Microwave Phase Sensing for Measuring Water Film Thickness on Road Pavements

Hydroplaning, as a result of water films on highway pavements, poses a significant threat to driving safety. Consequently, real-time, cost-effective <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> monitoring of water film thickness on road surfaces is crucial for early warning systems. Microwaves, when transmitted through water, undergo substantial phase delays, from which the thickness of the water film can be deduced. We have developed an equivalent impedance model to measure the water film thickness, based on transmission line theory. The amplitude and phase of microwave reflection signals that traverse the water film have been analytically determined. For validation purposes, both electromagnetic and circuit simulations were conducted. To facilitate this, processing circuits for a microstrip antenna and a phase detector were integrated onto printed circuit boards (PCBs). The entire measurement system, encompassing the microstrip antenna and RF (radio frequency) PCBs, was packaged using a polyphenylene oxide (PPO) substrate and an aluminum alloy shell. Experimental results demonstrate that this system can accurately measure water film thicknesses of up to 7 mm at a frequency of 2.4 GHz. Notably, the measured water film thickness displayed a variance of less than 0.1 mm in comparison with a high-precision profilometer.