Paper Strip Speed Measurement Using Correlation Technique

A method for determining the speed of a moving paper strip by applying a correlation technique is described.The inherent irregular transparency of a paper strip is observed with photocells through two slits that are placed at a distance along the direction of strip movement. The crosscorrelation function of these two photoelectric signals gives a way of determining the strip speed. The delay given to the upstream signal so as to make the correlation between the signals maximum informs the time taken for the movement of the strip from the upstream to the downstream detecting point.The observed signals are made binary according to whether they are greater or smaller than each mean value and then sampled. Upstream signal is delayed with a fixed stage shift register and is multiplied by the downstream signal. The delay time can be continuously varied by changing the shift pulse period, which is the same as that of sampling. Following up of the peak of the correlation function is performed simply by controlling the sampling frequency or the delay time so as to minimize the signal of the derivative of the correlation function.The shift pulse frequency thus controlled is directly proportional to the speed of the paper strip. The variance of fluctuation of the correlator output due to finite time averaging is evaluated in the case where the observed signal is binary whose points of zero crossing are distributed at random with its autocorrelation function φxx(τ)=e-2λ|τ|. And the fluctuation of the shift pulse frequency controlled by the correlator output is also estimated.Experimental results were in good agreement with the theoretical consideration.