LUNAM University

GER, France

LUNAM University

GER, France
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Cord A.,IFSTTAR | Chambon S.,LUNAM University
Computer-Aided Civil and Infrastructure Engineering | Year: 2012

The state of roads is continuously degrading due to meteorological conditions, ground movements, and traffic, leading to the formation of defects, such as grabbing, holes, and cracks. In this article, a method to automatically distinguish images of road surfaces with defects from road surfaces without defects is presented. This method, based on supervised learning, is generic and may be applied to all type of defects present in those images. They typically present strong textural information with patterns that show fluctuations at small scales and some uniformity at larger scales. The textural information is described by applying a large set of linear and nonlinear filters. To select the most pertinent ones for the current application, a supervised learning based on AdaBoost is performed. The whole process is tested both on a textural recognition task based on the VisTex image database and on road images collected by a dedicated road imaging system. A comparison with a recent cracks detection algorithm from Oliveira and Correia demonstrates the proposed method's efficiency. © 2011 Computer-Aided Civil and Infrastructure Engineering.

Janot A.,ONERA | Vandanjon P.-O.,LUNAM University | Gautier M.,University of Nantes
IEEE Transactions on Control Systems Technology | Year: 2014

This paper deals with the important topic of industrial robot identification. The usual identification method is based on the inverse dynamic identification model and the least squares technique. This method has been successfully applied on several industrial robots. Good results can be obtained, provided a well tuned derivative band-pass filtering of joint positions is used to calculate the joint velocities and accelerations. However, one cannot be sure whether or not the band-pass filtering is well tuned. An alternative is the instrumental variable (IV) method, which is robust to data filtering and is statistically optimal. In this paper, a generic IV approach suitable for robot identification is proposed. The instrument set is the inverse dynamic model built from simulated data calculated from simulation of the direct dynamic model. The simulation is based on previous estimates and assumes the same reference trajectories and the same control structure for both actual and simulated robots. Finally, gains of the simulated controller are updated according to IV estimates to obtain a valid instrument set at each step of the algorithm. The proposed approach validates the inverse and direct dynamic models simultaneously, is not sensitive to initial conditions, and converges rapidly. Experimental results obtained on a six-degrees-of-freedom industrial robot show the effectiveness of this approach: 60 dynamic parameters are identified in three iterations. © 2013 IEEE.

Gautier M.,University of Nantes | Janot A.,ONERA | Vandanjon P.-O.,LUNAM University
IEEE Transactions on Control Systems Technology | Year: 2013

Offline robot dynamic identification methods are mostly based on the use of the inverse dynamic model, which is linear with respect to the dynamic parameters. This model is sampled while the robot is tracking reference trajectories that excite the system dynamics. This allows using linear least-squares techniques to estimate the parameters. The efficiency of this method has been proved through the experimental identification of many prototypes and industrial robots. However, this method requires the joint force/torque and position measurements and the estimate of the joint velocity and acceleration, through the bandpass filtering of the joint position at high sampling rates. The proposed new method called DIDIM requires only the joint force/torque measurement, which avoids the calculation of the velocity and acceleration by bandpass filtering of the measured position. It is a closed-loop output error method where the usual joint position output is replaced by the joint force/torque. It is based on a closed-loop simulation of the robot using the direct dynamic model, the same structure of the control law, and the same reference trajectory for both the actual and the simulated robot. The optimal parameters minimize the 2-norm of the error between the actual force/torque and the simulated force/torque. This is a nonlinear least-squares problem which is dramatically simplified using the inverse dynamic model to obtain an analytical expression of the simulated force/torque, linear in the parameters. A validation experiment on a two degree-of-freedom direct drive rigid robot shows that the new method is efficient. © 2012 IEEE.

