1,140 research outputs found
Development of a machine vision system for automated structural assembly
Research is being conducted at the LaRC to develop a telerobotic assembly system designed to construct large space truss structures. This research program was initiated within the past several years, and a ground-based test-bed was developed to evaluate and expand the state of the art. Test-bed operations currently use predetermined ('taught') points for truss structural assembly. Total dependence on the use of taught points for joint receptacle capture and strut installation is neither robust nor reliable enough for space operations. Therefore, a machine vision sensor guidance system is being developed to locate and guide the robot to a passive target mounted on the truss joint receptacle. The vision system hardware includes a miniature video camera, passive targets mounted on the joint receptacles, target illumination hardware, and an image processing system. Discrimination of the target from background clutter is accomplished through standard digital processing techniques. Once the target is identified, a pose estimation algorithm is invoked to determine the location, in three-dimensional space, of the target relative to the robots end-effector. Preliminary test results of the vision system in the Automated Structural Assembly Laboratory with a range of lighting and background conditions indicate that it is fully capable of successfully identifying joint receptacle targets throughout the required operational range. Controlled optical bench test results indicate that the system can also provide the pose estimation accuracy to define the target position
Postbuckling response of long thick isotropic plates loaded in compression including higher order transverse shearing effects
Buckling and postbuckling results for aluminum plates loaded in compression are presented. The buckling results were plotted to show the effects of thickness on the stress coefficient. Buckling results are given for various length-to-width ratios. Postbuckling results for plates with transverse shearing flexibility are compared to results from classical theory for various width-to-thickness ratios. The plates are considered to be long with side edges simply supported, with edges free of stress and the plates are subjected to longitudinal compressive displacement. Characteristic curves indicating the average longitudinal direct stress resultant as a function of the applied displacements are calculated based on four different theories: Classical von Karman, first-order shear deformation, higher-order shear deformation, and three-dimensional flexibility. Present results indicate that the three-dimensional flexibility theory gives the lowest and therefore, most accurate results. The higher-order shear deformation theory has fewer unknowns than the three-dimensional flexibility but is not as accurate. The figures presented show that small differences occur in the maximum stress resultants and the transverse displacements calculated when the effects of transverse shear are included
Postbuckling response of long thick plates loaded in compression including higher order transverse shearing effects
Buckling and postbuckling results are presented for compression-loaded simply-supported aluminum plates and composite plates with a symmetric lay-up of thin + or - 45 deg plies composed of many layers. Buckling results for aluminum plates of finite length are given for various length-to-width ratios. Asymptotes to the curves based on buckling results give N(sub xcr) for plates of infinite length. Postbuckling results for plates with transverse shearing flexibility are compared to results from classical theory for various width-to-thickness ratios. Characteristic curves indicating the average longitudinal direct stress resultant as a function of the applied displacements are calculated based on four different theories: Classical von Karman theory using the Kirchoff assumptions, first-order shear deformation theory, higher-order shear deformation theory, and 3-D flexibility theory. Present results indicate that the 3-D flexibility theory gives the lowest buckling loads. The higher-order shear deformation theory has fewer unknowns than the 3-D flexibility theory but does not take into account through-the-thickness effects. The figures presented show that small differences occur in the average longitudinal direct stress resultants from the four theories that are functions of applied end-shortening displacement
Dimethylarginine Dimethylaminohydrolase Promotes Endothelial Repair After Vascular Injury
ObjectivesWe sought to determine if a reduction in asymmetric dimethylarginine (ADMA) enhances endothelial regeneration.BackgroundAsymmetric dimethylarginine is an endogenous inhibitor of nitric oxide synthase (NOS). Increased plasma levels of ADMA are associated with endothelial vasodilator dysfunction in patients with vascular disease or risk factors. Asymmetric dimethylarginine is eliminated largely by the action of dimethylarginine dimethylaminohydrolase (DDAH), which exists in 2 isoforms. Dimethylarginine dimethylaminohydrolase-1 transgenic (TG) mice manifest increased DDAH activity, reduced plasma and tissue ADMA levels, increased nitric oxide synthesis, and reduced systemic vascular resistance.MethodsThe left femoral arteries of DDAH1 TG mice and wild-type (WT) mice were injured by a straight spring wire, and regeneration of the endothelial cell (EC) monolayer was assessed. Endothelial sprouting was assayed with growth factor-reduced Matrigel.ResultsRegeneration of the EC monolayer was more complete 1 week after injury in TG mice (WT vs. TG: 40.0 ± 6.5% vs. 61.2 ± 6.4%, p < 0.05). The number of CD45 positive cells at the injured sites was reduced by 62% in DDAH TG mice (p < 0.05). Four weeks after injury, the neointima area and intima/media ratio were attenuated in DDAH TG mice (WT vs. TG: 0.049 ± 0.050 mm2vs. 0.031 ± 0.060 mm2, 3.1 ± 0.5 vs. 1.7 ± 0.2, respectively, p < 0.05). Endothelial cell sprouting from vascular segments increased in TG mice (WT vs. TG: 24.3 ± 3.9 vs. 39.0 ± 2.2, p < 0.05).ConclusionsWe find for the first time an important role for DDAH in EC regeneration and in neointima formation. Strategies to enhance DDAH expression or activity might be useful in restoring the endothelial monolayer and in treating vascular disease
