4,022 research outputs found
Using Acoustic Holography for Vibration Analysis
DisertaÄnĂ prĂĄce se zabĂœvĂĄ bezkontaktnĂ analĂœzou vibracĂ pomocĂ metod akustickĂ© holografie v blĂzkĂ©m poli. AkustickĂĄ holografie v blĂzkĂ©m poli je experimentĂĄlnĂ metoda, kterĂĄ rekonstruuje akustickĂ© pole v tÄsnĂ© blĂzkosti povrchu vibrujĂcĂho pĆedmÄtu na zĂĄkladÄ mÄĆenĂ akustickĂ©ho tlaku nebo akustickĂ© rychlosti v urÄitĂ© vzdĂĄlenosti od zkoumanĂ©ho pĆedmÄtu. KonkrĂ©tnĂ realizace tĂ©to metody zĂĄvisĂ na pouĆŸitĂ©m vĂœpoÄetnĂm algoritmu. VlastnĂ prĂĄce je zamÄĆena zejmĂ©na na rozbor algoritmĆŻ, kterĂ© vyuĆŸĂvajĂ k rekonstrukci zvukovĂ©ho pole v blĂzkosti vibrujĂcĂho objektu transformaci do domĂ©ny vlnovĂœch ÄĂsel (prostorovĂĄ transformace), kde probĂhĂĄ vlastnĂ vĂœpoÄet. V Ășvodu prĂĄce je vysvÄtlena zĂĄkladnĂ teorie metody akustickĂ© holografie v blĂzkĂ©m poli s popisem zĂĄkladnĂch vlastnostĂ a dĂĄle rozborem konkrĂ©tnĂch nejÄastÄji pouĆŸĂvanĂœch algoritmĆŻm pro lokalizaci a charakterizaci zdroje zvuku a pro nĂĄslednou vibraÄnĂ analĂœzu. StÄĆŸejnĂ ÄĂĄst prĂĄce se vÄnuje pokroÄilĂœm metodĂĄm zpracovĂĄnĂ, kterĂ© se snaĆŸĂ urÄitĂœm zpĆŻsobem optimalizovat pĆesnost predice zvukovĂ©ho pole v blĂzkosti vibrujĂcĂho pĆedmÄtu v reĂĄlnĂœch podmĂnkĂĄch. Jde zejmĂ©na o problematiku pouĆŸitĂ©ho mÄĆicĂho systĂ©mu s akustickĂœmi snĂmaÄi, kterĂ© nejsou ideĂĄlnĂ, a dĂĄle o moĆŸnost mÄĆenĂ v prostorĂĄch s difĂșznĂm charakterem zvukovĂ©ho pole. Pro tento pĆĂpad byla na zĂĄkladÄ literĂĄrnĂho prĆŻzkumu optimalizovĂĄna a ovÄĆena metoda vyuĆŸĂvajĂcĂ dvouvrstvĂ© mikrofonnĂ pole, kterĂ© umoĆŸĆuje oddÄlenĂ zvukovĂœch polĂ pĆichĂĄzejĂcĂch z rĆŻznĂœch stran a tedy ĂșspÄĆĄnĂ© mÄĆenĂ v uzavĆenĂœch prostorĂĄch napĆ. kabin automobilĆŻ a letadel. SouÄĂĄstĂ prĂĄce byla takĂ© optimalizace, rozĆĄĂĆenĂ a nĂĄslednĂ© ovÄĆenĂ algoritmĆŻ publikovanĂœch v poslednĂch letech pro mÄĆenĂ v reĂĄlnĂœch podmĂnkĂĄch za pouĆŸitĂ bÄĆŸnÄ dostupnĂœch akustickĂœch snĂmaÄĆŻ.The main aim of the thesis is application of near-field acoustic holography for non-contact vibration analysis. Near-field acoustic holography is an experimental technique for reconstruction of sound field close to the surface of the vibrating object based on measurement of sound pressure or acoustic particle velocity in certain distance from the examined object. Practical realization of this method depends on used calculation procedure. The thesis is focused on analysis of acoustic holography algorithms with transformation into wavenumber domain (spatial transformation) where the reconstruction of the sound field near vibrating object is calculated. The introductory part of the thesis describes the theory of near-field acoustic holography with general characteristics and with analysis of most common algorithms used for localization and characterization of sound source and consequent vibration analysis. Principal part of the thesis deals with advanced processing methods where these methods try to optimize the accuracy of prediction of sound field near vibrating object in real environment. In this study, real measurement conditions represent the measurement system with non-ideal acoustic sensors and also areas with reverberant sound field. Based on literature study, there has been optimized and verified the new method which uses double layer microphone array to separate incoming and outgoing sound field, thus allows successful measurement in confined space e.g. cabins of cars and airplanes. Part of the thesis has been also focused on optimization, extension and successive experimental validation of selected classical algorithms published in last decade for possible measurement in real conditions and with common acoustic sensors.
