52,177 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
Invisibility in non-Hermitian tight-binding lattices
Reflectionless defects in Hermitian tight-binding lattices, synthesized by
the intertwining operator technique of supersymmetric quantum mechanics, are
generally not invisible and time-of-flight measurements could reveal the
existence of the defects. Here it is shown that, in a certain class of
non-Hermitian tight-binding lattices with complex hopping amplitudes, defects
in the lattice can appear fully invisible to an outside observer. The
synthesized non-Hermitian lattices with invisible defects possess a real-valued
energy spectrum, however they lack of parity-time (PT) symmetry, which does not
play any role in the present work.Comment: to appear in Phys. Rev.
Discrete analogue computing with rotor-routers
Rotor-routing is a procedure for routing tokens through a network that can
implement certain kinds of computation. These computations are inherently
asynchronous (the order in which tokens are routed makes no difference) and
distributed (information is spread throughout the system). It is also possible
to efficiently check that a computation has been carried out correctly in less
time than the computation itself required, provided one has a certificate that
can itself be computed by the rotor-router network. Rotor-router networks can
be viewed as both discrete analogues of continuous linear systems and
deterministic analogues of stochastic processes.Comment: To appear in Chaos Special Focus Issue on Intrinsic and Designed
Computatio
Spin 1 inversion: a Majorana tensor force for deuteron alpha scattering
We demonstrate, for the first time, successful S-matrix to potential
inversion for spin one projectiles with non-diagonal yielding a
interaction. The method is a generalization of the
iterative-perturbative, IP, method. We present a test case indicating the
degree of uniqueness of the potential. The method is adapted, using established
procedures, into direct observable to potential inversion, fitting ,
, , and for d + alpha scattering over
a range of energies near 10 MeV. The interaction which we find is
very different from that proposed elsewhere, both real and imaginary parts
being very different for odd and even parity channels.Comment: 7 pages Revtex, 4 ps figure
Exact and approximate dynamics of the quantum mechanical O(N) model
We study a quantum dynamical system of N, O(N) symmetric, nonlinear
oscillators as a toy model to investigate the systematics of a 1/N expansion.
The closed time path (CTP) formalism melded with an expansion in 1/N is used to
derive time evolution equations valid to order 1/N (next-to-leading order). The
effective potential is also obtained to this order and its properties
areelucidated. In order to compare theoretical predictions against numerical
solutions of the time-dependent Schrodinger equation, we consider two initial
conditions consistent with O(N) symmetry, one of them a quantum roll, the other
a wave packet initially to one side of the potential minimum, whose center has
all coordinates equal. For the case of the quantum roll we map out the domain
of validity of the large-N expansion. We discuss unitarity violation in the 1/N
expansion; a well-known problem faced by moment truncation techniques. The 1/N
results, both static and dynamic, are also compared to those given by the
Hartree variational ansatz at given values of N. We conclude that late-time
behavior, where nonlinear effects are significant, is not well-described by
either approximation.Comment: 16 pages, 12 figrures, revte
Chaos in effective classical and quantum dynamics
We investigate the dynamics of classical and quantum N-component phi^4
oscillators in the presence of an external field. In the large N limit the
effective dynamics is described by two-degree-of-freedom classical Hamiltonian
systems. In the classical model we observe chaotic orbits for any value of the
external field, while in the quantum case chaos is strongly suppressed. A
simple explanation of this behaviour is found in the change in the structure of
the orbits induced by quantum corrections. Consistently with Heisenberg's
principle, quantum fluctuations are forced away from zero, removing in the
effective quantum dynamics a hyperbolic fixed point that is a major source of
chaos in the classical model.Comment: 6 pages, RevTeX, 5 figures, uses psfig, changed indroduction and
conclusions, added reference
Density waves and supersolidity in rapidly rotating atomic Fermi gases
We study theoretically the low-temperature phases of a two-component atomic
Fermi gas with attractive s-wave interactions under conditions of rapid
rotation. We find that, in the extreme quantum limit, when all particles occupy
the lowest Landau level, the normal state is unstable to the formation of
"charge" density wave (CDW) order. At lower rotation rates, when many Landau
levels are occupied, we show that the low-temperature phases can be
supersolids, involving both CDW and superconducting order.Comment: 4 pages, 1 figure, uses feynmp.st
A Comparative Study of the Valence Electronic Excitations of N_2 by Inelastic X-ray and Electron Scattering
Bound state, valence electronic excitation spectra of N_2 are probed by
nonresonant inelastic x-ray and electron scattering. Within the usual
theoretical treatments, dynamical structure factors derived from the two probes
should be identical. However, we find strong disagreements outside the dipole
scattering limit, even at high probe energies. This suggests an unexpectedly
important contribution from intra-molecular multiple scattering of the probe
electron from core electrons or the nucleus. These effects should grow
progressively stronger as the atomic number of the target species increases.Comment: Submitted to Physical Review Letters April 27, 2010. 12 pages
including 2 figure pages
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