Degradation feature extraction method for piezoelectric ceramic of ultrasonic motor based on DCT-SV cross entropy

Crack on piezoelectric ceramic is the main reason leading to failure of ultrasonic motors. A novel degradation feature extraction method based on discrete cosine transform (DCT) -singular value (SV) cross entropy was proposed in this paper. In order to improve the correlation with the crack, the DCT coefficients with the property of energy aggregation, were used to extract fault information. To avoid the influence of human factors in traditional DCT de-noising method, a matrix composed of DCT coefficients was constructed, and the SV cross entropy of the matrix was taken as the degradation feature for ultrasonic motor. A numerical simulated noise was added to the measured signal to verify the anti-noise performance of the feature. Analysis of the experimental results demonstrates that the proposed DCT-SV cross entropy is feasible and effective in indicating the degradation of piezoelectric ceramic in ultrasonic motor

Performance of a Boussinesq model for shoaling and breaking waves : a comparison with large scale laboratory data

In this thesis, a nonlinear model predicting hydrodynamics data for waves shoaling and breaking on a beach is reproduced and extensively tested with laboratory data. The model is based on the 1D Boussinesq equations as derived by Madsen et al. (1991) and Madsen and Sorensen (1992), with the free surface elevation and the depth-integrated velocity as variables. It allows slowly varying bathymetries and contains additional high order terms to improve the frequency dispersion for shorter wave periods, and thus also to improve the shoaling properties of the model. Wave breaking is modelled using the concept of a surface roller as formulated by Schäffer et al. (1993). It is assumed that bottom friction is negligible. A large scale laboratory experiment (Supertank), designed in particular to obtain data to test the validity of wave propagation models, provides the wave and current data. Wave evolution over a complex bathymetry is examined for 4 cases. The data include conditions for long and short waves, and regular and irregular waves. During the model evaluation, emphasis is put on the study of parameters of importance to sediment transport, including (orbital) velocity, undertow and wave shape prediction. The latter encompasses velocity and elevation skewness, kurtosis and asymmetry. It is found that, despite an overestimation of the depth-averaged horizontal velocity in some cases, the predicted velocity moments and undertow are in good agreement with the data. Using a bispectral analysis, it is shown that the nonlinear transfers of energy amongst the low order harmonics are well reproduced, but that errors are introduced in the treatment of the high order super-harmonics. As a result, the short waves tests are found to yield better results than those for long waves. A sensitivity analysis on the free parameters introduced in the simulation of wave breaking is carried out. It appears that the results are mostly sensitive to the critical xv avve fi-ont slope OB , and in particular that the elevation and velocity skewness and kurtosis predictions are very sensitive to this parameter.the University of Plymouth

The Optimization of Vibration Data Analysis for the Detection and Diagnosis of Incipient Faults in Roller Bearings

The rolling element bearing is a key component of many machines. Accurate and timely diagnosis of its faults is critical for proactive predictive maintenance. The research described in this thesis focuses on the development of techniques for detecting and diagnosing incipient bearing faults. These techniques are based on improved dynamic models and enhanced signal processing algorithms. Various common fault detection techniques for rolling element bearings are reviewed in this work and a detailed experimental investigation is described for several selected methods. Envelope analysis is widely used to obtain the bearing defect harmonics from the spectrum of the envelope of a vibration signal. This enables the detection and diagnosis of faults, and has shown good results in identifying incipient faults occurring on the different parts of a bearing. However, a critical step in implementing envelope analysis is to determine the frequency band that contains the signal component corresponding to the bearing fault (the one with highest signal to noise ratio). The choice of filter band is conventionally made via manual inspection of the spectrum to identify the resonant frequency where the largest change has occurred. In this work, a spectral kurtosis (SK) method is enhanced to enable determination of the optimum envelope analysis parameters, including the filter band and centre frequency, through a short time Fourier transform (STFT). The results show that the maximum amplitude of the kurtogram indicates the optimal parameters of band pass filter that allows both outer race and inner race faults to be determined from the optimised envelope spectrum. A performance evaluation is carried out on the kurtogram and the fast kurtogram, based on a simulated impact signal masked by different noise levels. This shows that as the signal to noise ratio (SNR) reaches as low as -5dB the STFT-based kurtogram is more effective at identifying periodic components due to bearing faults, and is less influenced by irregular noise pulses than the wavelet based fast kurtogram. A study of the accuracy of rolling-bearing diagnostic features in detecting bearing wear processes and monitoring fault sizes is presented for a range of radial clearances. Subsequently, a nonlinear dynamic model of a deep groove ball bearing with five degrees of freedom is developed. The model incorporates local defects and clearance increments in order to gain the insight into the bearing dynamics. Simulation results indicate that the vibrations at fault characteristic frequencies exhibit significant variability for increasing clearances. An increased vibration level is detected at the bearing characteristic frequency for an outer race fault, whereas a decreased vibration level is found for an inner race fault. Outcomes of laboratory experiments on several bearing clearance grades, with different local defects, are used herein for model validation purposes. The experimental validation indicates that the envelope spectrum is not accurate enough to quantify the rolling element bearing fault severity adequately. To improve the results, a new method has been developed by combining a conventional bispectrum (CB) and modulation signal bispectrum (MSB) with envelope analysis. This suppresses the inevitable noise in the envelope signal, and hence provides more accurate diagnostic features. Both the simulation and the experimental results show that MSB extracts small changes from a faulty bearing more reliably, enabling more accurate and reliable fault severity diagnosis. Moreover, the vibration amplitudes at the fault characteristic frequencies exhibit significant changes with increasing clearance. However, the vibration amplitude tends to increase with the severity of an outer race fault and decrease with the severity of an inner race fault. It is therefore necessary to take these effects into account when diagnosing the size of a defect

