3,591 research outputs found
gamma* gamma -> pi pi at large Q^2
The QCD analysis of the process gamma* gamma -> pi pi at large Q^2 and small
center-of-mass energy allows one to access a new hadronic observable describing
the exclusive transition from a q qbar or g g state to a pair of mesons. A
fruitful study may be envisaged at existing machines.Comment: 7 pages, to appear in the Proceedings of PHOTON 2000, Ambleside,
England, August 200
A System of Autonomously Flying Helicopters for Load Transportation
Die Arbeit beschreibt Entwurf, Umsetzung und Validierung eines autonomen
Lastentransportsystems, welches auf Basis mehrerer Modellhubschrauber
realisiert wurde. AbhÀngig von den Anforderungen der zu transportierenden
Last kann die Anzahl der verwendeten Hubschrauber individuell angepasst
werden. Die prÀsentierten Modelle und Regler wurden durch
Computersimulationen und reale Flugversuche verifiziert.
Zwei nichtlineare Modelle werden prĂ€sentiert: Ein Model fĂŒr Konfigurationen
bestehend aus einem Helikopter und einer Last (single-lift) und eines fĂŒr
Konfigurationen bestehend aus zwei bzw. mehreren Helikoptern und einer Last
(dual- und multi-lift). Neben diesen komplexen Modellen werden vereinfachte
Modelle vorgestellt, die fĂŒr den Reglerentwurf verwendet werden.
Ein generischer Orientierungsregler wird entwickelt, der fĂŒr die Regelung
aller beschriebenen Transportkonfigurationen verwendet werden kann. Durch
die Nutzung dieses Reglers vereinfacht sich der Entwurf der
Translationsregler erheblich.
Drei Translationsregler werden beschrieben: Ein Regler fĂŒr single-lift
Konfigurationen, der eine aktive UnterdrĂŒckung von Lastschwingungen
erlaubt, und ein verteilter Regler fĂŒr multi-lift Konfigurationen.
Weiterhin wird ein dual-lift Regler prÀsentiert, der eine Kombination der
anderen Regler darstellt. Die Regler fĂŒr dual- und multi-lift
Konfigurationen verwenden keine mechanischen Hilfskonstrukte wie
Abstandshalter.
Die Position der Last wird durch die Orientierung des Seils, gemessen nahe
dem Helikopterrumpf, bestimmt. Externe Störungen wie WindstöĂe können eine
Eigenschwingung des Seils anregen, welche die ermittelte Lastposition
verfÀlscht. Die Eigenschwingung des Seils sowie der Einfluss der
verwendeten Messeinrichtung werden analysiert. Auf Basis dieser Analyse
wird ein Lastbeobachter entwickelt und in mehreren Experimenten
verifiziert. Dieser Lastbeobachter ist von essentieller Wichtigkeit fĂŒr den
sicheren Betrieb des Lastentransportsystems, insbesondere bei schlechten
Wetterbedingungen.
Die entwickelten nichtlinearen Modelle des Systems wie auch die Regler der
single- und multi-lift Konfigurationen wurden durch Flugversuche validiert.
Dabei hat das System bewiesen, dass es auch bei sehr schlechten
Wetterbedingungen einsetzbar iThis work covers the design, realization and validation of an autonomous
load transportation system, utilizing several small size helicopters. The
number of participating helicopters is configurable for the described
system, depending on the requirements of the transported load. The
presented models and controllers have been validated in computer simulation
and flight experiments.
Two non-linear models are presented: One model covers single-lift and one
model covers dual- and multi-lift configurations. Simplified models are
introduced beside the complex models, which are utilized for the
translation controller design.
A generic orientation controller is presented, which is applicable for the
control of all presented slung load configurations. The utilization of this
controller significantly simplifies the design of the translation
controllers. The independence from the actual slung load configuration is
achieved through measurement of the rope force vector in the rope
attachment point, which is located on the helicopter fuselage.
