Narrow-band frequency analysis for laser-based glass thickness measurement

This paper presents a narrow-band frequency analysis approach for a new laser interferometric heterodyne system, which is used for noncontact glass bottle wall thickness measurement. The measurement signal consists of a number of spectral components, the strongest of which is a reliable representation of the above-mentioned thickness. A fast method for searching and locating this frequency is vital for real-time implementations. As the standard fast Fourier transform (FFT) proved to be ineffective for the given problem, we use a combination of a zero-padding discrete Fourier transform (DFT) with a coarse-to-fine technique to locate this frequency at a smaller processing cost. Considering also the Chirp-z transform, a comparison of the different methods under investigation demonstrates the effectiveness of the proposed approach for online thickness estimation.