594 research outputs found
Visual 3-D SLAM from UAVs
The aim of the paper is to present, test and discuss the implementation of Visual SLAM techniques to images taken from Unmanned Aerial Vehicles (UAVs) outdoors, in partially structured environments. Every issue of the whole process is discussed in order to obtain more accurate localization and mapping from UAVs flights. Firstly, the issues related to the visual features of objects in the scene, their distance to the UAV, and the related image acquisition system and their calibration are evaluated for improving the whole process. Other important, considered issues are related to the image processing techniques, such as interest point detection, the matching procedure and the scaling factor. The whole system has been tested using the COLIBRI mini UAV in partially structured environments. The results that have been obtained for localization, tested against the GPS information of the flights, show that Visual SLAM delivers reliable localization and mapping that makes it suitable for some outdoors applications when flying UAVs
Shot Noise in Nanoscale Conductors From First Principles
We describe a field-theoretic approach to calculate quantum shot noise in
nanoscale conductors from first principles. Our starting point is the
second-quantization field operator to calculate shot noise in terms of single
quasi-particle wavefunctions obtained self-consistently within density
functional theory. The approach is valid in both linear and nonlinear response
and is particularly suitable in studying shot noise in atomic-scale conductors.
As an example we study shot noise in Si atomic wires between metal electrodes.
We find that shot noise is strongly nonlinear as a function of bias and it is
enhanced for one- and two-Si wires due to the large contribution from the metal
electrodes. For longer wires it shows an oscillatory behavior for even and odd
number of atoms with opposite trend with respect to the conductance, indicating
that current fluctuations persist with increasing wire length.Comment: 4 pages, 4 figure
Critical flux pinning and enhanced upper-critical-field in magnesium diboride films
We have conducted pulsed transport measurements on c-axis oriented magnesium
diboride films over the entire relevant ranges of magnetic field 0 \alt H \alt
H_{c2} (where \hcu is the upper critical field) and current density 0 \alt j
\alt j_{d} (where is the depairing current density). The intrinsic
disorder of the films combined with the large coherence length and
three-dimensionality, compared to cuprate superconductors, results in a
six-fold enhancement of and raises the depinning current density
to within an order of magnitude of . The current-voltage
response is highly non-linear at all fields, resulting from a combination of
depinning and pair-breaking, and has no trace of an Ohmic free-flux-flow
regime.
Keywords: pair, breaking, depairing, superconductor, superconductivity, flux,
fluxon, vortex, mgb
Optically induced coherent intra-band dynamics in disordered semiconductors
On the basis of a tight-binding model for a strongly disordered semiconductor
with correlated conduction- and valence band disorder a new coherent dynamical
intra-band effect is analyzed. For systems that are excited by two, specially
designed ultrashort light-pulse sequences delayed by tau relatively to each
other echo-like phenomena are predicted to occur. In addition to the inter-band
photon echo which shows up at exactly t=2*tau relative to the first pulse, the
system responds with two spontaneous intra-band current pulses preceding and
following the appearance of the photon echo. The temporal splitting depends on
the electron-hole mass ratio. Calculating the population relaxation rate due to
Coulomb scattering, it is concluded that the predicted new dynamical effect
should be experimentally observable in an interacting and strongly disordered
system, such as the Quantum-Coulomb-Glass.Comment: to be published in Physical Review B15 February 200
Manipulating the Tomonaga-Luttinger exponent by electric field modulation
We establish a theoretical framework for artificial control of the power-law
singularities in Tomonaga-Luttinger liquid states. The exponent governing the
power-law behaviors is found to increase significantly with an increase in the
amplitude of the periodic electric field modulation applied externally to the
system. This field-induced shift in the exponent indicates the tunability of
the transport properties of quasi-one-dimensional electron systems.Comment: 7 pages, 3 figure
Evaluation of the low-lying energy levels of two- and three-electron configurations for multi-charged ions
Accurate QED evaluations of the one- and two-photon interelectron interaction
for low lying two- and three-electron configurations for ions with nuclear
charge numbers are performed. The three-photon interaction is
also partly taken into account. The Coulomb gauge is employed. The results are
compared with available experimental data and with different calculations. A
detailed investigation of the behaviour of the energy levels of the
configurations , near
the crossing points Z=64 and Z=92 is carried out. The crossing points are
important for the future experimental search for parity nonconserving (PNC)
effects in highly charged ions
Reflection and Ducting of Gravity Waves Inside the Sun
Internal gravity waves excited by overshoot at the bottom of the convection
