316 research outputs found

    A Combined Numerical and Experimental Approach for Rolling Bearing Modelling and Prognostics

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    Rolling-element bearings are widely employed components which cover a major role in the NVH behaviour of the mechanical systems in which they are inserted. Therefore, it is crucial to thoroughly understand their fundamental properties and accurately quantify their most relevant parameters. Moreover, their inevitable failure due to contact fatigue leads to the necessity of correctly describing their dynamic behaviour. In fact, they permit to develop diagnostic and prognostic schemes, which are heavily requested in the nowadays industrial scenario due to their increasingly important role in the development of efficient maintenance strategies. As a result, throughout the years several techniques have been developed by researchers to address different challenges related to the modelling of these components. Within this context, this thesis aims at improving the available methods and at proposing novel approaches to tackle the modelling of rolling-element bearings both in case of static and dynamic simulations. In particular, the dissertation is divided in three major topics related to this field, i.e. the estimation of bearing radial stiffness trough the finite-element method, the lumped-parameter modelling of defective bearings and the development of physics-based prognostic models. The first part of the thesis deals with the finite-element simulations of rolling-element bearings. In particular, the investigation aims at providing an efficient procedure for the generation of load-dependent meshes. The method is developed with the primary objective of determining the radial stiffness of the examined components. In this regard, the main contribution to the subject is the definition of mesh element dimensions on the basis of analytical formulae and in the proposed methodology for the estimation of bearing stiffness. Then, the second part describes a multi-objective optimization technique for the estimation of unknown parameters in lumped parameter models of defective bearings. In fact, it was observed that several parameters which are commonly inserted in these models are hardly measurable or rather denoted by a high degree of uncertainty. On this basis, an optimization procedure aimed at minimizing the difference between experimental and numerical results is proposed. The novelty of the technique lies in the approach developed to tackle the problem and its peculiar implementation in the context of bearing lumped-parameter models. Lastly, the final part of the dissertation is devoted to the development of physics-based prognostic models. Specifically, two models are detailed, both based on a novel degradation-related parameter, i.e. the Equivalent Damaged Volume (EDV). An algorithm capable of extracting this quantity from experimental data is detailed. Then, EDV values are used as input parameters for two prognostic models. The first one aims at predicting the bearing vibration under different operative conditions with respect to a given reference deterioration history. On the other hand, the objective of the second model is to predict the time until a certain threshold on the equivalent damaged volume is crossed, regardless of the applied load and the shaft rotation speed. Therefore, the original aspect of this latter part is the development of prognostic models based on a novel indicator specifically introduced in this work. Results obtained from all proposed models are validated through analytical methods retrieved from the literature and by comparison with data acquired on a dedicated test bench. To this end, a test rig which was set-up at the Engineering Department of the University of Ferrara was employed to perform two type of tests, i.e. stationary tests on bearings with artificial defects and run-to-failure tests on initially healthy bearings. The characteristics of acceleration signals acquired during both tests are extensively discussed.I cuscinetti a rotolamento sono componenti meccanici che influenzano in maniera considerevole il comportamento dinamico dei sistemi all’interno dei quali sono montati. Pertanto, è di fondamentale importanza possedere strumenti atti alla stima dei loro parametri più rilevanti e avere a disposizione modelli dedicati allo studio delle loro caratteristiche dinamiche. Questo aspetto è di estrema importanza soprattutto nell’ottica dello sviluppo di schemi di diagnostica e prognostica, i quali sono sempre più richiesti all’interno dello scenario industriale odierno. In questo contesto, questa tesi si propone di migliorare le tecniche numeriche già esistenti e di fornire nuovi approcci per la modellazione dei cuscinetti a rotolamento sia nel caso di problemi statici che dinamici. Nello specifico, il lavoro tratta in maniera dettagliata tre diversi argomenti relativi a questo tema, ossia la stima della rigidezza radiale tramite il metodo agli elementi finiti, la modellazione a parametri concentrati di cuscinetti con difetti e lo sviluppo di modelli di prognostica physics-based. La prima parte della tesi concerne la simulazione di cuscinetti a rotolamento tramite il metodo ad elementi finiti. In particolare, lo studio si propone di fornire una procedura per la generazione di griglie le cui dimensioni dipendano dal carico applicato. Il metodo è sviluppato con l’obbiettivo di stimare in maniera computazionalmente efficace la rigidezza radiale del componente in esame. Pertanto, il contributo principale sul tema dato da questa prima parte riguarda il metodo analitico che permette di definire a priori le dimensioni degli elementi che costituiscono la mesh e la metodologia utilizzata per la stima della rigidezza. La seconda parte descrive una procedura di ottimizzazione multi obbiettivo per la stima dei parametri incogniti all’interno dei modelli a parametri concentrati di cuscinetti con difetti. Questa esigenza nasce dall’osservazione che numerosi parametri tipicamente inseriti in questa tipologia di modelli sono difficilmente misurabili oppure caratterizzati da un alto grado di incertezza. Perciò, nella seconda parte viene introdotta una tecnica innovativa che consente di stimare i parametri di modello che minimizzano la differenza fra risultati numerici e sperimentali in diverse condizioni di funzionamento. Infine, l’ultima parte è dedicata allo sviluppo di modelli di prognostica physics-based. A tal riguardo, vengono dettagliati due modelli basati su un nuovo indicatore di degrado del cuscinetto, denominato Equivalent Damaged Volume (EDV). Questo indicatore viene calcolato durante il funzionamento del cuscinetto tramite un algoritmo dedicato. I valori così ottenuti sono poi utilizzati come dati di input per i due modelli prognostici. Il primo mira a predire la vibrazione del cuscinetto in condizioni operative diverse rispetto ad una storia di degrado di riferimento. Diversamente, il secondo modello permette di prevedere il tempo rimanente prima del superamento di una soglia critica di volume equivalente danneggiato, indipendentemente da carico applicato e velocità di rotazione. Dunque, l’aspetto originale di quest’ultima parte ricade nello sviluppo di tecniche prognostiche basate su un nuovo indicatore introdotto ad-hoc in questo lavoro. I risultati ottenuti da tutti i modelli proposti sono validati grazie a metodi analitici di letteratura e a dati acquisiti in laboratorio per mezzo di un banco prova installato presso il Dipartimento di Ingegneria dell’Università di Ferrara. Il banco prova è stato utilizzato per realizzare due tipologie di prove, ossia test stazionari su cuscinetti che presentano difetti artificiali e prove di tipo run-to-failure su cuscinetti inizialmente sani. Le caratteristiche dei segnali di accelerazione acquisiti in entrambe le prove sono discussi in maniera esaustiva

