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| 2. |
- Sjödahl, Mikael, et al.
(författare)
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Multi-spectral speckles : theory and applications
- 2012
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Ingår i: Proceedings of Speckle 2012. - Bellingham, Wash. : SPIE.
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Konferensbidrag (refereegranskat)abstract
- The objective of this paper is to discuss the properties and a few applications of multi-spectral speckles. The paper starts with a theoretical section where the correlation properties of multi-spectral speckles are detailed for the case of reflective imaging geometry. Both a free-space geometry and an imaging geometry are detailed. As an application example effects and possibilities provided by the theory in a measurement of surface shape of a generally shaped object from a dual-wavelength holographic recording are detailed. It is showed that the same phase profile is obtained from integration of speckle movements and phase unwrapping and they are therefore exchangeable quantities.
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| 3. |
- Erhart, Paul, 1978
(författare)
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A first-principles study of helium storage in oxides and at oxide-iron interfaces
- 2012
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 111:11
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Tidskriftsartikel (refereegranskat)abstract
- Density-functional theory calculations based on conventional as well as hybrid exchange-correlation functionals have been carried out to study the properties of helium in various oxides (Al2O3, TiO2, Y2O3, YAP, YAG, YAM, MgO, CaO, BaO, SrO) as well as at oxide-iron interfaces. Helium interstitials in bulk oxides are shown to be energetically more favorable than substitutional helium, yet helium binds to existing vacancies. The solubility of He in oxides is systematically higher than in iron and scales with the free volume at the interstitial site nearly independently of the chemical composition of the oxide. In most oxides, He migration is significantly slower and He-He binding is much weaker than in iron. To quantify the solubility of helium at oxide-iron interfaces two prototypical systems are considered (Fe-MgO, Fe-FeO-MgO). In both cases, the He solubility is markedly enhanced in the interface compared to either of the bulk phases. The results of the calculations allow to construct a schematic energy landscape for He interstitials in iron. The implications of these results are discussed in the context of helium sequestration in oxide dispersion strengthened steels, including the effects of interfaces and lattice strain.
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| 4. |
- Felicetti, Leonard, et al.
(författare)
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Coordinated attitude control for multiple heterogeneous satellites missions
- 2012
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Ingår i: AIAA/AAS Astrodynamics Specialist Conference 2012. - Reston, VA : American Institute of Aeronautics and Astronautics, AIAA. - 9781624101823
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Konferensbidrag (refereegranskat)abstract
- The paper investigates cooperative control strategies for spacecraft formations, also in case platforms are not homogeneous but differs in attitude control actuators. Specifically, either a common inertial or a time-varying pointing are considered as requirements for a formation of spacecraft, controlled either entirely by reaction wheels or partly by wheels and partly by thrusters. Two control strategies, namely the classical leader-follower or a more cooperative one, also labelled as behavioural based, where the kinematic state of each spacecraft is known to the others and enters in their command loop, are applied. In order to actually compute the actions, two controllers are considered: a classical proportionalderivative (PD) and an optimal one using the variable gain state dependent Riccati equation (SDRE). Numerical simulations to validate the approach are presented and, within this implementation, SDRE approach shows to succeed even in cases when PD fails
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| 5. |
- Felicetti, Leonard, et al.
(författare)
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Formation flying dynamics analysis by means of a virtual multibody approach
- 2012
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Ingår i: First IAA Conference on Dynamics and Control of Space Systems 2012. - San Diego, Calif : American Astronautical Society. - 9780877035879 ; , s. 577-596
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Konferensbidrag (refereegranskat)abstract
- The aim of this paper is to propose the multibody approach as an alternative method for the analysis of the relative motion in a satellite formation and the evaluation of the related control effort. In the suggested implementation the spacecraft are substituted by the joints of a multibody system in which the links, represented as virtual structural elements, reproduce the relative constraints in position and attitude among the platforms. The idea is to consider the commanded, time-varying orientation and length of the links such that the joints will eventually assume the relative geometry which is the formation's target state at a given time. The forces and torques to be provided to the real spacecraft belonging to the formation are related to the reaction torques and forces which are provided at the joints in the corresponding multibody representation. These reactions can be easily computed by available multibody codes, and can be inserted in a standard orbital propagator to compute the dynamical behaviour of a formation and to validate the approach.
