1,721,032 research outputs found
Analytical Solutions to Spacecraft Formation-Flying Guidance Using Virtual Motion Camouflage
No full textFormation-flying has been recognized as one of the enabling technologies for a variety of space mission concepts because of the potential benefits in cost reduction, enhanced flexibility, increased observational baseline, and better survivability and reliability. One important requirement of the formation-flying system is the ability to generate trajectories in real time reliably to maximize mission return. This paper presents simple and analytical open-loop guidance solutions to formation-flying reconfiguration trajectory design problems through the bioinspired virtual motion camouflage methodology and the pseudospectral discretization approach. The algorithms investigated here can be applied to different kinds of initial and final conditions as well as different linearized models considering the eccentricity effects and/or the J2 perturbation. Simulations are used to demonstrate the capabilities of the achieved analytical solutions for two formation-flying reconfiguration scenarios. Copyright © 2010 by Yunjun Xu. Published by the American Institute of Aeronautics and Astronautics, Inc
Subspace Optimal Control and Motion Camouflage
No full textSome insects can move towards female ones or their preys while minimizing the chance of being aware from their natural background using a technique called motion camouflage. In this paper, first as a prelude, an optimal steering law for the minimum time motion camouflage under a speed constraint is proposed. The result of the optimal steering law is then compared with the ones achieved numerically using the pseudospectral collocation and nonlinear programming. The findings from this comparison thereafter leads to a much deeper discussion of the proposed virtual motion camouflage subspace optimization method, which is expected to benefit widely experienced nonlinear constrained optimal control problems. Since the optimization is progressed in the subspace, the solution optimality is justified accordingly. Through this method, the dimension of the nonlinear constrained optimal control problem can be reduced and the rapidly achieved trajectory could be used as a good and feasible initial guess for further optimization. © 2008 by Yunjun Xu
Pre and Post Optimality Checking of the Virtual Motion Camouflage Based Nonlinear Constrained Subspace Optimal Control
No full textNonlinear constrained trajectory optimization remains an active field of research. Current popular methods end up with either solving a classical multi-dimensional two-point boundary value problem or a high dimensional nonlinear programming problem. Inspired by the motion camouflage phenomenon of insects like dragonflies, recently the authors proposed a subspace optimization method, the virtual motion camouflage method, in order to reduce the problem dimension. Thus the computational cost experienced in the widely used direct collocation and nonlinear programming methods can be reduced. The solution found through this approach is in the feasible region however the optimality of the solution is not guaranteed automatically in the original full search space. In this paper, two optimality checking ways extended from the Karush-Kuhn-Tucker necessary condition are proposed. The pre-optimality checking is proposed to judge the selection of the virtual prey motion and the reference point, while a post-optimality checking is suggested to test the solution obtained from the virtual motion camouflage method in the original search space. Two numerical examples are used to illustrate the capabilities of the new subspace optimal control algorithm. Copyright © 2009 by Yunjun Xu
A Quadrature Based Method of Moments for Nonlinear Filtering
No full textAccording to the nonlinear filtering theory, optimal estimates of a general continuousdiscrete nonlinear filtering problem can be obtained by solving the Fokker-Planck equation, coupled with a Bayesian update rule. This procedure does not rely on linearizations of the dynamical and/or measurement models. However, the lack of fast and efficient methods for solving the Fokker-Planck equation presents challenges in real time nonlinear filtering problems. In this paper, a direct quadrature method of moments is introduced to solve the Fokker-Planck equation efficiently and accurately. This approach involves representation of the state conditional probability density function in terms of a finite collection of Dirac delta functions. The weights and locations (abscissas) in this representation are determined by moment constraints and modified using the Baye\u27s rule according to measurement updates. As compared with finite difference methods that are typically used in solving the Fokker- Planck equation, the computational cost is much lower. As emonstrated by a numerical example this approach appears to be promising in the field of nonlinear filtering. Copyright © 2008 by Yunjun Xu. Published by the American Institute of Aeronautics and Astronautics. Inc
Unmanned Aerial Vehicle Formation Flight via a Hierarchical Cooperative Control Approach
No full textCooperative control is crucial for networked dynamical systems, such as unmanned aerial vehicles in a formation, to respond more quickly and to be capable of working in a cluttered environment. In this paper, a divide-and-conquer hierarchical approach is proposed in three levels to handle the design challenges associated with the mission consensus, different constraints, and precise feedback tracking. In the top level, the algorithm aims to guarantee the macro cooperative behaviors, such as formation consensus of multiple agents under simplified double-integrator dynamics and obstacles. In the middle level, the virtual motion camouflage based algorithm is employed in each individual vehicle to compute the near optimal trajectory considering realistic constraints that the top-level algorithm has to neglect. To enhance the robustness of the trajectory control, a nonlinear feedback controller is used in the bottom level for each vehicle to precisely track the desired trajectory commanded by the middle level. Simulation results demonstrate the formation control capabilities of the hierarchical control strategy under obstacle-laden environments and constraints. © 2011 by Yunjun Xu, Ming Xin, Jianan Wang
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
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
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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