1,720,988 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Superconducting non-contact linear slider for precision positioning in cryogenic environments
Get PDFIn this thesis, a novel device for precision positioning in a long stroke suitable for cryogenics environments has been proposed, designed, built and tested. The device is based on superconducting magnetic levitation. A set of high temperature superconductors allows a long permanent magnet to levitate stably over them. Furthermore, due to the high translational symmetry of the magnetic field applied on the superconductors for any position of the slider in its path, the superconductors not only provide stable levitation to the slider, but al so guide it. Therefore, a sliding kinematic pair is established between the permanent magnet and the superconductors. Finally, using a pair of coils, the position of the slider can be controlled with an open-Ioop control strategy of the current in the coils with nanometre resolution and reduced power consumption. Besides, a set of design rules has been proposed and experimentally verified at 77 K. Parameters of the performance of the mechanism such as the stroke, sensitivity, stiffness, natural frequency, run outs or the power consumption can be modified and optimized by an appropriate designo After this, two prototypes of a long stroke nanopositioner based on these design rules have been built and tested in a relevant environment (T~15 K, in a high vacuum < 10⁻⁶ Pa). Nanometre resolution in the positioning of a mass of about 170 g has been demonstrated in a stroke up to 18 mm. ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------En esta tesis doctoral se propone, diseña, construye y demuestra un dispositivo de alta resolución para posicionado en una carrera larga para entornos criogénicos. Este dispositivo está basado en levitación magnética superconductora de manera que, un conjunto de superconductores de alta temperatura (superconductores de tipo II) permiten a un imán permanente levitar de manera estable sobre ellos y, al mismo tiempo, ser guiado. De hecho, se establece un par cinemático de deslizamiento entre el imán y los superconductores gracias a la alta simetría traslacional del campo magnético aplicado en estos últimos. Además, la posición de la deslizadera puede ser controlada mediante una estrategia de control en bucle abierto de la corriente circulante en un par de bobinas diseñadas específicamente para esta tarea, obteniéndose una excelente resolución y un consumo de energía muy reducido. Así mismo, se proponen una serie de reglas de diseño que fueron verificadas a una temperatura de operación de 77 K. Estas reglas demuestran que hay una serie de parámetros característicos del desempeño del mecanismo como la sensibilidad, la rigidez, la frecuencia natural, las desviaciones o el consumo energético que pueden ser modificados mediante un diseño apropiado. Tras obtener estas reglas de diseño, un par de prototipos de un nanoposicionador de larga carrera han sido diseñados en consecuenCIa, construidos y probados en un ambiente relevante (T ~ 15 K, alto vacío < 10⁻⁶ Pa). Una resolución nanométrica en el posicionamiento de una masa de 170 gramos ha sido demostrada en una carrera de hasta 18 mm
Sistema experimental para la medición del ángulo de equilibrio entre imanes y superconductores
Get PDFLa expulsión del campo magnético del interior de un superconductor debido al efecto Meissner genera un conjunto de fuerzas que son capaces de hacer levitar un imán. Las fuerzas generadas por este efecto han sido estudiadas indirectamente en este proyecto midiendo el ángulo girado por un imán sometido a efecto Meissner- Oschenfeld en su posición de equilibrio. Las interacciones de las distintas superficies de la pastilla superconductora con el imán para diferentes posiciones relativas entre este y la pastilla hacen el ángulo de equilibrio girado por el imán varíe compensándose así los momentos generados por las fuerzas de origen electromagnético del superconductor sobre el imán. Con ello se corrobora un modelo teórico planteado por los profesores José Luís Pérez Díaz y Juan Carlos García Prada de la Universidad Carlos III de Madrid. Además el lector podrá consultar las condiciones de seguridad y precauciones a tener en cuenta cuando se emplea nitrógeno líquido, imanes y superconductores en el laboratorio, pudiéndose extrapolar algún conocimiento de este proyecto para el desarrollo de futuros experimentos similares. A partir del texto a continuación se ha extraído un artículo que a fecha de defensa del presente proyecto ha sido aceptado para discusión en la 55ª edición de la Conferencia Anual de Magnetismo y Materiales Magnéticos y para su posterior publicación en el Journal Applied of Physics. ___________________________________________________________________________The