Chris K. Mechefske

Detection of Induction Motor Faults: A Comparison of Stator Current, Vibration and Acoustic Methods:

In this paper we present the comparison results of induction motor fault detection using stator current, vibration, and acoustic methods. A broken rotor bar fault and a combination of bearing faults (inner race, outer race, and rolling element faults) were induced into variable speed three-phase induction motors. Both healthy and faulty signatures were acquired under different speed and load conditions. To address the detection capabilities of the above methods, comparisons are made in both the time and joint time-frequency domains. In the frequency domain, spectral differences are compared and characterized under constant speed conditions. To evaluate the detection sensitivities under non-stationary conditions (e.g. startup), a joint time-frequency method called the smoothed pseudo Wigner-Ville distribution (SPWVD) is employed to analyze non-stationary signatures. The SPWVD is a powerful technique for revealing non-stationary characteristics of motor signatures. Experimental results show that the stator current method is sensitive to the broken rotor bar fault while the vibration method is sensitive to bearing faults. The acoustic method is very attractive in that it contains less noise and interference within the analyzing frequency band. With the proper selection of monitoring and analysis methods, induction motor faults can be detected accurately under both stationary and non-stationary states.

Fault detection and diagnosis in low speed rolling element bearings Part I: The use of parametric spectra

Abstract An effective procedure for vibration condition monitoring of low speed (⩽100 RPM) rolling element bearings is described. The procedure incorporates fault detection using frequency domain trending indices with fault diagnosis using the frequency spectra. By using parametric models to generate frequency spectra, successful fault detection and diagnosis can be achieved from considerably shorter signal lengths than when using conventional procedures. The results presented here follow directly from earlier work done to compare parametric model based frequency spectra with FFT based frequency spectra, when being used to detect and diagnose faults in low speed rolling element bearings.

Modeling of a Fully Flexible 3PRS Manipulator for Vibration Analysis

In this paper we provide a vibration analysis model and the modeling method for a fully flexible 3-Parallel-Revolute-joint-and-Spherical-joint (3PRS) manipulator-a sliding-leg tripod with flexible links and joints. A series of tripod configurations are set by rigid kinematics for simulation and experiment. All the links are modeled by finite elements: triangular membranes combined with bending plates for the moving platform and spatial beams for the legs. The joint complication is overcome by modeling the joint constraints as virtual springs. The nodal coordinates are statically condensed in order to validate the model. Using eigenvalue sensitivity analysis in terms of the condensed coordinates, the stiffness parameters of the joint virtual springs are adjusted in the experimental configurations until the acceleration frequency response functions (FRFs) from the calculation agree with the ones from the impact tests. The adjusted joint parameters are interpolated linearly into a series of configurations in simulation. The analysis shows that the model with the modified joints proposed in this paper is more effective than the conventional model with ideal joints for predicting the system natural frequencies and their variations against different tripod configurations. The good agreement between the simulation and the experiment at resonant peaks of the FRFs indicates the effectiveness of the modeling method.

Gradient-induced acoustic and magnetic field fluctuations in a 4T whole-body MR imager

Both the acoustic and magnetic fluctuation frequency response functions for a Siemens AS25 body gradient coil inside a 4 Tesla whole‐body MR system were measured and analyzed in this study. In an attempt to correlate the acoustic noise inside the gradient coil with magnetic field oscillations, triangular and trapezoidal gradient impulses of varying amplitudes and widths were used to excite the gradient coil. The acoustic and magnetic responses to these inputs were measured. The results show the existence of discrete resonances in both acoustic and uniform magnetic field fluctuation spectra, while gradient magnetic field fluctuation spectra show no such resonances. In addition, the dominant amplitude peaks in spectra fluctuate similarly with respect to trapezoidal gradient impulse flat‐top widths. This implies that these phenomena are correlated, and that the trapezoidal impulse flat‐top width may be used as a way to suppress both acoustic noise and uniform magnetic field oscillations. Magn Reson Med 44:532–536, 2000.

Acoustic noise reduction in a 4 T MRI scanner.

High-field, high-speed magnetic resonance imaging (MRI) can generate high levels of noise. There is ongoing concern in the medical and imaging research communities regarding the detrimental effects of high acoustic levels on auditory function, patient anxiety, verbal communication between patients and health care workers and ultimately MR image quality. In order to effectively suppress the noise levels inside MRI scanners, the sound field needs to be accurately measured and characterized. This paper presents the results of measurements of the sound radiation from a gradient coil cylinder within a 4 T MRI scanner under a variety of conditions. These measurement results show: (1) that noise levels can be significantly reduced through the use of an appropriately designed passive acoustic liner; and (2) the true noise levels that are experienced by patients during echo planar imaging.

