Digital correlator for measuring the velocity of solid surfaces

Correlation techniques offer a good possibility for non-contact measurement of the velocity of solid surfaces. The whole system presented consists of an optical sensor unit, transforming the stochastic characteristics of the object into two signals with a delay time τ*, and of a correlation unit, extracting the delay time from the signals and calculating the velocity of the surface. The main algorithm of the digital correlator, representing an identification problem of the delay time τ*, is realized by a closed loop with the advantage of estimating only one point of the correlation function. The dynamic response is mainly dependent on the bandwidth of the sensor signals and the sampling frequency of the correlator, but not on its arithmetic speed. Former problems in self-adapting the correlator to different signal statistics and the velocity are solved using a second difference quotient of the correlation function. The mathematical description of the correlator is discussed both in the time-domain and the z-domain. Based on a linearized system this allows to determine the dynamic response and the stability of the closed loop correlator. Two different applications are presented: the measurement of the velocity of paper webs and the measurement of the length of textile yarns during a winding process