zone can be influenced by rotation and by the strong toroidal magnetic field
that is likely to be present in the solar tachocline. Using a simple Cartesian
model, we show how waves with a vertical component of propagation can be
reflected when traveling through a layer containing a horizontal magnetic field
with a strength that varies with depth. This interaction can prevent a portion
of the downward-traveling wave energy flux from reaching the deep solar
interior. If a highly reflecting magnetized layer is located some distance
below the convection zone base, a duct or wave guide can be set up, wherein
vertical propagation is restricted by successive reflections at the upper and
lower boundaries. The presence of both upward- and downward-traveling
disturbances inside the duct leads to the existence of a set of horizontally
propagating modes that have significantly enhanced amplitudes. We point out
that the helical structure of these waves makes them capable of generating an
alpha-effect, and briefly consider the possibility that propagation in a shear
of sufficient strength could lead to instability, the result of wave growth due
to over-reflection.Comment: 23 pages, 5 figures. Accepted for publication in Solar Physic
An epitaxial model for heterogeneous nucleation on potent substrates
© The Minerals, Metals & Materials Society and ASM International 2012In this article, we present an epitaxial model for heterogeneous nucleation on potent substrates. It is proposed that heterogeneous nucleation of the solid phase (S) on a potent substrate (N) occurs by epitaxial growth of a pseudomorphic solid (PS) layer on the substrate surface under a critical undercooling (ÎT ). The PS layer with a coherent PS/N interface mimics the atomic arrangement of the substrate, giving rise to a linear increase of misfit strain energy with layer thickness. At a critical thickness (h ), elastic strain energy reaches a critical level, at which point, misfit dislocations are created to release the elastic strain energy in the PS layer. This converts the strained PS layer to a strainless solid (S), and changes the initial coherent PS/N interface into a semicoherent S/N interface. Beyond this critical thickness, further growth will be strainless, and solidification enters the growth stage. It is shown analytically that the lattice misfit (f) between the solid and the substrate has a strong influence on both h and ÎT ; h decreases; and ÎT increases with increasing lattice misfit. This epitaxial nucleation model will be used to explain qualitatively the generally accepted experimental findings on grain refinement in the literature and to analyze the general approaches to effective grain refinement.EPSRC Centre for Innovative Manufacturing in Liquid Metal Engineerin
The effects of the composition of microporous layers on the permeability of gas diffusion layers used in polymer electrolyte fuel cells
The effects of the composition of the microporous layer (MPL) on the through-plane permeability of the gas diffusion layers (GDLs) used in polymer electrolyte fuel cells (PEFCs) have been thoroughly experimentally investigated in this paper. For a given PTFE loading in the MPL, the GDL permeability was found to decrease with increasing carbon loading and this is due to the increase in the thickness of the MPL. For all the investigated carbon loadings of the MPL, the permeability values of the GDLs were found to have common trends for the PTFE loadings ranging from 10 to 50% (by weight): the GDL permeability increases when the PTFE loading in the MPL is increased from 20 to 50%; the GDL permeability decreases when the PTFE loading in the MPL is increased from 10 to 20%; and the GDL permeability is a minimum at 20% PTFE loading present in the MPL. On the other hand, the permeability of the GDL was found to depend on the carbon loading of the MPL in the PTFE range 0â10%. The effects of the MPL composition on the MPL permeability were found to be similar to those on the GDL permeability. However, the permeability values of the MPLs of the same composition, which were supposed to be ideally the same, were found to significantly vary. This was attributed to the MPL penetration into the body of the carbon substrates
Using Simulation to Assess the Opportunities of Dynamic Waste Collection
In this paper, we illustrate the use of discrete event simulation to evaluate how dynamic planning methodologies can be best applied for the collection of waste from underground containers. We present a case study that took place at the waste collection company Twente Milieu, located in The Netherlands. Even though the underground containers are already equipped with motion sensors, the planning of container emptyingâs is still based on static cyclic schedules. It is expected that the use of a dynamic planning methodology, that employs sensor information, will result in a more efficient collection process with respect to customer satisfaction, profits, and CO2 emissions. In this research we use simulation to (i) evaluate the current planning methodology, (ii) evaluate various dynamic planning possibilities, (iii) quantify the benefits of switching to a dynamic collection process, and (iv) quantify the benefits of investing in fillâlevel sensors. After simulating all scenarios, we conclude that major improvements can be achieved, both with respect to logistical costs as well as customer satisfaction
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