Analytical formulation of the fractal dimension of filtered stochastic signals

International audienceThe aim of this study was to investigate the effects of a linear filter on the regularity of a given stochastic process in terms of the fractal dimension. This general approach, described in a continuous time domain, is new and is characterized by its simplicity. The framework of this problem is general since it emerges when a fractal process undertakes a transformation, as is the case in denoising or measurement processes

Estimation of the blood Doppler frequency shift by a time-varying parametric approach

International audienceDoppler ultrasound is widely used in medical applications to extract the blood Doppler flow velocity in the arteries via spectral analysis. The spectral analysis of non-stationary signals and particularly Doppler signals requires adequate tools that should present both good time and frequency resolutions. It is well-known that the most commonly used time-windowed Fourier transform, which provides a time-frequency representation, is limited by the intrinsic trade-off between time and frequency resolutions. Parametric methods have then been introduced as an alternative to overcome this resolution problem. However, the performances of those methods deteriorate when high non-stationarities are present in the Doppler signal. For the purpose of accurately estimating the Doppler frequency shift, even when the temporal flow velocity is rapid (high non-stationarity), we propose to combine the use of the time-varying auto-regressive method and the (dominant) pole frequency. This proposed method performs well in the context where non-stationarities are very high. A comparative evaluation has been made between classical (FFT based) and auto-regressive (both block and recursive) algorithms. Among recursive algorithms we test an adaptive recursive method as well as a time-varying recursive method. Finally, the superiority of the time-varying parametric approach in terms of frequencies tracking and of delay on the frequency estimate is illustrated on both simulated and in vivo Doppler signals

Contribution à l'étude de la surveillance de l'usure des outils de coupe en usinage

Notre travail porte sur l'analyse des signaux vibratoires en utilisant des approches temporelles et fréquentielles. L'objectif de cette analyse est de pouvoir identifier l'influence de l'usure d'un outil coupant sur les vibrations produites lors d'un processus d'usinage, puis de déterminer quels paramètres extraits de ces signaux traduisent l'état de l'usinage. Le choix de ces paramètres est validé par une approche expérimentale fondée sur le contrôle direct de l'état de l'outil de coupe caractérisé par la largeur de l'usure en dépouille

Analyse temps-fréquence appliquée aux signaux vibratoires relevés en tournage

En vue de suivre l'usure des outils de coupe en usinage à partir de signaux vibratoires, nous proposons dans cet article une méthode fondée sur une analyse conjointe en temps et en fréquence (Transformée de Fourier à Court Terme, Transformée en Ondelettes) des signatures vibratoires mesurées en tournage. Cette technique d'analyse permet la détermination de plusieurs indicateurs d'usure (niveau vibratoire, détection d'événements brusques, fréquences caractéristiques, élargissement spectrale...). L'évolution de ces descripteurs indique les points de transition de l'état d'usure de l'outil de coupe durant ses trois phases de vie : rodage, stabilisation et vieillissement

Commande optimale appliquée aux systèmes d'imagerie ultrasonore

Les systèmes d imagerie médicale ultrasonore ont considérablement amélioré le diagnostic clinique par une meilleure qualité des images grâce à des systèmes plus sensibles et des post-traitements. La communauté scientifique de l imagerie ultrasonore a consenti à un très grand effort de recherche sur les posttraitements et sur le codage de l excitation sans s intéresser, outre mesure, aux méthodes de commandeoptimale. Ce travail s est donc légitimement tourné vers les méthodes optimales basées sur l utilisationd une rétroaction de la sortie sur l entrée. Pour rendre applicable ces méthodes, ce problème complexe decommande optimale a été transformé en un problème d optimisation paramétrique sous-optimal et plussimple. Nous avons appliqué ce principe au domaine de l imagerie ultrasonore : l échographie, l imagerieharmonique native et l imagerie harmonique de contraste avec ou sans codage de la commande.La simplicité de l approche nous a permis, par une modification de la fonction de coût, de l adapter àl imagerie harmonique. Cette adaptation montre que la méthode peut être appliquée à l imagerie ultrasonoreen générale.Medical ultrasound imaging systems have greatly improved the clinical diagnosis by improvingthe image quality thanks to more sensitive systems and post-processings. The scientific community has madea great effort of research on post-processing and on encoding the excitation. The methods of the optimalcontrol have been neglected. Our work has focused on the optimal methods based on the feedback fromoutput to input. We have transformed the complex problem of optimal control into an easier suboptimalparametric problem. We apply the principle of optimal control to the ultrasound imaging, the ultrasoundharmonic imaging and to the constrast harmonic imaging with or without encoding.The simplicity of the method has allowed us to adapt it to harmonic imaging by a change in the costfunction. This adaptation shows that our method can usually be applied to the ultrasound imaging.TOURS-Bibl.électronique (372610011) / SudocSudocFranceF

Sur la structure cascade-du filtrage a plusieures entrees

Le filtrage de Wiener à n entrées - en prenant m entrées de référence (1<m<n) - caractérise un système linéaire et invariant dans le temps dont la synthèse peut Stre envisagée selon diverses configurations. Nous montrons, dans ce travail, que les structures du type cascade jouissent de propriétés intéressantes du point de vue de l'erreur d'estimation des fonctions de transfert élémentaires. Nous présentons ensuite, une structure "cascade récursive" qui améliore les performances du filtre au dépend d'un algorithme plus complexe et plus long. Enfin, les conditions de mise en oeuvre de ces configurations sont exposées

Fast techniques for time delay and Doppler estimation

International audienceThis paper considers the problem of estimating time delay and Doppler shift based on the maximum likelihood criteria. Joint estimation of time delay and Doppler stretch of a random signal received by two (or more) sensors is required in many signal processing applications. This problem can be viewed as the determination of the effects of a time-varying channel on the emitted signal. It is shown that discrete time-frequency (time-scale) techniques provide the correct system characterization. Theses techniques are simple, fast and efficient. Moreover, in areas where the measurements are repeatable, such as ultrasonic imaging and nondestructive evaluation, the process of averaging can be used to improve the SNR. In order to implement the simultaneous estimation of time delay and Doppler coefficient according to the maximum likelihood criterion, two practical problems are investigated: the interpolation method used to obtain a fine resolution of the estimates and the scaling procedure which is not a trivial problem

Distance Learning in Higher Education in France during the COVID-19 Pandemic Chapter 4

International audienceSince the beginning of the COVID-19 pandemic, students around the world have seen their schooling completely disrupted. Their teachers have had to reorganize in a hurry and even develop digital skills to be able to teach their classes synchronously but at a distance. Thanks to strong political incentives, both at national and university level, many tools have been massively deployed: digital learning management systems (e.g. Moodle), collaborative digital platforms (e.g. Google Meets, Microsoft Teams and Zoom) and even social networks (e.g. Facebook and Twitter). While this unprecedented health crisis has demonstrated the essential role of digital technologies in higher education, major questions arise regarding the quality of distance education, the learning process itself and the evaluation of knowledge and skills acquired at a distance. In this policy paper, about a hundred students from engineering schools and master's degrees in France were followed during several periods of confinement in order to provide some answers to these questions. The performance of the students in this very particular context is analyzed. The feelings of students, but also those of teachers, are also discussed, both on the use of digital technologies and on the digital transformation of higher educatio

Détection de ruptures utilisant une modélisation paramétrique adaptative dans un contexte différentiel

Décision et Interprétation Non Stationnaire, GDR ISIS, CNRS 199