The Effect of Amplitude Modulation on the Axial Resolution of Doppler-Based Ultrasonic Topography Measurement

Ultrasonic Doppler-based systems for surface topography measurements are attractive alternatives to the transit-time-based methods. Sensors used in Doppler systems are less dependent on the speed of the sound in air, although contemporary Doppler measurement systems are sensitive to the amplitude variation of the received signal. Amplitude variation significantly affects the measurement accuracy when the surface axial displacement range is comparable with the ultrasonic wavelength. This paper presents a theoretical and experimental study of the effect of amplitude modulation on the performance of the Doppler measurement techniques. A modified Doppler measurement system that significantly improves the measurement accuracy is also presented. The fabricated sensor has 72-μm measurement accuracy using 40-kHz transducers. This technique can also be employed in cost-effective displacement measurement applications that require high axial resolution.