1,720,972 research outputs found
Quasi-distributed optical fibre strain sensors
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Distributed grating sensors: an alternative to multiplexed grating arrays?
No full textFibre Bragg gratings have been widely used for smart structures sensing applications. Until recently all sensor systems using gratings have only measured the average value of a physical field. The field has been averaged either over the length of a grating or over a length of fibre between gratings. To obtain a spatial image of the physical field, these sensors have been multiplexed to form a sensor array. Recently, fully distributed images of physical fields have been measured along the entire length of a grating. Distributed sensors show great promise for the detection and location of small physical features within structures, such as cracks and hot spots. As the fabrication length of gratings continues to increase, there appears great potential for distributed grating sensors. Distributed grating sensors may be classified as either narrowband or broadband, according to the spectral width of the interrogation source. Both types of sensor are discussed and briefly compared. The various forms of averaging gating sensors are also discussed and their performance is compared with that of distributed grating sensors using two specific smart structures applications. The latest results of our broadband distributed sensor are presented. This distributed sensor may be viewed as an adaptive averaging sensor since the number of interrogation regions, their size, location and the spatial resolution may all be varied in response to the sensing information. Finally, a sensing network combining the advantages of both multiplexed and distributed sensors is demonstrated and discussed
Multiplexed point and stepwise-continuous fibre grating based sensors: practical sensor for structural monitoring?
No full textWe report on recent progress on developing a complimentary pair of optical-fibre-based sensor methods for structural monitoring, for potential application in aerospace composites or civil engineering structures. Particular emphasis is placed on a method for addressing arrays of grating sensors using an acousto-optic tunable filter (AOTF), and on a longer-gauge-length system based on optical-time-domain reflectometry (OTDR). The AOTF possesses the desired frequency-agile capability for random multiplexed access and has a wide tuning range. It is suitable for both dynamic and quasi-static strain sensing. The scheme involves frequency-shift-keying of the RF drive to an AOTF to track the wavelength changes of gratings. We are currently constructing a compact interrogation system based on the concept. For many sensing applications, it is desirable to have a system able to monitor the average strain over a longer length of the structure. This is being researched using a specially-designed high-resolution OTDR. It enables us to determine the optical path length between partially-reflected points (eg gratings) along the fibre. From the measured optical range of each individual discontinuity, changes in length in each intervening fibre section can be determined. In terms of distance, a resolution equivalent to 100µm over a 5m long fibre section has been achieved within a measurement time of 5sec. Work is being directed towards extending the number of sections monitored, and reducing the measurement time using improved processing algorithms, and to extend performance using communications components
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
Lock-in techniques for interrogation of long- and short-gauge length optical fiber sensor arrays
No full textTwo complimentary optical fiber strain sensors employing lock-in techniques are presented. The first system interrogates an array of long gauge length sensors, defined by broadband optical reflectors and employs multiplexing in the time domain. The second system operates over shorter gauge lengths using multiple narrowband reflectors and wavelength-division-multiplexing. The first system tracks minima in the amplitude response produced from the superposition of two sinusoidal subcarrier waves. The second uses an acousto-optic-tunable-filter (AOTF) to track the peak reflective wavelength of an array of Bragg gratings. Both systems are constructed using telecommunications components. Together, the systems may be used to examine both line-integrated strain (or temperature) over long gauge lengths and local strain at a number of selected discrete points of particular interest. Lock-in techniques using dithered signals are applicable to sensors having a transfer function containing at least one turning point. This may be a maximum or minimum when observed either in transmission or reflection. The sensor responds to the dither with an amplitude-modulated signal, which permits locking of the interrogation system to the turning point. This provides a real-time response and better noise performance than scanned measurements. High-resolution monitoring of time-varying strain is demonstrated using this method. The long gauge length system has demonstrated a resolution of 3 microstrain over discrete 5 m long sensing sections, with an interrogation time of 0.25 s. When multiplexed to interrogate an array of four sections, intersection crosstalk levels were below minus 50 dB. The short gauge length interrogation system has been demonstrated using both fiber Bragg gratings and an in-line Fabry-Perot cavity as the wavelength selective reflectors. A resolution below 1 microstrain was obtained using the gratings, whereas a resolution of 1.5 multiplied by 10-6 in optical path-length-difference was obtained when interrogating a Fabry-Perot cavity. Simultaneous monitoring of multiple Bragg gratings has also been demonstrated by multiplexing with different dither frequencies. The versatility and the high resolution make the lock-in systems ideal for smart structures applications
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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