Fault detection and diagnosis in low speed rolling element bearings Part II: The use of nearest neighbour classification

Abstract An effective procedure for automatic fault diagnosis in low speed (⩽100 RPM) rolling element bearings is described. The procedure involves the calculation of a statistical distance measure between vibration signals. The distance measure is then automatically used to distinguish between different fault conditions. A new trending index, based on the statistical distance measure, which may be used for fault detection, is also described.

Optimal Calibration of Parallel Kinematic Machines

In this paper, a new method for optimal calibration of parallel kinematic machines (PKMs) is presented. The basis of the methodology is to exploit the least error sensitive regions in the workspace to yield optimal calibration. To do so, an error model is developed that takes into consideration all the geometric errors due to imprecision in manufacturing and assembly. Based on this error model, it is shown that the error mapping from the geometric errors to the pose error of the PKM depends on the Jacobian inverse. The Jacobian inverse would introduce spurious errors that would affect the calibration results, if used without proper care. Hence, areas in the workspace with smaller condition numbers are selected for calibration. Simulations and experiments are presented to show the effectiveness of the proposed method. Calibration software based on the proposed method has been embedded in the tripod developed at the National Research Council of Canadas Integrated Manufacturing Technologies Institute.

Characterization of vibration and acoustic noise in a gradient-coil insert

High-speed switching of current in gradient coils within high magnetic field strength magnetic resonance imaging (MRI) scanners results in high acoustic sound pressure levels (SPL) in and around these machines. To characterize the vibration properties as well as the acoustic noise properties of the gradient coil, a finite-element (FE) model was developed using the dimensional design specifications of an available gradient-coil insert and the concentration of the copper windings in the coil. This FE model was then validated using experimentally collected vibration data. A computational acoustic noise model was then developed based on the validated FE model. The validation of the finite-element analysis results was done using experimental modal testing of the same gradient coil in a free-free state (no boundary constraints). Based on the validated FE model, boundary conditions (supports) were added to the model to simulate the operating condition when the gradient-coil insert is in place in an MRI machine. Vibration analysis results from the FE model were again validated through experimental vibration testing with the gradient-coil insert installed in the MRI scanner and excited using swept sinusoidal time waveforms. The simulation results from the computational acoustic noise model were also validated through experimental noise measurement from the gradient-coil insert in the MRI scanner using swept sinusoidal time waveform inputs. Comparisons show that the FE model predicts the vibration properties and the computational acoustic noise model predicts the noise characteristic properties extremely accurately.

MRI Gradient Coil Cylinder Sound Field Simulation and Measurement

High-field, high-speed Magnetic Resonance Imaging (MRI) generates high sound levels within and nearby the scanner. The mechanism and process that produces the gradient magnetic field (a cylindrical electro-magnet, called the gradient coil cylinder, which produces a spatially and temporally varying magnetic field inside a static background magnetic field) is the primary source of this noise. This noise can cause difficulties in verbal communication in and around the scanner, heightened patient anxiety, temporary hearing loss and possible permanent hearing impairment for health care workers and patients. In order to effectively suppress the sound radiation from the gradient coil cylinder the sound field within and nearby the gradient coil needs to be characterized This characterization may be made using an analytical solution of the sound pressure field, computational simulation, measurement analysis or some combination of these three methods. This paper presents the computational simulation and measurement results of a study of the sound radiation from a head and neck gradient coil cylinder within a 4 Tesla MRI whole body scanner. The measurement results for the sound pressure level distribution along the centerline of the gradient coil cylinder are presented. The sound pressure distributions predicted from Finite Element Analysis of the gradient coil movement during operation and subsequent Boundary Element Analysis of the sound field generated are also presented. A comparison of the measured results and the predicted results shows close agreement. Because of the extremely complex nature of the analytical solution for the gradient coil cylinder, a treatment of the analytical solution and comparison to the computational results for a simple cylinder vibrating in a purely radial direction are also presented and also show close agreement between the two methods thus validating the computational approach used with the more complex gradient coil cylinder.

A comprehensive experimental study of micro-perforated panel acoustic absorbers in MRI scanners

ObjectA comprehensive experimental study has been conducted to investigate the possibilities of using micro-perforated panel (MPP) acoustic absorbers in magnetic resonance imaging (MRI) scanners.Materials and methodThe experimental acoustic measurements include measurements in an impedance tube, measurements in an MRI scanner bore mock-up, and in situ measurements in an actual MRI scanner.ResultsThe experimental results are in good agreement with theoretical calculations. This study confirms that MPP acoustic absorbers have multiple absorption frequency bands and wider frequency bands at higher frequency ranges when they are used in cylindrically shaped ducts such as MRI scanner bores. It has also been found that the acoustic noise level in the scanner bore is significantly increased when the air gap depth behind the MPP is too large.ConclusionThis study shows that an MPP absorber, when properly designed, is effective in reducing the acoustic noise in MRI scanners. And, when designing an MPP absorber for MRI scanners, the air gap depth should be carefully considered.