    Paesaggio, cosa ?

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    Si le paysage n’est pas exclusivement un décor naturel, mais aussi le résultat d’une action humaine dans laquelle nous pouvons reconnaître nos émotions, notre histoire et notre culture, alors chacun aura une idée personnelle du paysage. Cependant, puisqu’il s’agit d’une valeur collective fondamentale – un bien commun – et que comme tel il doit être défendu et protégé, il est nécessaire d’établir quelques paramètres pour pouvoir l’évaluer, sinon d’une manière objective, du moins non subjective, afin de pouvoir l’offrir au législateur. Au paramètre naturaliste on pourra alors joindre des paramètres historiques et culturels tel que, par exemple, la valeur qui pourrait être attribuée aux fragments de l’archéologie industrielle, aux sédiments de territoire historiquement importants (comme les installations agricoles), ou bien encore aux transformations radicales des écosystèmes (tels les assèchements de marécages), et aussi des instruments plus spécifiquement écologiques, comme la stabilité globale d’un écosystème due à une transformation anthropique ; à partir de ce paramètre dérive en particulier la nécessité de protéger quelques-uns des paysages anthropiques qui caractérisent la plus grande partie des territoires européens

    Lagrangian phase transitions in nonequilibrium thermodynamic systems

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    In previous papers we have introduced a natural nonequilibrium free energy by considering the functional describing the large fluctuations of stationary nonequilibrium states. While in equilibrium this functional is always convex, in nonequilibrium this is not necessarily the case. We show that in nonequilibrium a new type of singularities can appear that are interpreted as phase transitions. In particular, this phenomenon occurs for the one-dimensional boundary driven weakly asymmetric exclusion process when the drift due to the external field is opposite to the one due to the external reservoirs, and strong enough.Comment: 10 pages, 2 figure

    Mesh Fixation Methods in Groin Hernia Surgery

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    No unanimous consent has been reached by surgeons in terms of a method for mesh fixation in laparoscopic and open surgery for inguinal hernia repair. Many different methods of fixation are available, and the choice of which one to use is still based on surgeons’ preferences. At present, tissue glues, sutures, and laparoscopic tacks are the most common fixating methods. In open technique, sutures have been the method of choice for their reduced costs and surgeons’ habits. Nevertheless, tissue glues have been demonstrated to be effective and safe. Similarly, tacks can be considered the most common means of fixation in laparoscopic hernia repair, but they are connected to a higher risk of complication and morbidity. In this chapter, we present these types of mesh fixation, their characteristics and potential risks, and advantages of their use