This paper provides a modal solution for the three-dimensional modeling of Lamb and SH waves excited by sources of arbitrary shape. This solution is applicable to elastic and viscoelastic plates, in the far-field as well as in the near-field regions, under the assumption of transverse isotropy about the thickness direction. The theoretical developments are conducted based on a semi-analytical finite element formulation. This formulation yields a one-dimensional modal problem, fast from a computational point of view, and allows to readily handle heterogeneous materials having depth-varying properties (multilayered, piecewise or continuously varying, functionally graded). The modal solution is shown to be expressed in terms of Hankel functions of multiple order thanks to a proper application of inverse transforms and Cauchy residue calculus. The link between the proposed formulation and a fully analytical approach is discussed. The solution of this paper is then successfully compared to literature results and degenerates to the point source case. Formulas are presented to calculate point source excitabilities from lines sources. These formulas remain valid for non-propagating modes, viscoelastic materials and account for the near-field contribution. Finally, the example of a viscoelastic bilayer waveguide excited by a rectangular source is considered in order to check the theoretical results. © 2014 Elsevier B.V.

Guillaume G.,LUNAM University | Picaut J.,LUNAM University
Journal of Sound and Vibration | Year: 2013

An absorbing layer formulation for transmission line matrix modeling is proposed. The approach consists in attenuating the incident pulse propagating toward the absorbing layer only, using an attenuation factor which gradually decreases as the sound wave propagates along the absorbing medium. The formulation of the damping function followed by the attenuation factor along the absorbing layer is depicted and discussed. The efficiency of the present formulation is validated by comparison with another absorbing layer model and virtual boundary conditions proposed in the literature. Numerical simulations are also given in order to evaluate the effects of both the attenuation factor and the depth on the absorbing layer efficiency. As expected, results are consistent with absorbing layer implementation in other numerical methods; firstly, the attenuation at the entrance of the absorbing layer must be gentle, and secondly the efficiency increases with the layer depth. Lastly, it is shown that the unwanted reflection seems to vanish over the time when increasing the layer depth, meaning that reflections continuously occur within the absorbing layer and not on the geometrical limits of the absorbing layer. Although the approach is dedicated to outdoor sound propagation modeling (only an example on urban acoustics application is given), the proposed formulation of absorbing layers can be applied in other domains of acoustics. However, its application in shielded areas should be avoided because unwanted reflections due to an insufficient attenuation can be significant in comparison with the ambient noise in such quiet environments. © 2013 Elsevier Ltd. All rights reserved.

Treyssede F.,LUNAM University
Journal of Computational Physics | Year: 2016

A numerical method is proposed to compute high-frequency low-leakage modes in structural waveguides surrounded by infinite solid media. In order to model arbitrary shape structures, a waveguide formulation is used, which consists of applying to the elastodynamic equilibrium equations a space Fourier transform along the waveguide axis and then a discretization method to the cross-section coordinates. However several numerical issues must be faced related to the unbounded nature of the cross-section, the number of degrees of freedom required to achieve an acceptable error in the high-frequency regime as well as the number of modes to compute. In this paper, these issues are circumvented by applying perfectly matched layers (PML) along the cross-section directions, a high-order spectral element method for the discretization of the cross-section, and an eigensolver shift suited for the computation of low-leakage modes. First, computations are performed for an embedded cylindrical bar, for which literature results are available. The proposed PML waveguide formulation yields good agreement with literature results, even in the case of weak impedance contrast. Its performance with high-order spectral elements is assessed in terms of convergence and accuracy and compared to traditional low-order finite elements. Then, computations are performed for an embedded square bar. Dispersion curves exhibit strong similarities with cylinders. These results show that the properties of low-leakage modes observed in cylindrical bars can also occur in other types of geometry. © 2016 Elsevier Inc.

Artoni R.,LUNAM University | Richard P.,LUNAM University
Physical Review Letters | Year: 2015

We report numerical simulations on granular shear flows confined between two flat but frictional sidewalls. Novel regimes differing by their strain localization features are observed. They originate from the competition between dissipation at the sidewalls and dissipation in the bulk of the flow. The effective friction at sidewalls is characterized (effective friction coefficient and orientation of the friction force) for each regime, and its interdependence with slip and force fluctuations is pointed out. We propose a simple scaling law linking the slip velocity to the granular temperature in the main flow direction which leads naturally to another scaling law for the effective friction. © 2015 American Physical Society.

