1,127 research outputs found
Nonlinear compensation techniques for magnetic suspension systems
In aerospace applications, magnetic suspension systems may be required to operate over large variations in air-gap. Thus the nonlinearities inherent in most types of suspensions have a significant effect. Specifically, large variations in operating point may make it difficult to design a linear controller which gives satisfactory stability and performance over a large range of operating points. One way to address this problem is through the use of nonlinear compensation techniques such as feedback linearization. Nonlinear compensators have received limited attention in the magnetic suspension literature. In recent years, progress has been made in the theory of nonlinear control systems, and in the sub-area of feedback linearization. The idea is demonstrated of feedback linearization using a second order suspension system. In the context of the second order suspension, sampling rate issues in the implementation of feedback linearization are examined through simulation
Mean curvature flow without singularities
We study graphical mean curvature flow of complete solutions defined on
subsets of Euclidean space. We obtain smooth long time existence. The
projections of the evolving graphs also solve mean curvature flow. Hence this
approach allows to smoothly flow through singularities by studying graphical
mean curvature flow with one additional dimension.Comment: 19 pages, 4 figure
Quasiparticle excitations in frustrated antiferromagnets
We have computed the quasiparticle wave function corresponding to a hole
injected in a triangular antiferromagnet. We have taken into account
multi-magnon contributions within the self consistent Born approximation. We
have found qualitative differences, under sign reversal of the integral
transfer t, regarding the multi-magnon components and the own existence of the
quasiparticle excitations. Such differences are due to the subtle interplay
between magnon-assisted and free hopping mechanisms. We conclude that the
conventional quasiparticle picture can be broken by geometrical frustration
without invoking spin liquid phases.Comment: 5 pages, 4 figures, presented at " At the Frontiers of the condensed
Matter II, Buenos Aires. June, 2004 ". To be published in Physica
Finite size Spin Wave theory of the triangular Heisenberg model
We present a finite size spin wave calculation on the Heisenberg
antiferromagnet on the triangular lattice focusing in particular on the
low-energy part of the excitation spectrum. For s=1/2 the good agreement with
the exact diagonalization and quantum Monte Carlo results supports the
reliability of the spin wave expansion to describe the low-energy spin
excitations of the Heisenberg model even in presence of frustration. This
indicates that the spin susceptibility of the triangular antiferromagnet is
very close to the linear spin wave result.Comment: 6 pages (LateX), 2 ps-figure
Characterization Rig for Antennas and Retroreflectors
Development of an antenna characterization system, software and hardware parts. As a result, the radiation pattern of an antenna is obtained together with other relevant values.Desarrollo de un sistema de caracterización de antenas, parte de software y hardware. Como resultado se obtiene el diagrama de radiación de una antena y otros valores relevantes.Desenvolupament d'un sistema de caracterització d'antenes, part de software i hardware. Com a resultat s'obté el diagrama de radiació d'una antena i altres valors rellevants
Five degree-of-freedom control of an ultra-precision magnetically-suspended linear bearing
The authors constructed a high precision linear bearing. A 10.7 kg platen measuring 125 mm by 125 mm by 350 mm is suspended and controlled in five degrees of freedom by seven electromagnets. The position of the platen is measured by five capacitive probes which have nanometer resolution. The suspension acts as a linear bearing, allowing linear travel of 50 mm in the sixth degree of freedom. In the laboratory, this bearing system has demonstrated position stability of 5 nm peak-to-peak. This is believed to be the highest position stability yet demonstrated in a magnetic suspension system. Performance at this level confirms that magnetic suspensions can address motion control requirements at the nanometer level. The experimental effort associated with this linear bearing system is described. Major topics are the development of models for the suspension, implementation of control algorithms, and measurement of the actual bearing performance. Suggestions for the future improvement of the bearing system are given
On the Presence of Thermal SZ Induced Signal in the First Year WMAP Temperature Maps
Using available optical and X-ray catalogues of clusters and superclusters of
galaxies, we build templates of tSZ emission as they should be detected by the
WMAP experiment. We compute the cross-correlation of our templates with WMAP
temperature maps, and interpret our results separately for clusters and for
superclusters of galaxies. For clusters of galaxies, we claim 2-5
detections in our templates built from BCS Ebeling et al. (1998), NORAS
(Boehringer et al. 2000) and de Grandi et al. (1999) catalogues. In these
templates, the typical cluster temperature decrements in WMAP maps are around
15-35 K in the RJ range (no beam deconvolution applied). Several tests
probing the possible influence of foregrounds in our analyses demonstrate that
our results are robust against galactic contamination. On supercluster scales,
we detect a diffuse component in the V & W WMAP bands which cannot be generated
by superclusters in our catalogues (Einasto et al. 1994, 1997), and which is
not present in the clean map of Tegmark, de Oliveira-Costa & Hamilton (2003).
Using this clean map, our analyses yield, for Einasto's supercluster
catalogues, the following upper limit for the comptonization parameter
associated to supercluster scales: y_{SC} < 2.18 \time s 10^{-8} at the 95%
confidence limit.Comment: MNRAS accepted. New section and minor changes include
The politics of the street: Street art, public writing and the history of political contest in Chile
This commentary focuses on the politics of public space in democracy and dictatorship. It delves into what Peter Winn calls the revolution ‘from below’ from the perspective of urban conflict, suggesting a political history that attends to urban and visual culture as a crucial arena of political practice. It suggests that the often-conflictive battle over public spaces was, and continues to be, a mechanism by which an unprecedented range of citizens entered into an ongoing debate over the boundaries of citizenship, practice, politics and that this practice was adapted, transformed and reimagined over the last five decades. The struggle over streets and walls continues to be central to Chilean political history, and urban space remains a field of ongoing contest and debate: the estallido of social unrest in contemporary Chile connected a new generation of activists to this longer history of creative politics of protest and protest art and gave them the opportunity to articulate new forms of intersectional political thought in public space, even in the face of state-sponsored violence. Studying these forms of unrest reveals that theirs is an incisive, intersectional critique of the limits of the ‘transition to democracy’, of neoliberal democracies and of the legacies of dictatorship
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