7,272 research outputs found
Algorithms for computing the multivariable stability margin
Stability margin for multiloop flight control systems has become a critical issue, especially in highly maneuverable aircraft designs where there are inherent strong cross-couplings between the various feedback control loops. To cope with this issue, we have developed computer algorithms based on non-differentiable optimization theory. These algorithms have been developed for computing the Multivariable Stability Margin (MSM). The MSM of a dynamical system is the size of the smallest structured perturbation in component dynamics that will destabilize the system. These algorithms have been coded and appear to be reliable. As illustrated by examples, they provide the basis for evaluating the robustness and performance of flight control systems
Metallic and insulating behaviour of the two-dimensional electron gas on a vicinal surface of Si MOSFETs
The resistance R of the 2DEG on the vicinal Si surface shows an unusual
behaviour, which is very different from that in the (100) Si MOSFET where an
unconventional metal to insulator transition has been reported. The crossover
from the insulator with dR/dT0 occurs at a low
resistance of R_{\Box}^c \sim 0.04h/e^2. At the low-temperature transition,
which we attribute to the existence of a narrow impurity band at the interface,
a distinct hysteresis in the resistance is detected. At higher temperatures,
another change in the sign of dR/dT is seen and related to the crossover from
the degenerate to non-degenerate 2DEG.Comment: 4 pages, 4 figure
Impurity relaxation mechanism for dynamic magnetization reversal in a single domain grain
The interaction of coherent magnetization rotation with a system of two-level
impurities is studied. Two different, but not contradictory mechanisms, the
`slow-relaxing ion' and the `fast-relaxing ion' are utilized to derive a system
of integro-differential equations for the magnetization. In the case that the
impurity relaxation rate is much greater than the magnetization precession
frequency, these equations can be written in the form of the Landau-Lifshitz
equation with damping. Thus the damping parameter can be directly calculated
from these microscopic impurity relaxation processes
Robust computer-aided synthesis and optimization of linear multivariable control systems with varying plant dynamics via AUTOCON
AUTOCON is an automated computer-aided design tool for the synthesis and optimization of linear multivariable control systems based upon user-defined control parameter optimization. Violations in stability and performance requirements are computed from constraints on Single Input/Single Output (SISO) open- and closed-loop transfer function frequency responses, and from constraints on the singular-value frequency responses of Multiple Input/Multiple Output (MIMO) transfer functions, for all critical plant variations. Optimum nonlinear programming algorithms are used in the search for local constrained solutions in which violations in stability and performance are caused either to vanish or be minimized for a proper selection of the control parameters. Classical control system stability and performance design can, in this way, be combined with modern multivariable robustness methods to offer general frequency response loop-shaping via a computer-aided design tool. Complete Nichols, Nyquist, Bode, singular-value Bode magnitude and transient response plots are produced, including user-defined boundary responses. AUTOCON is used to synthesize and optimize the lateral/directional flight control system for a typical high-performance aircraft
Adsorption and two-body recombination of atomic hydrogen on He-He mixture films
We present the first systematic measurement of the binding energy of
hydrogen atoms to the surface of saturated He-He mixture films.
is found to decrease almost linearly from 1.14(1) K down to 0.39(1) K, when the
population of the ground surface state of He grows from zero to
cm, yielding the value K cm
for the mean-field parameter of H-He interaction in 2D. The experiments
were carried out with overall He concentrations ranging from 0.1 ppm to 5 %
as well as with commercial and isotopically purified He at temperatures
70...400 mK. Measuring by ESR the rate constants and for
second-order recombination of hydrogen atoms in hyperfine states and we
find the ratio to be independent of the He content and to
grow with temperature.Comment: 4 pages, 4 figures, all zipped in a sigle file. Submitted to Phys.
Rev. Let
Fluctuation-dissipation considerations and damping models for ferromagnetic materials
The role of fluctuation-dissipation relations (theorems) for the
magnetization dynamics with Landau-Lifshitz-Gilbert and Bloch-Bloembergen
damping terms are discussed. We demonstrate that the use of the Callen-Welton
fluctuation-dissipation theorem that was proven for Hamiltonian systems can
give an inconsistent result for magnetic systems with dissipation
Fluctuations and Dissipation of Coherent Magnetization
A quantum mechanical model is used to derive a generalized Landau-Lifshitz
equation for a magnetic moment, including fluctuations and dissipation. The
model reproduces the Gilbert-Brown form of the equation in the classical limit.
The magnetic moment is linearly coupled to a reservoir of bosonic degrees of
freedom. Use of generalized coherent states makes the semiclassical limit more
transparent within a path-integral formulation. A general
fluctuation-dissipation theorem is derived. The magnitude of the magnetic
moment also fluctuates beyond the Gaussian approximation. We discuss how the
approximate stochastic description of the thermal field follows from our
result. As an example, we go beyond the linear-response method and show how the
thermal fluctuations become anisotropy-dependent even in the uniaxial case.Comment: 22 page
Enhanced shot noise in resonant tunnelling via interacting localised states
In a variety of mesoscopic systems shot noise is seen to be suppressed in
comparison with its Poisson value. In this work we observe a considerable
enhancement of shot noise in the case of resonant tunnelling via localised
states. We present a model of correlated transport through two localised states
which provides both a qualitative and quantitative description of this effect.Comment: 4 pages, 4 figure
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