Treyssede F.,LUNAM University | Laguerre L.,LUNAM University
Journal of the Acoustical Society of America | Year: 2013

In the analysis of elastic waveguides, the excitability of a given mode is an important feature defined by the displacement-force ratio. Useful analytical expressions have been provided in the literature for modes with real wavenumbers (propagating modes in lossless waveguides). The central result of this paper consists in deriving a generalized expression for the modal excitability valid for modes with complex wavenumbers (lossy waveguides or non-propagating modes). The analysis starts from a semi-analytical finite element method and avoids solving the left eigenproblem. Analytical expressions of modal excitability are then deduced. It is shown that the fundamental orthogonality property to be used indeed corresponds to a form of Auld's real orthogonality relation, involving both positive- and negative-going modes. Finally, some results obtained from the generalized excitability are compared to the approximate lossless expression. © 2013 Acoustical Society of America.

Lumbroso D.,LUNAM University | Lumbroso D.,HR Wallingford | Gaume E.,LUNAM University
Journal of Hydrology | Year: 2012

Direct current meter measurements are rarely available for extreme flash floods. Corresponding discharges are generally estimated using so-called " indirect" techniques such as the slope - area method. These methods are based on empirical hydraulic formulae that typically use Manning's equation, and have been calibrated and also widely tested for flow conditions that differ significantly from those encountered during flash floods. Recent work conducted in Europe, as part of the HYDRATE research project and other studies, has shown that the use of these formulae and their associated tabulated roughness values available in current guidance documents, without further verification, can lead to over-estimates of peak discharges in the case of flash floods. After having discussed the limitations of indirect methods based on Manning's formula, the paper illustrates how the uncertainty in indirect discharge estimates can be reduced through the analysis of various types of data that can be collected during post-event surveys and through consistency checks. Based on a review of current literature and on recent flash flood studies, this paper proposes simple guidelines to assist practitioners in estimating extreme discharges during post-event surveys. © 2011 Elsevier B.V.

Treyssede F.,LUNAM University
Journal of Sound and Vibration | Year: 2016

Elastic guided waves are of interest for the non-destructive evaluation of cables. Such structures are usually helical, multiwired and highly prestressed, which greatly complicates the understanding of wave propagation from a theoretical point of view. A remarkable feature is the occurrence of a missing frequency band in experimental time-frequency diagrams, sometimes referred to as notch frequency in the literature. The central frequency of this band increases under tensile loads. Recently, a numerical model has been proposed to compute the dispersion curves of prestressed helical seven-wire waveguides. Results have shown that the notch frequency indeed corresponds to a curve veering phenomenon between two longitudinal-like modes and that the increase of the notch under tensile loads is mainly due to interwire contact mechanisms. The main goal of this paper is to highlight the origin of this curve veering phenomenon, which is still unexplained up to the author's knowledge. This paper also provides further results which allow us to clarify the accuracy of numerical solutions as well as the influence of contact assumptions. First, the static part of the model, necessary to compute the prestress state including contact effects, is checked from reference analytical solutions. Owing to the importance of contact, the accuracy of results is discussed both in statics and in dynamics. The influence of slip contact conditions is outlined. Then, some numerical tests are conducted by varying the Poisson coefficient and the helix lay angle. These tests allows us to find out that the radial displacement constraint imposed on peripheral wires by the central one in the contact regions constitutes the main source of curve veering. More precisely, it is shown that a similar curve veering does occur for an uncoupled single peripheral wire when constrained by a radially blocked motion localized in its contact zone. Indeed, such a localized boundary condition completely breaks the circular symmetry of the wire cross-section, yielding coupling between longitudinal, flexural and torsional motion together with curve veering phenomena. © 2016 Elsevier Ltd.

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