1,720,968 research outputs found
R-FRF-based modal analysis: additional modal parameters for local diagnoses of perturbated systems
No full textContributi innovativi nel campo dell’Analisi Modale Operazionale finalizzati all’identificazione, al monitoraggio e alla damage detection in applicazioni ingegneristiche di frontiera = Novel contributions in the field of Operational Modal Analysis aimed at system identification, monitoring, and damage detection in challenging engineering applications
Full text linkIl presente programma di ricerca individuale si compone di contribuiti innovativi nel campo dell’Analisi Modale Operazionale (abbreviata in OMA) sviluppati in due branche fondamentali. La prima è dedicata alla formulazione della cosiddetta OMA vibro-acustica, quale metodologia semplice ed efficace per la caratterizzazione dinamica di microsistemi. Questa specifica metodologia di analisi modale output-only (la quale impiega un’eccitazione acustica, generata da altoparlanti, e sfrutta l’interferometria laser per la misura delle risposte) viene illustrata attraverso un esempio sperimentale relativo all’identificazione dei modi flessionali in-plane ed out-of-plane di un diapason in quarzo. In aggiunta, viene proposto lo sviluppo di una formulazione OMA detta “generalizzata”: essa mira a superare i principali svantaggi dell’OMA legati all’ipotesi di eccitazione ambientale che descrive le forze esterne quali rumori bianchi non correlati. Tale ipotesi viene difatti violata in tutti quei casi in cui i carichi operazionali sono caratterizzati da colorazione, componenti armoniche o qualsivoglia genere di correlazione (ne fanno esempio sistemi tipici dell’ingegneria meccanica quali veicoli o turbine eoliche). Nello specifico, la tecnica OMA proposta richiede una certa descrizione dei carichi agenti del sistema, risultando così applicabile a tutti quei sistemi per i quali certe caratteristiche degli ingressi sono note a priori. Si deriva, così, una decomposizione modale generalizzata delle funzioni di correlazione e delle densità spettrali di potenza relativamente alle sole risposte strutturali: tali modelli evidenziano una dipendenza non solo dai parametri modali ma anche da alcune caratteristiche spettrali degli ingressi e vengono quindi impiegati nello sviluppo di tecniche ad hoc per l’identificazione. La seconda branca di ricerca è dedicata ad una specifica classe di trasmissibilità, qui denominata come response-based frequency response functions (in breve R-FRFs). Essa approfondisce il processo di misura delle stesse R-FRFs per poi passare alla derivazione del relativo modello modale, impiegabile dai classici stimatori dell’analisi modale per l’identificazione di parametri modali aggiuntivi. Si dimostra, infatti, come i modi ricavabili dalle R-FRFs siano legati al sistema in esame quando considerato virtualmente soggetto a diverse tipologie di condizioni al contorno. Tale peculiarità denota un carattere locale di questi parametri modali addizionali, confermando la potenzialità delle R-FRFs nel campo della rilevazione di danni strutturali. Entrambi i filoni di ricerca sono corroborati da casi studio numerici e sperimentali, i quali offrono svariati scenari applicativi e risultati interessanti.In this dissertation, novel contributions in the field of Operational Modal analysis are presented in two principal branches. The former focuses on a vibro-acoustical OMA formulations as a simple and effective methodology for microsystems dynamic characterization. The particular output-only modal analysis methodology, that includes acoustical excitation via speakers and response measurements through a laser interferometer and microphone, is illustrated through the in-plane and out-of-plane flexural mode identification by experiment on high-quality factor quartz tuning fork (QTF). Additionally, a generalized OMA framework is proposed with the aim to overcome the main drawback of OMA approach consisting in the NExT assumption of uncorrelated white noises excitations. These hypotheses, in fact, are violated in all those cases in which the exerted environmental loads exhibit coloration, harmonic content or some kind of correlation, as in the cases of mechanical engineering systems like vehicles or wind turbines. Specifically, the proposed OMA technique requires some knowledge about the inputs acting on the system and, thus, it is applicable to systems for which something about the inputs is somehow known. The generalized modal structures of the output cross-correlation functions and power spectral densities are derived, as models showing the dependence not only by the modal parameters, but also by the input spectral characteristics, and employed in a customized identification technique. The second research offshoot is dedicated to a specific class of transmissibility functions, here called Response-based Frequency-Response-Functions (R-FRFs), and it comprises a first investigation on the estimation process of R-FRFs followed by a derivation of the relevant modal model, suitable for being tackled through frequency-domain estimators from the field of experimental and operational modal analysis, which let these additional modal parameters to be identified. It is demonstrated how modes retrieved from R-FRFs are related to the system under investigation, but, virtually, with a different set of boundary conditions. The particular properties give this additional modal parameters the advantage of being local, in turn confirming the significance of response-based frequency response functions in the field of damage detection. Both the research lines are corroborated by numerical and real-world experimental case studies that offer a number of application scenarios for results discussion
Operational-modal-analysis-based processing of no-next engineering applications datasets: A generalized power spectral density modal model formulation
No full textOperational Modal Analysis, OMA, even referred to as Output-only Modal Analysis, as opposed to the Input-Output technique, is a powerful technique used to identify the dynamic properties of a vibration system in steady working conditions. Starting from the only measured output signals, OMA allows achieving the estimation of resonance frequencies, damping ratios, and modes, i.e. the modal parameters. The main drawback of OMA approach consists in the NExT assumptions of uncorrelated white noises excitations. These hypotheses, in fact, are violated in all those cases in which the exerted environmental loads cannot be described as white noises, as in the cases of systems having rotating parts (machine tools, engines or wind turbines) or characterized by speed and/or time correlated inputs (road and rail vehicles). In this paper, we derive an OMA formulation not based on the NExT assumptions but incorporating the relationship between outputs, inputs, and modal parameters in a suitable way. Specifically, the proposed OMA technique requires some knowledge about the inputs acting on the system and, thus, it is applicable to systems for which something about the inputs is somehow known. We show the existence of a modal model of the output Power Spectral Densities, PSDs, which contain the dependence not only by the modal parameters, but also by the input PSDs. This model is referred to as the generalized PSD modal model. Examples of the usage of this approach are illustrated in the case of the identification of a lumped parameter system in the presence of both stochastic and harmonic excitations and in that of the rigid body modes of a road/railway vehicle from numerical data
