1,720,982 research outputs found
Modeling of orbital and attitude dynamics of nanosatellites controlled via active electrostatic charging
No full textThe large-scale exploration of airless bodies, such as asteroids and moons, is gaining interest, however it is limited by mobility issues: the lack of atmosphere, low gravity, and unknown soil properties pose difficult challenges for many forms of traditional locomotion. The environment in proximity of these bodies is also electrically charged due to interactions with solar wind and UV radiation. The EGlider (Electrostatic Glider) concept would be able to overcome these mobility issues by leveraging the natural environment, allowing operations in close proximity of the surface, while enabling long duration missions by minimizing propellant consumption. The EGlider is an advanced concept for small satellite mobility and propulsion, which relies on the electric fields naturally present around airless bodies in order to generate forces and torques useful for maneuvering. It does so by extending electrically charged appendages, which enable it to electrostatically soar above the surface. By differentially charging its electrodes it can also produce torques to control its attitude. The charges are maintained by continuous active ion or electron emission from the spacecraft, in order to cancel out the neutralizing influx of charges from surrounding plasma. An investigation of the spacecraft-plasma interaction was carried out. This included studying the effect of electrode geometry and calculating the charge-to-mass ratios required to enable several mission scenarios. Long, thin wire electrodes were identified to be the most power-efficient and would allow power-to-weight ratios achievable with current nanosatellite technologies. High electrode potential represents the main limiting factor for the system design. In order to test the feasibility of active control by means of differential charging, a simple 2D interaction model was developed, and a feedback controller to stabilize the vehicle was tested in a simulation environment. The results confirmed that good performance could be obtained for both position and attitude control. Finally, a dedicated software was developed for future simulation and testing of control strategies for the EGlider. This software allows to study the trajectory and attitude of an arbitrarily configured spacecraft in the proximity of an arbitrarily defined main airless body. The spacecraft can be assembled from basic parts, each with specified electrical, mass and optical properties. Efficient models allow to calculate gravitational and electrical interactions with the rotating main body and the local plasma field, as well as solar radiation pressure effects. Control models can be implemented as simple plug-in functions and easily tested. The preliminary validation campaign showed good matching with the reference cases that have been analyzed
Formation Pointing Dynamics of Tether-Connected Architecture for Space Interferometry
No full textAbstract The baseline pointing dynamics, i.e. the attitude of the instantaneous plane of rotation, of
a linear tethered interferometer with single baseline orbiting in an Earth trailing,
heliocentric orbit is analyzed. Two tether arms provide a passive control of the
spacecraft formation by keeping two light collectors and one combiner aligned while
spinning about the boresight of the interferometer. The interferometer optical path delay
variation under gravity gradient perturbation and unbalanced solar radiation torque is
investigated analytically for a fixed-length dumbbell model. Analytical results are
compared with results from numerical simulations of a visco-elastic massive tether model
in order to assess the effect of the tether dynamics on the pointing stability of the
formation. The study highlights the fundamental role of spin stabilization in
counteracting the effect of external perturbations upon the interferometer Optical Path Delay (OPD)
Modeling and simulation of trapping mechanisms of granular media in space
No full textThis paper describes the modeling and simulation of trapped granular media, within the context of the Granular Imager project. We describe the physics of trapped granular media in space, and the methodologies used to stably confine and shape such a medium using electromagnetic fields. The numerical models have also been validated with results in the literature, obtaining excellent agreement. The results of the numerical tests indicate that it is possible, with structural arrangements of rings and plates at different levels of electrostatic potential, to stably confine one or more charged particles, when driven by voltages that can be modulated in time and space
Dynamical Effects of Solar radiation Pressure on a Spinning Tethered System for Interferometry
No full text
Dynamics and Control of Helical Arrays in Low Earth Orbit
No full textSpacecraft formation flying is an anticipated critical technology, needed to enhance astrophysical and science missions in near-Earth and interplanetary environments. Enabling a set of distributed spacecraft to corporate together, collectively fulfilling a mission objective, has proven to have several benefits over the conventional large single entity spacecraft. Mission cost and risk are reduced, while the retrieval of scientific data is significantly increased. Augmented adaptability and flexibility will play a crucial role in future space missions which require radar apertures that are excessively large and not practical to built. The key strategic goal of our work is to develop active and passive radar remote sensing applications based on distributed array architectures. Distributed formations of low-cost Small-Sats, either deployable or free-flying, can deliver a comparable or greater mission capability than large monolithic spacecraft, but with significantly enhanced flexibility (adaptability, scalability, evolvability, and maintainability) and robustness (reliability, survivability, and fault-tolerance). This research is aligned with the NASA Technology Roadmap for Robotics and Autonomous Systems (TA4), in particular TA4.5 System-Level Autonomy (Activity Planning; Autonomous Guidance and Control; and Multi-Agent Coordination) and TA4.6 Autonomous Rendezvous and Docking. This paper outlines the design of a small-satellite helical formation, serving as a Synthetic Aperture Radar (SAR) in low Earth orbit. The macro topic treated in this paper is the analysis of the feasibility and problems related to the operation of autonomous satellite formations serving as a Synthetic Aperture Radar (SAR) in low Earth orbit. An earlier preliminary version of this work was presented recently [1]. The main objective is to control with great precision the relative 6DoF dynamics of the followers with respect to a leader satellite in order to allow a correct taking of the data required by the mission. The differential accelerations to which the formation satellites are subjected make it necessary to implement control techniques for their re-positioning. To ensure a long mission duration, the number of such correction maneuvers should be minimized. In an autonomous formation perspective, such corrections are computed by the spacecrafts themselves, which therefore need to be equipped with sufficient computational resources. In this paper the problems just presented are described in detail and some techniques to mitigate their effects are reported. In order to have results more similar to reality, a high precision dynamic propagation model has been created and validated with the NASA General Mission Analysis Tool (GMAT). This model includes harmonics of the gravitational potential up to order 21, drag, solar pressure and third-body perturbation (Moon and Sun) [2]. After defining the external environment in which the satellites operate, the problem of maintaining the desired configuration of the system is addressed through two different analyzes: uncontrolled dynamics stability analysis and active formation control. The study of uncontrolled formation stability aims to derive the initial conditions of the formation satellites that most closely minimize the relative drift between followers and leader. This allows to reduce the number of maneuvers required to maintain the formation given a fixed interval of time. Despite the careful choice of initial conditions, this drift, although minimal, will tend to alter the initial configuration until the formation is no longer operational. For these reasons, an active control solution, aiming to minimize the amount of fuel used to perform the correction maneuver, has been implemented. The optimal controller is presented in different variants, in particular two strategies, centralized and decentralized, have been implemented in the context of Sequential Convex Programming (SCP). Both types of control were analyzed considering possible un-modeled external factors. Some test cases are reported so that discussion and conclusions can be made regarding the limitations and issues associated with the methodologies implemented
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
Lagrangian modeling and simulation of the free surface-affected dynamics of underwater vehicles
No full textThe paper deals with the development of a novel simulator for underwater vehicles that takes into account the interaction between the submerged body and the free water surface. By using a Lagrangian approach, the potential flow theory is combined with the 6-DoF equations of motions, in order to derive a mathematical model of the system dynamics to be solved in time-domain. A numerical model is then specifically developed and an extensive simulation campaign is carried out, leading to the Submerged Bodies Simulator (SubBoS). The results highlight an extension of the prediction validity with respect to the state-of-art modeling, typically based on stringent hypotheses on body motion (e.g. small displacements, pure surge motion)
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