitized sensing output from COMSOL was played back via a multi-channel Digital-to-Analog Converter (DAC) connected to the wireless sensor testbed. In addition to the emulation of concrete structures, WiSeREmulator also allows users to choose prestored data collected from field experiments and synthesized data. A user-friendly Graphical User Interface (GUI) was developed that facilitates intuitive configurations of experimental settings, control of the on-set and progression of the experiments, and real-time visualization of experimental results. We have implemented WiSeREmulator in MATLAB. This work advances the state of the art in providing low cost solutions to evaluating Cyber Physical Systems such as wireless structural health monitoring networks.

Towards a reference cavitating vessel Part III—design and acoustic pressure characterization of a multi-frequency sonoreactor

A multi-frequency cavitation vessel (RV-multi) has been commissioned at the National Physical Laboratory (NPL, UK), with the aim of establishing a standard source of acoustic cavitation in water, with reference to which details of the cavitation process can be studied and cavitation measurement techniques evaluated. The vessel is a cylindrical cavity with a maximum capacity up to 17 L, and is designed to work at six frequency ranges, from 21 kHz to 136 kHz, under controlled temperature conditions. This paper discusses the design of RV-multi and reports experiments carried out to establish the reproducibility of the acoustic pressure field established within the vessel and its operating envelope, including sensitivity to aspects such as water depth and temperature. The acoustic field distribution was determined along the radial and depth directions within the vessel using a miniature hydrophone, for two input voltage levels under low power transducer excitation conditions (e.g. below the cavitation threshold). Particular care was taken in determining peak acoustic pressure locations, as these are critical for accompanying cavitation studies. Perturbations of the vessel by the measuring hydrophone were also monitored with a bottom-mounted pressure sensor.

Shear-wave elasticity imaging of a liver fibrosis mouse model using high-frequency ultrasound.

The objective of this study was to develop a high-frequency imaging platform for evaluating liver fibrosis in mice based on shear-wave elasticity imaging (SWEI). Although SWEI has been used to diagnose hepatic fibrosis clinically, it is performed at relatively low frequencies (<20 MHz). For preclinical ultrasound imaging in small animals, a high-frequency (>30 MHz) single-element transducer with mechanical scanning is often used. In this study we developed a new SWEI system based on a 40-MHz single-element transducer for imaging and a separate 20-MHz excitation transducer for producing the radiation force and the associated shear waves. Liver fibrosis was induced in ten C57BL/6 (B6) mice using carbon tetrachloride; the other ten mice served as the control group. Synchronizing the excitation beam (i.e., the beam from the excitation transducer) and the detection beam sequence (i.e., the beam from the imaging transducer) allows this mechanical-scanning setup to analyze the shear-wave dispersion relation. The liver viscoelastic properties were determined in vivo by measuring the shear-wave dispersion curve followed by fitting to the Voigt model. The mice were then killed and the fibrosis stage was evaluated (from F0 to F4) based on the METAVIR score. The measured mean values of liver elasticity and viscosity, respectively, ranged from 1.06 to 1.89 kPa and from 1.29 to 1.75 Pa∙s for normal F0 and fibrosis stages of F3 and F4. The Spearman coefficients for the correlations between the measured elasticity and viscosity at various fibrosis stages as assessed by the METAVIR score were 0.73 (p < 0.001) and 0.634 (p = 0.0013), respectively. We also found that the collagen content in the liver was linearly correlated with the measured elasticity (r(2) = 0.54, p < 0.001) and less strongly with the viscosity (r2 = 0.26, p = 0.022). Finally, the diagnosis performance of high-frequency SWEI was evaluated using multivariate receiver operating characteristic curve (ROC) analysis. The areas under the multivariate ROC curve for diagnosing fibrosis stages of F ≥ 3, F = 4, F0 vs. F3, F0 vs. F4, and F3 vs. F4 were 0.9, 0.98, 0.83, 1.0, and 0.96, respectively. Compared with traditional ROC analysis, an improved diagnosis performance was found for diagnosing fibrosis stages of F ≥ 3 and F0 vs. F3. These results demonstrate that the developed high-frequency SWEI platform can yield quantitative viscoelastic properties for diagnosing various fibrosis stages in mice. It is a promising tool for studying the progression of liver fibrosis in preclinical animal models both noninvasively and quantitatively.

Low-frequency broadband ultrasonic transducers for testing articles that are manufactured of large-structure and composite materials. Part 2. excitation of low-frequency ultrasonic wide-band signals

A number of methods for the electric excitation of piezoelectric transducers (PETs) that are characterized by multiple frequencies of vibration modes and are virtually single-mode are considered. It was found that in order to substantially extend the spectrum of a radiated elastic pulse it is necessary that the frequency components that are equal to the resonance frequencies of PETs be absent in the spectrum of the excitation pulse. Several designs of PETs with a “carrier” frequency of up to 300 kHz that radiate elastic pulses with controlled durations of one, two, three, or more periods of the carrier frequency, were developed.

High acoustic strains in Si through ultrafast laser excitation of Ti thin-film transducers.

The role of thin-film metal transducers in ultrafast laser-generated longitudinal acoustic phonons in Si (100) monocrystal substrates is investigated. For this purpose degenerate femtosecond pump-probe transient reflectivity measurements are performed probing the Brillouin scattering of laser photons from phonons. The influence of the metallic electron-phonon coupling factor, acoustical impedance and film thickness is examined. An optical transfer matrix method for thin films is applied to extract the net acoustic strain relative strength for the various transducer cases, taking into account the experimental probing efficiency. In addition, a theoretical thermo-mechanical approach based on the combination of a revised two-temperature model and elasticity theory is applied and supports the experimental findings. The results show highly efficient generation of acoustic phonons in Si when Ti transducers are used. This demonstrates the crucial role of the transducer's high electron-phonon coupling constant and high compressive yield strength, as well as strong acoustical impedance matching with the semiconductor substrate.

Investigation of the Half Bridge and Transformer Push–Pull Pulser Topologies for Ultrasonic Transducer Excitation

Comparison of two high power pulser topologies is presented. Pulser design was aimed for piezoelectric transducer excitation, yet it can also be used for electromagnetic acoustic transducer (EMAT) or capacitive micromachined ultrasonic transducers (CMUTs) excitation. Pulser can produce both single rectangular pulse and trains of rectangular arbitrary duration pulses. In order to achieve the economy of the electrical power consumption and speed both high-pulling and low-pulling elements are active switches. Energy per pulse was used to evaluate the amount of energy consumed. Two topologies were selected for evaluation: transformer output push–pull topology and half bridge output. Experimental investigation results are presented.