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.
A wide variety of devices based on piezoceramic elements, vibrating in several modes, have been proposed and developed for energy harvesting applications. The evaluation of the capability of ...converting mechanical energy into electric energy is a main issue in the device design. In this work, the dependence of the piezoceramic element performances on the aspect ratio, i.e., the ratio between thickness and diameter, is analyzed with the aim of maximizing the energy conversion. To the end, the effective electromechanic coupling factor (k eff ), which is a parameter that can be easily measured, is the investigated parameter. Results obtained with analytical and FE models have shown that the higher k eff value is obtained when the thickness and the diameter of the piezoceramic element are comparable. A first experimental validation of simulation results is finally provided.
In this work, an improved ultrasound technique for biometric recognition is proposed. It allows to acquire simultaneously two biometric characteristics: 3D Hand Geometry and 3D Palmprint.
An open ...research platform is employed as ultrasound imaging system. A commercial high frequency probe is moved in the elevation direction over the region of interest by an automated scanning system based on numeric controlled pantograph. Experimental results are presented and the advantages of the proposed technique are highlighted and discussed.
Wrist vein patterns are acquiring increasing con-sideration in biometric recognition application. Optical and near infrared technologies provide only 2D information. In this work, 3D wrist vein ...patterns, extracted from grayscale ultrasound images, are evaluated for biometric recognition purposes. Both water and gel were experimented with as coupling medium; water provided better image quality. A procedure for extracting 3D vascula pattern was derived and evaluated both by comparison with C-mode image at various depth and through verification experiments on a limited number of samples. Preliminary. results were encouraging and showed that 3D templates provide better recognition performances than 2D ones.
Commonly used high displacement ultrasonic actuators are composed of a Langevin transducer and of a sectional ultrasonic concentrator working as a displacement amplifier. In this work, a novel ...ultrasonic actuator, which exploits a displacement amplifier vibrating in a flexural mode, is proposed. Design and analysis of the actuator have been performed by using a FEM software. Performances of the proposed actuator have been evaluated by using a classical ultrasonic actuator, based on a stepped horn concentrator, as a benchmark. Simulated results have shown that the flexural amplifier exhibits a displacement amplification about 50% higher than that of the stepped horn; furthermore, due to its capability to absorb a higher electrical power, the whole actuator has shown a maximum displacement that is twice the maximum displacement of the stepped horn actuator. Simulated results have been validated by measurements carried out on two manufactured prototypes.
In this paper we report the design, fabrication process, and characterization of a 64-elements capacitive micromachined ultrasonic transducer (cMUT), 3 MHz center frequency, 100% fractional ...bandwidth. Using this transducer, we developed a linear probe for application in medical echographic imaging. The probe was fully characterized and tested with a commercial echographic scanner to obtain first images from phantoms and in vivo human body. The results, which quickly follow similar results obtained by other researchers, clearly show the great potentiality of this new emerging technology. The cMUT probe works better than the standard piezoelectric probe as far as the axial resolution is concerned, but it suffers from low sensitivity. At present this can be a limit, especially for in depth operation. But we are strongly confident that significant improvements can be obtained in the very near future to overcome this limitation, with a better transducer design, the use of an acoustic lens, and using well matched, front-end electronics between the transducer and the echographic system.
Multimodal biometric systems are becoming more and more popular because they allow to improve the overall performance of unimodal ones. Ultrasound systems have the peculiarity of collecting 3D image ...that contains information on different biometric characteristics. In the present work, a technique for collecting volumetric images of the palm based on an advanced ultrasound scanner (ULA OP 256) and compound plane waves is proposed and experimentally evaluated. Good quality images were achieved by transmitting 17 steered plane waves in the range ±3.15°. Image field depth was sufficiently high to extract both palmprint and vein patterns. Recognition capabilities are evaluated through palmprint verification and identification experiments on a homemade database composed of 216 samples from 47 users, obtaining very good results (EER=0.26% and identification rate=100%). It has also been experimentally verified that the proposed system allows to sensibly speed up the acquisition process without image quality loss. Further potentialities of the proposed approach are finally discussed.
Modeling of capacitive micromachined ultrasonic transducers (cMUTs) is based on a two-port network with an electrical and a mechanical side. To obtain a distributed model, a solution of the ...differential equation of motion of the diaphragm for each element of the transducer has to be found. Previous works omit the mechanical load of the cavity behind the diaphragm, i.e., the effect of the gas inside. In this paper, we propose a distributed model for cMUTs that takes this effect into account. A closed-form solution of the mechanical impedance of the membranes has been obtained, including the effect of the restoring forces because of the stiffness of the membrane and because of the compression of the air in the cavity. Simulation results based on the presented model are compared with the experimental data for two types of cMUTs reported in the recent literature. It is demonstrated that the compression of the air has a significant effect on the fundamental frequency of the air transducer, with a deviation of about 22% from the prediction of a model that does not consider the interaction between the vibrating diaphragm and the air cushion.
In this work a piezoelectric actuator is described whose stator is composed of two cylindrical steel axes fitted at the surface of a thin piezoelectric membrane. The slider is a beam pressed in ...contact with two rotors. Each rotor consists of a cylindrical permanent magnet, pressed in contact with the top surface of each axis, by means of magnetic forces. A travelling wave, at the natural flexural vibration frequency of the thin piezoelectric membrane, is excited via piezoelectric effect. The flexural displacement of the membrane is geometrically amplified by the axes, obtaining a wide precessional motion of the axes. The transmission mechanism of the proposed actuator is based on this motion. The actuator is able to give large displacements with a relatively high linear speed (1.21 m/s) and force (0.51 N) by using a commercial piezoelectric membrane (diameter 32 mm, thickness 0.2 mm), driven at relatively low voltage (±18 V). The very small thickness of the overall structure makes this actuator suitable for microsystem applications. A simple analytical approach of the transmission mechanism is reported and experimental measurements are discussed.
In this work, a matrix model of the axle vibration of a piezoelectric motor is proposed. The stator of this motor is composed of a thin piezoelectric membrane and a steel axle fitted at the center of ...the membrane. The rotor consists of a cylinder-shaped permanent magnet, pressed in contact with the other end of the axle by means of the magnetic forces. A travelling wave is excited in the membrane by using four electrodes and four, properly delayed, driving signals. The rotating flexural displacement of the membrane produces a wide precessional motion of the axle. In this way, a continuous slipping takes place between the axle and the rotor, and therefore, a torque is transmitted to the rotor. In this paper, the precessional motion of the axle is modeled as the composition of two transverse vibrations belonging to two perpendicular planes passing through the axle. The axle, vibrating in its transverse mode, is modeled as a two-port system: the input is the bending moment supplied by the membrane, and the output is the transverse force at the terminal end of the axle. With this model, we have computed the trasmission transfer function as a function of frequency, and the transversal displacement along the axle at its resonance frequency. The computed results are in reasonable agreement with experimental interferometric measurements carried out on a prototype.
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