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Published Articles

The Volume 9, No 2, June 2004

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Enhanced Unsupervised Noise Cancellation using Angular Resampling for Planetary Bearing Fault Diagnosis

F. Bonnardot, R. B. Randall, J. Antoni


In this paper, some techniques for bearing diagnosis are reviewed (unsupervised angular resampling and noise cancellation, envelope analysis) and applied in combination for the first time to solve a particularly difficult diagnostic problem. Unsupervised noise cancellation exploits the periodicity of gear signals. Since the vibrations from gears are periodic in the angular domain, we propose an enhanced method that uses an unsupervised ordertracking algorithm to perform noise cancellation in the angular domain rather than in the time domain. This method is then applied to bearing fault diagnosis of a planetary bearing in a helicopter gearbox. Due to random speed fluctuation, unsupervised noise cancellation initially did not separate the gear and bearing signals. However, the enhanced noise cancellation, which includes a pre-treatment to suppress speed fluctuation based on phase demodulation of gearmesh frequencies, without the need for a tachometer signal, provides better results. Finally the denoised signal was studied using the envelope analysis technique, and the bearing fault frequency was then detected. Without proper noise cancellation this was not readily detectable in the spectrum noise.

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A Review on Shape Memory Alloy Structures

Basavaraj S. Balapgol, Sudhakar A. Kulkarni, Kamal M. Bajoria


This paper presents a literature review on smart material NiTi shape memory alloys and their structural applications. Shape memory alloys are capable of recovering large strains as a result of martensitic transformation. The NiTi shape memory alloy is widely used because of its large memory strain, superelastic behaviour and greater thermal stability. The review includes the behaviours and the characteristics of the NiTi shape memory alloys, and also covers, in brief, structural applications and limitations.

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Spatial Measurements and Estimation of Acoustic Noise in a 4T MRI Scanner

C.K. Mechefske and W. Li


High-field, high-speed magnetic resonance imaging (MRI) scanners generate high-intensity acoustic noise which causes side effects such as discomfort, anxiety and hearing loss in patients and health care workers. For instance, the echo planar imaging (EPI) sequences may produce sound levels as high as 115 dB(A) under certain circumstances, requiring the exposure time to be less than 15 minutes per day. To reduce and control the noise levels during MRI scanning, an in-depth understanding of the sound field inside MRI scanners is highly desirable. This paper presents the results of noise measurements in a 4 Tesla scanner. The spatially distributed frequency response functions (FRFs) were also derived. The FRFs describe the distribution of the sound field inside the scanner and its transmission characteristics. To validate the FRFs, comparisons were made between acoustic responses estimated using the FRFs and acoustic responses measured directly from echo planar imaging (EPI) gradient coil excitation input sequences. The comparisons show that the acoustic responses from both analysis paths are similar to each other across all the frequency spectrum components of interest. The derived FRFs were then used to estimate the spatial noise distributions within the MRI and again comparisons with measured results showed a close similarity.

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Influence of Elasticity of a Control Device Mount on Vibration Control Performance for an Overhung Rotor System

Kazuki Mizutani and Ryojun Ikeura


This paper describes the influence of the elasticity of a control device mount on vibration control effects for an overhung rotor system. An overhung rotor is supported by a flexible bearing pedestal installed in four pairs of electromagnets, and the PD (proportional and derivative) control is performed in the system. These electromagnets give the control force to a flexible bearing and effectively reduce unbalanced vibrations of the overhung rotor indirectly. When the vibration control system is set up to industrial rotating machinery, a mount supporting the control system to the foundation may be assumed to be not rigid but elastic. The effect of stiffness and damping of the elastic mount on the vibration control performance is examined for the overhung rotor system by numerical simulations, and frequency response curves of the overhung rotor are calculated for several parameters. These simulations show that the vibration control performance of the overhung rotor becomes very low for a certain value of the elastic mount stiffness. Then, the effect of mount elasticity on the vibration control for overhung rotor is discussed in detail by the parametric study.

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Acoustic Radiation from Compressor Shells

Y. V. Siva Prasad, Chandramouli Padmanabhan and N. Ganesan


Structure-borne refrigeration compressor noise reduction has been a major concern for the industry. Structural modifications for noise reduction have often been carried out on the basis of a vibration or modal analysis of the compressor shell alone. Since there are two dense fluids inside a compressor shell, refrigerant and lubricating oil, significant dynamic interaction between the shell and the fluids is likely to occur. The major focus of this paper is to examine the transmission loss of such shells, considering the fluid-structure coupling. A semi-analytical finite element method is used to find the vibration response of the shell-fluid coupled system when excited by acoustic pulsations in the shell cavity caused by the reciprocating mechanism of the compressor. To calculate the acoustic and structural response, a special modal superposition technique based on adjoint eigenvectors is used since the finite element formulation leads to unsymmetric matrices. Once the structural vibration response has been computed, an axisymmetric boundary integral technique is used to predict acoustic pressures on the outside surface of the compressor shell. The transmission loss is then computed by comparing the acoustic pressures inside the cavity with the pressures on the outside surface. It is demonstrated that transmission loss is high for the first few modes of the shell and decreases with increase in the circumferential mode number. For a given circumferential mode the transmission loss is shown to increase with increase in axial mode number.

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Experimental and Spectral Finite Element Study of Plates with Shunted Piezoelectric Patches

M. Tawfik, A. Baz


The need for passive damping techniques arises from the complexities, added weight, and energy requirements associated with the implementation of various active control techniques. A novel passive damping approach for the attenuation and localisation of the vibration of plates is presented in this study. The introduction of distributed piezoelectric patches with passively shunted circuits is presented. A numerical model that describes the coupling of a shunted circuit with flexible plates is developed using a spectral finite element approach. The prediction of the model is validated against analytical and experimental results. The numerical and experimental results obtained demonstrate the feasibility of using piezoelectric patches with passive shunting as an effective means for damping out the plate vibration. Furthermore, it is shown that the proposed approach is capable of producing broadband attenuation and localisation of the vibration by introducing disorder into the passive shunting circuits.

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