72 research outputs found
A Lyapunov Approach to Control of Microgrids with a Network-Preserved Differential-Algebraic Model
We provide sufficient conditions for asymptotic stability and optimal resource allocation for a networkpreserved microgrid model with active and reactive power loads. The model considers explicitly the presence of constantpower loads as well as the coupling between the phase angle and voltage dynamics. The analysis of the resulting nonlinear differential algebraic equation (DAE) system is conducted by leveraging incremental Lyapunov functions, definiteness of the load flow Jacobian and the implicit function theorem
Stability of quantized time-delay nonlinear systems: A Lyapunov-Krasowskii-functional approach
Lyapunov-Krasowskii functionals are used to design quantized control laws for
nonlinear continuous-time systems in the presence of constant delays in the
input. The quantized control law is implemented via hysteresis to prevent
chattering. Under appropriate conditions, our analysis applies to stabilizable
nonlinear systems for any value of the quantization density. The resulting
quantized feedback is parametrized with respect to the quantization density.
Moreover, the maximal allowable delay tolerated by the system is characterized
as a function of the quantization density.Comment: 31 pages, 3 figures, to appear in Mathematics of Control, Signals,
and System
Coordination of passive systems under quantized measurements
In this paper we investigate a passivity approach to collective coordination
and synchronization problems in the presence of quantized measurements and show
that coordination tasks can be achieved in a practical sense for a large class
of passive systems.Comment: 40 pages, 1 figure, submitted to journal, second round of revie
Clinical results with the 31 mm CoreValve™ in large aortic annuli: The importance of implantation technique
The CoreValve Revalving System (CRS) (Medtronic Inc., Minneapolis, MN, USA) is currently available in four sizes: 23 mm, 26 mm, 29 mm and 31 mm. Aim of our study was to assess the acute clinical results after implantation of a 31 mm CRS
Controlled Synchronization of One Class of Nonlinear Systems under Information Constraints
Output feedback controlled synchronization problems for a class of nonlinear
unstable systems under information constraints imposed by limited capacity of
the communication channel are analyzed. A binary time-varying coder-decoder
scheme is described and a theoretical analysis for multi-dimensional
master-slave systems represented in Lurie form (linear part plus nonlinearity
depending only on measurable outputs) is provided. An output feedback control
law is proposed based on the Passification Theorem. It is shown that the
synchronization error exponentially tends to zero for sufficiantly high
transmission rate (channel capacity). The results obtained for synchronization
problem can be extended to tracking problems in a straightforward manner, if
the reference signal is described by an {external} ({exogenious}) state space
model. The results are applied to controlled synchronization of two chaotic
Chua systems via a communication channel with limited capacity.Comment: 8 pages, 2 figure
A population of gamma-ray emitting globular clusters seen with the Fermi Large Area Telescope
Globular clusters with their large populations of millisecond pulsars (MSPs)
are believed to be potential emitters of high-energy gamma-ray emission. Our
goal is to constrain the millisecond pulsar populations in globular clusters
from analysis of gamma-ray observations. We use 546 days of continuous
sky-survey observations obtained with the Large Area Telescope aboard the Fermi
Gamma-ray Space Telescope to study the gamma-ray emission towards 13 globular
clusters. Steady point-like high-energy gamma-ray emission has been
significantly detected towards 8 globular clusters. Five of them (47 Tucanae,
Omega Cen, NGC 6388, Terzan 5, and M 28) show hard spectral power indices and clear evidence for an exponential cut-off in the range
1.0-2.6 GeV, which is the characteristic signature of magnetospheric emission
from MSPs. Three of them (M 62, NGC 6440 and NGC 6652) also show hard spectral
indices , however the presence of an exponential cut-off
can not be unambiguously established. Three of them (Omega Cen, NGC 6388, NGC
6652) have no known radio or X-ray MSPs yet still exhibit MSP spectral
properties. From the observed gamma-ray luminosities, we estimate the total
number of MSPs that is expected to be present in these globular clusters. We
show that our estimates of the MSP population correlate with the stellar
encounter rate and we estimate 2600-4700 MSPs in Galactic globular clusters,
commensurate with previous estimates. The observation of high-energy gamma-ray
emission from a globular cluster thus provides a reliable independent method to
assess their millisecond pulsar populations that can be used to make
constraints on the original neutron star X-ray binary population, essential for
understanding the importance of binary systems in slowing the inevitable core
collapse of globular clusters.Comment: Accepted for publication in A&A. Corresponding authors: J.
Kn\"odlseder, N. Webb, B. Pancraz
Fermi Large Area Telescope Constraints on the Gamma-ray Opacity of the Universe
The Extragalactic Background Light (EBL) includes photons with wavelengths
from ultraviolet to infrared, which are effective at attenuating gamma rays
with energy above ~10 GeV during propagation from sources at cosmological
distances. This results in a redshift- and energy-dependent attenuation of the
gamma-ray flux of extragalactic sources such as blazars and Gamma-Ray Bursts
(GRBs). The Large Area Telescope onboard Fermi detects a sample of gamma-ray
blazars with redshift up to z~3, and GRBs with redshift up to z~4.3. Using
photons above 10 GeV collected by Fermi over more than one year of observations
for these sources, we investigate the effect of gamma-ray flux attenuation by
the EBL. We place upper limits on the gamma-ray opacity of the Universe at
various energies and redshifts, and compare this with predictions from
well-known EBL models. We find that an EBL intensity in the optical-ultraviolet
wavelengths as great as predicted by the "baseline" model of Stecker et al.
(2006) can be ruled out with high confidence.Comment: 42 pages, 12 figures, accepted version (24 Aug.2010) for publication
in ApJ; Contact authors: A. Bouvier, A. Chen, S. Raino, S. Razzaque, A.
Reimer, L.C. Reye
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