Microfluidic Microwave Sensor Loaded With Annular Microstrip Patch for Lubricating Oil Quality Inspection

In this article, a microwave sensor loaded with an annular microstrip patch resonator is designed and fabricated for lubricating oil quality inspection. The sensor operates in 6.1 to 6.9 GHz with an echo loss of −24 dB and has a size of $70\times70$ mm. The annular patch resonator designed in the sensor is used to transmit and receive electromagnetic signals, which has low cost and high sensitivity. According to the dielectric properties of the solution, the change in moisture, metal impurity content, and acid value will change the dielectric properties of the lubricating oil, which can lead to a change in the sensor’s resonant frequency. The sensor uses the changes in the resonant frequency of the ${S}_{{11}}$ to detect the water content, metal impurities, and acid value, which are the main components that affect the quality of lubricating oil. In this study, iron powder is used to simulate the metal impurities, and the acid value of the lubricating oil is changed by adding potassium hydroxide. The results show that the sensor’s resonant frequency decreases with the increase of water content, iron powder, and potassium hydroxide content in the solution. The sensor has an excellent wideband sensing performance, with a sensitivity of 2.08%.