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Modeling and motion analysis of autonomous paragliders. Department of Computer and System Sciences Technical Reports, no. 5, 2010
No full textThis report describes a preliminary study on modeling and
control of parafoil and payload systems with the twofold objective of developing tools for automatic testing and classification of parafoils and of devising autonomous paragliders able to accomplish long-range delivery or monitoring tasks. Three different models of decreasing complexity are derived and their accuracy compared by simulation
Dynamics and control of a paraglider for planetary exploration
No full textParagliders represent a light, low cost, space efficient means for autonomous transportation. They can be used for accurate delivering of payloads using inexpensive guidance and control modules. If equipped with proper sensor, it is possible to use paragliders for scientific observation during flight, such as chemical, thermal, meteorological, or biological analyses of the atmosphere. Aerial remote sensing of the surface can also be carried out, obtaining high-resolution imaging, measurements of seismic activity, and more generally collecting data on dangerous or inaccessible areas. Since they are inexpensive, light and versatile, they are interesting for planetary exploration, in particular if Mars, Venus, Titan and the Outer Planets are considered because of their atmosphere. The aim of the paper is to have a preliminary study of the behavior of a paraglider flying through the atmosphere of a planet. A scalability analysis to compare performance with results obtained when flying over Earth will be carried out, highlighting that at steady state all coordinates but the norm of velocity reach the same equilibrium point. A ratio between the velocity over the planet and over Earth will be derived as function of the gravitational acceleration and the atmosphere density. Finally, the performance of a line following algorithm developed in a previous work will be evaluated through simulations on Mars
Position and Attitude Determination of Orbiting Manipulators Using GNSS
No full textManipulator GNC problem calls for an effective,
robust control, which can actually provide good
performance only up to the level the kinematic state
(i.e. the position, velocity and attitude) is well
known. The present paper is devoted to the sensors,
which should provide such a state. Besides the
request for accuracy, a special attention, as for the
other components of the GNC loop, should be paid
to the extreme-operational environment they will
face onboard.
A typical solution for manipulators’ kinematic
sensing is represented by encoders, located at the
joints and able to provide the rotation between the
following arm members. The location of the end
effector is therefore computed by simple geometry
due to the knowledge of the arms’ length (Figure 1).
Different technical solutions do exist, for the observables nature (analog or digital), for the working principle (optical, electromagnetic) and for the measurements (incremental or absolute).
In order to provide a different, possibly better
solution, and also looking for a back-up sensing
technique, the chance to use GNSS is suggested. The idea is to locate receiving antennas at the joints, and even along the
arms to define the end effector’s relative position
Determination of kinematic state of an orbiting multibody using GNSS signals
No full textPrecise attitude determination of the members of a free-flying multi-body system is a not so immediate task, due essentially to the large motion of its appendages coupled with their relevant flexibility effects. In fact, sensors used to this aim in current projects, such as optical encoders usually positioned near the joints of each arm, are almost blind to these effects, and clusters of specific redundant sensors should therefore be required in order to reconstruct both elastic deformations and rigid motion. Satellite navigation systems (GNSS) offer a suitable and reliable solution to this problem. To Exploit the phase of the signal, instead of the traditional pseudo random code, ensures a very high accuracy of the order of magnitude of centimeter. Such a process requires the solution of an initial ambiguity problem, related to the number of integer wavelength included in the length of the member. The aim of the paper is to investigate the capability of this GNSS based technique to reconstruct the kinematics of a flexible multi-body system orbiting around the Earth. This analysis requires a simulation including both the multi-body dynamics and the navigation system constellation to define the satellites lines of sight at each time step. Concerning multi-body equations of motion, a Newtonian formulation is adopted in this work. A special attention is required about the choice of the state variables. As the internal forces are associated to the relative displacements between the bodies, which are small fractions of the distance of the multi-body spacecraft from the center of the Earth, the task of obtaining these forces from inertial coordinates could be impossible from a numerical point of view. So, the problem is reformulated in such a way that the equation of motion of the system contains global equations, with no internal forces, and local equations, with internal forces. In the latter only quantities of the same order of the spacecraft dimensions are present. Accuracies achievable in LEO orbit with current GPS and upcoming Galileo systems are evaluated to show the interest of the proposed technique
Theoretical, Numerical and Experimental Activities on Multibody Dynamics, Navigation and Control at Università di Roma La Sapienza
No full textControl-oriented modelization of a satellite with large flexible appendages and use of worst-case analysis to verify robustness to model uncertainties of attitude control
No full textThe design of large space structures (LSS) requires the use of design and analysis tools that include different disciplines. For such a kind of spacecrafts it is in fact mandatory that mechanical design and GNC design are developed within a common framework. One of the keypoints in the development of LSS is related to the dynamic phenomena. These phenomena usually lead to two different interpretations. The former one is related to the overall motion of the spacecraft, i.e. the motion of the center of gravity and motion around the center of gravity. The latter one is related to the local motion of the elastic elements that leads to oscillations. These oscillations have in turn a disturbing effect on the motion of the spacecraft. From an engineering perspective, the structural model of flexible spacecrafts is generally obtained via FEM involving thousands of DOFs. Many of them are not significant from the attitude control point of view. One of the procedures to reduce the structural DOFs is tied to the modal decomposition technique. In the present paper a technique to develop a control-oriented structural model will be proposed. Starting from a detailed FE model of the spacecraft and using a special modal condensation approach, a Continuous Model is defined. With this transformation the number of DOFs necessary to study the coupled elastic/rigid dynamic is minimized. The final dynamic model will be suitable for the control design implementation. In order to properly design a satellite controller, it is important to recall that the characteristic parameters of the satellite are uncertain. The effect that uncertainties has on control performance must be investigated. A possible solution is that, after the altitude controller is designed on the nominal model, a Verification and Validation (V&V) process is performed to guarantee a correct functionality under a large number of scenarios. The V&V process can be very lengthy and expensive: difficulty and cost do increase because of the overall system dimension that depends on the number of uncertainties. Uncertain parameters have to be parametrically investigated to determine an estimate of the worst case stability and performance of the control laws. A gridding approach can be used and the V&V can be obtained with the Worst-Case Analysis, i.e. an optimization process, to find an estimation of the true worst-case behavior, such to verify that the design is robust enough to meet the system performance specification under worst-case conditions and tolerances
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
KINEMATIC STATE DETERMINATION TECHNIQUES FOR ROBOTIC MANIPULATORS INTENDED FOR SPACE APPLICATIONS
No full textVariations 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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