1,721,045 research outputs found
Huygens' synchronization of dynamical systems : beyond pendulum clocks
Full text linkSynchronization is one of the most deeply rooted and pervasive behaviours in nature. It extends from human beings to unconscious entities. Some familiar examples include the fascinating motion of schools of fish, the simultaneous flashing of fireflies, a couple dancing in synchrony with the rhythm of the music, the synchronous firing of neurons and pacemaker cells, and the synchronized motion of pendulum clocks. In a first glimpse to these examples, the existence of selfsynchronization in nature may seem almost miraculous. However, the main "secret" behind this phenomenon is that there exists a communication channel, called coupling, such that the entities/systems can influence each other. This coupling can be, for instance, in the form of a physical interconnection or a certain chemical process. Although synchronization is a ubiquitous phenomenon among coupled oscillatory systems, its onset is not always obvious. Consequently, the following questions arise: How exactly do coupled oscillators synchronize themselves, and under what conditions? In some cases, obtaining answers for these questions is extremely challenging. Consider for instance, the famous example of Christiaan Huygens of two pendulum clocks exhibiting anti-phase or in-phase synchronized motion. Huygens did observe that there is a "medium" responsible for the synchronized motion, namely the bar to which the pendula are attached. However, despite the remarkably correct observation of Huygens, even today a complete rigorous mathematical explanation of this phenomenon, using proper models for pendula and flexible coupling bar, is still missing. The purpose of this thesis is to further pursue the nature of the synchronized motion occurring in coupled oscillators. The first part, addresses the problem of natural synchronization of arbitrary self-sustained oscillators with Huygens coupling. This means that in the analysis, the original setup of Huygens’ clocks is slightly modified in the sense that each pendulum clock is replaced by an arbitrary second order nonlinear oscillator and instead of the flexible wooden bar (called here Huygens’ coupling), a rigid bar of one degree of freedom is considered. Each oscillator is provided with a control input in order to guarantee steady-state oscillations. This requirement of having a control input to sustain the oscillations can be linked to Huygens’ pendulum clocks, where each pendulum is equipped with an escapement mechanism, which provides an impulsive force to the pendulum in order to keep the clocks running. Then, it is shown that the synchronized motion in the oscillators is independent of the kind of controller used to maintain the oscillations. Rather, the coupling bar, i.e. Huygens’ coupling is considered as the key element in the occurrence of synchronization. In particular, it is shown that the mass of the coupling bar determines the eventual synchronized behaviour in the oscillators, namely in-phase and anti-phase synchronization. The Poincaré method is used in order to determine the existence and stability of these synchronous motions. This is feasible since in the system there exists a natural small parameter, namely the coupling strength, which value is determined by the mass of the coupling bar. Next, the synchronization problem is addressed from a control point of view. First, the synchronization problem of two chaotic oscillators with Huygens’ coupling is discussed. It is shown that by driving the coupling bar with an external periodic excitation, it is possible to trigger the onset of chaos in the oscillators. The mass of the coupling bar is considered as the bifurcation parameter. When the oscillators are in a chaotic state, the synchronization phenomenon will not occur naturally. Consequently, it is demonstrated that by using a master-slave configuration or a mutual synchronization scheme, it is possible to achieve (controlled) synchronization. Secondly, the effect of time delay in the synchronized motion of oscillators with Huygens’ coupling is investigated. In this case, the wooden bar, is replaced by a representative dynamical system. This dynamical system generates a suitable control input for the oscillators such that in closed loop the system resembles a pair of oscillators with Huygens’ coupling. Under this approach, the oscillators do not need to be at the same location and moreover, the dynamical system generating the control input should be implemented separately, using for instance a computer. Consequently, the possibility of having communication time-delays (either in the oscillators or in the applied control input) comes into play. Then, the onset of in-phase and anti-phase synchronization in the coupled/controlled oscillators is studied as a function of the coupling strength and the time delay. In addition to computer simulations, the (natural and controlled) synchronized motion of the oscillators is validated by means of experiments. These experiments are performed in an experimental platform consisting of an elastically supported (controllable) rigid bar (in Huygens’ example the wooden bar) and two (controllable) mass-spring-damper oscillators (the pendulum clocks in Huygens’ case). A key feature of this platform is that its dynamical behaviour can be adjusted. This is possible due to the fact that the oscillators and the coupling bar can be actuated independently, then by using feedback, specific desirable oscillators’ dynamics are enforced and likewise the behaviour of the coupling bar is modified. This feature is exploited in order to experimentally study synchronous behaviour in a wide variety of dynamical systems. Another question considered in this thesis is related to the modeling of the real Huygens experiment. The models used in the first part of this thesis and the ones reported in the literature are simplifications of the real model: the coupling bar has been considered as a rigid body of one degree of freedom. However, in the real Huygens experiment, the bar to which the clocks are attached is indeed an infinite dimensional system for which a rigorous study of the in-phase and antiphase synchronized motion of the two pendula is, as far as is known, still never addressed in the literature. The second part of the thesis focuses on this. A Finite Element Modelling technique is used in order to derive a model consisting of a (finite) set of ordinary differential equations. Numerical results illustrating all the possible stationary solutions of the "true" infinite dimensional Huygens problem are included. In summary, the results contained in the thesis in fact reveal that the synchronized motion observed by Huygens extends beyond pendulum clocks
Modeling and control of thermomechanical systems: managing heat-induced deformation in extreme ultraviolet lithography
Full text linkReduced-order model updating for prediction of performance variables in mechanical structures
Full text link
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Variations on the Author
Full text link“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
Full text linkWe 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
Model updating for digital twins using Gaussian process inverse mapping models
Full text linkIn engineering dynamics, model updating is typically applied to minimize the mismatch between a physical system and its digital twin. This paper proposes to use inverse mapping models, based on Gaussian Processes (GPs). The latter are trained offline using simulated data, enabling fast online updating of physically interpretable parameter values in first-principles-based nonlinear dynamics models. The GPs infer parameter values based on time-domain features measured on the real system. Additionally, GPs enables uncertainty quantification of the inferred parameter values. A nonlinear multibody model is used to illustrate the capability of this method to update parameter values, with high computational efficiency, and extract corresponding uncertainty measures
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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
- …
