Experimental analysis of surface detection methods for two-medium imaging with a linear ultrasonic array.

In ultrasonic nondestructive testing, when using an ultrasonic linear array transducer for imaging objects with nonplanar surfaces, a coupling medium must be used. To compensate for the refraction at the coupler-object interface, its shape must be known. Two methods for surface detection of convex objects in immersion are proposed, using the same linear array transducer for surface detection and for SAFT imaging. One is based on imaging technique and the other is based on the time-of-flight of the echoes on the captured ultrasonic signals. The accuracy and performance of the two methods are compared experimentally with an existing fast method called pitch-catch. The proposed methods produced smaller errors in part of the tested configurations, with a slower performance compared with the pitch-catch method. After the surface detection, in the SAFT image formation phase, the delays are calculated through a simple and fast proposed technique to determine the fastest paths, following the Fermat's principle. Images are formed for nine distinct array element groups, and then combined using the effective aperture technique. The results show that the developed methods allow interactive two-medium image formation on a general-purpose CPU.

A FEM-based method to determine the complex material properties of piezoelectric disks.

Numerical simulations allow modeling piezoelectric devices and ultrasonic transducers. However, the accuracy in the results is limited by the precise knowledge of the elastic, dielectric and piezoelectric properties of the piezoelectric material. To introduce the energy losses, these properties can be represented by complex numbers, where the real part of the model essentially determines the resonance frequencies and the imaginary part determines the amplitude of each resonant mode. In this work, a method based on the Finite Element Method (FEM) is modified to obtain the imaginary material properties of piezoelectric disks. The material properties are determined from the electrical impedance curve of the disk, which is measured by an impedance analyzer. The method consists in obtaining the material properties that minimize the error between experimental and numerical impedance curves over a wide range of frequencies. The proposed methodology starts with a sensitivity analysis of each parameter, determining the influence of each parameter over a set of resonant modes. Sensitivity results are used to implement a preliminary algorithm approaching the solution in order to avoid the search to be trapped into a local minimum. The method is applied to determine the material properties of a Pz27 disk sample from Ferroperm. The obtained properties are used to calculate the electrical impedance curve of the disk with a Finite Element algorithm, which is compared with the experimental electrical impedance curve. Additionally, the results were validated by comparing the numerical displacement profile with the displacements measured by a laser Doppler vibrometer. The comparison between the numerical and experimental results shows excellent agreement for both electrical impedance curve and for the displacement profile over the disk surface. The agreement between numerical and experimental displacement profiles shows that, although only the electrical impedance curve is considered in the adjustment procedure, the obtained material properties allow simulating the displacement amplitude accurately.

Experimental and numerical characterization of the sound pressure in standing wave acoustic levitators.

A novel method for predictions of the sound pressure distribution in acoustic levitators is based on a matrix representation of the Rayleigh integral. This method allows for a fast calculation of the acoustic field within the resonator. To make sure that the underlying assumptions and simplifications are justified, this approach was tested by a direct comparison to experimental data. The experimental sound pressure distributions were recorded by high spatially resolved frequency selective microphone scanning. To emphasize the general applicability of the two approaches, the comparative studies were conducted for four different resonator geometries. In all cases, the results show an excellent agreement, demonstrating the accuracy of the matrix method.

Evaluation of therapeutic ultrasound equipment performance.

The therapeutic ultrasound (US) is one of the resources mostly used by physiotherapists; however the use of uncalibrated equipment results in inefficient or even harmful therapies to the patient. In this direction, the objective of this study was to evaluate the performance and the procedures of utilization and maintenance of US in use in clinics and Physical-therapy offices. A questionnaire with questions related to the procedures applied in service during the use of therapeutic ultrasound was applied to physiotherapists. The performance of 31 equipment of 6 different brands and 13 different models was evaluated according to the IEC 61689 norm. The parameters measured were: acoustic power; effective radiating area (AER); non-uniformity ratio of the beam (RBN); maximum effective intensity; acoustic frequency of operation, modulation factor and wave form on pulsate mode. As for the questionnaires, it was evident that the professionals are not concerned about the calibration of the equipment. The results demonstrated that only 32.3% of the equipment were in accordance with the norms for the variables power and effective radiation area. The frequency analysis indicated that 20% of the 3MHz transducers and 12.5% of the 1MHz contemplated the norms. In the pulsate mode, 12.7% presented relation rest/duration inside allowed limits. A great variation of the ultrasonic field was observed on the obtained images, which presented beams not centered, sometimes with bifurcation of its apex. The results allow concluding that, although used in therapeutic sessions with the population, none of the equipment presents all the analyzed variables inside technical norms.

Ultrasonic material characterization using large-aperture PVDF receivers.

This work describes the use of a large-aperture PVDF receiver in the measurement of liquid density and composite material elastic constants. The density measurement of several liquids is obtained with accuracy of 0.2% using a conventional NDE emitter transducer and a 70-mm-diameter, 52-microm P(VDF-TrFE) membrane with gold electrodes. The determination of the elastic constants is based on the phase velocity measurement. Diffraction can lead to errors around 1% in velocity measurement when using alternatively the conventional pair of ultrasonic transducers (1-MHz frequency and 19-mm-diameter) operating in through-transmission mode, separated by a distance of 100 mm. This effect is negligible when using a pair of 10-MHz, 19-mm-diameter transducers. Nevertheless, the dispersion at 10 MHz can result in errors of about 0.5%, when measuring the velocity in composite materials. The use of an 80-mm diameter, 52-microm-thick PVDF membrane receiver practically eliminates the diffraction effects in phase velocity measurement. The elastic constants of a carbon fiber reinforced polymer were determined and compared with the values obtained by a tensile test.

Ultrasonic measurement of density of liquids flowing in tubes.

This paper presents the implementation of the relative reflection method for the ultrasonic measurement of the density of liquids, which may be flowing in pipelines, at different temperatures. This technique will be shown to be valid for large-diameter tubes containing flowing liquids. It employs a double-element transducer, consisting of a piezoelectric ceramic transmitter and a large aperture PVDF membrane receiver, separated by a polymethylmethacrylate buffer rod. Between the receiver and the liquid is a PMMA reference rod. The density is obtained from the reflection coefficient of the reference rod-liquid interface and the transit time between this interface and a reflector placed in the opposite wall of the tube. The DET is calibrated once to account for temperature effects. The calibration is incorporated during signal processing, so that the actual density measurement is temperature compensated. In testing this method, a system was implemented and measurements of several liquids, stationary and flowing in a pipeline, were conducted. The error of measurements obtained by this method for distilled water, tap water, castor oil, and ethanol, when compared to data in the literature or obtained by a pycnometer, is less than 1.5%