1,720,958 research outputs found
Nonlinear damping in energy harvesters
No full textEnergy harvesting from ambient vibration has become an attractive topic in the recent years. Initial studies aimed to maximise the performance of small linear device for different excitation scenarios. These devices were assumed to be located in hostile and inaccessible environments and be able to provide energy for low powered sensors. Due to the limited size of the energy harvesters, however, the amount of power produced was small.More recently, many researchers have considered using nonlinear stiffness to improve the performance of these devices. This thesis, however, focuses on the use of nonlinear damping in energy harvesters. Nonlinear damping may be unwanted and introduced as the mechanism of the harvester, but can also be deliberately introduced to improve the dynamic range of the harvester.Typically ambient vibration generates a relative displacement between the suspended mass and the base in an energy harvester, which induces an electromotive force (EMF) in a circuit that is used to harvest electrical energy. It is possible to introduce nonlinear mechanical damping by having a circuit with a nonlinear resistance. Specifically, a load, in which the current is a third-power function of the voltage is compared with an equivalent linear load for three kinds of excitation such as harmonic, random white noise and random bandlimited noise. According to the numerical and analytical results, the cubic load provides more harvested power at resonance at low levels when compared to an equivalent linear load at the same level of excitation. As the frequency bandwidth of a random excitation becomes wider, to the limit of white noise, as the power generated by the cubic converges to the linear case. Electromagnetic transducer energy harvesters usually adopt a conversion mechanism of motion, such as ball screw or rack and pinion, which introduce a source of loss such as friction. Static friction is then added to the model and this is shown to affect the harvested power at low input levels. Another proposed strategy consists of adjusting the electric load according to the input level, which can also enlarge the dynamic range of performance of energy harvested compared to a device with constant load. To demonstrate the effectiveness of the level-dependent load, an energy harvesting device was designed and manufactured, which comprised of an oscillating beam sprung to the base, and attached to a generator. Across the terminals of the generator, an electric resistance is mounted and the voltage measured is used to compute the harvested power. Experiments are conducted by exciting the harvested with a harmonic input at resonance via a shaker. A level-dependent load and a constant load were separately tested; with results that are in good agreement with the simulations, it is shown that by adjusting the load according to the input level, the harvested power is increased compared to a linear constant load
Design of an electromagnetic-transducer energy harvester
No full textThis paper presents the design and the manufacturing of an electromagnetic-transducer energy harvester. The design considers the coupling between the mechanical vibrating behaviour, generated by a base excitation, and the electromagnetic conversion of energy, which is aimed to produce the voltage across a load resistance. The design is based on some constraints, which are related to the characteristics of the shaker and aimed to obtain the best performance of the device. Current tests show the presence friction at low input levels, which is associated with the gearbox. The output voltage and the harvested power of the device are studied experimentally for different values of load. By increasing the value of the load from zero (short circuit) to high values (open circuit) the swing angle increases, while the harvested power presents a peak associated with the electrical damping. Also, harmonic tests are run at resonance for different levels of excitation to demonstrate the effect of the nonlinearity on the voltage and the harvested power. A nonlinear load resistance, is then introduced as part of future work. The aim is to try to increase the harvested power with respect to the linear load, at low level of excitation
Level-dependent load in a pendulum like energy harvester
No full textTypically ambient vibration generates a relative displacement between the suspended mass and the base in an energy harvester, and the induced electromotive force (EMF) in an electrical circuit is used to harvest electrical energy. Electromagnetic energy harvesters can operate at an optimum load, which provides the maximum power. However, due to the physical constraints of these devices, such as the limited available volume, the throw is also limited and operating at the optimum load is not feasible. By using a constant load, the optimum conditions cannot be achieved and the harvested power therefore can be reduced dramatically. To overcome this issue, the present paper proposes a new strategy, which consists of adjusting the electric load according to the input level at steady state condition. If the electric load, which is assumed to be a pure electrical resistance, is tuned according to the level of the harmonic excitation, then the harvester can always operate at the optimum conditions. To demonstrate the effectiveness of the level-dependent load, an energy harvesting device in the form of a pendulum is designed and manufactured, which is comprised of an oscillating beam sprung to the base, and attached to a generator. Across the terminals of the generator, an electric resistance (digital potentiometer) is mounted and the voltage produced is converted by Joule effect into thermal power, which is considered here as the harvested power. Experiments are conducted by exciting the harvester with a harmonic input at resonance via a shaker. A level-dependent load and a constant load are separately tested. The experimental results are in good agreement with the simulations. It is shown that by adjusting the load according to the input level, the harvested power is increased compared to a linear constant load.</p
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
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
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