Calibration of Ultrasonic Method for Oil Film Thickness Measurement

The oil film thickness between the cylinder and the piston on a diesel engine can be used to evaluate the lubricant performance.Although ultrasonic method is an effective way to measure the oil film thickness due to the advantage of noninvasive measurement,its calibration is crucial during the application.The resonant model and the quasi-static spring model were introduced and theoretically analyzed.The relationship between the ultrasonic reflection coefficient,the incident frequency and the oil film thickness were obtained.Moreover,the errors of the resonant model and the quasi-static spring model were analyzed using the Taylor series.Through the analysis,the relationship between the error,resonant frequency and sampling frequency was presented,as well as the relationship between the error,ultrasonic frequency and reflection coefficient,and also the rational range of the ultrasonic reflection coefficient.Two calibration devices with micrometer structure and wedge-shaped gauge block structure were proposed and applied to calibrate the resonant model and the quasi-static spring model respectively.The results show the two devices can be used to calibrate the two models effectively.