Three translation controllers are described: A controller for single-lift
configurations, which allows the active compensation of load oscillations
and a distributed controller for multi-lift configurations. A dual-lift
translation controller is presented, which resembles a combination of
single- and multi-lift translation controller. The presented controllers
for dual- or multi-lift configurations do not utilize auxiliary constructs,
like spreader-bars.
The position of the load is estimated from the measured orientation of the
rope, close to the helicopter fuselage. External disturbances, like wind
gusts, are able to stimulate internal oscillations of the rope, which
disturb the estimated load position. The internal motion of the rope as
well as the influence of the used measurement device are analyzed and a
flexible rope model is presented. Based on the results a load motion
observer is developed and validated in several experiments. This load
motion observer is essential for the safe operation of the slung load
system, especially during bad weather conditions.
The derived non-linear models of the system as well as the proposed
controllers for single- and multi-lift configurations have been validated
in flight experiments. The system has been proven to be operable even in
presence of adverse weather conditions
Renormalization factor and effective mass of the two-dimensional electron gas
We calculate the momentum distribution of the Fermi liquid phase of the
homogeneous, two-dimensional electron gas. We show that, close to the Fermi
surface, the momentum distribution of a finite system with electrons
approaches its thermodynamic limit slowly, with leading order corrections
scaling as . These corrections dominate the extrapolation of the
renormalization factor, , and the single particle effective mass, , to
the infinite system size. We show how convergence can be improved analytically.
In the range , we get a lower renormalization factor and
a higher effective mass, , compared to the perturbative RPA values.Comment: 4 pages, 3 figure
The relaxation time of a chiral quantum R-L circuit
We report on the GHz complex admittance of a chiral one dimensional ballistic
conductor formed by edge states in the quantum Hall regime. The circuit
consists of a wide Hall bar (the inductor L) in series with a tunable resistor
(R) formed by a quantum point contact. Electron interactions between edges are
screened by a pair of side gates. Conductance steps are observed on both real
and imaginary parts of the admittance. Remarkably, the phase of the admittance
is transmission-independent. This shows that the relaxation time of a chiral
R-L circuit is resistance independent. A current and charge conserving
scattering theory is presented that accounts for this observation with a
relaxation time given by the electronic transit time in the c cuit
Sustainable Value: an application to the Swiss dairy farms of the mountainous area
The improvement of the sustainable performance of the agricultural sector is a priority of the Swiss agricultural policy. The sustainability of Swiss dairy farms located in the mountainous area might be critical as many of them show a weak performance in the use of their economic and/or social resources, and sometimes also of their environmental resources. An improvement of the sustainability of these farms prerequisites to better know on a large scale their sustainable performance and its determinants. For a representative sample of 480 dairy farms, we perform an assessment of their sustainable efficiency with the âsustainable valueâ, an approach to assess corporate sustainability based on the capital and opportunity cost theories. Using a linear regression, we analyze the determinants of the sustainable efficiency. The results show a tight positive relationship between sustainable performance and pure economic performance. The intensity of the use of intermediate consumptions is found to be the most important determinant of the sustainable efficiency. Farms with a high sustainable efficiency are those that use their intermediate consumptions in the most efficient way. The part of direct payments in the gross profit is shown to negatively affect the sustainable efficiency. The structural characteristics of the farms and the sociologic characteristics of the farmer managers are shown to hardly influence the sustainable efficiency.corporate sustainable performance, dairy farms, Switzerland, Farm Management, Livestock Production/Industries,
Finite-size analysis of the Fermi liquid properties of the homogeneous electron gas
We analyze the extrapolation to the thermodynamic limit of Fermi liquid
properties of the homogeneous electron gas in two and three dimensions. Using
field theory, we explicitly calculate finite-size effects of the total energy,
the renormalization factor, and the effective mass at the Fermi surface within
the random phase approximation (RPA) and discuss the validity for general
metallic systems.Comment: 6 page
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