259 research outputs found
On Selecting and Scheduling Assembly Plans Using Constraint Programming
This work presents the application of Constraint
Programming to the problem of selecting and sequencing assembly
operations. The set of all feasible assembly plans for a single product is
represented using an And/Or graph. This representation embodies some
of the constraints involved in the planning problem, such as precedence
of tasks, and the constraints due to the completion of a correct assembly
plan. The work is focused on the selection of tasks and their optimal
ordering, taking into account their execution in a generic multi-robot
system. In order to include all different constraints of the problem, the
And/Or graph representation is extended, so that links between nodes
corresponding to assembly tasks are added, taking into account the
resource constraints. The resultant problem is mapped to a Constraint
Satisfaction Problem (CSP), and is solved using Constraint
Programming, a powerful programming paradigm that is increasingly
used to model and solve many hard real-life problems
A Pomset-Based Model for Estimating Workcells' Setups in Assembly Sequence Planning
This paper presents a model based on pomsets (partially ordered multisets)
for estimating the minimum number of setups in the workcells in Assembly
Sequence Planning. This problem is focused through the minimization of
the makespan (total assembly time) in a multirobot system. The planning model
considers, apart from the durations and resources needed for the assembly tasks,
the delays due to the setups in the workcells. An A* algorithm is used to meet
the optimal solution. It uses the And/Or graph for the product to assemble, that
corresponds to a compressed representation of all feasible assembly plans. Two
basic admissible heuristic functions can be defined from relaxed models of the
problem, considering the precedence constraints and the use of resources separately.
The pomset-based model presented in this paper takes into account the
precedence constraints in order to obtain a better estimation for the second heuristic
function, so that the performance of the algorithm could be improved
A Genetic Algorithm for Assembly Sequence Planning
This work presents a genetic algorithm for assembly sequence planning.
This problem is more difficult than other sequencing problems that have
already been tackled with success using these techniques, such as the classic
Traveling Salesperson Problem (TSP) or the Job Shop Scheduling Problem
(JSSP). It not only involves the arranging of tasks, as in those problems, but
also the selection of them from a set of alternative operations. Two families of
genetic operators have been used for searching the whole solution space. The
first includes operators that search for new sequences locally in a predetermined
assembly plan, that of parent chromosomes. The other family of operators introduces
new tasks in the solution, replacing others to maintain the validity of
chromosomes, and it is intended to search for sequences in other assembly
plans. Furthermore, some problem-based heuristics have been used for generating
the individuals in the population
De Sitter Cosmic Strings and Supersymmetry
We study massive spinor fields in the geometry of a straight cosmic string in
a de Sitter background. We find a hidden N=2 supersymmetry in the fermionic
solutions of the equations of motion. We connect the zero mode solutions to the
heat-kernel regularized Witten index of the supersymmetric algebra.Comment: Version similar to the one accepted by General Relativity and
Gravitatio
Metal-insulator transition in the one-dimensional Holstein model at half filling
We study the one-dimensional Holstein model with spin-1/2 electrons at
half-filling. Ground state properties are calculated for long chains with great
accuracy using the density matrix renormalization group method and extrapolated
to the thermodynamic limit. We show that for small electron-phonon coupling or
large phonon frequency, the insulating Peierls ground state predicted by
mean-field theory is destroyed by quantum lattice fluctuations and that the
system remains in a metallic phase with a non-degenerate ground state and
power-law electronic and phononic correlations. When the electron-phonon
coupling becomes large or the phonon frequency small, the system undergoes a
transition to an insulating Peierls phase with a two-fold degenerate ground
state, long-range charge-density-wave order, a dimerized lattice structure, and
a gap in the electronic excitation spectrum.Comment: 6 pages (LaTex), 10 eps figure
Tidal torques. A critical review of some techniques
We point out that the MacDonald formula for body-tide torques is valid only
in the zeroth order of e/Q, while its time-average is valid in the first order.
