Showing papers by "Mines ParisTech published in 1992"

Adaptive nonlinear regulation: estimation from the Lyapunov equation

Abstract: A stabilizing adaptive controller for a nonlinear system depending affinely on some unknown parameters is presented. It is assumed that this system is feedback stabilizable. A key feature of the method is the use of the Lyapunov equation to design the adaptive law. A result on local stability, two different conditions for global stability, and a local result where the initial conditions of the state of the system only are restricted are given. >

Sample adequately to estimate variograms of soil properties

Abstract: SUMMARY The variogram is central in the spatial analysis of soil, yet it is often estimated from few data, and its precision is unknown because confidence limits cannot be determined analytically from a single set of data. Approximate confidence intervals for the variogram of a soil property can be found numerically by simulating a large field of values using a plausible model and then taking many samples from it and computing the observed variogram of each sample. A sampling distribution of the variogram and its percentiles can then be obtained. When this is done for situations typical in soil and environmental surveys it seems that variograms computed on fewer than 50 data are of little value and that at least 100 data are needed. Our experiments suggest that for a normally distributed isotropic variable a variogram computed from a sample of 150 data might often be satisfactory, while one derived from 225 data will usually be reliable.

Video-controlled tensile testing of polymers and metals beyond the necking point

Abstract: A novel technique has been developed to record the intrinsic plastic behaviour of ductile materials by monitoring the effective strain and the effective stress in the mid-plane of hour-glass-shaped tensile specimens. The method utilizes a computer-aided video extensometer which analyses the sample profile in real time. The effective strain is computed automatically from the minimum diameter, and the effective stress is deduced from the applied load by taking into account the stress triaxiality corresponding to the local radius of curvature of the sample profile. Furthermore, a digital closed-loop system controls the ram speed of the hydraulic tensile testing machine in such a way that the local effective strain rate is maintained at a constant value. It is shown that most polymeric and metallic materials are entitled to be investigated by this method, which gives access in real time to the constitutive plastic equation, up to strains far beyond the necking point. The capabilities of the technique are illustrated and discussed critically, with more details for two polymers of different structures: polyethylene and polycarbonate.

Intergranular crack tip oxidation mechanism in a nickel-based superalloy

Abstract: This paper is concerned with the intergranular crack tip oxidation mechanism in alloy 718 at elevated temperatures. The basic concept is based on the ability of the oxygen partial pressure to control the preferential formation of oxide layers at the crack tip. The time required to complete the build-up of the protective oxide type at the metal-oxide interface is considered a measure of the limits of the oxidation process. Identification by transmission electron microscopy of oxide scale formed along fracture surfaces during a low frequency fatigue crack process in alloy 718 at 650°C supports the proposed model concepts. An experimental program was carried out to investigate the role of passivation time in controlling the progressive process of crack tip oxidation. This was achieved by testing the influence of oxide buil-up during hold time at minimum load, as well as the effect of a minor high frequency cycle imposed on the hold time period. It was established that an increase in fatigue crack growth rate accompanies the increase in passivation time period. These results were interpreted on the basis of the oxidation formation concepts.

Dynamic feedback linearization of nonholonomic wheeled mobile robots

Abstract: Smooth time-varying laws can solve the stabilization problem of nonholonomic mechanical systems. The authors show that by means of dynamic state feedback, it is possible for three-wheeled mobile robots to track arbitrary fast trajectories not reduced to equilibrium points. Dynamical modeling of nonholonomic mechanical systems for the case of three-wheeled mobile robots is considered. Dynamic feedback allows solution of the tracking problem for an omnidirectional mobile robot with less motors than degrees of freedom. This is possible by choosing output functions depending on the mass repartition of the robot. >

Mobilization of Waterflood Residual Oil by Gas Injection for Water-Wet Conditions

Abstract: This paper reports that mechanisms by which waterflood residual oil is mobilized and recovered during tertiary gasflooding at quasistatic rates and strongly water-wet conditions were investigated with 2D glass micromodels. Two three-phase oil/water/gas systems were used in the displacement experiments. One system had a positive spreading coefficient, the other a negative coefficient. Results for the two systems were compared to determine the differences in displacement mechanisms and oil recovery efficiency. Displacement in both systems proceeds by a double-drainage mechanism where a gas/oil displacement is always associated with an oil/water displacement. The oil/water displacement leads to coalescence and reconnection of oil blobs. Oil recovery was significantly higher for the positive spreading system. The higher displacement efficiency resulted from flow through thin but continuous oil films that always separated the oil and water phases in the positive spreading system. The absence of oil films and the possibility of direct gas/water displacements reduced oil recovery for the negative spreading system.

