22,369 research outputs found
Selection of sampling rate for digital control of aircrafts
The considerations in selecting the sample rates for digital control of aircrafts are identified and evaluated using the optimal discrete method. A high performance aircraft model which includes a bending mode and wind gusts was studied. The following factors which influence the selection of the sampling rates were identified: (1) the time and roughness response to control inputs; (2) the response to external disturbances; and (3) the sensitivity to variations of parameters. It was found that the time response to a control input and the response to external disturbances limit the selection of the sampling rate. The optimal discrete regulator, the steady state Kalman filter, and the mean response to external disturbances are calculated
Integral Constraints On cosmological Perturbations and their Energy
We show the relation between Traschen's integral equations and the energy,
and ``position of the centre of mass'', of the matter perturbations in a
Robertson-Walker spacetime. When the perturbations are ``localised'' we get a
set of integral constraints that includes hers. We illustrate them on a simple
example.Comment: 19 pages, Tex file, submitted to Classical and Quantum Gravit
Single-growth embedded epitaxy AlGaAs injection lasers with extremely low threshold currents
A new type of strip-geometry AlGaAs double-heterostructure laser with an embedded optical waveguide has been developed. The new structure is fabricated using a single step of epitaxial growth. Lasers with threshold currents as low as 9.5 mA (150 µm long) were obtained. These lasers exhibit operation in a single spatial and longitudinal mode, have differential quantum efficiencies exceeding 45%, and a characteristic temperature of 175° C. They emit more than 12 mW/facet of optical power without any kinks
Synthesizing Finite-state Protocols from Scenarios and Requirements
Scenarios, or Message Sequence Charts, offer an intuitive way of describing
the desired behaviors of a distributed protocol. In this paper we propose a new
way of specifying finite-state protocols using scenarios: we show that it is
possible to automatically derive a distributed implementation from a set of
scenarios augmented with a set of safety and liveness requirements, provided
the given scenarios adequately \emph{cover} all the states of the desired
implementation. We first derive incomplete state machines from the given
scenarios, and then synthesis corresponds to completing the transition relation
of individual processes so that the global product meets the specified
requirements. This completion problem, in general, has the same complexity,
PSPACE, as the verification problem, but unlike the verification problem, is
NP-complete for a constant number of processes. We present two algorithms for
solving the completion problem, one based on a heuristic search in the space of
possible completions and one based on OBDD-based symbolic fixpoint computation.
We evaluate the proposed methodology for protocol specification and the
effectiveness of the synthesis algorithms using the classical alternating-bit
protocol.Comment: This is the working draft of a paper currently in submission.
(February 10, 2014
Four-dimensional lattice results on the MSSM electroweak phase transition
We present the results of our large scale 4-dimensional (4d) lattice
simulations for the MSSM electroweak phase transition (EWPT). We carried out
infinite volume and continuum limit extrapolations and found a transition whose
strength agrees well with perturbation theory. We determined the properties of
the bubble wall that are important for a successful baryogenesis.Comment: 5 pages, 3figures. Talk presented at Johns Hopkins Workshop on
Nonperturbative Quantum Field Theory Methods and their Applications (19-21
August 2000.
When the Milky Way turned off the lights: APOGEE provides evidence of star formation quenching in our Galaxy
Quenching, the cessation of star formation, is one of the most significant
events in the life cycle of galaxies. We show here the first evidence that the
Milky Way experienced a generalised quenching of its star formation at the end
of its thick disk formation 9 Gyr ago. Elemental abundances of stars
studied as part of the APOGEE survey reveal indeed that in less than 2
Gyr the star formation rate in our Galaxy dropped by an order-of-magnitude.
Because of the tight correlation between age and alpha abundance, this event
reflects in the dearth of stars along the inner disk sequence in the
[Fe/H]-[/Fe] plane. Before this phase, which lasted about 1.5 Gyr, the
Milky Way was actively forming stars. Afterwards, the star formation resumed at
a much lower level to form the thin disk. These events are very well matched by
the latest observation of MW-type progenitors at high redshifts. In late type
galaxies, quenching is believed to be related to a long and secular exhaustion
of gas. In our Galaxy, it occurred on a much shorter time scale, while the
chemical continuity before and after the quenching indicates that it was not
due to the exhaustion of the gas. While quenching is generally associated with
spheroids, our results show that it also occurs in galaxies like the Milky Way,
possibly when they are undergoing a morphological transition from thick to thin
disks. Given the demographics of late type galaxies in the local universe, in
which classical bulges are rare, we suggest further that this may hold true
generally in galaxies with mass lower than or approximately , where
quenching could be directly a consequence of thick disk formation. We emphasize
that the quenching phase in the Milky Way could be contemporaneous with, and
related to, the formation of the bar. We sketch a scenario on how a strong bar
may inhibit star formation.Comment: 17 pages, 8 figures. Published versio
A new look at the kinematics of the bulge from an N-body model
(Abridged) By using an N-body simulation of a bulge that was formed via a bar
instability mechanism, we analyse the imprints of the initial (i.e. before bar
formation) location of stars on the bulge kinematics, in particular on the
heliocentric radial velocity distribution of bulge stars. Four different
latitudes were considered: , , , and
, along the bulge minor axis as well as outside it, at
and . The bulge X-shaped structure comprises
stars that formed in the disk at different locations. Stars formed in the outer
disk, beyond the end of the bar, which are part of the boxy peanut-bulge
structure may show peaks in the velocity distributions at positive and negative
heliocentric radial velocities with high absolute values that can be larger
than 100 , depending on the observed direction. In some
cases the structure of the velocity field is more complex and several peaks are
observed. Stars formed in the inner disk, the most numerous, contribute
predominantly to the X-shaped structure and present different kinematic
characteristics. Our results may enable us to interpret the cold high-velocity
peak observed in the APOGEE commissioning data, as well as the excess of
high-velocity stars in the near and far arms of the X-shaped structure at
= and =. When compared with real data, the kinematic
picture becomes more complex due to the possible presence in the observed
samples of classical bulge and/or thick disk stars. Overall, our results point
to the existence of complex patterns and structures in the bulge velocity
fields, which are generated by the bar. This suggests that caution should be
used when interpreting the bulge kinematics: the presence of substructures,
peaks and clumps in the velocity fields is not necessarily a sign of past
accretion events.Comment: 21 pages, 18 figures. Accepted for publication in A&
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