An Experimental Study of Contactless Fluid Height Estimation Using Surface Acoustic Waves

The surface acoustic wave-based devices have been a viable technology for surface defect detection in many applications. The propagation of surface acoustic waves over a thick solid plate is highly influenced by the presence of fluid on the surface. At the solid-fluid interface, a Leaky Rayleigh Wave is present, and it radiates energy into the fluid, causing a signification attenuation on the surface acoustic wave amplitude. Due to the difference in the acoustic impedance between solid and fluid at the interface, a refracted wave (longitudinal wave) is propagating upwards into the liquid layer until it hits the fluid-air boundary and returns to the solid surface. Based on the arrival time of the first reflected wave and speed of sound in water, the water height can be estimated. In this study, the Pulse-Echo Technique is utilized, and the surface acoustic waves are generated on a thick 1018 steel surface via a 5MHz transducer coupled through an angle beam wedge.  The wedge was custom-designed to convert the longitudinal wave actuated from the transducer into the surface acoustic wave at the wedge-solid surface. A 3D printed container, made of PLA material, is inserted on the solid surface 77mm distance from the wedge, and it is filled with the fluid (Deionized water). The time-domain responses of the signal were recorded at five different volumes (0, 400µl, 600µl, 1000µl, and 1800µl). The results indicate that the average drop in the reflected wave amplitude from the edge due to water loading is -62.5% compared to the case of the empty container. The results further show the percentage of error in estimating the water height for 400 µl, 600 µl,1000 µl, and 1800 µl were all within 10% for all fluid volumes. With the aid of parametric CAD software, both the position and distance of the entire traveling wave in the fluid layer were verified.

Moreover, the localized-time frequency components of the reflected wave from the edge was obtained via the Short-Time Fourier Transform technique with a frequency resolution of 1MHz. A slight reduction in the central frequency of the signal occurred due to fluid increasing, which can be estimated at 0.5MHz. We also believe that this method can be useful for many applications, where some of the fluid parameters need to be assessed or understanding the ultrasonic wave behavior in the fluid, such as measuring the reflected angle at the interface between liquid and other media.