156 research outputs found
Ab-initio self-energy corrections in systems with metallic screening
The calculation of self-energy corrections to the electron bands of a metal
requires the evaluation of the intraband contribution to the polarizability in
the small-q limit. When neglected, as in standard GW codes for semiconductors
and insulators, a spurious gap opens at the Fermi energy. Systematic methods to
include intraband contributions to the polarizability exist, but require a
computationally intensive Fermi-surface integration. We propose a numerically
cheap and stable method, based on a fit of the power expansion of the
polarizability in the small-q region. We test it on the homogeneous electron
gas and on real metals such as sodium and aluminum.Comment: revtex, 14 pages including 5 eps figures v2: few fixe
Multi-robot spot-welding cells: An integrated approach to cell design and motion planning
The necessity to manage several vehicle models on the same robotized assembly cell has made the cell design and the robot off-line motion planning two fundamental activities. Industrial practice and state-of-the-art methods focus on the technical issues of each activity, but no integrated approach has been yet proposed, resulting in a lack of optimality for the final cell configuration. The paper introduces a formalization of the whole process and proposes a heuristic multi-stage method for the identification of the optimal combination of cell design choices and motion planning. The proposed architecture is depicted through a real case for welding application
Multi-robot spot-welding cell design: Problem formalization and proposed architecture
The multi-robot cell design for car-body spot welding is faced by industry as a sequence of tasks, where researches are focused on issues of the
problem as a whole. In authors’ knowledge, none work in literature have suggested any formalization for the complete process. This paper tries
to bridges the gap proposing coherent process formalization, and presenting a corresponding innovative architecture for the automatic optimal
cell design. Specifically, the formalization involves the identification and allocation of the resources in terms of a set of decisional variables (e.g.
robot model/positioning/number, welding gun models/allocation/number, welding point allocation etc.); then, the design optimization process
minimizes the investment costs granting the cycle time. The multi-loop optimization architecture integrates both new algorithms and existent
procedures from different fields. Test-bed showing its feasibility is reported
robotic am system for plastic materials tuning and on line adjustment of process parameters
Abstract Additive Manufacturing (AM) techniques based on thermoplastic polymer extrusion allow the manufacture of complex parts, but their slow printing speed limits their use for mass production. To overcome this drawback, an industrial screw-based extruder has been mounted on an anthropomorphic robot, realizing a flexible AM platform for big objects. The most important process parameters have been set by a suitable experimental campaign, ensuring a regular deposited layer geometry. A closed-loop control has been implemented to further improve the process parameter setting based on data measured during the deposition, in this way compensating the material withdrawal or other unexpected defects
Tungsten Oxide Nanowires Chemical Sensors
AbstractTungsten oxide nanowires have been synthetized by a simple thermal oxidation route in an oxygen atmosphere, at low vacuum pressure. The morphology of the nanostructures was investigated by scanning electron microscope (SEM).Chemical sensors were fabricated using tungsten oxide nanowires as active layer, directly deposited on the transducer. Gas sensingperformances were evaluated towards some target chemical species (CO, NO2, NH3, Acetone, Ethanol). Firstly, a temperature screening was performed in presence of a fixed gas concentration, to determine the optimal working temperature of the sensors. Then, calibration curves for some chemical species were estimated. The influence of relative humidity was taken into account. Fabricated devices seem very promising for the detection of ammonia and carbon monoxide
The Orion Region: Evidence of enhanced cosmic-ray density in a stellar wind forward shock interaction with a high density shell
Context. In recent years, an in-depth gamma-ray analysis of the Orion region
has been carried out by the AGILE and Fermi-LAT (Large Area Telescope) teams
with the aim of estimating the H2-CO conversion factor, XCO. The comparison of
the data from both satellites with models of diffuse gamma-ray Galactic
emission unveiled an excess at (l,b)=[213.9, -19.5], in a region at a short
angular distance from the OB star k-Ori. Possible explanations of this excess
are scattering of the so-called "dark gas", non-linearity in the H2-CO
relation, or Cosmic-Ray (CR) energization at the k-Ori wind shock.
Aims. Concerning this last hypothesis, we want to verify whether cosmic-ray
acceleration or re-acceleration could be triggered at the k-Ori forward shock,
which we suppose to be interacting with a star-forming shell detected in
several wavebands and probably triggered by high energy particles.
Methods. Starting from the AGILE spectrum of the detected gamma-ray excess,
showed here for the first time, we developed a valid physical model for
cosmic-ray energization, taking into account re-acceleration, acceleration,
energy losses, and secondary electron contribution.
Results. Despite the characteristic low velocity of an OB star forward shock
during its "snowplow" expansion phase, we find that the Orion gamma-ray excess
could be explained by re-acceleration of pre-existing cosmic rays in the
interaction between the forward shock of k-Ori and the CO-detected,
star-forming shell swept-up by the star expansion. According to our
calculations, a possible contribution from freshly accelerated particles is
sub-dominant with respect the re-acceleration contribution. However, a simple
adiabatic compression of the shell could also explain the detected gamma-ray
emission. Futher GeV and TeV observations of this region are highly recommended
in order to correctly identify the real physical scenario.Comment: 8 pages, 5 figures, accepted by A&
Validation of an extended approach to multi-robot cell design and motion planning
According to both industrial practice and literature, multi-robot cell design and robot motion planning for vehicle spot welding are two sequential activities, managed by different functional units through different software tools. Due to this sequential computation, the whole process suffers from inherent inefficiency. In this work, a new methodology is proposed, that overcomes the above inefficiency through the simultaneous resolution of design and motion planning problems. Specifically, three mathematical models were introduced that (i) select and positions the resources, (ii) allocate the tasks to the resources and (iii) identify a coordinated robot motion plan. Based on the proposed methodology, we built three ad-hoc cases with the goal to highlight the relations between design, motion planning and environment complexity. These cases could be taken as reference cases so on. Moreover, results on an industrial case are presented
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