5,893 research outputs found
RCTS: A flexible environment for sensor integration and control of robot systems; the distributed processing approach
Most robot systems lack a suitable hardware and software environment for the efficient research of new control and sensing schemes. Typically, engineers and researchers need to be experts in control, sensing, programming, communication and robotics in order to implement, integrate and test new ideas in a robot system. In order to reduce this time, the Robot Controller Test Station (RCTS) has been developed. It uses a modular hardware and software architecture allowing easy physical and functional reconfiguration of a robot. This is accomplished by emphasizing four major design goals: flexibility, portability, ease of use, and ease of modification. An enhanced distributed processing version of RCTS is described. It features an expanded and more flexible communication system design. Distributed processing results in the availability of more local computing power and retains the low cost of microprocessors. A large number of possible communication, control and sensing schemes can therefore be easily introduced and tested, using the same basic software structure
Emergence of topological electronic phases in elemental lithium under pressure
Lithium, a prototypical simple metal under ambient conditions, has a
surprisingly rich phase diagram under pressure, taking up several structures
with reduced symmetry, low coordination numbers, and even semiconducting
character with increasing density. Using first-principles calculations, we
demonstrate that some predicted high-pressure phases of elemental Li also host
topological electronic structures. Beginning at 80 GPa and coincident with a
transition to the Pbca phase, we find Li to be a Dirac nodal line semimetal. We
further calculate that Li retains linearly-dispersive energy bands in
subsequent predicted higher pressure phases, and that it exhibits a Lifshitz
transition between two Cmca phases at 220 GPa. The Fd-3m phase at 500 GPa forms
buckled honeycomb layers that give rise to a Dirac crossing 1 eV below the
Fermi energy. The well-isolated topological nodes near the Fermi level in these
phases result from increasing p-orbital character with density at the Fermi
level, itself a consequence of rising 1s core wavefunction overlap, and a
preference for nonsymmorphic symmetries in the crystal structures favored at
these pressures. Our results provide evidence that under pressure, bulk 3D
materials with light elements, or even pure elemental systems, can undergo
topological phase transitions hosting nontrivial topological properties near
the Fermi level with measurable consequences; and that, through pressure, we
can access these novel phases in elemental lithium.Comment: 5 pages, 5 figures, accepted for publicatio
A wind-tunnel investigation of sonic-boom pressure distributions of bodies of revolution at Mach 2.96, 3.83, and 4.63
Measurements of sonic boom pressure distribution of bodies of revolution at Mach 2.96, 3.83, and 4.63 in Unitary Plan wind tunne
Quality assurance in stereotactic radiosurgery/radiotherapy according to DIN 6875-1
The new DIN (' Deutsche Industrie- Norm') 6875- 1, which is currently being finalised, deals with quality assurance ( QA) criteria and tests methods for linear accelerator and Gamma Knife stereotactic radiosurgery/ radiotherapy including treatment planning, stereotactic frame and stereotactic imaging and a system test to check the whole chain of uncertainties. Our existing QA program, based on dedicated phantoms and test procedures, has been refined to fulfill the demands of this new DIN. The radiological and mechanical isocentre corresponded within 0.2 mm and the measured 50% isodose lines were in agreement with the calculated ones within less than 0.5 mm. The measured absorbed dose was within 3%. The resultant output factors measured for the 14-, 8- and 4- mm collimator helmet were 0.9870 +/- 0.0086, 0.9578 +/- 0.0057 and 0.8741 +/- 0.0202, respectively. For 170 consecutive tests, the mean geometrical accuracy was 0.48 +/- 0.23 mm. Besides QA phantoms and analysis software developed in- house, the use of commercially available tools facilitated the QA according to the DIN 6875- 1 with which our results complied. Copyright (C) 2004 S. Karger AG, Basel
A three-dimensional scalar field theory model of center vortices and its relation to k-string tensions
In d=3 SU(N) gauge theory, we study a scalar field theory model of center
vortices that furnishes an approach to the determination of so-called k-string
tensions. This model is constructed from string-like quantum solitons
introduced previously, and exploits the well-known relation between string
partition functions and scalar field theories in d=3. Center vortices
corresponding to magnetic flux J (in units of 2\pi /N) are composites of J
