A Microwave Sensor with Split-Ring Resonator Structure for the Detection of Lubricating Oil Contaminants

The degree of lubricating oil contamination can directly reflect the wear state of mechanical equipment. This work proposes a microwave sensor based on microstrip line coupling split-ring resonator for detecting contaminants in lubricating oil. It consists of a printed circuit board and a microfluidic channel. The sensor can effectively distinguish the pollutant type by the resonance frequency’s offset direction, count the contaminant by the number of pulses, and estimate the contaminant size through the offset amplitude. The detection limits of the sensor are stainless steel particles with a diameter of 70 μm and air bubbles of 150 μm, with an accuracy of 94% and 93.3%, respectively. The sensor has an average sensitivity of 7.28 kHz/μm for detecting metal particles and 3.09 kHz/μm for detecting air bubbles. Furthermore, the sensor has a fast response speed and a high signal-to-noise ratio. This method provides a novel approach to detecting the health of mechanical equipment.