Synthesis of Nanocrystalline Alumina–Metal Composites by Room‐Temperature Ball‐Milling of Metal Oxides and Aluminum

Abstract: The synthesis of nanometer-sized α-Al2O3−M composites (nanocomposites) is described for M = Fe, V, Cr, Mn, Co, Ni, Cu, Zn, Nb, Mo, W, Si, and Fe alloys. The synthesis is performed by solid-state reduction reaction which occurs during room-temperature ball-milling of mixtures of a metal oxide (MxOy) and aluminum (Al2y/3). A complete reduction is reached for milling times of ≅ 1.5 h in a planetary mill. The crystallite sizes of alumina and metal obatined, for powders processed in a vibratory mill, are in general about 10 nm.

Mathematical model coupling phase transformations and temperature evolutions in steels

Abstract: A mathematical model for calculating phase transformations in steels during rapid heating and cooling is presented. It is based on a rule of additivity. The isothermal kinetics are modelled by Johnson-Mehl-Avrami law. The model describes the kinetics of austenitization during heating, the state of austenite at the end of heating (carbon content, grain size), the kinetics of transformations during cooling, the final microstructure and hardness. The model is worked out firstly on dilatometric specimens without thermal gradients in order to validate the modelling and the input data. Then the application of the model to massive cylinders heated up and cooled down with high thermal gradients is presented.

Rainfall estimation in the Sahel: the EPSAT-NIGER experiment

Abstract: EPSAT-NIGER (Estimation of Precipitation by SATellite—NIGER experiment) has been designed to improve the understanding of the precipitation systems of Sudano-Sahelian Africa and to develop operational rainfall estimation algorithms for this region. It is based on the combined use of a very dense raingauge network (93 gauges over a study area of 16 000 km2) and a C-band weather radar system. The experiment is scheduled to last three years, 1990–1992. The network pattern, a regular grid with nodes spaced at 12.5 km and a 16 gauge target area where the distance between stations is decreased to 1 km, has allowed for some preliminary studies on the rainfall distribution at various space and time scales. Whereas the long term average rainfall gradient is uniform, rainfall increasing north to south, a single rainy season can be markedly different. The local variability may be extremely large. That variability is enhanced at smaller sampling time steps and the computation of reference areal rainfall for ...

A hardware implementation of pure ESTEREL

Abstract: esterel is a synchronous concurrent programming language dedicated to reactive systems (controllers, protocols, man-machine interfaces etc.). esterel has an efficient standard software implementation based on well-defined mathematical semantics. We present a new hardware implementation of the pure synchronization subset of the language. Each program generates a specific circuit that responds to any input in one clock cycle. When the source program satisfies some statically checkable dynamic properties, the circuit is shown to be semantically equivalent to the source program. The hardware translation has been effectively implemented on the programmable active memory perle0 developed by J Vuillemin and his group at Digital Equipment.

Influence of calcium concentration on the dentin wettability by an adhesive.

Abstract: Plane dentin surfaces were abraded perpendicular to the radicular axis of sound human molars. They were cleaned to reveal the tubules, and the morphological features of each surface were studied microscopically. For a first series of teeth, the Vickers microhardness of the dentin surfaces was measured and the calcium and phosphorus composition was determined by electron microprobe analysis. For a second series, the microhardness and wettability of the surface by the Scotchbond adhesive were compared. Positive correlations were found between the following parameters: degree of minerality, dentin compactness, hardness, and spreading capability of the adhesive.

