Waypoint Following for Differentially Driven Wheeled Robots with Limited Velocity Perturbations

In this paper a unified motion control strategy dedicated for the waypoint following task realized by a differentially driven robot is presented. It is assumed that the vehicle moves with limited velocities and accelerations in order to reduce excessive slip and skid effects. In order to include operational constraints, a motion planner is combined with a universal stabilizer taking advantage of transverse functions. To improve tracking precision translated transverse functions are deployed and a new adaptive technique for the controller tuning is proposed. During the motion planning stage an auxiliary trajectory connecting points in the configuration space and satisfying assumed phase constraints is generated. The resulting motion execution system has been implemented on a laboratory-scale skid-steering mobile robot, which served as platform for experimental validation of presented algorithms.     

Finite element analysis of a deep excavation: A case study from the Bangkok MRT

The Bangkok metropolitan area, located on a thick river soft clay deposit, has recently started a construction project on a mass rapid transit underground railway (MRT). This paper presents a finite element study on the Bangkok MRT underground construction project. The excavation of Sukhumvit Station was selected as the case study for the FEM numerical modelling in this paper. The numerical study focuses on the initial input on the ground conditions and the constitutive soil models. The geotechnical parameters were selected based on the soil investigation reports carried out for the purpose of the construction. The parameters selected for the constitutive models used in the FEM analysis were calibrated against the laboratory testing results. Finally, all the FEM simulations were compared with the data from field investigations.     

PATTERN FORMATION IN SOLID FILM GROWTH DURING CVD

This paper deals with the evolution of a gas-solid interface during growth of amorphous CVD films. The aim of the analysis is to show that a comprehensive gas-solid CVD model may simplify significantly under conditions of two limiting cases: kinetic and diffusion-limited growth. The linear stability behavior of the simplified model was found to be almost identical to that of the original and more comprehensive model. It was found that planar film growth was inherently stable under kinetically controlled deposition conditions, but under diffusion-limited conditions, planar film stability depended on the magnitude of the dimensionless group 9Pe, which represents a Damkdhler number of deposition. Numerical solution of the simplified model showed that an increase in 9pe adversely affected the uniformity of film deposition. Predicted morphological phenomena were found to be very similar to experimental observations (Viljoen, et al,1994).     

Influence of forward and multiple light scatter on the measurement of beam attenuation in highly scattering marine environments

Using three-dimensional Monte Carlo radiative transfer simulations, we examine the effect of beam transmissometer geometry on the relative error in the measurement of the beam-attenuation coefficient in an aquatic environment characterized by intense light scattering, especially within submerged bubble clouds entrained by surface-wave breaking. We discuss the forward-scattering error associated with the detection of photons scattered at small angles (< 1 degrees) and the multiple-scattering error associated with the detection of photons scattered more than once along the path length of the instrument. Several scattering phase functions describing bubble clouds at different bubble void fractions in the water are considered. Owing to forward-scattering error, a beam-attenuation meter (beam transmissometer) with a half-angle of receiver acceptance of 1.0 degrees and a path length of 0.1 m can underestimate the true beam attenuation within the bubble cloud by more than 50%. For bubble clouds with a beam attenuation of as much as 100 m(-1), the multiple-scattering error is no more than a few percent. These results are compared with simulations for some example phase functions that are representative of other scattering regimes found in natural waters. The forward-scattering error for the Petzold phase function of turbid waters is 16% for a typical instrument geometry, whereas for the Henyey-Greenstein phase function with the asymmetry parameter of 0.7 and 0.9 the error range is 8-28%.     

Modeling the optical properties of mineral particles suspended in seawater and their influence on ocean reflectance and chlorophyll estimation from remote sensing algorithms

The optical properties of mineral particles suspended in seawater were calculated from the Mie scattering theory for different size distributions and complex refractive indices of the particles. The ratio of the spectral backscattering coefficient to the sum of the spectral absorption and backscattering coefficients of seawater, b(b)(lambda)/[a(lambda) + b(b)(lambda)], was analyzed as a proxy for ocean reflectance for varying properties and concentrations of mineral particles. Given the plausible range of variability in the particle size distribution and the refractive index, the general parameterizations of the absorption and scattering properties of mineral particles and their effects on ocean reflectance in terms of particle mass concentration alone are inadequate. The variations in the particle size distribution and the refractive index must be taken into account. The errors in chlorophyll estimation obtained from the remote sensing algorithms that are due to the presence of mineral particles can be very large. For example, when the mineral concentration is 1 g m(-3) and the chlorophyll a concentration is low (0.05 mg m(-3)), current global algorithms based on a blue-to-green reflectance ratio can produce a chlorophyll overestimation ranging from approximately 50% to as much as 20-fold.     

Interference colours of soap bubbles

In this article, we present a new versatile method for calculating the interference colour of any configuration of thin, optically isotropic, dielectric films. This method is free from common weakness of previous methods, namely the poor accuracy for large angles of incidence of light. This method gives full control over the accuracy and speed of computations, and allows for adjustments of the parameters according to the physical and chemical properties of the film, in order to present the optimal results. We also present a formula to model the thickness of thin films made up of liquids, such as soap bubbles.     

Fabrication and properties of novel composite laser ceramics with a surface-gain structure

To suppress heat effects in a high-power laser system, it has been proposed to use a few composite laser gain media to improve the heat management ability. The surface-gain structure has the advantages of both slab and thin-disk configurations, which has huge potential in high-power laser. For the first time, novel composite laser ceramics with surface-gain structure have been fabricated by tape casting and sintering technology. The in-line transmittances of surface-gain ceramics in 1100 and 400 nm are 83.5% and 79.1%, respectively. According to the laser ablation–coupled plasma mass spectrometry, the diffusion distance of Yb³⁺ is about 150 µm. The laser experiment yields a low repetition rate pulse of 1.9 J with a pump absorption energy of 11 J and an optical-to-optical conversion efficiency of 17.2%.     

Random Multimode Optical Media: I. Mode Coupling Process in Slab Waveguide with Stochastic Wall Perturbations

Abstract

The scope of the paper is to present in explicit form the mode coupling coefficients in slab waveguide, in the case when both boundaries of guiding core or gradient refractive index distribution are perturbed by random deviations from an ideal model. The central idea is to present the relationships between the processes, depicting the waveguide deformations and the estimators of the mode coupling processes. The correlation functions and the spectral density functions of both groups of processes are linearly interdependent; we examine the complex coefficients of these relations. In particular, the TE modes are analysed in detail in step-index slab. For the case of parabolic profile (n(x) = A(1 ? Bx²), within the core) the method which derives the statistic integral form of coupling coefficients is presented (also for guided TE modes). All modes are assumed to be monochromatic, i.e. time-dependence is exp (?iωt).     

Squeezing in Finite Superpositions of Photon-number States without the Vacuum

Abstract

We show that a finite superposition of photon-number states without the vacuum state can lead to squeezed fluctuations. The properties of these states are discussed.     

Determination of metastable atom concentration by use of electrostatic probe technique

A method for determination of the metastable atom concentration in high pressure (>100 Torr) high density (> 10¹²cm²³); helium plasma from current–voltage characteristics of a single electrostatic probe is described. It is shown, that the flux of metastable atoms to the probe is controlled by ion sheath thickness and consequently by probe bias. The method for calculation of metastables concentration from the negative part of the current–voltage probe characteristics is proposed. The metastables concentrations measured in pulsed microwave discharge are in agreement with values calculated from the metastable balance equation.