Experimental behavior of graphite-epoxy Y-stiffened specimens loaded in compression
An experimental investigation of the behavior of graphite-epoxy Y-stiffened specimens loaded in compression is presented. Experimental results are presented for element specimens with a single stiffener and for panel specimens with three stiffeners. Response and failure characteristics of the specimens are described. Effects of impact damage on structural response for both specimen configurations are also presented. Experimental results indicate that impact location may significantly affect the residual strength of the Y-stiffened specimens. The failure results indicate that the critical failure mode is buckling of the stiffener webs for Y-stiffened element specimens and buckling of the stiffener webs and other stiffener blades for the Y-stiffened panel specimens
Comment on `conservative discretizations of the Kepler motion'
We show that the exact integrator for the classical Kepler motion, recently
found by Kozlov ({\it J. Phys. A: Math. Theor.\} {\bf 40} (2007) 4529-4539),
can be derived in a simple natural way (using well known exact discretization
of the harmonic oscillator). We also turn attention on important earlier
references, where the exact discretization of the 4-dimensional isotropic
harmonic oscillator has been applied to the perturbed Kepler problem.Comment: 6 page
Long-time behaviour of discretizations of the simple pendulum equation
We compare the performance of several discretizations of the simple pendulum
equation in a series of numerical experiments. The stress is put on the
long-time behaviour. We choose for the comparison numerical schemes which
preserve the qualitative features of solutions (like periodicity). All these
schemes are either symplectic maps or integrable (preserving the energy
integral) maps, or both. We describe and explain systematic errors (produced by
any method) in numerical computations of the period and the amplitude of
oscillations. We propose a new numerical scheme which is a modification of the
discrete gradient method. This discretization preserves (almost exactly) the
period of small oscillations for any time step.Comment: 41 pages, including 18 figures and 4 table
Generalized stacking fault energy surfaces and dislocation properties of aluminum
We have employed the semidiscrete variational generalized Peierls-Nabarro
model to study the dislocation core properties of aluminum. The generalized
stacking fault energy surfaces entering the model are calculated by using
first-principles Density Functional Theory (DFT) with pseudopotentials and the
embedded atom method (EAM). Various core properties, including the core width,
splitting behavior, energetics and Peierls stress for different dislocations
have been investigated. The correlation between the core energetics and
dislocation character has been explored. Our results reveal a simple
relationship between the Peierls stress and the ratio between the core width
and atomic spacing. The dependence of the core properties on the two methods
for calculating the total energy (DFT vs. EAM) has been examined. The EAM can
give gross trends for various dislocation properties but fails to predict the
finer core structures, which in turn can affect the Peierls stress
significantly (about one order of magnitude).Comment: 25 pages, 12 figure
Nucleon-deuteron elastic scattering as a tool to probe properties of three-nucleon forces
Faddeev equations for elastic Nd scattering have been solved using modern NN
forces combined with the Tucson-Melbourne two-pion exchange three-nucleon
force, with a modification thereof closer to chiral symmetry and the Urbana IX
three-nucleon force. Theoretical predictions for the differential cross section
and several spin observables using NN forces only and NN forces combined with
three-nucleon force models are compared to each other and to the existing data.
A wide range of energies from 3 to 200 MeV is covered. Especially at the higher
energies striking three-nucleon force effects are found, some of which are
supported by the still rare set of data, some are in conflict with data and
thus very likely point to defects in those three-nucleon force models.Comment: 30 pages, 14 Postscript figures; now minor changes in figures and
reference
Three-Nucleon Force Effects in Nucleon Induced Deuteron Breakup: Predictions of Current Models (I)
An extensive study of three-nucleon force effects in the entire phase space
of the nucleon-deuteron breakup process, for energies from above the deuteron
breakup threshold up to 200 MeV, has been performed. 3N Faddeev equations have
been solved rigorously using the modern high precision nucleon-nucleon
potentials AV18, CD Bonn, Nijm I, II and Nijm 93, and also adding 3N forces. We
compare predictions for cross sections and various polarization observables
when NN forces are used alone or when the two pion-exchange Tucson-Melbourne
3NF was combined with each of them. In addition AV18 was combined with the
Urbana IX 3NF and CD Bonn with the TM' 3NF, which is a modified version of the
TM 3NF, more consistent with chiral symmetry. Large but generally model
dependent 3NF effects have been found in certain breakup configurations,
especially at the higher energies, both for cross sections and spin
observables. These results demonstrate the usefulness of the kinematically
complete breakup reaction in testing the proper structure of 3N forces.Comment: 42 pages, 20 ps figures, 2 gif figure
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