1,374 research outputs found
Blockwise Subspace Identification for Active Noise Control
In this paper, a subspace identification solution is provided for active noise control (ANC) problems. The solution is related to so-called block updating methods, where instead of updating the (feedforward) controller on a sample by sample base, it is updated each time based on a block of N samples. The use of the subspace identification based ANC methods enables non-iterative derivation and updating of MIMO compact state space models for the controller. The robustness property of subspace identification methods forms the basis of an accurate model updating mechanism, using small size data batches. The design of a feedforward controller via the proposed approach is illustrated for an acoustic duct benchmark problem, supplied by TNO Institute of Applied Physics (TNO-TPD), the Netherlands. We also show how to cope with intrinsic feedback. A comparison study with various ANC schemes, such as block filtered-U, demonstrates the increased robustness of a subspace derived controlle
Impact of time-variant turbulence behavior on prediction for adaptive optics systems
For high contrast imaging systems, the time delay is one of the major
limiting factors for the performance of the extreme adaptive optics (AO)
sub-system and, in turn, the final contrast. The time delay is due to the
finite time needed to measure the incoming disturbance and then apply the
correction. By predicting the behavior of the atmospheric disturbance over the
time delay we can in principle achieve a better AO performance. Atmospheric
turbulence parameters which determine the wavefront phase fluctuations have
time-varying behavior. We present a stochastic model for wind speed and model
time-variant atmospheric turbulence effects using varying wind speed. We test a
low-order, data-driven predictor, the linear minimum mean square error
predictor, for a near-infrared AO system under varying conditions. Our results
show varying wind can have a significant impact on the performance of wavefront
prediction, preventing it from reaching optimal performance. The impact depends
on the strength of the wind fluctuations with the greatest loss in expected
performance being for high wind speeds.Comment: 10 pages, 8 figures; Accepted to JOSA A March 201
Early-Stuart Funeral Elegies from Manuscript
This document is a collection of English funeral elegies from the years 1603 to 1640, which survive in manuscript but were not published, either in their own time or more recently. It served as the basis for James Doelman, The Daring Muse of the Early Stuart Funeral Elegy (Manchester University Press, 2021)
Adiabatic stability under semi-strong interactions: The weakly damped regime
We rigorously derive multi-pulse interaction laws for the semi-strong
interactions in a family of singularly-perturbed and weakly-damped
reaction-diffusion systems in one space dimension. Most significantly, we show
the existence of a manifold of quasi-steady N-pulse solutions and identify a
"normal-hyperbolicity" condition which balances the asymptotic weakness of the
linear damping against the algebraic evolution rate of the multi-pulses. Our
main result is the adiabatic stability of the manifolds subject to this normal
hyperbolicity condition. More specifically, the spectrum of the linearization
about a fixed N-pulse configuration contains essential spectrum that is
asymptotically close to the origin as well as semi-strong eigenvalues which
move at leading order as the pulse positions evolve. We characterize the
semi-strong eigenvalues in terms of the spectrum of an explicit N by N matrix,
and rigorously bound the error between the N-pulse manifold and the evolution
of the full system, in a polynomially weighted space, so long as the
semi-strong spectrum remains strictly in the left-half complex plane, and the
essential spectrum is not too close to the origin
Blooming in a non-local, coupled phytoplankton-nutrient model
Recently, it has been discovered that the dynamics of phytoplankton concentrations in an ocean exhibit a rich variety of patterns, ranging from trivial states to oscillating and even chaotic behavior [J. Huisman, N. N. Pham Thi, D. M. Karl, and B. P. Sommeijer, Nature, 439 (2006), pp. 322–325]. This paper is a first step towards understanding the bifurcational structure associated with nonlocal coupled phytoplankton-nutrient models as studied in that paper. Its main subject is the linear stability analysis that governs the occurrence of the first nontrivial stationary patterns, the deep chlorophyll maxima (DCMs) and the benthic layers (BLs). Since the model can be scaled into a system with a natural singularly perturbed nature, and since the associated eigenvalue problem decouples into a problem of Sturm–Liouville type, it is possible to obtain explicit (and rigorous) bounds on, and accurate approximations of, the eigenvalues. The analysis yields bifurcation-manifolds in parameter space, of which the existence, position, and nature are confirmed by numerical simulations. Moreover, it follows from the simulations and the results on the eigenvalue problem that the asymptotic linear analysis may also serve as a foundation for the secondary bifurcations, such as the oscillating DCMs, exhibited by the model
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