Signalverarbeitungsverfahren und virtuelle Instrumente zur Messung von elektrischen Signalen und zur Fehlerdiagnose an Maschinen

Es wird in dieser Arbeit gezeigt, dass sich die Chirp-z-Transformation (CZT) und das Zeropadding-Verfahren (ZP), kombiniert mit der Anwendung geeigneter Fensterfunktionen, zur Bestimmung der Größen von Spannungs- und Stromsignalen bezüglich der Zuverlässigkeit bei kurzen Messzeiten eignen. Die allgemeinen Bedingungen für die Anwendung der CZT bzw. des ZP bei reellen Multifrequenz-Signalen unter dem Einsatz der Fensterfunktionen dargestellt. Der Einfluß eines reellen weißen Rauschsignals auf die geschätzten Größen eines reellen Multifrequenz-Signals wurde untersucht. Das TDA-Verfahren wird zur Verarbeitung der harmonischen Komponenten der Ausgangssignale eines Frequenzumformers untersucht. Zur Detektion und Messung der interharmonischen Komponenten wird ein digitales Differenz-Filter vorgestellt. Die Anwendbarkeit des TDA-Verfahrens und des digitalen Differenz-Filters wird durch die Ergebnisse aus Simulationen und praktischen Anwendungen gezeigt. Virtuelle Instrumente wurden zur Fehldiagnose und Schwingungsanalyse an Planetengetrieben entwickelt. Die praktischen Anwendungen zeigen, dass das Spektrum der demodulierten hochfrequenten mechanischen Schwingungen – Hüllkurvenspektrum – eine spektrale Struktur bei den Frequenzen enthält, die mit den Fehlern auf Innenring- oder Außenringlaufbahn in Verbindung stehen.It will be shown that the chirp-z-transform and zero-padding, combined with a appropriate window, are suitable for the estimation of the parameters of the voltage and current signals with moderate computational complexity. Through the analysis of the spectral interference in the discrete time Fourier transform the conditions for the simultaneous use of the windows and the chirp-z-transform or zero-padding will be presented. The statistical errors of the estimations will be discussed. The time domain averaging will be here investigated to process harmonic components in the output signals of frequency converters. For the detection and measurement of interharmonics a digital difference-filter will be proposed. Simulations and field test results are provided to illustrate the utility of the time domain averaging and the difference-filter. Based on the analysis of the faults of gears and rolling bearings the virtual instruments for the fault diagnosis and vibration analysis in epicyclic gearboxes are developed. The practical applications show that the spectrum of the demodulated high-frequency vibration – envelope spectrum – contains a pattern of spectral lines at frequencies which can be related to the faults on the inner or outer race of rolling bearings

Does Industry-Specific Expertise Improve Board Functioning? Evidence from Forced Bank CEO Turnovers

This study investigates whether independent directors’ expertise in the industry in which the firm operates improves board functioning. To assess the quality of board functioning, I examine firm performance following a CEO turnover. Using a sample of 173 bank CEO turnovers from 1995 to 2010, I find that the market responds more favorably to forced CEO turnover decisions when they are made by a board with more independent financial industry experts. I document that following a forced bank CEO turnover, improvements in bank performance are positively related to independent financial industry expertise on the board, while bank-risk taking is negatively correlated with such expertise. This is likely because a properly functioning board is particularly important when a forced CEO turnover becomes necessary, and industry-specific expertise greatly improves boards’ ability to locate a superior successor CEO and to monitor and advise new management in such a crisis situation. I do not find that board independence has a similar impact on bank performance or risk-taking

RULES BASED MODELING OF DISCRETE EVENT SYSTEMS WITH FAULTS AND THEIR DIAGNOSIS

Failure diagnosis in large and complex systems is a critical task. In the realm of discrete event systems, Sampath et al. proposed a language based failure diagnosis approach. They introduced the diagnosability for discrete event systems and gave a method for testing the diagnosability by first constructing a diagnoser for the system. The complexity of this method of testing diagnosability is exponential in the number of states of the system and doubly exponential in the number of failure types. In this thesis, we give an algorithm for testing diagnosability that does not construct a diagnoser for the system, and its complexity is of 4th order in the number of states of the system and linear in the number of the failure types. In this dissertation we also study diagnosis of discrete event systems (DESs) modeled in the rule-based modeling formalism introduced in [12] to model failure-prone systems. The results have been represented in [43]. An attractive feature of rule-based model is it\u27s compactness (size is polynomial in number of signals). A motivation for the work presented is to develop failure diagnosis techniques that are able to exploit this compactness. In this regard, we develop symbolic techniques for testing diagnosability and computing a diagnoser. Diagnosability test is shown to be an instance of 1st order temporal logic model-checking. An on-line algorithm for diagnosersynthesis is obtained by using predicates and predicate transformers. We demonstrate our approach by applying it to modeling and diagnosis of a part of the assembly-line. When the system is found to be not diagnosable, we use sensor refinement and sensor augmentation to make the system diagnosable. In this dissertation, a controller is also extracted from the maximally permissive supervisor for the purpose of implementing the control by selecting, when possible, only one controllable event from among the ones allowed by the supervisor for the assembly line in automaton models

Molecular Dynamic Simulation of the Effect of Initial Surface Temperature on Arc Erosion Due to Ion Bombardment

This study focuses on the effects of initial surface temperature on arc erosion caused by ion bombardment. The simulation results show that higher surface temperature leads to a greater number of lost Cu atoms and an increased size of the erosion crater. This is due to the ability of the incident ions to have a greater sputtering yield at higher temperatures. Moreover, the Cu atoms tend to agglomerate and form clusters after ion bombardment while leaving the surface

Turbulent Flow Overtopping a Dam - A CFD Modeling Study

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv