Two-wire 4-20 ma electronics for a fiber optic vortex...

Details Two-wire 4-20 ma electronics for a fiber optic vortex... Two-wire 4-20 ma electronics for a fiber optic vortex...

G - Physics

01

F

73/59

G01F 1/32 (2006.01) G08C 19/02 (2006.01)

Patent

CA 1281557

IMPROVED TWO-WIRE 4-20 mA ELECTRO- NICS FOR A FIBER OPTIC VORTEX SHEDDING FLOWMETER ABSTRACT OF THE DISCLOSURE A method and apparatus for processing optically generated signals to form a two-wire 4-20 mA signal comprises generating a control signal having pulses at a selected frequency to drive a light emitter which generates light pulses, transmitting the light pulses to a light detector over a transmission line having variable attenuation to form a sensor signal and ampli- fying the sensor signal in an operational amplifier. The variations and attentuations follow a process vari- able to be measured. To save power the operational amplifier has a low-current mode into which it is switched whenever no pulse is present in the sensor signal. The amplifier is switched into its high-current mode only when a pulse is present in the sensor signal. Switching is controlled by the control signal for the light emitter. Peaks in the signal from the operations amplifier are sampled and held and then subject to low pass filtering to remove the selected frequency compon- nent and leave a cyclic filtered signal. The operational amplifier also receives a signal to drive it toward ground using a feedback clamping signal which changes slowly with respect to the cyclic filter signal. The filter signal is used to trigger a multivibrator to form a pulse signal having pulses with fixed length and ampli- tude for each cycle of the filter. The pulse signal is then averaged with respect to its voltage and sub- jected to zero and span adjustments. The voltage sig- nal is then converted to a two-line 4-20 mA current signal.

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