US 3,948,098 AGranted

Vortex flow meter transmitter including piezo-electric sensor

AssigneeThe Foxboro Company

InventorsDavid A. Richardson, Robert J. Robinson

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Abstract

Flow-metering apparatus of the vortex-shedding type wherein an obstruction in a flowing fluid develops a series of vortices which are sensed by a pressure-responsive piezo-electric element to produce an alternating voltage having a frequency corresponding to the fluid flow rate. The piezo-electric voltage signal is processed by electronic circuitry to provide for precise detection of the zero-crossings of the voltage alternations. The resulting frequency signal controls the charge/discharge characteristics of a set of capacitors, with the average charging current corresponding in magnitude to the frequency of vortex shedding. This average current controls an output amplifier in a feedback arrangement to produce a relatively high-powered d-c flow signal over a two-wire transmission line leading to a remote station. In another arrangement, a high-powered pulsating flow signal is developed for transmission over a three-wire transmission line leading to a remote station.

Claims19

Dependent claims

A piezo-electric flow-rate converter for transforming variable-frequency pressure pulses from a vortex-shedding flow meter into a d-c output signal to be transmitted through a two-wire transmission line to a central station which includes, in series between the two wires of said transmission line, a source of d-c voltage and a current-sensing means to produce an effect determined by the magnitude of d-c current flowing through said two-wire transmission line;

said flow-rate converter comprising:
piezo-electric means responsive to said pressure pulses to develop a corresponding voltage signal varying in frequency with the applied pressure-pulse frequency;
first amplifier means having an input to which said variable-frequency voltage signal is coupled and including means to produce an alternating control signal at the frequency of said a-c voltage;
controllable current-source means having an input coupled to the output of said first amplifier means to receive said alternating control signal, said current-source means including means to develop a low-level continuous d-c signal current the magnitude of which is directly proportional to the frequency of said control signal;
second amplifier means having an input coupled to the output of said current-source means to receive said low-level continuous d-c signal current, said second amplifier means including an output circuit with first and second output terminals to be connected respectively to the two wires of said two-wire transmission line, said output circuit including current-varying means for controlling the magnitude of a relatively high-level d-c output current flowing through said terminals and powered by said source of d-c voltage;
a feedback resistor forming part of said converter and connected in the output of said second amplifier means to develop a feedback signal responsive to the current flowing through said output circuit terminals;
negative feedback circuit means coupling said feedback resistor to said input of said second amplifier means so as to effect correspondence between changes in the d-c output current flowing through said output terminals and changes in said low-level d-c signal current resulting from changes in the fluid flow rate;
and
power circuit means coupled to said output terminals to produce, from the electrical power of said source of d-c voltage, d-c supply voltages for said first and second amplifier means and said controllable current-source means.

6 more elements

A field-mountable vortex-meter flow transmitter comprising:

a piezo-electric sensor responsive to the vortex pressure pulses for developing an alternating voltage signal corresponding to the fluid flow rate;
frequency-responsive electronic means having an input to which said alternating voltage signal is directed and including means to develop a low-level continuous d-c current signal the magnitude of which is porportional to the frequency of said alternating voltage;
output power amplifier means;
an output circuit for said power amplifier means, said output circuit including first and second output terminals for connection respectively to the separate wires of a two-wire transmission line leading to a remote area having a d-c power supply and a current-sensing device in series with each other and with said transmission line;
said output power amplifier means including means to control the magnitude of the d-c output current flowing through said first and second output terminals from the d-c power supply at said remote area;
a feedback resistor connected in series with said output circuit and including means to develop a feedback signal responsive to said d-c output current;
summing circuit means connecting said low-level d-c current signal and said feedback signal together to the input of said output power amplifier to develop a composite amplifier input signal representing a comparison of said low-level d-c current signal and said feedback signal, the d-c output current flowing through said first and second output terminals being continuously controlled by said output power amplifier means so that said output current tracks said low-level d-c current signal developed by said frequency-responsive electronic means;
power circuit means connected to said output circuit and including means to supply operating power from said d-c power supply to said frequency-responsive electronic means and to said output power amplifier;
the average d-c current flowing through said first and second output terminals and sensed by said current-sensing device indicating by its magnitude the frequency of said alternating voltage and thereby indicating the flow rate of the fluid being measured by the vortex flow-meter.

6 more elements

Transmitter apparatus for use in industrial process instrumentation systems for developing a d-c measurement signal corresponding to the magnitude of a process condition such as flow rate and suited for transmission over a relatively long transmission line to a remote receiving station having a supply source of d-c voltage to furnish power for developing said measurement signal;

said transmitter apparatus comprising:
condition-responsive means comprising a transducer responsive to said process condition and including signal-generating means for developing an alternating voltage having a frequency continuously varying with and proportional to the magnitude of said process condition;
controllable d-c current-source means coupled to the output of said condition-responsive means and responsive to said alternating voltage signal, said current-source means including means for developing uniform-sized increments of electrical energy and for releasing said increments of energy at a rate proportional to the frequency of said alternating voltage signal, whereby to produce a d-c current signal having an average value proportional to the frequency of said alternating voltage signal;
amplifier means having a summing input terminal connected to the output of said current-source means to receive said d-c current signal therefrom;
an output circuit for said amplifier means comprising first and second terminals to be connected respectively to the individual wires of a two-wire transmission line leading to said remote receiving station, said amplifier means output circuit including current-varying means to controllably vary the flow of current from said source of d-c voltage through said transmission line;
and
a feedback circuit coupling the output of said amplifier means to said summing junction to establish correspondence between said average d-c current and the d-c current flowing in said two-wire output circuit, whereby the current in said two-wire transmission line corresponds to the magnitude of said process condition.

4 more elements

Dependencies are inferred from the claim text.