Physics

En este trabajo se presenta el modelo biomecánico de una prótesis de pierna. Con el objetivo de estudiar el cambio de velocidad en la unión prótesis-muñón al momento del impacto del pie con el suelo, está se modeló como un sistema resorte-amortiguador, permitiendo evidenciar la necesidad de construir la unión muñón-prótesis con dispositivos de impedancia mecánica variable. Además se desarrolló un simulador con el objetivo de hacer representaciones virtuales de un paciente con prótesis. Para ello se modeló al paciente como un robot bípedo planar, el simulador permite estudiar el efecto de las fuerzas de impacto con el suelo de la unión prótesis-muñón como una etapa anterior a la implementación real de la misma. Copyright c 2014 CEA. Publicado por Elsevier España, S.L. Todos los derechos reservados.

The aim of this article is to present a statistical comparison of the electric energy expenditure between two techniques for the generation of gait patterns in biped robots. The first one is to minimize the sum of the squared torques applied to the joints of the robot, and the second one is based on the cart-table model. For the experiments, we measured the energy delivered by the battery of the robot to the servomotors. We applied the two aforementioned methods for three velocities (0.5, 1.0, and 1.3 m/min). Additionally, each combination of method and velocity was performed by the robot 10 times. The energy expenditure for each method was compared by applying the Wilcoxon test. In all comparisons, the value of p was lower than 0.004, indicating that the differences were statistically significant. The optimization approach leads to a reduction in energy expenditure that ranged from 9.16 % to 13.35 %. The conclusion is that all the effort required to implement an approach that requires a complete dynamic model of the robot allows a significant reduction in energy consumption.

En este trabajo se estudia el modelo cinemático y dinámico de una bicicleta robótica (Arduino Engineering Kit), con el propósito de desarrollar algoritmos de control para su estabilización automática. Se muestra el modelo matemático de la bicicleta mediante la formulación Euler-Lagrange. Este permite el diseño y la implementación de dos estrategias de control para garantizar el equilibrio automático. El estudio de este sistema dinámico se presenta como una excelente oportunidad para integrar las destrezas adquiridas por los estudiantes en materias como física, programación y matemáticas, con el propósito de diseñar, modelar y controlar sistemas dinámicos. Descriptores: Enseñanza; ecuación de Lagrange; sistemas dinámicos; simulación. In this paper, a kinematic and dynamic model study of a robotic bicycle (Arduino Engineering Kit) is carried out to develop control algorithms for the automatic stabilization of the bicycle. The mathematical model of the bicycle is shown by the Euler-Lagrange formulation. This model allows the design and implementation of two control strategies to ensure automatic balance. The study of this dynamic system is presented as an excellent opportunity to integrate the skills acquired by students in subjects such as physics, programming, and mathematics with the purpose of designing, modeling, and controlling dynamic systems.

Simulation is a virtual representation of a dynamic system. For the case of mechanical systems, the simulator is used to calculate the reaction forces between its base and the ground and other constraints. The intermittent nature of these forces and the mathematical inequalities that they must satisfy lead to models described by hybrid algebraic differential equations. In this paper, a simulator was developed for a seven degrees of freedom planar biped robot, which was modeled using the Euler-Lagrange formulation. This model allowed the design and implementation of a control strategy for balance management, and the monitoring of articular reference paths are tested in the simulator before proceeding to implementation on the actual prototype.

En este trabajo se estudia la estabilidad de un péndulo invertido de dos grados de libertad y de base móvil, el cual se modeló mediante la formulación Euler-Lagrange. Este modelo permitió diseñar e implementar una estrategia de control para el seguimiento de trayectorias articulares de referencia. Los sistemas mecánicos de bases fijas y móviles permiten ilustrar un concepto fundamental en las ciencias físicas, que es la diferencia entre limitaciones tecnológicas y limitaciones fundamentales. Descriptores: Enseñanza; ecuación de Lagrange; sistemas mecánicos; simulación. In this paper, we study the stability of an inverted pendulum with two degrees of freedom and a mobile base, which was modeled using the Euler-Lagrange formulation. This model is used to design and implement a control strategy for tracking joint reference trajectories. The mechanical systems of fixed and mobile base illustrate a fundamental concept in the physical sciences, which is the difference between technological and fundamental limitations.

En este trabajo se propone una herramienta para la enseñanza de la mecánica clásica, modelando sistemas dinámicos con las ecuaciones de Lagrange, a partir de la simulación en Matlab . Este enfoque permite a los estudiantes de ingeniería y ciencias básicas, desarrollar competencias en programación, análisis numérico, con el uso de la física como herramienta fundamental para modelar y simular sistemas dinámicos complejos, como por ejemplo: robots industriales, robots bípedos y cualquier sistema mecánico con el propósito de analizar y estudiar sus propiedades cinemáticas y dinámicas. Esta herramienta fue probada en el curso de Sistemas Dinámicos de la Universidad del Cauca, demostrando ser una herramienta de fácil y rápido aprendizaje por los estudiantes del programa de Ingeniería en Automática Industrial.