Implementation of a digital optical phase conjugation system and its application to study the robustness of turbidity suppression by phase conjugation
In this work, we report a novel high capacity (number of degrees of freedom) open loop adaptive optics method, termed digital optical phase conjugation (DOPC), which provides a robust optoelectronic optical phase conjugation (OPC) solution. We showed that our prototype can phase conjugate light fields with ~3.9 x 10^(â3) degree accuracy over a range of ~3 degrees and can phase conjugate an input field through a relatively thick turbid medium (ÎŒ_sl ~13). Furthermore, we employed this system to show that the reversing of random scattering in turbid media by phase conjugation is surprisingly robust and accommodating of phase errors. An OPC wavefront with significant spatial phase errors (error uniformly distributed from â Ï/2 to Ï/2) can nevertheless allow OPC reconstruction through a scattering medium with ~40% of the efficiency achieved with phase error free OPC
Holographic Imaging of Crowded Fields: High Angular Resolution Imaging with Excellent Quality at Very Low Cost
We present a method for speckle holography that is optimised for crowded
fields. Its two key features are an iterativ improvement of the instantaneous
Point Spread Functions (PSFs) extracted from each speckle frame and the
(optional) simultaneous use of multiple reference stars. In this way, high
signal-to-noise and accuracy can be achieved on the PSF for each short
exposure, which results in sensitive, high-Strehl re- constructed images. We
have tested our method with different instruments, on a range of targets, and
from the N- to the I-band. In terms of PSF cosmetics, stability and Strehl
ratio, holographic imaging can be equal, and even superior, to the capabilities
of currently available Adaptive Optics (AO) systems, particularly at short
near-infrared to optical wavelengths. It outperforms lucky imaging because it
makes use of the entire PSF and reduces the need for frame selection, thus
leading to higher Strehl and improved sensitivity. Image reconstruction a
posteriori, the possibility to use multiple reference stars and the fact that
these reference stars can be rather faint means that holographic imaging offers
a simple way to image large, dense stellar fields near the diffraction limit of
large telescopes, similar to, but much less technologically demanding than, the
capabilities of a multi-conjugate adaptive optics system. The method can be
used with a large range of already existing imaging instruments and can also be
combined with AO imaging when the corrected PSF is unstable.Comment: Accepted for publication in MNRAS on 15 Nov 201
Autofocus for digital Fresnel holograms by use of a Fresnelet-sparsity criterion
We propose a robust autofocus method for reconstructing digital Fresnel holograms. The numerical reconstruction
involves simulating the propagation of a complex wave front to the appropriate distance. Since the latter value is difficult to determine manually, it is desirable to rely on an automatic procedure for finding the optimal distance to achieve high-quality reconstructions. Our algorithm maximizes a sharpness metric related to the sparsity of the signalâs expansion in distance-dependent waveletlike Fresnelet bases. We show results from simulations and experimental situations that confirm its applicability
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