Processing optically generated signals for vortex shedding flowmeter

摘要

A control signal having pulses at a selected frequency is generated (U7) to drive a light emitter (10) producing light pulses which are transmitted to a light detector (20) over a transmission line (15) having variable attenuation to form a sensor signal. The sensor signal is amplified (U1). The operational amplifier (U1) is switched into a high-current made only when a pulse is present in the sensor signal. Switching is controlled by the control signal for the light emitter (10). - Peaks in the signal from the amplifier (U7) are sampled and held (C1,CR1,S1) and then subjected to low pass filtering (R15,C2) to remove the selected frequency component and leave a cyclic filtered signal. The latter triggers or multivibrator (U10) to form a pulse signal having pulses of fixed length and amplitude of each cycle of the filtered signal. The pulse signal is then averaged w.r.t. its voltage, subjected to zero adjustment and span adjustment and then converted to a two-line 4-20mA current signal.