    Crisis Behavior in Autism Spectrum Disorders: A Self-Organized Criticality Approach

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    The Autism Spectrum Disorder (ASD) represents a set of life-long disorders. In particular, subjects with ASD can display momentary behaviors of acute agitation and aggressiveness called crisis behaviors. These events are problematic for the subject and care providers but little is known about their occurrence, namely, possible relations among intensity, frequency, and duration. A group of ASD subjects (n=33) has been observed for 12 months reporting data on each crisis (n=1137 crises). Statistical analysis did not find significant results, while the relation between crisis duration and frequency showed a good fit to a "power law" curve, suggesting the application of Self-Organized Criticality (SOC) model. The SOC is used to describe natural phenomena as earthquakes, bank failures of rivers, mass extinctions, and other systems where a type of "catastrophic events" is necessary to maintain a critical equilibrium. In a sense, subjects at risk of crisis behavior seem to fit the same model as seismic zones at risk of earthquakes. The employment of the same strategies, as those successfully developed for known SOC systems, could lead to important insights for ASD management. Moreover, the SOC model offers possible interpretations of crisis behavior dynamics suggesting that they are unpredictable and, in a sense, necessary

    Investigation on the occurrence of Echinococcus multilocularis in Central Italy

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    <p>Abstract</p> <p>Background</p> <p>Recent studies on geographic distribution of <it>Echinococcus multilocularis </it>in Europe show that it has a wider range than previously thought. It is unclear, however, if the wider distribution is due to its recent spreading or to a lack of previous data from the new areas. Italy, previously considered <it>E. multilocularis</it>-free, is now part of these new areas: infected foxes (the main definitive host of the tapeworm) have been observed in a Northern Alpine territory. Thus, more surveys need to be done in other Italian regions in order to monitor the spreading of <it>E. multilocularis</it>. The aim of the present study was to look for this parasite in 283 foxes collected in an Apennine area of Central Italy by different diagnostic methods.</p> <p>Results</p> <p>The foxes were heavily parasitized by 11 helminthic genera, but none of the animals was infected by <it>E. multilocularis </it>neither by <it>E. granulosus </it>(harboured adult worms or their DNA). Low specificity was observed in commercially available ELISA kits for the detection of <it>E. multilocularis </it>antigens in the faeces. Molecular diagnostics were sensitive and specific for the detection and identification of tapeworm eggs in faeces, but less sensitive, although specific, to adult tapeworms in the intestinal content.</p> <p>Conclusion</p> <p>Preliminarily, we can say that no <it>E. multilocularis </it>could be found in the study area. These data will enable us to follow temporal changes of the spatial distribution of the parasite in the study area of the Central Apennines. Due to its low specificity the ELISA kit for <it>E. multilocularis </it>coproantigens is not suitable for epidemiological surveys, whereas molecular diagnostics applied to faecal samples give useful results. Finally, absence of <it>E. granulosus </it>in foxes living in the endemic areas studied confirms the thought that this tapeworm prefers a different definitive host.</p

    Macroscopic fluctuation theory

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    Stationary non-equilibrium states describe steady flows through macroscopic systems. Although they represent the simplest generalization of equilibrium states, they exhibit a variety of new phenomena. Within a statistical mechanics approach, these states have been the subject of several theoretical investigations, both analytic and numerical. The macroscopic fluctuation theory, based on a formula for the probability of joint space-time fluctuations of thermodynamic variables and currents, provides a unified macroscopic treatment of such states for driven diffusive systems. We give a detailed review of this theory including its main predictions and most relevant applications.Comment: Review article. Revised extended versio

    Improving the quality of user generated data sets for activity recognition

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    It is fully appreciated that progress in the development of data driven approaches to activity recognition are being hampered due to the lack of large scale, high quality, annotated data sets. In an effort to address this the Open Data Initiative (ODI) was conceived as a potential solution for the creation of shared resources for the collection and sharing of open data sets. As part of this process, an analysis was undertaken of datasets collected using a smart environment simulation tool. A noticeable difference was found in the first 1–2 cycles of users generating data. Further analysis demonstrated the effects that this had on the development of activity recognition models with a decrease of performance for both support vector machine and decision tree based classifiers. The outcome of the study has led to the production of a strategy to ensure an initial training phase is considered prior to full scale collection of the data
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