Condition-based maintenance is becoming increasingly important in hydraulic systems. However, anomaly detection for these systems remains challenging, especially since that anomalous data is scarce and labeling such data is tedious and even dangerous. Therefore, it is advisable to make use of unsupervised or semi-supervised methods, especially for semi-supervised learning which utilizes unsupervised learning as a feature extraction mechanism to aid the supervised part when only a small number of labels are available. This study systematically compares semi-supervised learning methods applied for anomaly detection in hydraulic condition monitoring systems. Firstly, thorough data analysis and feature learning were carried out to understand the open-sourced hydraulic condition monitoring dataset. Then, various methods were implemented and evaluated including traditional stand-alone semi-supervised learning models (e.g., one-class SVM, Robust Covariance), ensemble models (e.g., Isolation Forest), and deep neural network based models (e.g., autoencoder, Hierarchical Extreme Learning Machine (HELM)). Typically, this study customized and implemented an extreme learning machine based semi-supervised HELM model and verified its superiority over other semi-supervised methods. Extensive experiments show that the customized HELM model obtained state-of-the-art performance with the highest accuracy (99.5%), the lowest false positive rate (0.015), and the best F1-score (0.985) beating other semi-supervised methods.

翻译：基于状态的维护在液压系统中正变得日益重要。然而，这些系统的异常检测仍然具有挑战性，特别是因为异常数据稀缺且此类数据的标注过程繁琐甚至危险。因此，采用无监督或半监督方法是可取的，尤其是对于半监督学习而言，它利用无监督学习作为特征提取机制，在仅有少量标签可用时辅助监督学习部分。本研究系统比较了应用于液压状态监测系统异常检测的半监督学习方法。首先，进行了深入的数据分析和特征学习以理解开源液压状态监测数据集。随后，实现并评估了多种方法，包括传统的独立半监督学习模型（如单类SVM、鲁棒协方差）、集成模型（如孤立森林）以及基于深度神经网络的模型（如自编码器、分层极限学习机（HELM））。特别地，本研究定制并实现了一种基于极限学习机的半监督HELM模型，并验证了其相对于其他半监督方法的优越性。大量实验表明，定制的HELM模型取得了最先进的性能，具有最高的准确率（99.5%）、最低的误报率（0.015）和最佳的F1分数（0.985），优于其他半监督方法。