527 research outputs found
A South African review of harmonic emission level assessment as per IEC61000-3-6
Large-scale renewable power producing plants are being integrated into South African networks.
Network operators need to ensure that Renewable Power Plants (RPP) do not negatively affect the
power quality levels of their networks, as harmonics amongst others could become a concern.
IEC 61000-3-6 details a method for allocating voltage harmonic emission limits for distorting loads.
This method works well for the allocation of emission limits; however it does not address the
management of harmonic emissions once a plant is connected to the network. The management of
harmonic emissions requires that network operators measure or quantify the emissions from loads and
generators to determine compliance. Post-connection quantification of harmonic levels and
compliance is a challenge for network operators. The question asked is “How should a network
operator measure/quantify the harmonic emissions of a load/generator to establish compliance with the
calculated limits as per IEC 61000-3-6”.
This paper reviews within a South African context methods of assessing harmonic emission levels and
then evaluates these methods by means of field data. Opportunities for improvement are identified
and operational requirements discussed
Mechanical cell-matrix feedback explains pairwise and collective endothelial cell behavior in vitro
In vitro cultures of endothelial cells are a widely used model system of the
collective behavior of endothelial cells during vasculogenesis and
angiogenesis. When seeded in an extracellular matrix, endothelial cells can
form blood vessel-like structures, including vascular networks and sprouts.
Endothelial morphogenesis depends on a large number of chemical and mechanical
factors, including the compliancy of the extracellular matrix, the available
growth factors, the adhesion of cells to the extracellular matrix, cell-cell
signaling, etc. Although various computational models have been proposed to
explain the role of each of these biochemical and biomechanical effects, the
understanding of the mechanisms underlying in vitro angiogenesis is still
incomplete. Most explanations focus on predicting the whole vascular network or
sprout from the underlying cell behavior, and do not check if the same model
also correctly captures the intermediate scale: the pairwise cell-cell
interactions or single cell responses to ECM mechanics. Here we show, using a
hybrid cellular Potts and finite element computational model, that a single set
of biologically plausible rules describing (a) the contractile forces that
endothelial cells exert on the ECM, (b) the resulting strains in the
extracellular matrix, and (c) the cellular response to the strains, suffices
for reproducing the behavior of individual endothelial cells and the
interactions of endothelial cell pairs in compliant matrices. With the same set
of rules, the model also reproduces network formation from scattered cells, and
sprouting from endothelial spheroids. Combining the present mechanical model
with aspects of previously proposed mechanical and chemical models may lead to
a more complete understanding of in vitro angiogenesis.Comment: 25 pages, 6 figures, accepted for publication in PLoS Computational
Biolog
A hybrid POMDP-BDI agent architecture with online stochastic planning and plan caching
This article presents an agent architecture for controlling an autonomous agent in stochastic, noisy environments. The architecture combines the partially observable Markov decision process (POMDP) model with the belief-desire-intention (BDI) framework. The Hybrid POMDP-BDI agent architecture takes the best features from the two approaches, that is, the online generation of reward-maximizing courses of action from POMDP theory, and sophisticated multiple goal management from BDI theory. We introduce the advances made since the introduction of the basic architecture, including (i) the ability to pursue and manage multiple goals simultaneously and (ii) a plan library for storing pre-written plans and for storing recently generated plans for future reuse. A version of the architecture is implemented and is evaluated in a simulated environment. The results of the experiments show that the improved hybrid architecture outperforms the standard POMDP architecture and the previous basic hybrid architecture for both processing speed and effectiveness of the agent in reaching its goals
Direct magneto-optical compression of an effusive atomic beam for high-resolution focused ion beam application
An atomic rubidium beam formed in a 70 mm long two-dimensional
magneto-optical trap (2D MOT), directly loaded from a collimated Knudsen
source, is analyzed using laser-induced fluorescence. The longitudinal velocity
distribution, the transverse temperature and the flux of the atomic beam are
reported. The equivalent transverse reduced brightness of an ion beam with
similar properties as the atomic beam is calculated because the beam is
developed to be photoionized and applied in a focused ion beam. In a single
two-dimensional magneto-optical trapping step an equivalent transverse reduced
brightness of A/(m sr eV) was
achieved with a beam flux equivalent to nA. The
temperature of the beam is further reduced with an optical molasses after the
2D MOT. This increased the equivalent brightness to A/(m sr eV). For currents below 10 pA, for which disorder-induced
heating can be suppressed, this number is also a good estimate of the ion beam
brightness that can be expected. Such an ion beam brightness would be a six
times improvement over the liquid metal ion source and could improve the
resolution in focused ion beam nanofabrication.Comment: 10 pages, 8 figures, 1 tabl
Theory and particle tracking simulations of a resonant radiofrequency deflection cavity in TM mode for ultrafast electron microscopy
We present a theoretical description of resonant radiofrequency (RF)
deflecting cavities in TM mode as dynamic optical elements for
ultrafast electron microscopy. We first derive the optical transfer matrix of
an ideal pillbox cavity and use a Courant-Snyder formalism to calculate the 6D
phase space propagation of a Gaussian electron distribution through the cavity.
We derive closed, analytic expressions for the increase in transverse emittance
and energy spread of the electron distribution. We demonstrate that for the
special case of a beam focused in the center of the cavity, the low emittance
and low energy spread of a high quality beam can be maintained, which allows
high-repetition rate, ultrafast electron microscopy with 100 fs temporal
resolution combined with the atomic resolution of a high-end TEM. This is
confirmed by charged particle tracking simulations using a realistic cavity
geometry, including fringe fields at the cavity entrance and exit apertures
Age-Related Changes in Frontal Network Structural and Functional Connectivity in Relation to Bimanual Movement Control
Changes in both brain structure and neurophysiological function regulating homotopic as well as heterotopic interhemispheric interactions (IHIs) are assumed to be responsible for the bimanual performance deficits in older adults. However, how the structural and functional networks regulating bimanual performance decline in older adults, as well as the interplay between brain structure and function remain largely unclear. Using a dual-site transcranial magnetic stimulation paradigm, we examined the age-related changes in the interhemispheric effects from the dorsolateral prefrontal cortex and dorsal premotor cortex onto the contralateral primary motor cortex (M1) during the preparation of a complex bimanual coordination task in human. Structural properties of these interactions were assessed with diffusion-based fiber tractography. Compared with young adults, older adults showed performance declines in the more difficult bimanual conditions, less optimal brain white matter (WM) microstructure, and a decreased ability to regulate the interaction between dorsolateral prefrontal cortex and M1. Importantly, we found that WM microstructure, neurophysiological function, and bimanual performance were interrelated in older adults, whereas only the task-related changes in IHI predicted bimanual performance in young adults. These results reflect unique interactions between structure and function in the aging brain, such that declines in WM microstructural organization likely lead to dysfunctional regulation of IHI, ultimately accounting for bimanual performance deficits
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