So the formula cannot be used for analysis in higher orders of e/Q. This
necessitates corrections in the theory of tidal despinning and libration
damping.
We prove that when the inclination is low and phase lags are linear in
frequency, the Kaula series is equivalent to a corrected version of the
MacDonald method. The correction to MacDonald's approach would be to set the
phase lag of the integral bulge proportional to the instantaneous frequency.
The equivalence of descriptions gets violated by a nonlinear
frequency-dependence of the lag.
We explain that both the MacDonald- and Darwin-torque-based derivations of
the popular formula for the tidal despinning rate are limited to low
inclinations and to the phase lags being linear in frequency. The
Darwin-torque-based derivation, though, is general enough to accommodate both a
finite inclination and the actual rheology.
Although rheologies with Q scaling as the frequency to a positive power make
the torque diverge at a zero frequency, this reveals not the impossible nature
of the rheology, but a flaw in mathematics, i.e., a common misassumption that
damping merely provides lags to the terms of the Fourier series for the tidal
potential. A hydrodynamical treatment (Darwin 1879) had demonstrated that the
magnitudes of the terms, too, get changed. Reinstating of this detail tames the
infinities and rehabilitates the "impossible" scaling law (which happens to be
the actual law the terrestrial planets obey at low frequencies).Comment: arXiv admin note: sections 4 and 9 of this paper contain substantial
text overlap with arXiv:0712.105
Quantum Monte Carlo and variational approaches to the Holstein model
Based on the canonical Lang-Firsov transformation of the Hamiltonian we
develop a very efficient quantum Monte Carlo algorithm for the Holstein model
with one electron. Separation of the fermionic degrees of freedom by a
reweighting of the probability distribution leads to a dramatic reduction in
computational effort. A principal component representation of the phonon
degrees of freedom allows to sample completely uncorrelated phonon
configurations. The combination of these elements enables us to perform
efficient simulations for a wide range of temperature, phonon frequency and
electron-phonon coupling on clusters large enough to avoid finite-size effects.
The algorithm is tested in one dimension and the data are compared with
exact-diagonalization results and with existing work. Moreover, the ideas
presented here can also be applied to the many-electron case. In the
one-electron case considered here, the physics of the Holstein model can be
described by a simple variational approach.Comment: 18 pages, 11 Figures, v2: one typo correcte
Branching Fractions for D0 -> K+K- and D0 -> pi+pi-, and a Search for CP Violation in D0 Decays
Using the large hadroproduced charm sample collected in experiment E791 at
Fermilab, we have measured ratios of branching fractions for the two-body
singly-Cabibbo-suppressed charged decays of the D0:
(D0 -> KK)/(D0 -> Kpi) = 0.109 +- 0.003 +- 0.003,
(D0 -> pipi)/(D0 -> Kpi) = 0.040 +- 0.002 +- 0.003, and
(D0 -> KK)/(D0 -> pipi) = 2.75 +- 0.15 +- 0.16. We have looked for
differences in the decay rates of D0 and D0bar to the CP eigenstates K+K- and
pi+pi-, and have measured the CP asymmetry parameters
A_CP(K+K-) = -0.010 +- 0.049 +- 0.012 and
A_CP(pi+pi-) = -0.049 +- 0.078 +- 0.030, both consistent with zero.Comment: 10 Postscript pages, including 2 figures. Submitted to Phys. Lett.
Asymmetries between the production of D+ and D- mesons from 500 GeV/c pi- nucleon interactions as a function of xF and pt**2
We present asymmetries between the production of D+ and D- mesons in Fermilab
experiment E791 as a function of xF and pt**2. The data used here consist of
74,000 fully-reconstructed charmed mesons produced by a 500 GeV/c pi- beam on C
and Pt foils. The measurements are compared to results of models which predict
differences between the production of heavy-quark mesons that have a light
quark in common with the beam (leading particles) and those that do not
(non-leading particles). While the default models do not agree with our data,
we can reach agreement with one of them, PYTHIA, by making a limited number of
changes to parameters used
- …