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An efficient numerical method for time domain electromagnetic wave propagation in co-axial cables
No full textInternational audienceIn this work we construct an efficient numerical method to solve 3D Maxwell's equations in coaxial cables. Our strategy is based upon an hybrid explicit-implicit time discretization combined with edge elements on prisms and numerical quadrature. One of the objective is to validate numerically generalized Telegrapher's models that are used to simplify the 3D Maxwell equations into a 1D problem. This is the object of the second part of the article
Effects of Hot-Water Aging on the Compression Properties of E-Glass/Epoxy Composites at Varying Strain Rates
No full textInternational audienceThe effects of hot-water aging on the quasi-static and dynamic compression properties of unidirectional E-glass/epoxy laminates were investigated. E-glass/epoxy specimens were aged in water at 60°C for 4900 h and then aged and unaged specimens were tested in compression at a rate of 1.3·10–3 s–1 and by a split Hopkinson pressure bar apparatus at varying strain rates. Their diffusion behavior was successfully described by the two-stage model whose parameters were found by the nonlinear regression method. The strain-rate-sensitivity of aged and unaged E-glass-reinforced epoxy specimens in the longitudinal direction was studied. Their dynamic and static compression properties were compared for specimens with the same dimensions. Empirical models were proposed to predict dynamic properties as functions of strain rate. SEM micrographs showed a low degradation of the resin matrix and fiber-matrix interface at hot-water aging for a time up to 4900 h
AutoExpe.jl : Ne coder que les méthodes de résolution
No full textInternational audienceAutoExpe.jl est un package julia permettant d'automatiser la réalisation d'expérimentations numériques et la génération de tableaux de résultats afin de se concentrer sur l'essentiel : l'implémentation des méthodes de résolution et la comparaison de leurs performances.Lien : https://github.com/ZacharieALES/AutoExp
Interval Extension of Neural Network Models for the Electrochemical Behavior of High-Temperature Fuel Cells
No full textInternational audienceIn various research projects, it has been demonstrated that feedforward neural network models (possibly extended toward dynamic representations) are efficient means for identifying numerous dependencies of the electrochemical behavior of high-temperature fuel cells. These dependencies include external inputs such as gas mass flows, gas inlet temperatures, and the electric current as well as internal fuel cell states such as the temperature. Typically, the research on using neural networks in this context is focused only on point-valued training data. As a result, the neural network provides solely point-valued estimates for such quantities as the stack voltage and instantaneous fuel cell power. Although advantageous, for example, for robust control synthesis, quantifying the reliability of neural network models in terms of interval bounds for the network’s output has not yet received wide attention. In practice, however, such information is essential for optimizing the utilization of the supplied fuel. An additional goal is to make sure that the maximum power point is not exceeded since that would lead to accelerated stack degradation. To solve the data-driven modeling task with the focus on reliability assessment, a novel offline and online parameterization strategy for interval extensions of neural network models is presented in this paper. Its functionality is demonstrated using real-life measured data for a solid oxide fuel cell stack that is operated with temporally varying electric currents and fuel gas mass flows
Receptive field estimation in large visual neuron assemblies using a super-resolution approach
No full textInternational audienceComputing the spike-triggered average (STA) is a simple method to estimate linear receptive fields (RFs) in sensory neurons. For random, uncorrelated stimuli the STA provides an unbiased RF estimate, but in practice, white noise at high resolution is not an optimal stimulus choice as it usually evokes only weak responses. Therefore, for a visual stimulus, images of randomly modulated blocks of pixels are often used. This solution naturally limits the resolution at which an RF can be measured. Here we present a simple super-resolution technique that can be overcome these limitations. We define a novel stimulus type, the shifted white noise (SWN), by introducing random spatial shifts in the usual stimulus in order to increase the resolution of the measurements. In simulated data we show that the average error using the SWN was 1.7 times smaller than when using the classical stimulus, with successful mapping of 2.3 times more neurons, covering a broader range of RF sizes. Moreover, successful RF mapping was achieved with brief recordings of light responses, lasting only about one minute of activity, which is more than 10 times more efficient than the classical white noise stimulus. In recordings from mouse retinal ganglion cells with large scale multi-electrode arrays, we successfully mapped 21 times more RFs than when using the traditional white noise stimuli. In summary, randomly shifting the usual white noise stimulus significantly improves RFs estimation, and requires only short recordings
Robust Feedback Control for Discrete-Time Systems Based on Iterative LMIs with Polytopic Uncertainty Representations Subject to Stochastic Noise
