Pneumatic actuator control: Solution based on adaptive twisting and experimentation

An adaptive version of the twisting algorithm is proposed, which actually presents a new second-order sliding-mode algorithm. Due to the dynamic adaptation of the gains the controller design does not require complete information on the bounds of uncertainties and perturbations. It automatically decreases the gains and respectively also the dangerous oscillations due to a too large discontinuous-control magnitude. Thus, both the performance and the accuracy of the closed-loop system are improved. In order to show the feasibility of the approach, the methodology is successfully applied to control the position of a pneumatic actuator in an experimental setup.     

Failure mechanisms in granular media: a discrete element analysis

This paper attempts to numerically validate the concept of diffuse failure using a discrete element method. First, the theoretical background is reviewed, and it is shown how the kinetic energy of a system, initially at rest after a loading history, is likely to abruptly increase under the effect of disturbances. The vanishing of the second-order work thus constitutes a basic ingredient, related to both the pioneering work of Hill (J Mech Phys Solids (6):236–249, 1958) and the notion of bifurcation applied to geomechanics (Vardoulakis and Sulem in Bifurcation analysis in geomechanics, Chapman & Hall Publisher, London, 1995). Discrete numerical simulations were performed on homogeneous three-dimensional specimens, and the three basic conditions that must be satisfied in order to observe a failure mechanism are numerically checked. Finally, this work illustrates the phenomena that are likely to affect in situ slopes, for instance, when the loading (due to weather conditions or human activities) meets the three basic conditions for a failure mechanism to develop.     

Defect analysis and excitons diffusion in undoped homoepitaxial diamond films after polishing and oxygen plasma etching

Several 65 μm thick epitaxial diamond films prepared on (100) Ib substrates by high power pulsed microwave plasma assisted chemical vapour deposition (HP-MPCVD) are studied as a function of surface treatments by cathodoluminescence (CL) and photoluminescence (PL) spectroscopies. They are either asgrown, or polished, or etched by a microwave oxygen plasma, or after applying subsequently the two last processes. In CL spectra, bands due to defects occur at 2.3 eV, 3.07 eV, 3.7 eV and 4.7 eV, the last one being specific of polished surfaces, with nearly no contrast in the luminescence image. A fundamental result consists in demonstrating that the defects induced by polishing can be removed by oxygen plasma etching. Additionally, in order to assess how luminescence spectra originate from a peculiar depth or not, a bevelled sample is studied. PL spectra are acquired on the sample side while CL spectra are measured at several points on the bevel top side till to the Ib substrate. Comparison of the two sets of result show that the H3 signal originates from the Ib substrate even if it is present in the CL spectra of the film. An analysis of the change in the intensity of the TO free exciton line, defect bands and H3 signal, along decreasing photon energies, as a function of the thickness of the remaining HP-PMPCVD film, is performed with the help of a model taking the diffusion of the unrecombined excitonic pairs and the re-excited photoluminescence into account. CL images recorded at specific wavelengths, which do not show inverted contrast, are also assessed. From these data, the exciton diffusion length is evaluated to 11 μm in the major part of the epitaxial layer except for the first 20 μm close to the Ib substrate where it decreases down to 2 μm. This study sheds light on the interpretation of luminescence spectra excited by an electron beam in undoped diamond layers. Defects bands due to damages induced by polishing and etching processes are also documented.     

具有国际化魅力的法国美食

去年十二月份,厨师Franck Page先生加入北京佛雄食品店时正值北方大雪纷飞的日子.但按照Franck家乡的习俗,遇见()雪天意味着好兆头.在黎巴嫩著名的五星级饭店Vendome Intercontinental积累了丰富经验之后.Franck决定对自己的职业生涯做一次转折,于是他加入了享誉全球的著名法国集团佛雄,该集团刚刚在北京开办了第一家分部.Franck就这样来到了北京佛雄食品店.     

A first order method for the Cauchy problem for the Laplace equation using BEM

The purpose is to propose an improved method for inverse boundary value problems. This method is presented on a model problem. It introduces a higher order problem. BEM numerical simulations highlight the efficiency, the improved accuracy, the robustness to noisy data of this new approach, as well as its ability to deblur noisy data.     

Study of the R-(Zr,W)-(O,N) (R = Y, Nd, Sm, Gd, Yb) oxynitride system

The replacement of tantalum by the couple Zr/W within the RTa-O-N systems (R = Y, Nd, Sm, Gd, Yb), enables the preparation of novel oxide and oxynitride phases in the R-Zr-W-O-N system. R₂Zr_2−xW_xO_7+x oxides exhibit the fluorite-type (x < 0.9) and scheelite (x ∼ 1) structures. Corresponding oxynitride compositions are of the fluorite-type and show different colors, for example in the case of ytterbium: pale yellow (x = 0.2 or 0.25), green (x = 0.5-0.8) and brown for the tungsten-rich samples (x = 0.9, 1). Photocatalytic activity measurements have been performed to investigate the overall water splitting behavior of these colored phases.     

Optic flow regulation: the key to aircraft automatic guidance

We have developed a visually based autopilot which is able to make an air vehicle automatically take off, cruise and land, while reacting appropriately to wind disturbances (head wind and tail wind). This autopilot consists of a visual control system that adjusts the thrust so as to keep the downward optic flow (OF) at a constant value. This autopilot is therefore based on an optic flow regulation loop. It makes use of a sensor, which is known as an elementary motion detector (EMD). The functional structure of this EMD was inspired by that of the housefly, which was previously investigated at our Laboratory by performing electrophysiological recordings while applying optical microstimuli to single photoreceptor cells of the insect's compound eye.

We built a proof-of-concept, tethered rotorcraft that circles indoors over an environment composed of contrasting features randomly arranged on the floor. The autopilot, which we have called OCTAVE (Optic flow based Control sysTem for Aerial VEhicles), enables this miniature (100 g) rotorcraft to carry out complex tasks such as ground avoidance and terrain following, to control risky maneuvers such as automatic take off and automatic landing, and to respond appropriately to wind disturbances. A single visuomotor control loop suffices to perform all these reputedly demanding tasks. As the electronic processing system required is extremely light-weight (only a few grams), it can be mounted on-board micro-air vehicles (MAVs) as well as larger unmanned air vehicles (UAVs) or even submarines and autonomous underwater vehicles (AUVs). But the OCTAVE autopilot could also provide guidance and/or warning signals to prevent the pilots of manned aircraft from colliding with shallow terrain, for example.     

Photodétecteurs en intégration monolithique sur guides optiques : modélisation et vérifications expérimentales

The authors developed a model on optical guided structures with complex refractive indices, on 9 desktop-computer. This model allows to predict the coupling efficiency of photodetectors monolithically integrated with optical waveguides. An oscillatory behaviour of the absorption coefficient versus the absorbing layer thinkness has been pointed out. Comparisons with experimental resuylts obtained with device made in different laboratories show a good agrement