Two-wire 4-20 mA electronics for a fiber optic vortex shedding f

Details Two-wire 4-20 mA electronics for a fiber optic vortex shedding f Two-wire 4-20 mA electronics for a fiber optic vortex shedding f

1986-02-03

1988-05-03

Masinick, Michael A.

Telecommunications

Receiver or analog modulated signal frequency converter

Signal selection based on frequency

250214A, 455610, 455612, 455617, H04B 900

Patent

active

047425741

ABSTRACT:
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 amplifying the sensor signal in an operational amplifier. The variations and attentuations follow a process variable 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 componnent 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 amplitude for each cycle of the filter. The pulse signal is then averaged with respect to its voltage and subjected to zero and span adjustments. The voltage signal is then converted to a two-line 4-20 mA current signal.

REFERENCES:
patent: 4628197 (1986-12-01), Thompson

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