Experimental observation of exceptional points in coupled pendulums

The concept of exceptional point (EP) is demonstrated experimentally in the case of a simple mechanical system consisting of two coupled pendulums. Exceptional points correspond to specific values of the system parameters that yield defective eigenvalues. These spectral singularities which are typical of non-Hermitian system means that both the eigenvalues and their associated eigenvectors coalesce. The existence of an EP requires an adequate parameterization of the dynamical system. For this aim, the experimental device has been designed with two controllable parameters which are the length of one pendulum and a viscous-like damping which is produced via electromagnetic induction. Thanks to the observation of the free response of the coupled pendulums, most EP properties are experimentally investigated, showing good agreements with theoretical considerations. In contrast with many studies on EPs, mainly in the field of physics, the novelty of the present work is that controllable parameters are restricted to be real-valued, and this requires the use of adequate search algorithms. Furthermore, it offers the possibility of exploiting the existence of EPs in time-domain dynamic problems

A high-wavenumber boundary-element method for an acoustic scattering problem

In this paper we show stability and convergence for a novel Galerkin boundary element method approach to the impedance boundary value problem for the Helmholtz equation in a half-plane with piecewise constant boundary data. This problem models, for example, outdoor sound propagation over inhomogeneous flat terrain. To achieve a good approximation with a relatively low number of degrees of freedom we employ a graded mesh with smaller elements adjacent to discontinuities in impedance, and a special set of basis functions for the Galerkin method so that, on each element, the approximation space consists of polynomials (of degree $\nu$) multiplied by traces of plane waves on the boundary. In the case where the impedance is constant outside an interval $[a,b]$, which only requires the discretization of $[a,b]$, we show theoretically and experimentally that the $L_2$ error in computing the acoustic field on $[a,b]$ is ${\cal O}(\log^{\nu+3/2}|k(b-a)| M^{-(\nu+1)})$, where $M$ is the number of degrees of freedom and $k$ is the wavenumber. This indicates that the proposed method is especially commendable for large intervals or a high wavenumber. In a final section we sketch how the same methodology extends to more general scattering problems

Plant Responses to Extreme Climatic Events: A Field Test of Resilience Capacity at the Southern Range Edge

The expected and already observed increment in frequency of extreme climatic events may result in severe vegetation shifts. However, stabilizing mechanisms promoting community resilience can buffer the lasting impact of extreme events. The present work analyzes the resilience of a Mediterranean mountain ecosystem after an extreme drought in 2005, examining shoot-growth and needle-length resistance and resilience of dominant tree and shrub species (Pinus sylvestris vs Juniperus communis, and P. nigra vs J. oxycedrus) in two contrasting altitudinal ranges. Recorded high vegetative-resilience values indicate great tolerance to extreme droughts for the dominant species of pine-juniper woodlands. Observed tolerance could act as a stabilizing mechanism in rear range edges, such as the Mediterranean basin, where extreme events are predicted to be more detrimental and recurrent. However, resistance and resilience components vary across species, sites, and ontogenetic states: adult Pinus showed higher growth resistance than did adult Juniperus; saplings displayed higher recovery rates than did conspecific adults; and P. nigra saplings displayed higher resilience than did P. sylvestris saplings where the two species coexist. P. nigra and J. oxycedrus saplings at high and low elevations, respectively, were the most resilient at all the locations studied. Under recurrent extreme droughts, these species-specific differences in resistance and resilience could promote changes in vegetation structure and composition, even in areas with high tolerance to dry conditions.This study was supported by Ministerio de Ciencia e Innovación (Spanish Government) Projects CGL2008-04794 and CGL2011-29910 to R.Z., and by grant FPU-MEC (AP2005-1561) to A. H

Impact de la robotisation des machines sur l'activité d'un opérateur

Adapting the robot to the man with whom it interacts in order to improve working conditions and efficiency is the final objective of this work. In order to achieve this it is essential to understand Man by watching him work and interact with the robot in order to adapt the said robot by modifying its mode of interaction and its degree of automation. Three research questions are addressed in this article: Is human activity influenced by its interaction with the robot? How does it cooperate in the event of a blocking event? And is the robot's perception of the human being altered in these cases? 23 subjects participated in an experiment using a tracking robot as part of a parcel collection task. Four indicators based on the analysis of the subjects' activity were constructed. They quantify the operator's strategy and cooperation with the robot. The results show that the increase in the degree of autonomy of the assistant has led to a change in the leaders' strategy, that the type of cooperation used by the leader is identifiable by the analysis of his activity and that the evolution of the robot's level of acceptance is linked to the degree of autonomy of the robot. The level of cooperation between a person and a robot is therefore quantifiable based on the observation of their behaviour. The real-time adaptation of the robot to the person co-operating with it is within the realm of feasibility.

Véhicule autonome en environnement extérieur: une approche sûre basée sur modèle numérique de terrain.

International audienceThis paper is about preservation of physical integrity of mobile robot for real outdoor applications encountered in agricultural field. Unlike other approaches that try to detect and to avoid obstacle, we consider the Allowable Speed Grid. This grid depends on the vehicle capabilities, its dynamics constraints, its speed and the 3D rendering of the environment. We made a dynamic study to estimate the acceleration of inertial center of vehicle taking into account geometry of the environment and the trajectory to follow. Then, we establish a perception strategy to efficiently use the computing resources of the navigation system. The objective is to focus the attention where the danger may potentially appear. Then, we propose a decision process which consists to select the speed by optimising a criteria linked to the mission objective

Modélisation de l'évaluation sensorielle du degré de croûtage de saucissons par l'expert par approche symbolique floue

In the field of food industry, the control of the sensory quality of the foodstuffs constitutes a key issue. In this paper we propose a method to establish a link between sensors and human perception of quality of foodstuffs through an appropriate model. More particularly the problem tackled is the reproduction in an instrumental way of expert sensory evaluation concerning the crusting of the sausage. First, the reasoning processes carried out by the experts to determine the degree of sausage crusting from an image of a slice were collected. Three explanatory features observed on the image were considered by experts: lengths of the black zones and of the dark red zones on the periphery of the slice, and color of the center of the slice. These three sensory features were associated with three physical quantities resulting from processing carried out on the luminance component of the image of the slice acquired by a camera. Second, the expertise on the evaluation of the crusting has been capitalized in the form of fuzzy symbolic rules linking classes of the three features and classes of crusting. The software instrument implemented on a PC gives convincing results, very close to the evaluations provided by the experts.Dans les procédés alimentaires, le contrôle de la qualité sensorielle des produits est clé. Dans ce papier nous proposons une méthode pour établir le lien entre les capteurs et la perception visuelle humaine des produits. Plus particulièrement nous traitons de la reproduction par mesure instrumentale de l'évaluation sensorielle visuelle des experts du degré de croûtage du saucisson sec. Dans une première étape le raisonnement des experts est collecté. Sur la base de cette expertise, un premier module de l'algorithme construit consiste à reproduire par analyse d'image les invariants utilisés par les experts pour construire la mesure de croûtage. Le deuxième module est dédié à la fusion des invariants pour reconstruire la mesure de croûtage à l'aide de fonctions issues de la théorie des ensembles flous. L'outil ainsi construit donne de bons résultats, très proches de l'évaluation des experts