Showing papers by "École Polytechnique Fédérale de Lausanne published in 1989"

Sensitivity of grating couplers as integrated-optical chemical sensors

Abstract: Grating couplers on planar waveguides can be used as integrated-optical sensors responding to (1) changes in the refractive index of a liquid sample covering the waveguide (differential refractometer) and (2) the adsorption and desorption, respectively, of molecules out of a gaseous or liquid sample on the waveguide (gas or chemical sensor). A theory of the sensor sensitivities is developed; conditions for the waveguide parameters in order to obtain high sensitivities are derived. It is shown that effects (1) and (2) can be distinguished by measurements of the effective index changes of both the TE0 and the TM0 modes. In the analysis both nonporous and microporous waveguiding films are considered.

Modeling of equiaxed microstructure formation in casting

Abstract: A general micro/macroscopic model of solidification for 2-D or 3-D castings, valid for both dendritic and eutectic equiaxed alloys, is presented. At the macroscopic level, the heat diffusion equation is solved with an enthalpy formulation using a standard FEM implicit scheme. However, instead of using a unique relationship between temperature and enthalpy (i.e., a unique solidification path), the specific heat and latent heat contributions, whose sum equals the variation of enthalpy at a given node, are calculated using a microscopic model of solidification. This model takes into account nucleation of new grains within the undercooled melt, the kinetics of the dendrite tips or of the eutectic front, and a solute balance at the scale of the grain in the case of dendritic alloys. The coupling between macroscopic and microscopic aspects is carried out using two time-steps, one at the macroscopic level for the implicit calculation of heat flow, and the other, much finer, for the microscopic calculations of nucleation and growth. This micro/macroscopic approach has been applied to one-dimensional and axisymmetric castings of Al-7 pct Si alloys. The calculated recalescences and grain sizes are compared with values measured for one-dimensional ingots cast under well-controlled conditions. Furthermore, the influence of casting conditions on temperature field, undercooling, grain size, and microstructural spacings is shown to be predicted correctly from axisymmetric calculations with regard to the expected experimental behavior.

Simulation of object and human skin formations in a grasping task

Abstract: This paper addresses the problem of simulating deformations between objects and the hand of a synthetic character during a grasping process. A numerical method based on finite element theory allows us to take into account the active forces of the fingers on the object and the reactive forces of the object on the fingers. The method improves control of synthetic human behavior in a task level animation system because it provides information about the environment of a synthetic human and so can be compared to the sense of touch. Finite element theory currently used in engineering seems one of the best approaches for modeling both elastic and plastic deformation of objects, as well as shocks with or without penetration between deformable objects. We show that intrinsic properties of the method based on composition/decomposition of elements have an impact in computer animation. We also state that the use of the same method for modeling both objects and human bodies improves the modeling both objects and human bodies improves the modeling of the contacts between them. Moreover, it allows a realistic envelope deformation of the human fingers comparable to existing methods. To show what we can expect from the method, we apply it to the grasping and pressing of a ball. Our solution to the grasping problem is based on displacement commands instead of force commands used in robotics and human behavior.

Tabu search: a tutorial and an application to neural networks

Abstract: Tabu Search is a general heuristic procedure for global optimization. Based on simple ideas it has been extremely efficient in getting almost optimal solutions for many types of difficult combinatorial optimization problems. The principles of Tabu Search are discribed and illustrations are given. An example of problem type where the use of Tabu Search has drastically cut down the computational effort is presented; it consists of the learning process of an associative memory represented by a neural network.

Thirty compounds tested in the Drosophila wing spot test.

Abstract: The Drosophila wing somatic mutation and recombination test (SMART) was evaluated for its suitability in genotoxicity screening by testing 30 chemicals. Of the 2 crosses used, the mwh-flr 3 cross turned out to be more convenient than the previously used mwh-flr cross. Based on the experience gained with both acute exposures and chronic exposures of different duration, we suggest that the optimal strategy in genotoxicity screening is to start with chronic exposure of 3-day-old larvae for 48 h (that is, until pupation). Only for unstable compounds and very volatile compounds and gases are acute treatments, including inhalation, recommended. In general, a qualitative evaluation of the genotoxicity of a compound in the wing assay is possible with as few as 1–2 different exposure concentrations. A more quantitative evaluation of genotoxicity, based upon dose-response data, can often be achieved with as few as 3–4 concentrations. The results reported here were obtained in 2 different laboratories, demonstrating that the wing spot test is easily transferable to other laboratories. The experience gained indicates that the assay has now been developed to an extent that a coordinated international comparative validation study is desirable.

