A Multifrequency Lock-In System for Oil Particle Detection and Identification

In this paper, we present a novel multi-frequency lock-in system for the detection and identification of metal wear particles in lubricating oil, addressing the limitations of conventional single-frequency methods that struggle with simultaneous discrimination of multiple metal types. The system constructs 8-dimensional feature vectors for each material using the ratio of peak-to-peak response voltage to frequency (Rpp/f) at eight discrete frequencies. A compact multi-frequency lock-in amplifier (16 × 10 × 4.5 cm) is designed. Integrating multi-frequency signal generation and parallel demodulation, it achieves high channel consistency and rapid processing speed, facilitating one-time detection for the multi-frequency information. The system shows sub-100-μm detection sensitivity and good distinction ability. The frequency-domain characteristics are obtained for seven types of metal particles and a clear distinction is observed between iron alloys, nickel, copper and aluminum. Particles with the same material but different sizes show similar (Rpp/f) trends. Further experiments are conducted to investigate the effects of oxidation and the superposition of different particles. This work provides a promising solution for online monitoring of mechanical equipment through frequency-domain response analysis of material-specific electromagnetic characteristics.