I. Introduction
Rotating machinery plays an increasing role in electric power, manufacturing, transportation, and other industries [1], [2]. As a safety-critical part of rotating machinery, various faults of rotor-bearing system during its service life may cause severe security accidents [3], [4]. Due to automatic detecting capability, more and more intelligent fault diagnosis techniques have been proposed and applied to health monitoring of rotor-bearing system and other industrial equipment [5], [6].