This paper proposes human motion capture to generate movements for theright leg in swing phase of a biped robot restricted to the sagittal plane.Such movements are defined by time functions representing the desiredangular positions for the joints involved. Motion capture performed witha Microsoft KinectTMcamera and from the data obtained joint trajec-tories were generated to control the robot’s right leg in swing phase. Theproposed control law is a hybrid strategy; the first strategyis based ona computed torque control to track reference trajectories,and the secondstrategy is based on time scaling control ensuring the robot’s balance. Thiswork is a preliminary study to generate humanoid robot trajectories frommotion capture.

En este artículo se realizó el modelado y control de un sistema de levitación magnética ( MAGLEV). Para ello se construyó un prototipo con materiales de bajo costo con el objetivo de realizar prácticas experimentales para la enseñaanza de sistemas de control en el programa de Ingeniería Física. Los resultados muestran que es posible utilizar el sistema para controlar la posición de una bola mediante una estrategia de control lineal.

Tactile maps are efficient tools to improve spatial understanding and mobility skills of visually impaired people. Their limited adaptability can be compensated with haptic devices which display graphical information, but their assessment is frequently limited to performance-based metrics only which can hide potential spatial abilities in O&M protocols. We assess a low-tech tactile mouse able to deliver three-dimensional content considering how performance, mental workload, behavior, and anxiety status vary with task difficulty and gender in congenitally blind, late blind, and sighted subjects. Results show that task difficulty coherently modulates the efficiency and difficulty to build mental maps, regardless of visual experience. Although exhibiting attitudes that were similar and gender-independent, the females had lower performance and higher cognitive load, especially when congenitally blind. All groups showed a significant decrease in anxiety after using the device. Tactile graphics with our device seems therefore to be applicable with different visual experiences, with no negative emotional consequences of mentally demanding spatial tasks. Going beyond performance-based assessment, our methodology can help with better targeting technological solutions in orientation and mobility protocols.

This paper presents an integrated analysis of factors affecting the dynamics of an Andean basin. The study was conducted by using Landsat images, SRTM DEM data from NASA, infrastructure information (cities and roads proximity) and the resulting hydrological data analysis, over a characteristic basin of Colombian valleys. Land cover recognition was validated with in-situ data, this allowing to build a temporal variation profile of the land cover over a 26 years period. This information was integrated by clustering to obtain uniform regions according to variables used, resulting in the generation of different trend maps. The proposed approach allows to establish the importance of using information as a measure of anthropogenic affectation levels, thus it is possible to make more specific action plans and efficiently compensate the environmental degradation effects.

The lignin is an extremely challenging natural material with wide and interesting applications, during the last two decades, great efforts have been dedicated to the study of its optical properties. About this subject have occurred many controversial opinions because of the nature and interactions of this material with other components of the vegetal cellular wall. In this work we will explore and simulate the electronic absorption and vibrations spectrums based on the semiempirical geometrical and quantum mechanics optimizations of amorphous two and tree units softwoods lignin fragments of Coniferyl Alcohol. These fragments were previously obtained using the semiempirical methods, Modified Neglect of Diatomic Overlap MNDO and Zerner's spectroscopic version of the Intermediate Neglect of Differential Overlap in configuration interactions mode: ZINDO/S CI.

The effects of traps and shallow acceptors on the continuous-wave steady-state photoluminescence of GaAs-(Ga,Al)As quantum-well wires are studied at room temperature. The analysis is based on a quantum-mechanical calculation of the transition rates of radiative recombinations of excited-conduction electrons with free and bound (at acceptors) holes, and on a phenomenological treatment of the nonradiative rates associated with transitions involving conduction electrons falling into traps, and trapped electrons recombining with free holes. The various steady-state radiative and nonradiative e-h recombination lifetimes as function of the cw laser intensity are then obtained, as well as the dependence of the conduction-electron quasi-Fermi level (or chemical potential), and carrier densities on the laser intensity. We have also studied the laser-intensity dependence of various recombination efficiencies and of the integrated photoluminescence intensity. Finally, trap and impurity effects...

The binding energies of hydrogenic donor in both finite and infinite GaAs-(Ga,Al)As spherical quantum dots are calculated as a function of the donor position for different radii within the effective-mass approximation. It is observed an enhancement of the binding energy of donors in quantum dots when compared to results in quantum wells and quantum-well wires, which is an expected consequence of the higher geometrical electronic confinement in these systems. The density of impurity states as a function of the donor binding energy was also calculated. As a general feature it presents structures associated with special impurity positions that may be important in the understanding of the absorption and photoluminescence experiments of doped quantum dots.

