1,720,958 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
Control system design by convex optimization
Get PDFThis thesis deals with the Boyd-Barratt paradigm for feedback controller design. The Boyd-Barratt approach combines the Youla parameterization with convex optimization. In this thesis, their paradigm is accepted in its entirety, but a completely different numerical approach is adopted. An algorithm due to Akilov and Rubinov, which is in essence an abstract rendition of one of the famous algorithms of Remez, is used instead. This completely circumvents the need to compute derivatives or subdifferentials, which can be a difficult task. Instead, certain linear functionals must be computed, and this is generally quite straightforward. An attractive feature of the approach is that the code is much shorter and more elegant. The Boyd-Barratt paradigm has the disadvantage that an infinite dimensional Banach space must be truncated to a finite dimensional subspace prior to optimizing. This thesis also applies certain primal-dual techniques from functional analysis to study the implications of this truncation. Primal-dual theory is used to show th a t the true optimal solution lies within the solution of two semi-infinite linear programming problems, namely the dual problem with finitely many variables and the primal problem with finitely many variables. Also, it is shown th a t the alignment property is closely related to the cost of truncation. These results provide an analysis of the effect of truncation
On the robustness of the linear quadratic regulator via perturbation analysis of the Riccati equation
Get PDFThe linear quadratic regulator (LQR) has been shown to have very attractive stability robustness properties. However, some authors have shown that LQR may suffer from poor robustness when special perturbations in its state-space formulation were introduced. This thesis continues the study of the stability robustness of LQ regulators. To acquire good stability robustness, weight selection is first investigated. For general cost weighting matrices, a new lower bound on the minimum singular value of the return difference is proposed. New guaranteed stability margins are also presented. This gives a formal mathematical basis for guidelines for the designer to improve stability robustness. As the weight on the plant’s inputs approaches zero, the exact bound on the perturbations which ensures stability is compared with the guaranteed margins. It is shown that the stability robustness properties are preserved in a general sense. Then, a numerical analysis of the conditioning of the continuous-time algebraic Riccati equation (CARE) is presented. The condition numbers of the CARE are utilized to measure the sensitivity of LQR subject to parameter changes. It is shown that the condition numbers grow sig-
nificantly when the weighting parameters on the system state and input matrices approach zero and infinity. This has application to the applied control situation
because it can be used to detect hidden vulnerabilities in LQR systems
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
Modelling of signal uncertainty and control objectives in robust controller design
Get PDFThis work develops a new paradigm for optimal robust controller synthesis in the frequency domain. A detailed examination is made of the engineering motivation and engineering efficacy underlying the various strands of robust control theory. The modelling of (a) signal uncertainty and (b) control system objectives in both Tioo and C\ control theories is considered in particular detail. Based on this examination, a theory which can fa irly be described as ‘a m odified 7ioo control theory’ or ‘a frequency domain C\ control theory’ is proposed. New signal sets for the modelling of uncertain signals are introduced. It is argued that these models more faithfully capture the way in which uncertain signals act on real physical systems. It is shown that by adopting these new models for uncertain signals, control theory can be used to
non-conservatively minimise maximum tracking errors in the time domain, in the SISO case. In the MIMO case, the problem of optimally synthesising a controller to non-conservatively minimise tracking errors in the time domain leads to a modest variation on existing control theory, requiring the usual norm to be modified
slightly. It is argued th a t the proposed paradigm in general achieves a better quality of control and more fa ith fu lly expresses the true objectives of feedback control systems. The proposed development is seen to also extend naturally to Ti.2 control theory, and indeed provides a new deterministic justification for the 7^2 control problem in the MIMO case.
The question of design transparency in the synthesis of optimal robust controllers for multivariable systems is considered in detail. The implications of the proposed paradigm for transparency of design and weighting function selection are detailed. A decoupling design procedure for robust controller synthesis is proposed which, under certain restrictive conditions, allows the calculation of super-optimal robust controllers on a loop by loop basis. The usefulness of a classical decoupling approach to MIMO control system design in the context of multivariable robust control theory is demonstrated.
A number of design examples are presented which show how the ideas and methods developed in this work can be applied to realistic control problems
Computation and applications of the structured singular value
Get PDFThis work develops a number of new algorithms for the computation of the convex estimate, ¿tco, of the structured singular value, fj. The basis for these algorithms lies in an application of the geometric form of the Hahn-Banach theorem Dual methods are shown to be more useful than their primal counterparts when determining Ha. In particular, a dual method based on a 1-norm optimisation strategy is shown to out perform its 2-norm based counterpart in terms of accuracy, reliability and speed. It is proven that this 1-norm dual method converges to /¿co. The algonthm has been successfully validated using a large selection of random, pseudo random and practically motivated problems. Improvements to the basic algorithm that significantly reduce computation times are outlined and analysed in some detail.
This work presents new applications for \i /x provides a novel way of analysing, nonconservatively, the effect of uncertainty in component values on filter performance. It is shown that the problems of computing (1) maximum filter gam, (11) minimum gain and (in) the maximum Euclidean deviation from nominal performance on a polar plot can all be determined using fi theory. The necessary formulations required to deal with these particular problems are developed. In contrast to a grid search or probabilistic approach, using /i provides frequency response information that fully addresses the cross coupling effects of component uncertainty in a rigorous and repeatable fashion. This process has been automated for Butterworth and Chebychev filters of arbitrary order. Third order examples are used to illustrate the approach.
L\ methods are used to develop new tuning rules for PID controllers This is achieved using the operator which can reasonably be described as a time domain analogue of /i. Used in conjunction with an application of the robust performance theorem, is used to find the PID settings that best satisfy a given time domain performance objective
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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