Overcoming the natural excitation assumption: a strategy to deal with colouration and time/space correlation in output-only modal analysis
No full textModal analysis through response-based FRFs: Additional modes for local diagnoses
No full textThe importance and role of a specific class of global transmissibility matrices (globalTFs), here named response-based frequency response functions (R-FRFs), in the areas of the identification and continuous monitoring of structures, is discussed and expanded in the present paper. The R-FRFs, as specialized frequency response functions, have been recently introduced in the literature, and, as originally proved, they are able to inherently provide local poles related to the system under investigation, but, virtually, with a different set of boundary conditions; i.e. as if some of the original degrees of freedom, arbitrarily chosen by the analyst, were constrained to ground. In this paper, such a concept is extended, including mode shapes. Herein, we show that theR-FRFs are also able to provide local modes associated with the aforementioned local poles. In this regard, we provide a parametric model of the R-FRFs matrix, suitable for being tackled through frequency-domain estimators from the field of experimental and operational modal analysis, which let these additional modal
parameters to be identified. Such a conceptual extension is carried out by both a theoretical and a numerical point of view. We process data sets from numerical and real-world experimental case studies and discuss the corresponding results. The estimated poles and modes are employed to detect structural modifications, in turn confirming the significance of response-based frequency response functions in the field of damage detection and structural healthmonitoring (SHM)
A generalized operational modal analysis framework for challenging no-next engineering applications
No full textOperational Modal Analysis (OMA) allows systems structural identification in working conditions, moving from output data only. Although OMA appears as a revolutionary technique, its main drawback is that, to be applicable, one must assume that the excitations are modelled as uncorrelated white noises. These assumptions, in fact, are heavily binding in all those cases in which the present environmental actions cannot be described as white noises: systems having rotating parts (machine tools, engines or wind turbines) or characterised by speed and/or time correlated inputs (road and rail vehicles). In this paper we perform the modal estimation applying a frequency domain procedure based on the description of a generalized modal model of output PSDs (Power Spectral Densities). Specifically, the proposed OMA technique requires a limited knowledge about features related to the source of excitation and, therefore, it is applicable to systems for which something about the inputs is somehow known
Road Roughness Identification in Vehicle Dynamics: The Role of Measurements in Ensuring System Observability
No full textDynamical systems, as, specifically, vehicles’ models, often rely on unknown inputs and on states that may not be directly measured. This represents a challenging research topic, for which various methods and solutions have been proposed. The present study focuses on the recovery of road roughness and the estimation of vertical dynamics states using a quarter-car model and two Kalman filtering-based approaches, even comparing their performance in terms of accuracy, robustness, and computational efficiency. The application of these algorithms may face issues due to certain measurements that are not readily available, impacting system observability. This aspect is thoroughly investigated by collecting different methods scattered in the scientific literature and introducing a new parameter derived from the entries of the observability matrix. Through numerical simulations and a carefully designed experiment, in this effort, we identify critical measurements and determine the most effective method for estimating quantities that are not directly measurable
Effect of porosity and pore size distribution on elastic modulus of foams
Full text linkIn this study, we investigate the impact of pore distribution and size on the mechanical response of elastic porous materials. Two-dimensional porous media with convex porosity are considered (e.g., foams and sponges). These structures are numerically generated by subtracting randomly dispersed circular holes from a solid square domain. The in-plane position vector of pore centers is given by a uniform probability density function (PDF). The pore diameters are, instead, drawn from a non-uniform two-parameter continuous probability distribution. The influence of the void ratio, i.e., the volume fraction of voids, and the parameters of the diameter PDF are studied over a wide range of values, including the percolation threshold, where the effective elastic modulus of the material abruptly drops to zero. This transition takes place at a critical value of the void ratio, which turns out to be unaffected by the parameters of the pore size distribution. Our results demonstrate that the elastic response of these structures is primarily governed by the void ratio, with the correlation length playing a secondary role. While the pore size distribution may affect the correlation length, it has a minimal impact on the foam's overall behavior
Operational modal analysis for the evaluation and classification of rail-head irregularities and the simultaneous identification of train modal parameters
No full textOn the estimation process of a particular class of global transmissibility functions: the R-FRFs
No full textThe importance and the role of a specific class of transmissibility functions (i.e. R-FRFs - Response-based Frequency-Response-Functions), within the realm of modal identification and continuous monitoring of structures, have been highlighted in previous works of the research group. The ability of the R-FRFs to incorporate modal data essentially related to subparts of a system, when thought subjected to certain virtual boundary conditions, makes these functions deserving of being measured through suitable estimators, whose definition and performance analysis comprise the main objective of this paper. The specific formal representation of the R-FRFs generally requires a simultaneous measurement of groups of these functions arranged in response-based transfer matrices. In this sense, we perform the assessment of R-FRFs by investigating the estimators for MIMO linear systems, extending known concepts and introducing new ideas. Specifically, we propose the definition of output-only and input-output estimators, including, in the latter case, the advantage of using the measured exciting forces, when available. Therefore, in this work, we present a detailed performance comparison of the proposed estimators by making usage of data from simulated and experimental case studies. The analysis carried out leads to conclusions aimed at identifying the best estimators for measuring R-FRFs
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