The melanin is a group of biological pigments commonly found in living beings, it can be classified in three groups: eumelanin human beings, pheomelanins in animals, and allomelanins in the vegetal kingdom. There is a special interest in the eumelanin because this biopolymer exhibits the typical properties of a semiconductor. Eumelanin is also responsible of the main cellular photoprotection mechanisms in the human beings. The exact structural pattern of eumelanin is not completely known yet, the planar molecules 5, 6-indolquinine and its reduced forms semiquinone and hydroquinone outline the greater part of the biological pigment. In this work the structural, electronic and optical properties of monomers in various charged states (0, -1) and dimmers of eumelanin in vacuum, are found for the neutral state of charge, by means of the Semiempirical methods MNDO, AM1, PM3 and ZINDO/S-CI

Using the effective mass approximation and variational method we have computed the effects of hydrostatic pressure on the absorption and photoluminescence spectra in spherical quantum dot GaAs-(Ga, Al) As, considering a finite confinement potential of this particular work we show the optical transitions in quantum of various sizes in the presence of hydrogenic impurities and hydrostatic pressure effects. Our first result describes the spectrum of optical absorption of 500 A QD for different values of hydrostatic pressure P = 0, 20 and 40 Kbar. The absorption peaks are sensitive to the displacement of the impurity center to the edge of the quantum dot and even more when the hydrostatic pressure changes in both cases showing that to the extent that these two effects are stronger quantum dots respond more efficiently. Also this result can be seen in the study of the photoluminescence spectrum as in the case of acceptor impurities consider them more efficiently capture carriers or electrons that pass from the conduction band to the valence band. Density states with randomly distributed impurity show that the additional peaks in the curves of the density of impurity states appear due to the presence of the additional hydrostatic pressure effects.

The PTCDA (3,4,9,10-Perylene-tetracarboxylic dianhydride) and the NTCDA (1,4,5,8-Naphtalenetetracarboxylic dianhydride) are aromatic, stable, planar and highly symmetric with unusual electrical properties. The PTCDA is a semiconductor organic crystalline of particular interest due to its excellent properties and electronic potential that are used in optoelectronic devices and the NTCDA it is monoclinic and its space group is similar to that of the PTCDA. Recently, alternate layers of PTCDA and NTCDA were growth forming multiple structures of quantum wells showing a new class of materials with new optic lineal properties. Some have assured that their big utilities would be centered in the construction of diodes and of possible guides of waves. We have carried out calculations semi-empirical of the electronic structures and of optic properties of the PTCDA and of the NTCDA that show us that they are structures highly orderly polymeric, semiconductors in a negative load state (charge state=-2).

We have studied spin-polarized currents in resonant tunneling diodes containing diluted magnetic semiconductor layers with different geometries. The spin polarization effects on the resonant current are studied within Green's function formalism following the diagrammatic technique for non-equilibrium processes as proposed by Keldysh, using the one-band tight-binding modeling. The magnetic impurities can be located inside the well or the barrier layers of a diode sample. The resonant I Â V curves are analyzed as a function of the magnetic potential strength induced by the magnetic ions.

A quantum-mechanical calculation of radiative recombinations in cylindrical GaAs-͑Ga,Al͒As quantum-well wires excited by a cw laser in a photoluminescence experiment under quasistationary excitation conditions is performed. We work within the effective-mass approximation and the parabolic-band model for describing both electrons and holes, and consider, in the steady state, the interband absorption, and some radiative recombination mechanisms, such as recombination of electrons with free holes and with holes bound at acceptors. Carrier densities and electron-hole recombination decay times are calculated at room temperature and as functions of the laser intensity. For doped quantum-well wires, it is shown that the presence of acceptors substantially modifies the dependence on the laser intensity of the above quantities.

The ground-state energy and the binding energy of shallow hydrogenic impurities in spherical GaAs-(Ga,A1)As quantum dots have been calculated as functions of the radius of the dot. The binding energy has been calculated following a variational procedure within the e8'ective-mass approximation. We have used a finite confining potential well with depth determined by the discontinuity of the band gap in the quantum dot and the cladding. Calculations were also performed for an infinite confining potential. For the infinite potential we11 we found that the impurity binding energy increases as the dot radius decreases whereas in the finite potential-well situation, the binding energy reaches a peak value as the dot radius decreases and then diminishes to a limiting value corresponding to the radius for which there are no bound states in the well. We found that the strong electronic confinement in these quantum dots rejects itself in the ground-state energy and in the impurity binding energies, which are higher than those found in GaAs-(Ga, A1)As quantum wells and quantum-well wires.