Sensitivity-Enhanced Multiport-Network-Based Sensor for Moisture Detection in Lubricant Oil

In this article, a multiport-network-based microwave sensor integrated with an interference circuit is proposed to detect the moisture content in lubricant oil for the purpose of phase-sensitivity enhancement. First, a composite right-/left-handed (CRLH) transmission lines (TLs)-based sensing element is developed to carry moisture content information characterized by a change in the transmission phase. Then, two CRLH TLs-based sensing elements in the differential configuration are integrated into an interference circuit, and the coupling coefficients of the hybrid coupler in the interference circuit are investigated to analyze the power distribution ratio between the reference and sensing branches and obtain an output phase-sensitivity enhancement. Subsequently, the output phase from the interference circuit is translated into power readings through the multiport circuit, and then the corresponding relationship between them is analyzed and utilized to estimate the moisture content in lubricant oil. Finally, a LabVIEW-based real-time measurement system is developed to characterize the moisture content in lubricant oil with the volume fraction from 0.2% to 15%. The measured results exhibit an improvement in phase sensitivity from 1.03°/(v%) to 3.82°/(v%) compared to the CRLH TLs-based sensing elements itself. Both theoretical analysis and experimental results validate the sensitivity enhancement scheme by use of a multiport-network-based topological structure.