Fault diagnosis of ball bearings using machine learning methods

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Abstract

Ball bearings faults are one of the main causes of breakdown of rotating machines. Thus, detection and diagnosis of mechanical faults in ball bearings is very crucial for the reliable operation. This study is focused on fault diagnosis of ball bearings using artificial neural network (ANN) and support vector machine (SVM). A test rig of high speed rotor supported on rolling bearings is used. The vibration response are obtained and analyzed for the various defects of ball bearings. The specific defects are considered as crack in outer race, inner race with rough surface and corrosion pitting in balls. Statistical methods are used to extract features and to reduce the dimensionality of original vibration features. A comparative experimental study of the effectiveness of ANN and SVM is carried out. The results show that the machine learning algorithms mentioned above can be used for automated diagnosis of bearing faults. It is also observed that the severe (chaotic) vibrations occur under bearings with rough inner race surface and ball with corrosion pitting.

Introduction

Condition monitoring of rotating machinery helps in early detection of faults and anticipation of problems in time, so as to prevent complete failure. Bearing vibration can generate noise and degrade the quality of a product line. Severe vibrations of bearings can even cause the entire system to function incorrectly and that results in downtime for the system and economic loss to the customer. Rolling bearings defects may be categorized as point or local defects and distributed defects. The vibrations are generated by geometrical imperfections on the individual bearing components and these imperfections are caused by irregularities during the manufacturing process as well as wear and tear. The various distributed defects are surface roughness, waviness, misaligned races, and off-size rolling elements. The local defects include cracks, corrosion pitting, brinnelling and spalls on the rolling surfaces. McFadden and Smith, 1985, McFadden and Smith, 1984 have developed the models for vibration produced by a single and multiple point defects on the inner race of the rolling element bearing under radial load based on high-frequency resonance technique. Prabhakar, Mohanty, and Sekhar (2002) have considered single and multiple point defects on inner race, outer race and the combination faults and used discrete wavelet transform (DWT) to detect these faults on bearings. Kankar, Harsha, Pradeep, and Sharma Satish (2009) have applied response surface methodology (RSM), to investigate the effects of various defects on the non-linear vibrations of rotor bearing system.

Various artificial intelligent (AI) techniques such as hidden Markov models (HMM) (Li, Wu, He, & Fulei, 2005), artificial neural networks (ANN) (Vyas & Satishkumar, 2001) and support vector machines (SVM) (Widodo and Yang, 2007, Yuan and Chu, 2007) have been used in the fault diagnosis of machines. Zhitong, Jiazhong, Hongpingn, Guoguang, and Ritchie (2003) have carried out fault detection of induction motor using SVM technique for detecting broken rotor bars. In their experiment, induction motor was experimented with no fault, one broken bar, two broken bars and three broken bars. They used stator current to obtain the signal and calculated the frequency spectrum for fault detection. Samanta (2004) has compared the performance of gear fault detection using ANN and SVM. The time-domain vibration signal of a rotating machine with normal and defective gears were processed for feature extraction. The results compare the effectiveness of both types of classifiers without and with GA-based selection of features and the classifier parameters. The main difference between ANNs and SVMs is in the principle of risk minimization (RM) (Gunn, 1998). In case of SVMs, structural risk minimization (SRM) principle is used for minimizing an upper bound on the expected risk whereas in ANNs, traditional empirical risk minimization (ERM) is used for minimizing the error on the training data.

This paper is mainly focused on bearing fault classification using two machine learning methods ANN and SVM as both can work with non-linear classifications. Vibration data are collected by piezoelectric accelerometers as a time domain signals for the healthy bearing and bearing with different faults. Defects are considered as crack in outer race, inner race with rough surface and corrosion pitting in balls. The signals obtained are processed for machine condition diagnosis as shown in the flow chart Fig. 1. Features are extracted from the time domain signal by statistical method. These features are fed to a supervised attribute filter that can be used to select features. Selected features with the known output are used for training and testing of ANN and SVM.

Section snippets

Support vector machine

Support vector machine (SVM) is a supervised machine learning method based on the statistical learning theory. It is a useful method for classification and regression in small-sample cases such as fault diagnosis. Pattern recognition and classification using SVM is described in brief (Cristianini & Shawe-Taylor, 2000).

A simple case of two classes is considered, which can be separated by a linear classifier. Fig. 2 shows triangles and squares stand for these two classes of sample points. Hyper

Artificial neural network

Artificial neural network (ANN) is an interconnected group of artificial neurons. These neurons use a mathematical or computational model for information processing. ANN is an adaptive system that changes its structure based on information that flows through the network (Zurada, 1999).

A single neuron consists of synapses, adder and activation function. Bias is an external parameter of neural network. Model of a neuron shown in Fig. 3 can be represented by following mathematical modelyk=ϕi=1pwki

Experimental setup and data acquisition

Experimental tests are carried on a test rig to generate training and test data. The rig is connected to a data acquisition system through proper instrumentation. A variety of faults are simulated on the bearing at various speeds. Various parameters of bearing used for the study are listed in Table 1. Accelerometers are used for picking up the vibration signals from various stations on the rig. Signatures for healthy bearings operation establish the baseline data. This baseline data can then be

Feature extraction and selection

A wide set of features are calculated from the vibration signals using statistics. These statistical features are explained below.

  • (a)

    Range: Range refers to the difference between maximum and minimum value of a signal.

  • (b)

    Mean value: Average value of a signal is termed as mean value.

  • (c)

    Standard deviation: Standard deviation is measure of energy content in the vibration signalStandard deviation=nx2-(x)2n(n-1)

  • (d)

    Skewness: Skewness is a measure of symmetry, or more precisely, the lack of symmetrySkewness=n(n-1

Results

ANN/SVM training and classification of faults is carried out using WEKA software. Training vectors are already compiled and are put as an input. The defects considered in the study are classified using ANN/SVM as shown in Table 3. Total 73 cases are considered for testing, which have 15, 14, 15, 14, 15 cases of ball with corrosion pitting, bearing with rough inner race surface, combined bearing component defects, healthy bearings and bearing with outer race crack respectively. From Table 3, we

Conclusions

This study presents a procedure for detection of bearing fault by classifying them using two machine learning methods, namely, ANNs and SVMs. Features are extracted from time-domain vibration signals using statistical techniques. Procedure incorporates most appropriate features selection by a filtering algorithm, which uses a density-based cluster to generate cluster membership values. The roles of different vibration signals, obtained with or without loader and at various speeds, are

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