I. Introduction
With the rapid development of condition monitoring technologies, numerous data containing abundant information of equipment are generated every day. How to extract useful information by conducting a thorough analysis of these data is becoming a hot research subject. Anomaly detection, or outlier detection is an important part of data analysis and data mining technologies. It aims to detect observations which deviate so much from others that they are suspected of being generated by different mechanisms [1]. Accurate and timely anomaly detection is critical for reliable and affordable operation of major industrial equipment. Accurate and timely alerts for anomalies can let operators allocate additional monitoring resources, schedule preventive maintenance, minimize downtime, and reduce costly expenses for unexpected maintenance events [2].