1,720,999 research outputs found
Adaptive control of launch vehicles in atmospheric flight
Full text linkThe research focuses on the development of innovative control techniques for attitude control of a launch vehicle (LV) during the atmospheric flight, and to investigate their possible benefits in terms - for instance - of improved disturbance rejection capability, as well as, a means for reducing the burden of recurrent activities of mission integration and flight program software finalization.
In this respect, a complete nonlinear mathematical model of the launch vehicle dynamics, comprehensive of all relevant aspects for the attitude control problem is first developed. Next, linearized equations of motion are derived under the assumption of small deviations of the vehicle motion from a reference trajectory. The time-invariant linear model is used to synthesize a baseline controller (BC) that features two proportional-derivative (PD) components for attitude and translational motion control, plus filters to phase-stabilize and notch the bending modes by using classical, frequency-based, analysis, and control design techniques.
Among several advanced algorithms referenced in the literature, the Adaptive Augmenting Control (AAC) has been selected and implemented in the LV flight control system in order to retain the functionality and proven record of success of classically designed linear control systems, while consistently and predictably improving their performance and robustness in expanded flight and/or uncertainty parameter envelopes. AAC adjusts the action of a baseline PID-type controller by means of a forward loop gain multiplicative adaptive law that, basically, on-line modulates BC output either to minimize the error with respect to a reference model or to limit undesirable high-frequency response in the control path.
In order to fully exploit the AAC potentiality, an effective and reliable tuning procedure for AAC gains is developed, where a robust design optimization (RDO) problem is formulated, and the goal is to maximize a statistical metric that describes FCS performance measured over a set of LV simulations.
Finally, an analysis of the effects of uncertainties on bending mode characteristics is carried out. Variations of bending mode parameters have a significant and negative impact on AAC performance and, consequently, on LV stability. In this respect, the use of adaptive filters is investigated in order to further improve flight control system robustness. An adaptive notch filter is designed, the parameters of which are updated continuously by an adaptation algorithm that uses the pitch rate sensor output so as to estimate the unknown parameters of the filter and precisely match the actual bending mode frequency
Adaptive attitude control of launch vehicles in atmospheric flight
No full textThe inclusion of an adaptive augmenting control (AAC) component in the flight control system (FCS) of launch vehicles can be highly effective for enhancing control stability and robustness with respect to parametric uncertainties and dealing with off-nominal conditions, so as to extend the envelope of failures and flight anomalies that can be managed by the vehicle control system. In this paper the adoption of an adaptive notch filter in a control architecture consisting of proportional-derivative (PD) elements, bending filters and AAC is proposed and discussed. The main goal of the study is to investigate the feasibility of implementation and the possible benefits of filter adaptation, such as overcoming critical limitations that degrades the AAC effectiveness for large uncertainties on elastic mode characteristics. To this end, the frequency of first bending mode is estimated during the flight in order to adapt the design parameters of the notch filter. Adaptive control performance is evaluated by simulation of vehicle motion in the atmospheric flight phase in selected stressing cases. Results of Monte Carlo simulations are also discussed for a broader assessment of the effects of adaptive filter on the robustness of integrated FCS
Opportunities and limitations of adaptive augmented control for launch vehicle attitude control in atmospheric flight
No full textThis paper investigates the benefits and the possible issues related to the use of an Adaptive Augmented Control system for launch vehicle attitude control in atmospheric flight. A time-frozen analysis is conducted, assuming a linear timeinvariant model of the rocket rigid body dynamics plus flexibility and parasite TVC dynamics for two launch-vehicle configurations, representative of a small launch vehicle and a large one, whose data are available in literature. The AAC system architecture is recalled and critically analyzed. An automatic gain tuning procedure is devised for the AAC adaptation gains, while refined tuning guidelines are provided for spectral damper filters. Eects of the AAC on the stability and robustness with respect to the baseline, that is non-augmented, controlled system are discussed by means of numerical simulations carried out in the time domain as well as using tools from linear system analysis in the frequency domain
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
Optimization of UAV Robust Control Using Genetic Algorithm
Full text linkA hybrid methodology combining the use of a robust LQR servomechanism (RSLQR) and a genetic algorithm (GA) for the design of the flight control system (FCS) of a lightweight unmanned aerial vehicle is the subject of this paper. The objective is to develop a systematic design approach based on a proven technique that provides improved time response and robust steady-state performance of the control system, so as to reduce the burden of trial-and-error procedures. The design of the inner loops of the UAV autopilot is formulated as an optimization problem where the GA is used to determine the weights of the RSLQR synthesis. The process is aimed at maximizing a weighted sum of an appropriately defined multi-objective fitness function, evaluated through a series of nonlinear simulations, so as to fully engage the control system in complex maneuvers, such as combined changes in altitude and heading at different flight speeds. The performance of the proposed control design approach is evaluated using analytical tools for linear systems, software-in-the-loop simulations, and Monte Carlo campaigns. The comparison between the new controller and a classical FCS with internal PID loops on attitude angles for stability and control augmentation is analyzed and discussed using an accurate vehicle model with an extended Kalman filter for output reconstruction
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
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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