J.M. Rouvaen

Acoustical image reconstruction in parallel‐processing analog electronic systems

Ultrafast cardiac‐valve ultrasonic tomography requires parallel multichannel processing of received echoes. In parallel processing the level of secondary ’’ghost’’ images due to spatial undersampling is much higher than in slower series processors which use a selective field insonification. The paper describes a 20‐channel moving‐focus parallel‐processing analog electronic system, which is realized in our laboratory. It is shown analytically that the secondary‐image level is reduced and remains unchanged when the receiver angular aperture (aperture relative to distance) is limited and kept constant during the whole observation time.

Ultrasonic spectroscopy of composite materials

Abstract Composite materials are more and more used in high technology industries owing to their unique mechanical properties. For such heterogeneous and quasi-periodic materials, classical non-destructive evaluation (NDE) methods prove very unsatisfactory, and even often inapplicable. Our aim here is to investigate the particular case of ultrasonic NDE of fibre-reinforced composite materials. When using the classical A-scan echography method, it is difficult to interpret time diagrams. This is mainly due to the heterogeneous nature of the materials, the complexity of the diagrams being especially increased by the occurrence of distributed defects (like porosities). Using spectral analysis techniques (ultrasonic spectroscopy), information may be extracted from these intricate time diagrams, In particular, the occurrence of a selective absorption frequency in both the transmitted and reflected energy spectra is evident. This absorption phenomenon is clearly related to the material structure and periodicity and advantage may then be taken of its characteristics (centre frequency, bandwidth and relative depth of the absorption dip in the spectrum) to characterize the material and identify the presence of defects of porosities.

Measurement of the thickness of thin layers by ultrasonic interferometry

An ultrasonic interferometer working in a pulsed mode is described in this paper. It allows for the measurement of coating thicknesses as thin as 5 μm with a 5% precision over different substrates at a very high rate (up to 1000 times per second). The optimal conditions for this interferometric measurement are defined theoretically and the probe characteristics have been optimized technologically. This, together with the design of a very large frequency bandwidth (from 90 to 510 MHz) electronic setup, leads to interesting performances. The advantages of the system for achieving thickness measurements are discussed and comparisons are made with other systems.

Combining two radar techniques to implement a collision avoidance system

Many microwave collision avoidance radars have been tested using FMCW (frequency modulated continuous wave) radar and pulsed radar working around 10, 35 and 60 GHz. The aim of this paper is to describe an efficient radar system, termed bimodal, obtained by the combination of these two systems. The FMCW sensor is used for close detection and easier speed calculation, and the pulsed one for large distances and multi-target separation. The bimodal system has been used to improve the performances of the two sensors by retaining the advantages of each. The new radar is able to detect obstacles between 2 and 150 m with low emitted power. The whole system is controlled by a decision program unit. An accuracy of m has been achieved.

Simultaneous generation of longitudinal and shear bulk ultrasonic waves in solids

The ability to generate acoustic longitudinal and shear bulk waves in solid materials using a single transducer is a suitable feature required in several devices and systems involving ultrasonic waves. The problem addressed here is that of the, possibly wideband, multimode buffer delay line, usable, for example, in non-destructive evaluation applications. The solution considered for this purpose is to use rotated, Y-cut, single-layer or two-layer transducers. The theory pertinent to this kind of transducer is briefly recalled and the results of the numerical analysis using two different delay line materials and three different piezoelectric materials are reported in detail. One experimental realization is also described.

On a general treatment of acousto‐optic interactions in linear anisotropic crystals

In arbitrary anisotropic crystals exhibiting optical activity, the normal modes of light propagation are elliptically polarized. These modes are studied using a new gyration pseudotensor affecting the relative dielectric impermeability tensor, defined here for convenience. The propagation of light inside a first‐order acoustically perturbated medium is then considered. The equations are solved by using a normal‐mode expansion of the electric‐displacement vector, assuming a stationary regime of diffraction and slowly spatially varying amplitudes. A coupled first‐order partial derivative set is so obtained, which describes the acousto‐optic interactions. The case of acoustic plane bulk wave modulation is finally emphasized and all the classical approximate results, deduced from theories with more restricted areas of applications, are retrieved using suitable assumptions in our general equations.

Visualization of thick specimens using a reflection acoustic microscope

A reflection acoustic microscope may be used to image structural details under the apparent surface of a thick sample. To design such an apparatus, the geometrical parameters of the acoustic lenses must be carefully defined. The acoustic field distribution has been computed in the previous structure for this purpose. Some preliminary experimental results are also reported here.

Evaluation of coding's methods for the development of a radar sensor for localization and communication dedicated to guided transport

To explore possibilities of new exploitation modes of automatic guided transport, our laboratories designed formerly a sensor radar called DIREP (Detection et Identification des Rames En Panne: detection and identification of broken-down trains). This system is founded on the principle of a co-operative radar using a transponder inside targets. It is based on a numerical correlation receiver and has a very broad band from 50 to 100 MHz. which remains unexploited. Thus we propose to benefit from this band to establish high flow communication. The proposed system is made of microwave transmitting and receiving equipment fixed on each train, one ahead and other behind. The two trains exchange data and specific signals coded that make the system able: to deduce the distance between the trains; to identify the state of the train (broken-down or not); and to allow the transmission of pictures or audio-video records with a high data flow. The aim of this work is to propose technical solutions for multiplexing communication data and localization code in order to allow high data flow transfer. Simulations are computed to evaluate the systems performance: BER, computing time and complexity.

Signal-processing methods for analysing the structure of carbon-epoxy-resin composite materials

The non-destructive evaluation of composite materials is needed in order to check their mechanical viability. Several classical ultrasonic signal-processing methods may be used to perform this task, such as resonance spectroscopy and extraction of features from time echograms. Neural-network processing methods may allow the evaluation of structural parameters (ply thickness, interface quality and so on) from ultrasonic echograms. The aim of this work is to implement such techniques and to apply them to ultrasonic echograms obtained from theoretical models and industrial samples, so that meaningful comparisons may be performed in order to assess the validity of the neural-network method. The neural-network based method appears very flexible and less computationally intensive in the recall phase than are the other two methods. It gives a precision sufficient for most purposes and so constitutes a good candidate for evaluation of composite-material (and more generally multi-layer) structures.

Acoustic microscopy: a tool for non-destructive evaluation of ceramics

Abstract The focusing properties of the lenses used in the scanning acoustic microscope (SAM) have been widely studied for the particular case of surface imaging. When focusing into the bulk of a sample the aberration of the lens become negligible with respect to that induced by the sample surface traversal. A more detailed study allows evaluation of the acoustic field distribution, using an angular spectrum expansion. This formalism, previously developed by several independent authors, is used for optimizing the geometrical parameters of the acoustic lens for non-destructive testing of ceramic materials. The deduced theoretical performances are compared with experimental findings at a working frequency of 100 MHz. Characteristic images for some ceramic materials are presented, which give valuable information about the geometry of the defects and the structure of the matrix material.A comparison with alternative non-destructive testing methods is also given.