Digital signal processing method for multilayered media thickness measurement

An ultrasonic thickness measurement system for the 20- to 110-μm range has been developed for thin paint layers on metallic or nonmetallic substrates. In the case of a multilayered sample this system generally furnishes only the sum of the different thicknesses. Thus, a digital signal-processing method has been developed to extract the different values of thickness from the power spectral density. This method is based on a power cepstrum analysis, defined as the Fourier transform of the logarithm power spectral density of the radio-frequency signal. This technique gives, in the case of three layers, the three thicknesses and their linear combinations. Experimental results confirm the effectiveness of the proposed method