Analysis of Infragravity Frequency Sediment Transport on Macrotidal Beaches

Many cross-shore sediment transport models use simple treatments of infragravity frequency (0.005- 0.05Hz) processes. For example, infragravity waves have been assumed to provide solely a 'drift velocity' for transport of sediment mobilised by incident frequency waves (0.05-0.5Hz) and be 100% reflected at the shoreline. Furthermore, numerous models calculate broken incident wave heights on the basis of water depth only. This work investigates both the processes underlying infragravity frequency variations in the crossshore velocity field, and the resulting effect of such variations on sediment suspension and transport. Data were selected from three beach experiments in order to compare observations from a range of energetic conditions and positions in the nearshore. Experiments conducted on a dissipative beach at Llangennith, and an intermediate beach at Spurn Head, form part of the pre-existing British Beach And Nearshore Dynamics dataset. The third deployment, at a dissipative site at Perranporth (Cornwall), provided new data for analysis. At Llangennith, high swell waves (significant wave height 3m) were observed, and the measurements come from an infragravity wave dominated saturated surf zone. At Perranponh, locally generated wind wave heights were 2m and measurements came from an incident wave dominated saturated surf zone. Conditions at Spurn Head saw swell wave heights of 1.5m, and observations were made in both an incident wave dominated non-saturated surf zone and the incident wave shoaling zone. Analysis of the data revealed that, in the surf zone, the nature of the infragravity wave field was dependent upon the distribution of energy between higher (>0.02Hz) and lower (<0.02Hz) infragraviiy frequencies. Lower frequency infragravity waves were found to shoal as free waves, while higher frequency infragravity waves were dissipated near to shore on low gradient beaches. Inftagravity wave reflection coefficients showed a dependence on frequency and beach slope (parameterised by the Iribarren number), varied between 50-90% for lower infragravity frequencies, and could be less than 50% for higher infragraviiy frequencies. Incident wave heights were modulated in the shoaling zone with a 'groupy' form. Modulation was also observed in the surf zone, but in the form of individual large waves occurring at low frequency. In the shoaling zone and very close to shore, non-linear interactions occurred between the incident and infragravity components, and calculated phase values between modulated incident waves and infragravity waves indicated a phase shift from a value of less than 180° in the shoaling zone toward 0° close to shore. However, the two signals were not significantly correlated for much of the surf zone. High velocities resulting from a combination of the mean, infragravity and incident wave components drove sediment suspension. Large suspension events occurring at infragravity frequencies were correlated with incident wave groupiness in the shoaling zone, and in high energy conditions with infragravity waves near to the swash zone. Such variations in suspension were related not only to velocity magnitude, but the duration for which a threshold for suspension was exceeded. The bed response to forcing also varied during a tide, possibly as a result of changing bed conditions (e.g. due to bedforms). The infragravity contribution to suspension was independent of the magnitude of suspended sediment concentration, and increased from approximately 30% at the breaker line to 90% in an infragravity wave dominated inner surf zone. The contribution of the infragravity component to transport did not show a similar behaviour, due to phase effects, which produced a reversal in the transport direction between higher and lower infragravity frequencies. Comparison of the observations of sediment transport with energetics predictors identified several cases where the observed transport was qualitatively different from the model prediction as a result of sediment transport thresholds being exceeded at, or for, infragravity timescales