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Comparison of the effectiveness of minimizing cost function parameters for active control of vibrational energy transmission in a lightly damped structure
No full textThe success of an active control of vibration system depends upon both the cost function used and the positions of the controlling actuators. The cost function used also affects the best actuator positions since their performance is judged on the attenuation of this parameter. However, the physical success will be dependent on how well the cost function represents the actual physical vibration. Sometimes the most meaningful cost function can be calculated in a theoretical model but is difficult to measure in practice, and a compromise to a more practical one is often made. In this paper four cost functions are considered with the aim of reducing the vibration transmitted from the base to the end of a lightweight cantilever two-dimensional structure, and their performances compared with a view to evaluating the true success in using other cost function parameters in reducing the vibrational energy.Of the four cost functions studied, two are energy-based: one representing the total vibrational energy and one using only the flexural energy level. The other two cost functions are based on velocity measurements: the sum of the squares of the translational velocity components, and one additionally using rotational velocity measurements. An initial study confirms that the total vibrational energy is the cost function which most comprehensively represents the beam vibration and is used as the reference in a comparison of the other cost functions.Then, a ranking of the best actuator positions on the structure is determined to achieve the best reductions in each cost function. For each of these sets of actuator positions the consequential attenuation in the total vibrational energy is evaluated whilst minimizing the other cost functions. Thus, the effectiveness of these cost functions in reducing the total vibrational energy is evaluated
Robustness of optimal design solutions to reduce vibration transmission in a lightweight 2D structure, part II: application of active vibration control techniques
No full textThis is the second paper which considers the reduction of the vibration transmission along a lightweight cantilever structure consisting of 40 rigidly joined beams over a frequency band. In the first paper [1] the reduction was achieved by allowing the geometry of the structure to be altered, such that the structure provided an inherently better vibration isolation. In this paper, the variation reduction over a band of frequencies is achieved using feedforward active vibration control (AVC) techniques applied to the original structure geometry. The success of AVC depends strongly on the position of the actuators. The actuator positions on the structure which achieve the best reductions in vibration transmission are found for one, two and thee actuators. A robustness analysis is then performed to show the sensitivity of each of the best solutions to small geometric perturbations. These solutions are the most practical, being less sensitive to small geometric changes that might occur, for example, as manufacturing tolerances. This is achieved by applying a sufficient number of random perturbations to determine the statistical distribution of the performance. A probability limit is then applied in order to predict a likely average minimum performance criterion, In addition to considering the robustness of the performance, the control effort required to achieve active control must be considered. If this increases significantly when the structure is perturbed the demand may not be met be a practical system, and the predicted performance cannot be obtained
Robustness of optimal design solutions to reduce vibration transmission in a lightweight 2D structure, part 1: geometric design
No full textA two-dimensional lightweight cantilever structure is studied, comprising 40 rigidly joined beams, of which the geometry is optimized to reduce vibration transmission over a given bandwidth. In this paper, the optimization is achieved by using genetic algorithms. Ten optimized design candidates were achieved for each of three cases resulting from minimizing an objective function (the vibration transmission between two points on the structure) which is calculated (i) using a single frequency, (ii) the frequency average over a narrow bandwidth, and (iii) the frequency average over a broad frequency range. All the candidates show performance improvements and normally the best performance is taken to be the best candidate. This paper then considers the sensitivity of each optimal candidate to small changes in the geometry of the structure. If the performance of a structure is too sensitive to perturbations its practical application is limited or may not be realizable in practice. The robustness of the optimized candidates is studied in order to find those candidates which are least sensitive to changing design parameters. This is achieved by perturbing the positions of the joints by an ensemble of sets of random numbers. The statistical effect on the objective function is investigated, and some candidates are seen to be more robust to such perturbations than others and generally the greater the bandwidth over which the structure is optimized the more robust the design. A selection criterion is then applied which enables the best candidates to be identified on grounds of both nominal optimized performance and robustness. Finally, the advantage of using genetic algorithms over traditional 'hill-climbing' optimization methods is shown, on the grounds of both nominal performance and robustness
On reducing vibration transmission in a two-dimensional cantilever truss structure using geometric optimization and active vibration control techniques
Full text linkFour optimization strategies were used to improve the average vibration isolation between the base and the end of a 10-m long two-dimensional (2D) cantilever truss structure. These were combinations of optimizing the structure geometry and the application of active vibration control (AVC) with optimal actuator positions. A power distribution analysis to investigate the mechanisms by which each strategy achieves reductions in the vibration transmission is reported. The trade-off is also explored between the freedom allowed in the size of the geometric changes and the number of actuators used in an AVC system to achieve a given level of vibration attenuation
Design of experiment algorithms for assembled products
No full textDesigning experiments to identify improvement in products that are assembled from manufactured components does not readily fit into conventional design of experiments methods and can be costly. Efficient methods are explored for determining designs for engineering problems where some, or all, of the factors of interest are (a) not easily set to prescribed values and (b) are dependent on a combination of properties of several components. The methods involve taking a sample of each type of component, measuring the relevant features and then finding a design that specifies an optimal set of assembled products for experiment. Three examples from manufacturing industry are presented to illustrate the approach. Two different algorithms for finding designs are described, an exchange algorithm and a genetic algorithm, and a comparison of their performances is made on the three examples
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
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