Using Modal Parameters for Structural Health Monitoring (original) (raw)
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Machine Vibration Monitoring for Diagnostics through Hypothesis Testing
Nowadays, the subject of machine diagnostics is gathering growing interest in the research field as switching from a programmed to a preventive maintenance regime based on the real health conditions (i.e., condition-based maintenance) can lead to great advantages both in terms of safety and costs. Nondestructive tests monitoring the state of health are fundamental for this purpose. An effective form of condition monitoring is that based on vibration (vibration monitoring), which exploits inexpensive accelerometers to perform machine diagnostics. In this work, statistics and hypothesis testing will be used to build a solid foundation for damage detection by recognition of patterns in a multivariate dataset which collects simple time features extracted from accelerometric measurements. In this regard, data from high-speed aeronautical bearings were analyzed. These were acquired on a test rig built by the Dynamic and Identification Research Group (DIRG) of the Department of Mechanical and Aerospace Engineering at Politecnico di Torino. The proposed strategy was to reduce the multivariate dataset to a single index which the health conditions can be determined. This dimensionality reduction was initially performed using Principal Component Analysis, which proved to be a lossy compression. Improvement was obtained via Fisher's Linear Discriminant Analysis, which finds the direction with maximum distance between the damaged and healthy indices. This method is still ineffective in highlighting phenomena that develop in directions orthogonal to the discriminant. Finally, a lossless compression was achieved using the Mahalanobis distance-based Novelty Indices, which was also able to compensate for possible latent confounding factors. Further, considerations about the confidence, the sensitivity, the curse of dimensionality, and the minimum number of samples were also tackled for ensuring statistical significance. The results obtained here were very good not only in terms of reduced amounts of missed and false alarms, but also considering the speed of the algorithms, their simplicity, and the full independence from human interaction, which make them suitable for real time implementation and integration in condition-based maintenance (CBM) regimes.
ABSTRACT: This paper attempts to summarise and review the recent research and developments in diagnostics and prognostics of mechanical systems implementing Condition Monitoring with emphasis on models, algorithms and technologies for data processing and maintenance decision-making. Realising the increasing trend of using multiple sensors in condition monitoring, the authors also discuss different techniques for multiple sensor data fusion. The paper concludes with a brief discussion on current practices, possible future trends of Condition Monitoring with a brief outline on the novelty of the current research work.
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Modal Analysis in Rotating Machinery
Paper Reference Number: CONAT20044078, 2004
The reliability and efficiency of every rotating machine depends on the designer's ability to predict correctly a range of its dynamic characteristics, including stability, vibration response levels and fatigue. For these tasks, reliable structural dynamics models are essential. In general, the properties of the rotating elements (rotors, disc etc) are time varying as seen by a stationary observer, and in principle to obtain the complete set of left and right hand eigenvectors needed to describe the structure completely. It is well known that most vibrations in rotating machinery are induced by rotating related sources. For example, rotating unbalance is the major source of vibration synchronous to the rotational speed O; misalignment and cracks in shafts cause the vibration iO (i is an integer); ball bearings defects cause the vibration nO (n is a real number); and so forth. As a consequence, forced vibration analysis of a rotating equipment subject to synchronous or asynchrono...
Structural Integrity Assessment of Rotating Systems
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Cracks are the most serious defect menacing the structural integrity of a rotating system. Monitoring, detection and identification of cracks in rotors is a serious safety task for every rotating machine. Crack identification in rotating shafts could be realized by applying either vibration or model-based methods. Vibration methods are based on direct signal measurements such as response or eigenfrequencies; the alteration of these measurements could be assigned to the crack. Model-based methods consider in the place of the crack, or other defects, equivalent loads that give the same result as the crack or the defects do.
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It is important to figure out the dynamic characteristics of a mechanical system for the purpose of safe design. The most widely used method to grasp the inherent dynamic characteristics is Modal analysis which gives us information of modal parameters, such as natural frequency, damping ratio and mode shape vector. However, as mechanical systems get larger and more complicated, difficulties on how to set boundary and excitation conditions arise in modal analysis. Surely, the actual vibration of mechanical systems varies with surrounding conditions, which are temperature, rpm, air flow, pressure, load and so on. Therefore, another method known as Operating Deflection Shape (ODS) is needed to deal with those situations that Modal Analysis cannot do. ODS shows structural vibrations under real operating conditions. Also, rapid measurement not influencing the dynamic characteristics of the structure is required for ODS. It is desirable to use a Laser Scanning Vibrometer (LSV) which has n...
FAULT DETECTION AND CONDITION MONITORING OF MACHINE-STRUCTURE: REVIEW
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Engineers and researchers, particularly in the aerospace and offshore oil industries, began to utilise vibration-based damage detection during the late 1970s and early 1980s (Farrar and Doebling 1999). For the detection of crack in the machine and civil structures many efforts have been reported so far. In the recent decade many methods have been applied for the detection of crack for example using changes in modal parameters, mode shape techniques, finite element methods, visual dot pattern of acoustic and vibration signal etc. Vibration behavior of cracked structures, in particulars cracked rotors has received considerable attention in the last two decades.