56,889 research outputs found
Spacecraft formation-flying using potential functions
A group of small spacecraft able to change their relative position and attitude through the use of the potential function method is discussed. The spacecraft shapes, sizes and manoeuvring capabilities are not identical, although all are assumed to manoeuvre using continuous thrusters. A hyperbolic form of the attractive potential function is used to reduce actuator effort by using natural orbital motion to approaching the goal configuration. A superquadric repulsive potential with 3D a rigid object representation is then used to provide an accurate representation of the shape of spacecraft in the potential function. As the spacecraft start away from their goal, a parabolic attractive potential is inefficient as the control force increases with distance from the goal. Using a hyperbolic attractive potential, the control force is independent of the distance to goal, ensuring smooth manoeuvring towards the goal with a bound actuator effort
Spectral isometries on non-simple C*-algebras
We prove that unital surjective spectral isometries on certain non-simple
unital C*-algebras are Jordan isomorphisms. Along the way, we establish several
general facts in the setting of semisimple Banach algebras.Comment: 7 pages; paper available since July 201
Sato-Crutchfield formulation for some Evolutionary Games
The Sato-Crutchfield equations are studied analytically and numerically. The
Sato-Crutchfield formulation is corresponding to losing memory. Then
Sato-Crutchfield formulation is applied for some different types of games
including hawk-dove, prisoner's dilemma and the battle of the sexes games. The
Sato-Crutchfield formulation is found not to affect the evolutionarily stable
strategy of the ordinary games. But choosing a strategy becomes purely random
independent on the previous experiences, initial conditions, and the rules of
the game itself. Sato-Crutchfield formulation for the prisoner's dilemma game
can be considered as a theoretical explanation for the existence of cooperation
in a population of defectors.Comment: 9 pages, 3 figures, accepted for Int. J. Mod. Phys.
Step-Indexed Logical Relations for Probability (long version)
It is well-known that constructing models of higher-order probabilistic
programming languages is challenging. We show how to construct step-indexed
logical relations for a probabilistic extension of a higher-order programming
language with impredicative polymorphism and recursive types. We show that the
resulting logical relation is sound and complete with respect to the contextual
preorder and, moreover, that it is convenient for reasoning about concrete
program equivalences. Finally, we extend the language with dynamically
allocated first-order references and show how to extend the logical relation to
this language. We show that the resulting relation remains useful for reasoning
about examples involving both state and probabilistic choice.Comment: Extended version with appendix of a FoSSaCS'15 pape
A generic framework for the development of standardised learning objects within the discipline of construction management
E-learning has occurred in the academic world in different forms since the early 1990s. Its use varies from interactive multimedia tools and simulation environments to static resources within learning management systems. E-learning tools and environments are no longer criticised for their lack of use in higher education in general and within the construction domain in particular. The main criticism, however, is that of reinventing the wheel in order to create new learning environments that cater for different educational needs. Therefore, sharing educational content has become the focus of current research, taking e-learning into a whole new era of developments. This era is enabled by the emergence of new
technologies (online and wireless) and the development of educational standards, such as SCORM (Sharable Content Object Reference Model) and LOM (Learning Object Metadata)
for example. Accordingly, the broad definition of the construction domain and the interlocking nature of subjects taught within this domain, makes the concept of sharing content most appealing.
This paper proposes a framework developed to describe the various steps required in order to enable the application of e-learning metadata standards and ontology for sharable learning objects to serve the construction discipline. The paper further describes the application of the
proposed framework to a case study for developing an online environment for learning objects that are standardised, sharable, transparent and that cater for the needs of learners, educators and curricula developers in Construction Management. Based on the framework, a
learning objects repository is developed incorporating educational and web standards. The repository manages objects as well as metadata using ontology and offers a set of services such as storing, retrieving and searching of learning objects using Semantic Web technologies. Thus, it increases the reusability, sharability and interoperability of learning objects
The effects of material combination and surface roughness in lubricated silicon nitride/steel rolling contact fatigue
Four kinds of commercially finished 12.7 min HIPed silicon nitride bearing balls with surface roughness values R-a ranging from 0.002 to 0.016 mum were tested using a four-ball rolling configuration. They were rolling against two types of steel testing balls with different surface roughness and hardness, in fully lubricated condition at a maximum compressive stress of 6.58 GPa and at a speed of 10 000 rpm for over 135 million stress cycles. Rolling track surfaces were examined by microscope, SEM, 3-D surface analysis and interference profilometry. Experiment results show that the composite surface roughness are most influential. The shape of the surface topography of silicon nitride are not very sensitive. The slight difference. in steel hardness may lead to significant differences in steel fatigue life
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