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| 6. |
- Felicetti, Leonard, et al.
(författare)
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Modeling the formationkeeping control with multibody codes
- 2012
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Ingår i: 2012 IEEE Aerospace Conference; Big Sky, MT. - Piscataway, NJ : IEEE Communications Society. - 9781457705564
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Konferensbidrag (refereegranskat)abstract
- Formation Flying control involves the computation of relative kinematics and dynamics among a number of orbiting platforms. Formations are not the only space application in which several components operate coordinately at the same time. "Multibody" is the scheme usually adopted to model the robotic arms of the space manipulators or large space platforms as the International Space station, and multibody can be also seen as a set of components orbiting together. A number of software codes have been developed during the years to represent and simulate this scheme, taking into account the differential forces acting on each member. This paper proposes to build on this effort to test a different way for evaluating the control of spacecraft formations. The formation spacecraft will be represented by the joints of the multibody. The links, represented as structural element with infinite stiffness, virtually reproduce the relative constraints in position and attitude among the platforms. The idea is to consider the orientation and the length of the links such that the joints (spacecraft) will actually assume the relative geometry which is the desired state at a given time. The forces and torques to be provided to the real spacecraft belonging to the formation are related to the reaction torques and forces which are provided at the joints in the corresponding multibody representation. These reactions can be easily computed by available multibody codes, and the values found can be applied to a standard orbital propagator to compute the dynamical behavior and to validate the approach. The advantage stays with the quick, easy computation of the inverse kinematics, which is routinely performed by multibody software. The solution should be useful to both the cases of keeping an already acquired configuration, like large distributed antennas virtually built by several spacecraft, as well as to the rigid reorientation of a formation, like in some astronomical missions
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| 7. |
- Felicetti, Leonard, et al.
(författare)
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Space webs dynamics and configuration control by means of reaction wheels
- 2012
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Ingår i: AIAA/AAS Astrodynamics Specialist Conference 2012. - Reston, VA : American Institute of Aeronautics and Astronautics, AIAA. - 9781624101823
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Konferensbidrag (refereegranskat)abstract
- Space webs are large, deployable systems composed by several spacecraft connected by tethers. Their dynamics is commanded by the directional effects of the gravity gradient on the specific configuration selected. The paper investigates webs' attitude dynamics by means of a model including six degrees of freedom for the spacecraft, and a number of elements, able to support tensile stresses and described only by their position, for the tethers. This model, applied to a planar, axial symmetrical configuration, allows for feasible numerical simulations. Specifically, the capability to command web's attitude by means of momentum exchange devices is investigated, and the possibility to transfer the angular momentum from a wheel actuator located in a central platform to the overall system is proofed.
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| 8. |
- Palmerini, Giovanni B., et al.
(författare)
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Design of debris removal missions performed by robotic graspers
- 2012
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Ingår i: 63rd International Astronautical Congress 2012. - Paris : International Astronautical Federation. - 9781622769797 ; , s. 6356-6366
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Konferensbidrag (refereegranskat)abstract
- The well known increase of the orbiting debris, leading to a critical condition in which additional launches could be precluded, calls for mitigation and removal practices. First, and maybe easier to accomplish with respect to other concepts under study, some missions should be probably carried out in a close fiiture to grasp large unused orbiting objects, like upper stages or idle spacecraft that already ended their operational lifetime. The focus on large objects, even if they are a limited subset of orbiting spent bodies, helps in two ways: The reduction of the cross section for possible impacts, and, more remarkably, the reduction of the number and size of additional debris to be generated in a possible collision. As a result, these targets can justify the cost and the complexity of removal missions which, even if almost traditional in the approach and not-too-far from current operational capabilities, still pose significant technical problems. The paper aims to present the operational sequence of a removal mission to be performed by a robotic spacecraft. The issues relevant for the different phases are discussed, with a special focus on the grasping operations, when the robotic arms of the servicing spacecraft, after the determination of the relative kinematic state of the target, should carcfully embraces and precisely catch, in a safe area, the orbiting body. Such an approach should bypass obstacles like solar panels and avoid the break-up of the target, possibly degraded due to its long exposure to space environment. The results of simulations under reasonable, engineering hypothesis for the mission's scenario are presented, with the estimate of torques and forces to be exerted by the robotic arms. The attitude issues for the servicing spacecraft, as well as the vibration behaviour for an accurate end-effector positioning during robotic arms manoeuvres are considered. The confidence in the findings of these numerical studies is strengthened by the know-how gained with the related experimental activities performed during recent years in the labs at Sapienza Universita' di Roma by means of dedicated, small test-beds
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| 9. |
- Dykin, Victor, 1985, et al.