A New Algorithm to Implement Causal Ordering

Abstract: This paper presents a new algorithm to implement causal ordering Causal ordering was first proposed in the ISIS system developed at Cornell University The interest of causal ordering in a distributed system is that it is cheaper to realize than total ordering The implementation of causal ordering proposed in this paper uses logical clocks of Mattern-Fidge (which define a partial order between events in a distributed system) and presents two advantages over the implementation in ISIS: (1) the information added to messages to ensure causal ordering is bounded by the number of sites in the system, and (2) no special protocol is needed to dispose of this added information when it has become useless The implementation of ISIS presents however advantages in the case of site failures

Quasiparticle energies in semiconductors: Self-energy correction to the local-density approximation.

Abstract: A model self-energy correction to the local-density approximation is derived from the GW approximation of the self-energy operator Excitation energies calculated in diamond, Si, Ge, GaAs, and AlAs compare favorably with those obtained with the full GW approximation Results for GaP are close to experimental data It is shown that the model consists of a ``scissor'' operator and additional nonrigid corrections

Primary structure of the cAMP-dependent phosphorylation site of the plasma membrane calcium pump

Abstract: The primary structure of a region of the erythrocyte plasma membrane calcium pump which is phosphorylated by the cAMP-dependent protein kinase has been determined. The sequence is A-P-T-K-R-N-S-S(P)-P-P-P-S-P-D. The site is located between the calmodulin binding domain and the C-terminus of the ATPase. The ATPase is phosphorylated only at this site by the cAMP-dependent protein kinase, and the phosphorylation is inhibited by calmodulin. The effect of the phosphorylation is to decrease the Km for Ca2+ of the purified ATPase from about 10 microM to about 1.4 microM and to increase the Vmax of ATP hydrolysis about 2-fold.

Evidence for enhancement of critical current by intergrain Ag in YBaCuO-Ag ceramics

Abstract: We report the evidence for enhancement of critical current density Jc by ∼50%, which occurs when ∼10 wt. % Ag is added to Y1Ba2Cu3O7−δ ceramics. The maximal Jc (∼700 A/cm2 at T=77 K) appears simultaneously with maximum YBaCuO compactness in the samples. The silver fills the intergranular space (holes) without Cu substitution, and the critical temperature Tc is not reduced from the bulk value (∼91 K). Normal‐state resistivity of Ag‐YBaCuO samples is decreased by an order of magnitude, and samples exhibit improved contact resistance and resistance to water. While the critical density is improved by adding ∼10 wt. % Ag, it decreases at higher Ag concentrations.

Grading of construction timeber by ultrasound

Abstract: The resistance of spruce beams was characterized with the aid of nondestructive ultrasonic measurements. The modulus of elasticity and the modulus of rupture in flexure for beams of commercial sized section can be estimated by observing the speed of propagation of a longitudinal wave applied to the longitudinal axis of the beam. The use of simple ultrasonic equipment and two experimental transducers (in direct and dry contact) permit measurements to be performed in industry. The technique constitutes the basis of an objective concept for classification of sawn timber guaranteeing a better reliability and profitability with regard to the visual grading rules used in Western Europe.

Piezoelectric properties of III-V semiconductors from first-principles linear-response theory.

Abstract: We present a novel ab initio approach to piezoelectricity. The piezoelectric tensor is given by the stress induced by a homogeneous electric field. The perturbation is treated self-consistently by linear response, thus avoiding both supercells and numerical differentiation. We calculate the piezoelectric constants of none III-V semiconductors: as a by-product we also provide the first systematic study of zone-center phonons, internal strain parameters, effective charges, and dielectric constants. Our results agree very well with experiments where available, and allow predictions where they are not.