1. State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China; Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China 2. High-Tech Research Institute of Xi’an, Xi’an 710025, China
The fault diagnosis of bearings is crucial in ensuring the reliability of rotating machinery. Deep neural networks have provided unprecedented opportunities to condition monitoring from a new perspective due to the powerful ability in learning fault-related knowledge. However, the inexplicability and low generalization ability of fault diagnosis models still bar them from the application. To address this issue, this paper explores a decision-tree-structured neural network, that is, the deep convolutional tree-inspired network (DCTN), for the hierarchical fault diagnosis of bearings. The proposed model effectively integrates the advantages of convolutional neural network (CNN) and decision tree methods by rebuilding the output decision layer of CNN according to the hierarchical structural characteristics of the decision tree, which is by no means a simple combination of the two models. The proposed DCTN model has unique advantages in 1) the hierarchical structure that can support more accuracy and comprehensive fault diagnosis, 2) the better interpretability of the model output with hierarchical decision making, and 3) more powerful generalization capabilities for the samples across fault severities. The multiclass fault diagnosis case and cross-severity fault diagnosis case are executed on a multicondition aeronautical bearing test rig. Experimental results can fully demonstrate the feasibility and superiority of the proposed method.
Distance between the feature and each classification hyperplane
H(p, q)
Cross-entropy loss function
Loss function of the tree-structured decision layer
K
Number of sample categories
Overall prediction
L
Feature dimension of the fully-connected layer
N
Number of samples
p(·)
Probability distribution of the predicted output
True labels of the pre-trained network
True labels of the tree-structured decision layer
Path probabilities of the tree-structured decision layer
P (subclass)
Probability of correct prediction for seed nodes
P (superclass)
Probability of correct prediction of leaf nodes
q(·)
Probability distribution of the actual output
Predicted probabilities of the pre-trained network
Predicted probabilities of the tree-structured decision layer
R
Dimension of the TFD matrix
s(t)
Signal in time t
swj
Weight vector of the jth leaf note
wj
Weight vector of the jth vector in weight matrix W of the fully-connected layer
Weight vector of the jth tree-structured decision layer after fine-tuning
W
Weight matrix
x
Input features of the Softmax classifier in the cross-entropy loss
x
Input feature vector of the tree-structured decision layer
Prediction probabilities by the Softmax classifier
Predicted probability for the jth category
Prediction scope corresponding to K categories
Weight adjusting the pre-trained decision and tree-structured decision
Mother wavelet
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