Bearing fault diagnosis based on wavelet transform and fuzzy inference
Introduction
Condition monitoring of rotating machinery is important in terms of system maintenance and process automation. Rolling element bearing failures are one of the foremost causes of failures in rotating machinery. This necessitates the development, implementation, and deployment of on-line diagnostic monitoring systems that are independent of operating conditions.
In most machine fault diagnosis and prognosis systems, the vibration of the rotating machine (motor, gearbox, etc.) is directly measured by an accelerometer, in some few cases, by an acoustic pickup. Some techniques use the stator currents of the electrical motor as the input signals for fault detection [1]. Fault signal detection and recognition are often accomplished by pattern recognition using a neural network [2], [3], RBF network [4], Gaussian mixture model network [5], [6], fuzzy logic network [5], Bayesian classifier [7], vector correlation or vector distance measure [8]. Commonly used feature generation methods include the short-time Fourier transform (STFT) [2], wavelet time-scale decomposition [2], [9], [10], cumulant spectrum [8], etc.
The discrete wavelet transform (DWT) provides an efficient method for generating feature vectors. The DWT coefficients can be used to generate statistical parameters from each resolution level of the transform. This method of feature extraction has been used to recognise signals from RF transmitters with a back propagation neural network [9] and ground vehicles with vector correlation and distance pattern matching [11]. Acoustic analysis methods have been developed to detect and classify underwater objects using wavelets with a neural network and quadratic Bayesian classifiers [12]. The discriminative feature extraction recogniser, which combines a feature extractor and classifier, is presented in [13]. This network optimises both a feature extraction process and a classification process by pattern production and adaptation. As an alternative to the back propagation neural network, a supervised radial basis function network is used. A new network type called “wave-net” [14] adapts the RBF network concept, and uses wavelets as the basis functions for the network. This network has been used for speaker identification [15]. Liu and Ling have applied the principle of mutual information to the identification of wavelets that carry significant information of machinery faults, instead of the “best matching” criterion used in matching pursuit [16]. Altmann and Mathew have used ANFIS for automated selection of wavelet packets containing bearing fault related features [17]. Peng et al., proposed a fusion fault diagnosis method based on the wavelet transform, genetic algorithms and neural networks [10]. Xu and Chan have done very similar work [18].
In this paper, a new technique for localised bearing fault diagnosis is developed using the discrete wavelet transform (DWT). In this method, experimental vibration signals for normal and faulty bearings are pre-processed to obtain a (0,1) normal distribution where the wavelet transform was used to process the normalised data. Then a feature vector is defined using the components from the DWT. By using selected segments from the available experimental data, typical sample feature vectors are generated for both normal bearings and bearings with different types of faults under different load conditions. Then different pattern classification methods have been studied in the decision making stage, including the neural-fuzzy inference system, which is believed to be most suitable for complex situations due to its adaptability and the capability of the network to realise a non-linear approximation.
Section snippets
Experimental system
The ball bearings are installed in a motor driven mechanical system, as shown in Fig. 1. A 2 hp, three-phase induction motor (Reliance Electric 2HP IQPreAlert motor), was connected to a dynamometer and a torque sensor by a self-aligning coupling. The dynamometer is controlled so that desired torque load levels can be achieved. An accelerometer with a bandwidth up to 5000 Hz and a 1 V/g output is mounted on the motor housing at the drive-end of the motor to acquire the vibration signals from the
Preprocessing test data
By examining the magnitude of the vibration data under operating conditions with severe bearing faults, it is possible to distinguish the normal data from different types of fault data. However, this is not always applicable because the signal morphology that results from a fault changes over time as the fault progresses from initiation to failure. Thus, some faults will be undetectable until failure is imminent. Because the early detection and isolation of faults is important for
Brief review of the wavelet theory
The Wavelet Transform is defined as the integral of the signal s(t) multiplied by scaled, shifted versions of a basic wavelet function ψ(t)—a real-valued function whose Fourier Transform satisfies the admissibility criteria [21], [22], [23]:where a is the so-called scaling parameter, b is the time localisation parameter. Both a and b can be continuous or discrete variables.
Multiplying each coefficient by an appropriately scaled and shifted wavelet yields the
Vector distance and correlation coefficient
As seen in the previous tables, two simple pattern classification methods can be used to make diagnostic decisions: (1) the Euclidean vector distance, and (2) the vector correlation coefficient method.
Discussions and conclusions
A new scheme has been developed for the diagnosis of defects in ball bearings. The technique is based on statistical analysis, the discrete wavelet transform, and pattern classification techniques such as neuro-fuzzy inference. By using vibration data collected from an AC motor driven system with different faulted bearings installed, this diagnostic strategy was evaluated. The signals were normalised to (0,1) standard random variables, and then the wavelet transforms were performed using the
Acknowledgements
This work was supported in part by the Office of Naval Research under agreement N00014-98-3-0012 and the National Science Foundation, Grant ECS-9906218.
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Current affiliation: E&ES Department, the Alstom Power Plant Laboratories, Windsor, CT 06095, USA.