elementary J=1 constituent vortices that come in N-1 types, with repulsion
between like constituents and attraction between unlike constituents. The
scalar field theory involves N scalar fields \phi_i (one of which is
eliminated) that can merge, dissociate, and recombine while conserving flux mod
N. The properties of these fields are deduced directly from the corresponding
gauge-theory quantum solitons. Every vacuum Feynman graph of the theory
corresponds to a real-space configuration of center vortices. We study
qualitatively the problem of k-string tensions at large N, whose solution is
far from obvious in center-vortex language. We construct a simplified dynamical
picture of constituent-vortex merging, dissociation, and recombination, which
allows in principle for the determination of vortex areal densities and
k-string tensions. This picture involves point-like "molecules" (cross-sections
of center vortices) made of constituent "atoms" that combine and disassociate
dynamically in a d=2 test plane . The vortices evolve in a Euclidean "time"
which is the location of the test plane along an axis perpendicular to the
plane. A simple approximation to the molecular dynamics is compatible with
k-string tensions that are linear in k for k<< N, as naively expected.Comment: 21 pages; RevTeX4; 4 .eps figure
Four Dimensional CFT Models with Rational Correlation Functions
Recently established rationality of correlation functions in a globally
conformal invariant quantum field theory satisfying Wightman axioms is used to
construct a family of soluble models in 4-dimensional Minkowski space-time. We
consider in detail a model of a neutral scalar field of dimension 2. It
depends on a positive real parameter c, an analogue of the Virasoro central
charge, and admits for all (finite) c an infinite number of conserved symmetric
tensor currents. The operator product algebra of is shown to coincide
with a simpler one, generated by a bilocal scalar field of
dimension (1,1). The modes of V together with the unit operator span an
infinite dimensional Lie algebra whose vacuum (i.e. zero energy lowest
weight) representations only depend on the central charge c. Wightman
positivity (i.e. unitarity of the representations of ) is proven to be
equivalent to .Comment: 28 pages, LATEX, amsfonts, latexsym. Proposition 2.3, and Conjecture
in Sec. 6 are revised. Minor errors are correcte
Granivory of invasive, naturalized, and native plants in communities differentially susceptible to invasion
Seed predation is an important biotic filter that can influence abundance and spatial distributions of native species through differential effects on recruitment. This filter may also influence the relative abundance of nonnative plants within habitats and the communities’ susceptibility to invasion via differences in granivore identity, abundance, and food preference. We evaluated the effect of postdispersal seed predators on the establishment of invasive, naturalized, and native species within and between adjacent forest and steppe communities of eastern Washington, USA that differ in severity of plant invasion. Seed removal from trays placed within guild-specific exclosures revealed that small mammals were the dominant seed predators in both forest and steppe. Seeds of invasive species (Bromus tectorum, Cirsium arvense) were removed significantly less than the seeds of native (Pseudoroegneria spicata, Balsamorhiza sagittata) and naturalized (Secale cereale, Centaurea cyanus) species. Seed predation limited seedling emergence and establishment in both communities in the absence of competition in a pattern reflecting natural plant abundance: S. cereale was most suppressed, B. tectorum was least suppressed, and P. spicata was suppressed at an intermediate level. Furthermore, seed predation reduced the residual seed bank for all species. Seed mass correlated with seed removal rates in the forest and their subsequent effects on plant recruitment; larger seeds were removed at higher rates than smaller seeds. Our vegetation surveys indicate higher densities and canopy cover of nonnative species occur in the steppe compared with the forest understory, suggesting the steppe may be more susceptible to invasion. Seed predation alone, however, did not result in significant differences in establishment for any species between these communities, presumably due to similar total small-mammal abundance between communities. Consequently, preferential seed predation by small mammals predicts plant establishment for our test species within these communities but not between them. Accumulating evidence suggests that seed predation can be an important biotic filter affecting plant establishment via differences in consumer preferences and abundance with important ramifications for plant invasions and in situ community assembly
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