1,720,995 research outputs found
Investigating the Correlation Between Microstructure and Machinability of Hot Forged Rings at Room Temperature
No full textAnalysis and Performance Evaluation of ZEM/ZEV Guidance and Its Sliding Robustification for Autonomous Rendezvous in Relative Motion
No full textDevising closed-loop guidance algorithms for autonomous relative motion is an important problem within the field of orbital dynamics. However, very few closed-loop algorithms have been devised that can autonomously generate feedback trajectories to execute rendezvous in relative motion (e.g. Lopez and McInnes, 1995, JGCD). In this paper, we explore the application of the generalized Zero-Effort-Miss/Zero-Effort-Velocity (ZEM/ZEV) feedback guidance (Guo et al., 2013, JGCD) and its robustified version known as Optimal Sliding Guidance (OSG, Wibben and Furfaro, 2016, ASR) to the problem of closed-loop spacecraft rendezvous guidance. The ZEM/ZEV feedback guidance has been studied extensively and can be found in the literature for intercept, rendezvous, terminal guidance and landing applications. Such analytical closed-loop guidance has been originally conceived by Battin who devised an energy optimal, feedback acceleration command for powered planetary descent. Ebrahimi et al. (2008, AA) introduced the ZEV concept, as a partner for the well-known ZEM and integrated it with a sliding surface for missile guidance with fixed-time propulsive maneuvers. Furfaro et al. (2011, Advances in Astronautical Sciences) extended the idea to the problem of lunar landing guidance and set the basis for the theoretical development of a robust closed-loop algorithm for precision landing. ZEM/ZEM feedback guidance is attractive because of its analytical simplicity as well as potential for quasi-optimal fuel performance. When robustified by a time-dependent sliding term, the resulting OSG can be proven to be Globally Finite-Time Stable (GFTS) in spite of perturbation with known upper bound. Here, we study the guided relative motion of two spacecraft for which one of them is executing an autonomous rendezvous via the ZEM/ZEV feedback guidance and its robustifed OSG counterpart. When augmented via time-dependent sliding, the application of Lyapunov stability theory for non-autonomous systems provides the sufficient conditions for GFTS. Indeed, the OSG can be demonstrated to be GFTS for any linear and non-linear relative motion model (e.g. rendezvous in circular orbit or in highly eccentric orbit). Starting from the classical Clohessy-Wiltshire (CW) model, we systematically analyze the ability of the ZEM/ZEV feedback guidance to execute quasi-optimal closed-loop maneuvers and its ability to correct disturbances for precision guidance. Comparison with the OSG counterpart will provide an assessment of the need for robustification as function of different rendezvous conditions (e.g. rendezvous in highly elliptical orbit) and different thrusting constraints (e.g. limited thrust)
Real-time prediction of geometrical distortion of hot-rolled steel rings during cooling
No full textThe paper deals with the application of neural network modelling to the real-time prediction of the geometrical distortion of hot rolled steel rings during cooling from rolling to room temperature. The neural network model was designed and developed to be part of a new modular system for the in-line monitoring and real-time control of the geometrical quality of rings, even those with a complex profile, during hot and warm ring rolling operations. The data utilised to train the neural network were generated by numerical simulations of the cooling phase. In order to do these simulations, an FIE model capable of coupling thermal, mechanical and metalllurgical events was accurately calibrated. The proposed model was then applied to an industrial case that is described in the paper
Robust Precise on Board Orbit Determination Exploiting T-RAIM for LEO-PNT
No full textThe rapid development of LEO-PNT constellations offers an opportunity to enhance GNSS services with additional signals from LEO satellites. These signals aim to improve performance and reliability for GNSS users. With various global initiatives, LEO-PNT systems utilize cost-effective, multi-tiered architectures that rely on GNSS spaceborne receivers for ODTS. These receivers use P2OD technology and PPP-like corrections to achieve real-time decimeter-level orbit reconstruction and unbiased nanosecond-level time synchronization. However, the interdependency between LEO and MEO layers can lead to cascading faults. This paper explores the extension of RAIM-like capabilities to spaceborne receivers, specifically T-RAIM, to mitigate these risks. The research validates the successful integration of T-RAIM with advanced HAS driven P2OD techniques in LEO scenarios, using a Hardware-in-the-Loop testing environment. The results show that an architecture with T-RAIM can preserve the nominal positioning and timing accuracy, even amidst MEO clock faults, ensuring ongoing system functionality without necessitating P2OD restarts-events that could interrupt the LEO-PNT services. The proposed loosely coupled integration seeks to handle the computational load and complexity while accommodating the limited resources of spaceborne receivers, aiming to offer a robust yet viable LEO-PNT solution
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
GOCE Italy: Scientific tasks and first results
No full textESA Living Planet Symposium
Bergen, 27/06/2010-02/07/201
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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