1,108 research outputs found
Suppression of biodynamic interference in head-tracked teleoperation
The utility of helmet-tracked sights to provide pointing commands for teleoperation of cameras, lasers, or antennas in aircraft is degraded by the presence of uncommanded, involuntary heat motion, referred to as biodynamic interference. This interference limits the achievable precision required in pointing tasks. The noise contributions due to biodynamic interference consists of an additive component which is correlated with aircraft vibration and an uncorrelated, nonadditive component, referred to as remnant. An experimental simulation study is described which investigated the improvements achievable in pointing and tracking precision using dynamic display shifting in the helmet-mounted display. The experiment was conducted in a six degree of freedom motion base simulator with an emulated helmet-mounted display. Highly experienced pilot subjects performed precision head-pointing tasks while manually flying a visual flight-path tracking task. Four schemes using adaptive and low-pass filtering of the head motion were evaluated to determine their effects on task performance and pilot workload in the presence of whole-body vibration characteristic of helicopter flight. The results indicate that, for tracking tasks involving continuously moving targets, improvements of up to 70 percent can be achieved in percent on-target dwelling time and of up to 35 percent in rms tracking error, with the adaptive plus low-pass filter configuration. The results with the same filter configuration for the task of capturing randomly-positioned, stationary targets show an increase of up to 340 percent in the number of targets captured and an improvement of up to 24 percent in the average capture time. The adaptive plus low-pass filter combination was considered to exhibit the best overall display dynamics by each of the subjects
Ab-initio Molecular Dynamics study of electronic and optical properties of silicon quantum wires: Orientational Effects
We analyze the influence of spatial orientation on the optical response of
hydrogenated silicon quantum wires. The results are relevant for the
interpretation of the optical properties of light emitting porous silicon. We
study (111)-oriented wires and compare the present results with those
previously obtained within the same theoretical framework for (001)-oriented
wires [F. Buda {\it et al.}, {\it Phys. Rev. Lett.} {\bf 69}, 1272, (1992)]. In
analogy with the (001)-oriented wires and at variance with crystalline bulk
silicon, we find that the (111)-oriented wires exhibit a direct gap at whose value is largely enhanced with respect to that found in bulk
silicon because of quantum confinement effects. The imaginary part of the
dielectric function, for the external field polarized in the direction of the
axis of the wires, shows features that, while being qualitatively similar to
those observed for the (001) wires, are not present in the bulk. The main
conclusion which emerges from the present study is that, if wires a few
nanometers large are present in the porous material, they are
optically active independently of their specific orientation.Comment: 14 pages (plus 6 figures), Revte
Experimental identification and validation of models in micro and macro plasticity
For micro-macro approaches to finite plasticity, one needs experimental results on both scales, the engineering scale (macro scale) and the crystal scale (micro scale). Since we know that a monocrystal behaves different from a crystallite embedded in a polycrystal, one is also interested in data obtained on the micro scale of a polycrystal. Such data is needed not only for the identification of the material parameters like hardening variables, but also for the validation of these models. In this paper, experiments on both scales and, in parallel, FEM-simulations are presented, in order to compare the results of both approaches. The specimens stem from a rolled sheet of the deep-drawing steel DC04. On the micro scale indenter tests have been performed and the orientation changes in the volume below the indent have been measured using micron-resolution 3D x-ray microscopy (Larson et al., 2004, 2008). On the macro scale the usual tension tests and additional shear tests in different directions (Bouvier etal.,2006) have been performed. In corresponding simulations, the micro-macro transition is performed by a full constrained Taylor-model and, in order to overcome the drawbacks of the Taylor-model, the RVE technique has been applied
Massey products in symplectic manifolds
The paper is devoted to study of Massey products in symplectic manifolds.
Theory of generalized and classical Massey products and a general construction
of symplectic manifolds with nontrivial Massey products of arbitrary large
order are exposed. The construction uses the symplectic blow-up and is based on
the author results, which describe conditions under which the blow-up of a
symplectic manifold X along its submanifold Y inherits nontrivial Massey
products from X ot Y. This gives a general construction of nonformal symplectic
manifolds.Comment: LaTeX, 48 pages, 2 figure
Dynamic Heterogeneity and DNA Methylation in Embryonic Stem Cells
Cell populations can be strikingly heterogeneous, composed of multiple cellular states, each exhibiting stochastic noise in its gene expression. A major challenge is to disentangle these two types of variability and to understand the dynamic processes and mechanisms that control them. Embryonic stem cells (ESCs) provide an ideal model system to address this issue because they exhibit heterogeneous and dynamic expression of functionally important regulatory factors. We analyzed gene expression in individual ESCs using single-molecule RNA-FISH and quantitative time-lapse movies. These data discriminated stochastic switching between two coherent (correlated) gene expression states and burst-like transcriptional noise. We further showed that the “2i” signaling pathway inhibitors modulate both types of variation. Finally, we found that DNA methylation plays a key role in maintaining these metastable states. Together, these results show how ESC gene expression states and dynamics arise from a combination of intrinsic noise, coherent cellular states, and epigenetic regulation
Mode tracking issues in structural optimization
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76633/1/AIAA-12747-342.pd
Quantum entanglement in photosynthetic light harvesting complexes
Light harvesting components of photosynthetic organisms are complex, coupled,
many-body quantum systems, in which electronic coherence has recently been
shown to survive for relatively long time scales despite the decohering effects
of their environments. Within this context, we analyze entanglement in
multi-chromophoric light harvesting complexes, and establish methods for
quantification of entanglement by presenting necessary and sufficient
conditions for entanglement and by deriving a measure of global entanglement.
These methods are then applied to the Fenna-Matthews-Olson (FMO) protein to
extract the initial state and temperature dependencies of entanglement. We show
that while FMO in natural conditions largely contains bipartite entanglement
between dimerized chromophores, a small amount of long-range and multipartite
entanglement exists even at physiological temperatures. This constitutes the
first rigorous quantification of entanglement in a biological system. Finally,
we discuss the practical utilization of entanglement in densely packed
molecular aggregates such as light harvesting complexes.Comment: 14 pages, 7 figures. Improved presentation, published versio
Internal transitions of quasi-2D charged magneto-excitons in the presence of purposely introduced weak lateral potential energy variations
Optically detected resonance spectroscopy has been used to investigate
effects of weak random lateral potential energy fluctuations on internal
transitions of charged magneto-excitons (trions) in quasi two-dimensional
GaAs/AlGaAs quantum-well (QW) structures. Resonant changes in the ensemble
photoluminescence induced by far-infrared radiation were studied as a function
of magnetic field for samples having: 1) no growth interrupts (short range
well-width fluctuations), and 2) intentional growth interrupts (long range
monolayer well-width differences). Only bound-to-continuum internal transitions
of the negatively charged trion are observed for samples of type 1. In
contrast, a feature on the high field (low energy) side of electron cyclotron
resonance is seen for samples of type 2 with well widths of 14.1 and 8.4 nm.
This feature is attributed to a bound-to-bound transition of the spin-triplet
with non-zero oscillator strength resulting from breaking of translational
symmetry.Comment: 16 pages, 3 figures, submitted to Physical Review
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