Determination of the wall slip velocity in the flow of a SBR compound

Abstract: Rubber compounds are known to exhibit slip at the wall in particular flow conditions. The slip velocity is usually determined by using the classical Mooney method. The rheological behavior of a styrene butadiene rubber (SBR) compound was studied with three different rheometers. Biconical rotational, capillary and slit die rheometers were used to define the true viscous behavior of the compound and the slip velocity. It was shown that it was impossible to apply the Mooney method to our experimental data. New characterizations were thus developed for both capillary and slit die experiments. They were based on the dependency of the slip velocity on the local flow gap. Contrarily to the Mooney method, they provided physically acceptable results and led to a power-law relationship between wall slip, wall shear stress and local geometry of the flow.

Inorganic nitrogen dynamics in the River Seine downstream from Paris (France)

Abstract: The River Seine, below Paris, receives the effluents from a large sewage treatment plant, increasing the ammonium concentration up to 6 mgN1− in late summer Careful measurement of ammonium, nitrate and organic nitrogen during the downriver travel of the water masses over 100 km below the outfall, along with direct determination of nitrification and benthic fluxes, allowed to establish a budget of nitrogen transport and transformations in this reach of the river Nitrification is shown to start after a distinct period of several days required for the growth of a significant nitrifying bacterial population Denitrification is active in the upper layer of bottom sediments but absent from the water column Comparison of our data with those published for the period 1973–1976 shows that the nitrate load carried by the river has increased not only because of higher runoff of agricultural nitrate in the upstream part of the watershed, but also as a result of the severe reduction in the rate of denitrification processes, owing to the restoration of better oxygen conditions

The orthorhombic approximant phases of the decagonal phase

Abstract: Two distinct orthorhombic phases form in the Al65Cu20Fe10Cr5 and Al63Cu17.5Co17.5Si2 alloys and represent an important class of approximant structures of the decagonal phase. Their structural units (convex pentagon, concave pentagon and rhombus) as revealed by high resolution electron microscopy (HREM) images can also be used to construct the Penrose tiling. We propose that one single orthorhombic phase may not be able to account for the transformation towards the high-temperature decagonal phase. For this transformation to be possible, the microcrystalline structure must satisfy a delicate balance among building blocks that is required to achieve a Penrose tiling. Two concepts are distinguished: the approximant phase and the approximation state. An analysis of the orientation relationships between a CsCl type of structure and orthorhombic phases suggests that such orthorhombic phases are three-dimensional superstruc-tures based on the CsCl unit cell. Furthermore, this relationship leads to the d...

Road tracking, lane segmentation and obstacle recognition by mathematical morphology

Abstract: Presents the work performed at the CMM as part of the European PROMETHEUS project. The research deals with the road edge detection, lane segmentation and obstacle recognition in a dynamic scene acquired by a monochrome monocular camera. The image processing is based on morphological segmentation tools. The experiments on over a thousand images show that the approach works well on difficult cases such as dense traffic, and roads without land markers. >

Adaptive control of an overhead crane using dynamic feedback linearization and estimation design

Abstract: The authors propose an indirect adaptive scheme for incompletely controlled mechanical systems such as overhead cranes or, more generally, classical rigid manipulators ended by a simple pendulum. In the case of known parameters a dynamic state feedback produces full linearization. An adaptive version is obtained by a simple estimation method together with a certainty equivalence law, taking advantage of the particular structure of the equations. Global stability is discussed, and simulation results for the overhead crane are reported. >

Sorption of uranyl ions by a modified chitosan: Kinetic and equilibrium studies

Abstract: The uranium sorption of an ascorbic chitosan derivative polymer (N.D.T.C. N‐[2‐(1,2‐dihydroxyethyl) tetra‐hydrofuryl] chitosan) was shown to be three to four times greater than chitosan under the same conditions, reaching 600 mgU.g−1 polymer (i.e. 2.5 mmolU.g−1 polymer). The effect of variation of the classic parameters was studied: pH, metal concentration and particle size. pH and metal concentration, which induced a change in metal solution chemistry, were found to be the controlling parameters, mainly affecting intraparticle diffusion; the diffusion rate is strongly affected by ionic size of solute, as opposed to a limited effect on external mass transfer. pH plays an important part in equilibrium studies, and sorption isotherms are mainly a function of pH. The Freundlich model shows a better correlation coefficient than the Langmuir model when fitted to experimental results; the uptake mechanism involves a monolayer sorption with molecular interactions between molecules sorbed on surface.