1,720,966 research outputs found
Drag-free control design for the LISA space mission
Full text linkL'abstract è presente nell'allegato / the abstract is in the attachmen
Guidance and Control in Autonomous Debris Removal Space Missions via Adaptive Nonlinear Model Predictive Control
Full text linkSpace debris orbiting around the Earth are becoming a major problem that could impair the future of space exploration. Among the different approaches to this problem that have been proposed in recent years, this work focuses on a possible innovative solution, consisting in an autonomous spacecraft that performs a rendezvous maneuver, collects a debris of unknown mass and then moves to a parking orbit. When the spacecraft collects a debris of unknown mass, the dynamics of the system may change substantially, and this may affect the control stability and performance of the spacecraft. In this paper, a control system is designed, capable of handling situations with time-varying and uncertain parameters, as it occurs in space debris removal missions. A control strategy based on an Adaptive Nonlinear Model Predictive Control (ANMPC) is considered. The unknown mass of the debris is treated as an uncertain parameter and is estimated by means of two different methods (Recursive Average and Extended Kalman Filter (EKF)). Then, the estimated mass is used to update the internal model of the ANMPC, which later solves an on-line optimization problem, providing an optimal trajectory and control action for reaching the debris and then the parking orbit. The simulations carried out show that the proposed control system is able to effectively accomplish the requested task
LISA L3 Gravity Wave Observatory: Nonlinear Modelling and Preliminary DFAC Architecture
No full textRecently, we witnessed the revolutionary discovery of gravitational waves (GW) by a ground-based laser interferometric observatory: a potentially game-changing observation tool in astronomy. Hence, the opportunity of setting up a space-based GW observatory, including their low-frequency spectrum not accessible from the ground, is gaining more and more support.
In this framework, the Laser Interferometric Space Antenna (LISA) mission has been proposed within the European Space Agency selection of the L3 launch opportunity. Hence, LISA might be the first space mission scanning the sky to retrieve both polarisations of the GWs simultaneously and to measure their source parameters in a bandwidth spanning from 10-4 to 10-1 Hz. The latest LISA mission concept, nominally lasting 4 years in science mode, encompasses three identical satellites in a triangular constellation, in an Earth-trailing heliocentric orbit about 50 Mkm from the Earth, whose three arms, averagely long 2.5 Mkm, are endowed with six optical links for laser interferometry. This aims to measure, with high accuracy, the distance variations among the free-flying test masses hosted in the three spacecrafts. To this purpose, each spacecraft is drag-free controlled, in order to follow its two test masses, along each of the two interferometric axes.
In this paper, we first review the general aspects of the LISA mission, including those successfully tested in the LISA Pathfinder experiment. Then, an overall nonlinear model is proposed to describe the LISA constellation dynamics. Possible architectures and methodologies are finally discussed, for the LISA Drag-Free Attitude Control System (DFACS)
The LISA DFACS: Preliminary Model Predictive Control Design for the Test Mass Release Phase
Full text linkThis paper presents a preliminary control design for the test mass release phase of the LISA space mission. LISA will feature a triangular constellation of three spacecraft dedicated to the detection of gravitational waves. Each spacecraft carries two cubic free-falling test masses needed for the scientific experiment, which are initially locked by a clamp mechanism. When the plungers are retracted, the free-floating test masses are captured electrostatically by means of an electrostatic suspension system. However, the low actuation authority and the critical initial conditions, make the attitude and translation control of the test mass a difficult task. Model Predictive Control (MPC) appears to be a suitable technique for this application because of its ability to deal with the state constraints and the input saturations. In the present paper, the TM release context is briefly analysed, then the preliminary MPC design and simulation results are shown
The LISA DFACS: Overview of the Control Design Activities for the Drag-Free Mode
No full textThis paper presents the main design steps for the drag-free control of the next LISA mission. This work was carried out during the feasibility study Assessment and Preliminary Prototyping of a drag free control system for the L3 gravity wave observatory supported by the European Space Agency. After a brief introduction about the spacecraft, the main modelling results are reported. Functional and performance requirements are shown and then the control architecture and the H-inf mixed sensitivity control design are discussed. Finally, simulation results are presented
The LISA DFACS: A nonlinear model for the spacecraft dynamics
Full text linkIn the last few years, the observation of gravitational waves by means of LIGO and Virgo interferometers and the success of LISA Pathfinder, gave a significant boost to the development of space-based gravitational wave observatories. The European Space Agency confirmed LISA as the third large class mission of the Cosmic Vision program. The present work is part of the Drag Free and Attitude Control System (DFACS) preliminary prototyping study, which aims at the development of mathematical models and advanced controllers for the science phases of the LISA mission. Nonlinear modelling is a fundamental step for the derivation of linearized and decoupled models as well as for the development of suitable linear and nonlinear controllers. In this paper, an analytical nonlinear model is derived, which describes all the relevant dynamics of a LISA spacecraft, representing an effective compromise between accuracy and complexity. The model is extensively validated through linearization analysis and Monte Carlo simulations
A finite-time local observer in the original coordinates for nonlinear control systems
Full text linkIn this article, by coupling sliding-mode differentiators with a tool capable of inverting, in finite time, the suspension of the observability map, a local finite-time observer for nonlinear control systems is proposed. Differently from other approaches, this observer does not rely on a change of coordinates and provides the state estimates in the original coordinates
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
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