I. Introduction
IN past years, due to the great merits of increasing machinery reliability, enhancing operational safety, and reducing maintenance cost, the field of prognostics and health management (PHM) has been attracting growing attention in both academic researches and real productions. A number of industrial scenarios have largely benefited from effective PHM applications [1]-[3], such as intelligent manufacturing, the automotive industry, the aero-space industry, and so on. Generally, existing popular PHM methods include physics-of-failure-based and data-driven approaches. In the recent years, with the rapid development of computing power and algorithms, the intelligent data-driven PHM techniques have been popularly applied in system predictive maintenance tasks, which require little prior expertise in advance and facilitate industrial applications. Through exploration of condition monitoring signals by using artificial intelligence, data-driven methods have achieved promising PHM performance, and have been more effective to meet the rising industrial demands of high reliability and efficiency.