31,894 research outputs found
Methods and strategies of object localization
An important property of an intelligent robot is to be able to determine the location of an object in 3-D space. A general object localization system structure is proposed, some important issues on localization discussed, and an overview given for current available object localization algorithms and systems. The algorithms reviewed are characterized by their feature extracting and matching strategies; the range finding methods; the types of locatable objects; and the mathematical formulating methods
Imaging and Nulling with the Space Interferometry Mission
We present numerical simulations for a possible synthesis imaging mode of the
Space Interferometer Mission (SIM). We summarize the general techniques that
SIM offers to perform imaging of high surface brightness sources, and discuss
their strengths and weaknesses. We describe an interactive software package
that is used to provide realistic, photometrically correct estimates of SIM
performance for various classes of astronomical objects. In particular, we
simulate the cases of gaseous disks around black holes in the nuclei of
galaxies, and zodiacal dust disks around young stellar objects. Regarding the
first, we show that a Keplerian velocity gradient of the line-emitting gaseous
disk -- and thus the mass of the putative black hole -- can be determined with
SIM to unprecedented accuracy in about 5 hours of integration time for objects
with H_alpha surface brigthness comparable to the prototype M 87. Detections
and observations of exo-zodiacal dust disks depend critically on the disk
properties and the nulling capabilities of SIM. Systems with similar disk size
and at least one tenth of the dust content of beta Pic can be detected by SIM
at distances between 100 pc and a few kpc, if a nulling efficiency of 1/10000
is achieved. Possible inner clear regions indicative of the presence of massive
planets can also be detected and imaged. On the other hand, exo-zodiacal disks
with properties more similar to the solar system will not be found in
reasonable integration times with SIM.Comment: 28 pages, incl. 8 postscript figures, excl. 10 gif-figures Submitted
to Ap
A robust and efficient implementation of LOBPCG
Locally Optimal Block Preconditioned Conjugate Gradient (LOBPCG) is widely
used to compute eigenvalues of large sparse symmetric matrices. The algorithm
can suffer from numerical instability if it is not implemented with care. This
is especially problematic when the number of eigenpairs to be computed is
relatively large. In this paper we propose an improved basis selection strategy
based on earlier work by Hetmaniuk and Lehoucq as well as a robust convergence
criterion which is backward stable to enhance the robustness. We also suggest
several algorithmic optimizations that improve performance of practical LOBPCG
implementations. Numerical examples confirm that our approach consistently and
significantly outperforms previous competing approaches in both stability and
speed
Normal approximation under local dependence
We establish both uniform and nonuniform error bounds of the Berry-Esseen
type in normal approximation under local dependence. These results are of an
order close to the best possible if not best possible. They are more general or
sharper than many existing ones in the literature. The proofs couple Stein's
method with the concentration inequality approach.Comment: Published by the Institute of Mathematical Statistics
(http://www.imstat.org) in the Annals of Probability
(http://www.imstat.org/aop/) at http://dx.doi.org/10.1214/00911790400000045
Supersymmetric Vertex Models with Domain Wall Boundary Conditions
By means of the Drinfeld twists, we derive the determinant representations of
the partition functions for the and supersymmetric vertex
models with domain wall boundary conditions. In the homogenous limit, these
determinants degenerate to simple functions.Comment: 19 pages, 4 figures, to be published in J. Math. Phy
Motor current signal analysis using a modified bispectrum for machine fault diagnosis
This paper presents the use of the induction motor current to identify and quantify common faults within a two-stage reciprocating compressor. The theoretical basis is studied to understand current signal characteristics when the motor undertakes a varying load under faulty conditions. Although conventional bispectrum representation of current signal allows the inclusion of phase information and the elimination of Gaussian noise, it produces unstable results due to random phase variation of the sideband components in the current signal. A modified bispectrum based on the amplitude modulation feature of the current signal is thus proposed to combine both lower sidebands and higher sidebands simultaneously and hence describe the current signal more accurately. Based on this new bispectrum a more effective diagnostic feature namely normalised bispectral peak is developed for fault classification. In association with the kurtosis of the raw current signal, the bispectrum feature gives rise to reliable fault classification results. In particular, the low feature values can differentiate the belt looseness from other fault cases and discharge valve leakage and intercooler leakage can be separated easily using two linear classifiers. This work provides a novel approach to the analysis of stator current for the diagnosis of motor drive faults from downstream driving equipment
The Occurrence Rate of Earth Analog Planets Orbiting Sunlike Stars
Kepler is a space telescope that searches Sun-like stars for planets. Its
major goal is to determine {\eta}_Earth, the fraction of Sunlike stars that
have planets like Earth. When a planet 'transits' or moves in front of a star,
Kepler can measure the concomitant dimming of the starlight. From analysis of
the first four months of those measurements for over 150,000 stars, Kepler's
science team has determined sizes, surface temperatures, orbit sizes and
periods for over a thousand new planet candidates. In this paper, we
characterize the period probability distribution function of the super-Earth
and Neptune planet candidates with periods up to 132 days, and find three
distinct period regimes. For candidates with periods below 3 days the density
increases sharply with increasing period; for periods between 3 and 30 days the
density rises more gradually with increasing period, and for periods longer
than 30 days, the density drops gradually with increasing period. We estimate
that 1% to 3% of stars like the Sun are expected to have Earth analog planets,
based on the Kepler data release of Feb 2011. This estimate of is based on
extrapolation from a fiducial subsample of the Kepler planet candidates that we
chose to be nominally 'complete' (i.e., no missed detections) to the realm of
the Earth-like planets, by means of simple power law models. The accuracy of
the extrapolation will improve as more data from the Kepler mission is folded
in. Accurate knowledge of {\eta}_Earth is essential for the planning of future
missions that will image and take spectra of Earthlike planets. Our result that
Earths are relatively scarce means that a substantial effort will be needed to
identify suitable target stars prior to these future missions.Comment: Accepted for publication in the Astrophysical Journal. 19 pages, 8
figures. Minor text revisions, as requested by the scientific editor.
Included an additional figure. No changes in the scientific result
Gearbox fault diagnosis under different operating conditions based on time synchronous average and ensemble empirical mode decomposition
In this paper, a new method is proposed by combining ensemble empirical mode decomposition (EEMD) with order tracking techniques to analyse the vibration signals from a two stage helical gearbox. The method improves EEMD results in that it overcomes the potential deficiencies and achieves better order spectrum representation for fault diagnosis. Based on the analysis, a diagnostic feature is designed based on the order spectra of extracted IFMs for detection and separation of gearbox faults. Experimental results show this feature is sensitive to different fault severities and robust to the influences from operating conditions and remote sensor locations
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