(författare)
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Development of a reduced-order model to investigate global/regional/local oscillations in BWRs and study of new stability indicators
- 2012
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- This report presents the results of some investigations performed at Chalmers University of Technology within the Nordic Thermal-Hydraulic Network (NORTHNET) in the area of stability of Boiling Water Reactors (BWRs). Such systems are known to pos- sibly become unstable under certain conditions, and thus the characterization of their stability properties is of prime importance. Due to the complexity of the problem at hand, a Reduced-Order Model (ROM) was developed, as an alternative to using three- dimensional solvers. The main advantage of using a ROM is that ROMs represent fast running models aimed at catching in a qualitative manner the physical phenomena of im- portance. In addition, the relative simplicity of ROMs compared with three-dimensional solvers leads to the possibility of providing some physical insight into the stability mech- anisms. The ROM developed in this project is unique since it has the ability to model global, regional, and local oscillations, and it is based on four heated channels. The four heated channels are necessary to properly represent the possible excitation of the two first azimuthal modes in case of regional oscillations. A careful examination of the ROM demonstrated that the stability behavior of the system is entirely defined by so-called Cmn-coefficients (assuming that there is no pure density wave oscillation). These coefficients represent the effect of a change of the void fraction on pairs of eigenmodes of the nuclear core. When these coefficients are positive, the system is clearly unstable because of the corresponding positive void feedback. When negative, it was demonstrated, both using the developed ROM and SIMULATE-3K, that the system becomes less stable for Cmn coefficients becoming more negative. A closer examination of the dependence of the Decay Ratio (DR) on the Cmn coefficients using the ROM also demonstrated that for small negative values of the coefficients, a non-monotonic relationship between the DR and the Cmn coefficients exists. Nevertheless, for realistic values of the void reactivity feedback, such a non-monotonic behavior cannot be noticed, because the Cmn coefficients are sufficiently negative. As a consequence, the estimation of the Cmn coefficients opens up the possibility of using such coefficients as a qualitative measure of core stability in a predictive manner. This could be used for instance as a means to compare the relative stability of several core loadings without the need of running lengthy time-dependent three-dimensional core calculations, and could be of great help to nuclear engineers when designing cores.
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| 10. |
- Uittenbogaard, Adriaan Cornelis, et al.
(författare)
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Space-Time Clusters of crime in Stockholm, Sweden
- 2012
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Ingår i: Review of European Studies. - : Canadian Center of Science and Education. - 1918-7173 .- 1918-7181. ; 4:5, s. 148-156
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Tidskriftsartikel (refereegranskat)abstract
- The aim of the study is to detect geographical clustering of offences over time using Kulldorff’s scan test(SaTScan version 9.01; Kulldorff, 2010) and police recorded data over Stockholm city, the capital of Sweden.This technique has a rigorous inference theory for identifying statistically significant clusters. The space–timescan statistics are used in a single retrospective analysis using data from 1st January 2006 to 31st December2009. A four years’ dataset is collapsed into ‘one year’. All space-time dimensions of the data are kept except‘year’. Clusters over the hours of the day, weekday and weekend and by seasons were tested. Total populationbut also day-time and night-time populations were used as reference. Findings show clear distinct patterns ofconcentration for violence (assault and threat) and property crimes (theft, robbery and burglary) over time andspace. Whilst property crimes tend to happen more often in the afternoons in the center and regional commercialcenters in the southern and western parts of Stockholm, violence takes place more often in the night, and isheavily concentrated in large parts of the city center. Weekends are more targeted than weekdays for bothoffences. Regardless of day of the week, the main urban core of the city contains the most likely cluster thatextends to commercial and socially disorganized areas in the west and south Stockholm. Whilst property crimelevels do not show significant differences over the seasons, violent crimes levels do (winter and summer). Themost likely clusters tend to be fairly constant in space over time. The article ends with implications of the resultsfor both research and practice.
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