expulsion of the magnetic flux due to Meissner effect in a superconductor creates forces that allows a permanent magnet to levitate over the superconductor. These forces have been studied in this project indirectly measuring the equilibrium angle by a permanent magnet under Meissner- Ochsenfeld effect. The interaction of the different superconductor surfaces with the permanent magnet for different relative positions between the magnet and the superconductor makes the equilibrium angle to change, reaching the new equilibrium position. The experimental results are in agreement to a theoretical model proposed by Doctors José Luís Pérez Díaz and Juan Carlos García Prada from Universidad Carlos III de Madrid. The reader also can find the necessary measures to avoid the hazards of using liquid nitrogen, permanent magnets and superconductors.Ingeniería Industria
Design of a bearingless motor for health applications
Get PDFEn este proyecto de fin de grado se presenta el diseño electromagnético y mecánico detallado de un dispositivo de asistencia ventricular (LVDA) basado en una bomba cardiaca de flujo continuo equipada con cojinetes magnéticos activos. La combinación de la alta eficiencia energética de los actuadores electromagnéticos con la utilización de la tecnología sin contacto de cojinetes magnéticos activos permitirá potencialmente un funcionamiento preciso, eficiente y con menores daños sanguíneos en pacientes que se beneficien de esta terapia para casos de insuficiencia cardiaca grave.
El dispositivo propuesto, de 40 mm de diámetro y menos de 40 mm de altura, presenta dos grados de libertad de control: uno dedicado al giro del motor eléctrico y otro para el control de la inestabilidad axial del conjunto del motor y el rodamiento magnético. El dispositivo es capaz de suministrar un par máximo de 14,89 mNm empleando una densidad de corriente de hasta 8 A/mm2 y un rango de control axial de hasta 0,54 mm, siendo a su vez estable en el resto de grados de libertad del sistema.The electromagnetic and mechanical design of a left ventricular-assist device (LVAD) based on a continuous flow electromagnetic blood pump equipped with active magnetic bearings is presented in this final bachelor project. The combination of the high efficiency offered by electromagnetic actuators and the utilization of active magnetic bearings will allow a fine-working, efficient and less damaging operation in patients with heart failure problems who benefit from this therapy.
The proposed device, with a diameter of 40 mm and a height of less than 40 mm, presents two controllable degrees of freedom: one related to the spin of the electric motor and another one for the control in the unstable axial axis of the assembled motor and magnetic bearings. The device is capable of offering a maximum torque of 14.89 mNm using a current density of 8 A/mm2 and an axial controlled range of 0.54 mm, while being stable in the rest of the system degrees of freedom.Grado en Ingeniería en Tecnologías Industriale
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Sistema experimental para la caracterización de componentes electrónicos pasivos a temperaturas criogénicas
Get PDFEl presente proyecto fin de carrera versa sobre el estudio experimental del comportamiento de componentes electrónicos pasivos en temperaturas criogénicas. Se ha propuesto un método económico y eficiente para caracterizar estos componentes a bajas temperatura utilizando nitrógeno líquido LN2 a presión atmosférica (-195,8 °C). Se han investigado diferentes componentes electrónicos que incluyen resistencias, condensadores y diodos de diversos tipos, comparándose las prestaciones entre ellos. Algunos de los componentes estudiados han demostrado unas excelentes propiedades a temperaturas criogénicas, mientras que otros no tanto. Este trabajo puede resultar útil para investigadores que necesiten construir circuitos electrónicos en ambientes criogénicos. _______________________________________________________________________________This final project deals with the experimental study of passive electronic components in cryogenic temperatures. It has proposed an economical and efficient method to characterize these components at low temperature using liquid nitrogen (LN2) at atmospheric pressure (-195.8 ° C). Have been investigated various electronic components including resistors, capacitors and diodes of various types, comparing the performance between them. Some of the compounds studied have shown excellent properties at cryogenic temperatures, while others not so much. This work can be useful to researchers who need to build electronic circuits in cryogenic environments.Ingeniería Técnica en Electrónic
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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