Circuit arrangement for deriving the measured variable from the signals of sensors of a flow meter

Abstract

In a circuit arrangement for deriving the measured variable from the signals (S to S ) of at least two sensors ( ) of a flow meter, which flow meter comprises one or several parallel fluid lines ( ) and means ( ) for exciting oscillations of a predetermined fundamental frequency (&ohgr;) in the fluid line(s), the sensors ( ) detect the oscillations and the sensor signals (S to S ) are supplied by way of a respective A-D converter ( ) to a digital processing unit (P) having a computation circuit ( ), in which their phase difference (&PHgr;) is determined as a measure of the flow. In order to be largely independent of unwanted changes in the fundamental frequency (&ohgr;) of the sensor signals (S , S ), and to measure the flow with little effort and with as few errors as possible, provision is made for the processing unit (P) between the A-D converter ( ) of each sensor signal (S , S ) and the computation circuit ( ) to comprise a digital multiplier circuit (M) and a digital filter arrangement (F) downstream thereof, for the digital sensor signals (S , S ) to be multiplied in the multiplier circuit (M) with respective digital signals (I, R) phase-displaced by 90° with respect to one another that represent sinusoidal oscillations of identical maximum amplitude (x) and of a frequency (&ohgr;+&Dgr;&ohgr;) that varies by a slight difference frequency (&Dgr;&ohgr;) from the fundamental frequency (&ohgr;), and for the pass band of the filter arrangement (F) to be matched to the difference frequency (&Dgr;&ohgr;).