No full textInternational audienceThis paper deals with the design of linear observer-based state feedback controllers with constant gains for a class of nonlinear discrete-time systems in the form of a quasi-linear representation in presence of stochastic noise. For taking into account nonlinearities in the design of linear observer-based state feedback controllers, a polytopic modeling approach is investigated. An optimization problem is formulated to reduce the sensitivity of the controlled system towards stochastic input, state, and output noise with a predefined covariance. Due to the nonlinearities, the separation principle does not hold, thus, the controller and the observer have to be designed simultaneously. For this purpose, a Lyapunov-based method is used, which provides, in addition to the controller and observer gains, a stability proof for the nonlinear closed loop in a predefined polytopic domain. In general, this leads to nonlinear matrix inequalities. To solve these nonlinear matrix inequalities efficiently, we propose an approach based on linear matrix inequalities (LMIs) with a superposed iteration rule. When using this iterative LMI approach, a minimization task can be solved additionally, which desensitizes the closed loop to stochastic noise. The proposed method additionally enables the consideration of different linear closed loop structures by a unified Lyapunov-based framework. The efficiency of the proposed approach is demonstrated and compared with a classical LQG approach for a nonlinear overhead traveling crane
Distributed Personalized Gradient Tracking with Convex Parametric Models
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Transverse Cracking Induced Acoustic Emission in Carbon Fiber-Epoxy Matrix Composite Laminates
No full textInternational audienceTransverse cracking induced acoustic emission in carbon fiber/epoxy matrix composite laminates is studied both experimentally and numerically. The influence of the type of sensor, specimen thickness and ply stacking sequence is investigated. The frequency content corresponding to the same damage mechanism differs significantly depending on the sensor and the stacking sequence. However, the frequency centroid does not wholly depend on the ply thickness except for the inner ply crack and a sensor located close enough to the crack. Outer ply cracking exhibits signals with a low-frequency content, not depending much on the ply thickness, contrary to inner ply cracking, for which the frequency content is higher and more dependent on the ply thickness. Frequency peaks and frequency centroids obtained experimentally are well captured by numerical simulations of the transverse cracking induced acoustic emission for different ply thicknesses
Spectral theory for Maxwell's equations at the interface of a metamaterial. Part II: Limiting absorption, limiting amplitude principles and interface resonance
No full textInternational audienceThis paper is concerned with the time-dependent Maxwell's equations for a plane interface between a negative material described by the Drude model and the vacuum, which fill, respectively, two complementary half-spaces. In a first paper, we have constructed a generalized Fourier transform which diagonalizes the Hamiltonian that represents the propagation of transverse electric waves. In this second paper, we use this transform to prove the limiting absorption and limiting amplitude principles, which concern, respectively, the behavior of the resolvent near the continuous spectrum and the long time response of the medium to a time-harmonic source of prescribed frequency. This paper also underlines the existence of an interface resonance which occurs when there exists a particular frequency characterized by a ratio of permittivities and permeabilities equal to −1 across the interface. At this frequency, the response of the system to a harmonic forcing term blows up linearly in time. Such a resonance is unusual for wave problem in unbounded domains and corresponds to a non-zero embedded eigenvalue of infinite multiplicity of the underlying operator. This is the time counterpart of the ill-posdness of the corresponding harmonic problem
Accélération matérielle de la vérification de sûreté et vivacité sur des architectures reconfigurables
No full textModel-Checking is an automated technique used in industry for verification, a major issue in the design of reliable systems, where performance and scalability are critical. Swarm verification improves scalability through a partial approach based on concurrent execution of randomized analyses. Reconfigurable architectures promise significant performance gains. However, existing work suffers from a monolithic design that hinders the exploration of reconfigurable architecture opportunities. Moreover, these studies are limited to safety verification. To adapt the verification strategy to the problem, this thesis first proposes a hardware verification framework, allowing to gain, through a modular architecture, a semantic and algorithmic genericity, illustrated by the integration of 3 specification languages and 6 algorithms. This framework allows efficiency studies of swarm algorithms to obtain a scalable safety verification core. The results, on a high-end FPGA, show gains of an order of magnitude compared to the state-of-the-art. Finally, we propose the first hardware accelerator for safety and liveness verification. The results show an average speed-up of 4875x compared to software.Le Model-Checking est une technique automatisée, utilisée dans l’industrie pour la vérification, enjeu majeur pour la conception de systèmes fiables, cadre dans lequel performance et scalabilité sont critiques. La vérification swarm améliore la scalabilité par une approche partielle reposant sur l’exécution concurrente d’analyses randomisées. Les architectures reconfigurables promettent des gains de performance significatifs. Cependant, les travaux existant souffrent d’une conception monolithique qui freine l’exploration des opportunités des architectures reconfigurable. De plus, ces travaux sont limités a la verification de sûreté. Pour adapter la stratégie de vérification au problème, cette thèse propose un framework de vérification matérielle, permettant de gagner, au travers d’une architecture modulaire, une généricité sémantique et algorithmique, illustrée par l’intégration de 3 langages de spécification et de 6 algorithmes. Ce cadre architectural permet l’étude de l’efficacité des algorithmes swarm pour obtenir un cœur de vérification de sûreté scalable. Les résultats, sur un FPGA haut de gamme, montrent des gains d’un ordre de grandeur par rapport à l’état de l’art. Enfin, on propose le premier accélérateur matériel permettant la vérification des exigences de sûreté et de vivacité. Les résultats démontrent un facteur d’accélération moyen de 4875x par rapport au logiciel