Recent years have witnessed increasing popularity and development of deep learning spanning through various fields. Deep networks, and in particular convolutional neural network (CNN) have also achieved many state-of-the-art competition results in the intelligent fault diagnosis of mechanical systems. However, most of the existing studies have been performed with the assumption that the same distribution holds for both the training data and the test data, which is not in accord with situations in real diagnosis tasks. To tackle this problem, a transfer learning framework based on pre-trained CNN, which leverages the knowledge learned from the training data to facilitate diagnosing a new but similar task, is presented in this work. First, the CNN is trained on large datasets to learn the hierarchical features from the raw data. Then, the architecture and weights of the pre-trained CNN are transferred to new tasks with proper fine-tuning instead of training a network from scratch. To adapt the pre-trained CNN in a specific case, three transfer learning strategies are discussed and compared to investigate the applicability as well as the significance of feature transferability from the different levels of a deep structure. The case studies show that the proposed framework can transfer the features of the pre-trained CNN to boost the diagnosis performance on unseen machine conditions in terms of diverse working conditions and fault types.
Keywords: Convolutional neural networks; Fine tuning; Intelligent fault diagnosis; Mechanical systems; Transfer learning.
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