Electric signal transmitter for vibrating-wire sensor

• US 4,118,977 A
• Filed: 10/13/1976
• Issued: 10/10/1978
• Est. Priority Date: 10/13/1976
• Status: Expired due to Term

• Current Assignee: The Foxboro Company (Schneider Electric SE)
• Original Assignee: The Foxboro Company (Schneider Electric SE)
• Inventors: Richardson, David A., Simpson, Donald C., Olsen, Everett O.
• Primary Examiner(s): Ruehl, Charles A.
• Application Number:
  US05/732,129
• Time in Patent Office: 727 Days
• Field of Search: 73/141 R, 73/517 AV, 73/DIG. 1, 73/67.2, 177/210 FP, 324/80, 235/193.5, 331/156
• US Class Current: 73/862.59
• CPC Class Codes: G01L 1/10 by measuring variations of frequency of stressed vibrating elements, e.g. of stressed strings using resistance strain gauges G01L1/22
• CPC Class Codes: Y10S 73/01 Vibration

Abstract

A force-measuring instrument of the vibrating-wire type wherein the mechanical components are located at the force-measuring location and the vibrating wire is coupled through a two-wire transmission line to electronic circuitry at a distant location. The electronic circuitry includes means to excite the vibrating wire and to produce a d-c measurement output signal corresponding to the frequency of vibrations. The electronic circuitry further includes function-generating means to establish a closely-linear relationship between the output current and the force applied to the wire. Means also are incorporated in the electronic circuitry for adjusting the zero and span of the instrument, and this can be done without interaction between the adjustments.

Claims

1. Transmitter apparatus for use in an industrial process instrumentation system to produce a d-c measurement signal responsive to the vibration frequency of a vibrating-wire force-sensor the wire of which is tensioned in accordance with an unknown force;

and wherein said d-c signal is connected through a two-wire line to a signal responsive load and a d-c source which furnishes over said two-wire line all of the power for said transmitter apparatus;

said transmitter apparatus comprising;

a first pair of terminals to receive respective leads coupled to said vibrating wire;

a second pair of terminals connected respectively to the leads of said two-wire line to receive power from said d-c source and to carry the d-c measurement signal;

power supply means coupled to said second pair of terminals to develop at least one supply voltage;

electronic circuit means coupled to said first pair of terminals to produce vibratory movement of said wire at a frequency in accordance with the tension thereof and to develop an alternating signal corresponding to that frequency;

function-generating means responsive to said alternating signal and operable to produce a control signal which corresponds to a multiple function of the frequency of said alternating signal;

amplifier means having input and output circuits;

means coupling said control signal to said amplifier means input circuit;

means coupling said amplifier means output circuit to said second pair of terminals to control the d-c current flowing through those two terminals in accordance with the control signal applied to said input circuit;

a feedback signal coupling said output circuit of said amplifier means to said input circuit to establish correspondence between said control signal and the d-c current flowing in said two-wire output circuit; and

means coupling said power supply means to said electronic circuit means, to said function-generating means, an to said amplifier means, to furnish operating supply voltages thereto.

2. Apparatus as claimed in claim 1, wherein said electronic circuit means comprises an oscillator the frequency of which matches that of the wire vibration;

frequency-to-analog conversion means coupled to said oscillator for developing a d-c analog signal proportional to the frequency of oscillation;

multiplier means having two inputs responsive respectively to a frequency signal and an analog signal, said multiplier means including means for producing an output analog signal corresponding to the multiplication of the values represented by the input frequency signal and the input analog signal supplied thereto;

means for directing said d-c signal from said conversion means to one input of said multiplier means; and

means for directing to the other input of said multiplier means a frequency signal corresponding to said alternating signal;

whereby the output analog signal can serve as at least a component of said control signal for said amplifier means.

3. Apparatus as claimed in claim 2, wherein said amplifier means input circuit comprises a summing terminal to which said output analog signal is coupled;

and

means for supplying to said terminal a d-c signal responsive proportionately to the frequency of said alternating signal as an additional component of said control signal;

whereby said control signal provides a transfer function proportional to the sum of two factors one of which corresponds to the square of said frequency and the other corresponds to a linear function of said frequency.

4. Apparatus as claimed in claim 3, including means to supply to said summing junction a current component of fixed magnitude.

5. Apparatus as claimed in claim 1, wherein said function-generating means comprises means to include in said control signal a first component proportional to the square of the frequency, and a second component linearly proportional to the frequency.

6. Apparatus as claimed in claim 5, wherein said function-generating means includes means to include in said control signal a component of fixed magnitude.

7. Apparatus as claimed in claim 1, wherein said function-generating means includes frequency-to-analog converter means for developing a d-c analog signal proportional to the frequency of said alternating signal;

multiplier means responsive to said d-c analog signal and to said alternating signal for producing a component in said control signal proportional to the product thereof;

means to incorporate with said d-c analog signal a second component which is fixed in magnitude; and

means to vary said second component magnitude to provide for adjustment of the instrument zero-setting.

8. Apparatus as claimed in claim 7, including gain-adjusting means coupled to said multiplier means to provide for adjustment of the instrument span.

9. Industrial process instrument apparatus for developing at one station an electrical signal responsive to the value of a process condition at a field measurement location remote from said one station, said process condition being represented at said measurement location by a force proportional to the value of such condition;

said instrument apparatus comprising;

a relatively thin, elongate element at said measurement location, said element being formed of a material dimensioned to provide lateral bending flexibility;

means for supporting said element in a tensioned state providing for resonant lateral vibration of said element, the tension force of said element being variable in accordance with changes in the value of said condition, thereby to alter correspondingly the resonant vibratory frequency of said element;

a two-wire transmission line with the leads at one end thereof coupled to said elongate element to provide for supplying excitation power to said wire;

said two-wire transmission line extending from said field measurement location to said one station;

electronic means including wire-excitation means at said one station for producing oscillatory electrical energy to be supplied to said vibratory element and to produce a corresponding signal responsive to frequency;

means at said one station coupling the other end of said transmission line to said electronic means, the resonant characteristics of said vibratory element being reflected through said transmission line at an impedance level to interact with said electronic means to control the frequency thereof at the resonant vibratory frequency of said element with electrical energy from said electronic means being coupled through said transmission line to said vibratory element to maintain said element vibrating at said resonant frequency; and

means for producing an output measurement signal at said one station corresponding to the oscillatory frequency of said electronic means.

10. Apparatus as claimed in claim 9, wherein said wire-excitation means comprises an oscillator to which said transmission line is coupled so that the oscillator frequency is controlled by the resonant characteristics of said element as reflected through said transmission line.

11. Apparatus as claimed in claim 10, wherein the leads at said one transmission line end are connected to longitudinally-spaced points of said element.

12. Apparatus as claimed in claim 11, including a transformer coupling said transmission line to said vibratable element.

13. Apparatus as claimed in claim 12, wherein said transformer is at said remote location.

14. Apparatus as claimed in claim 13, wherein said transformer has a relatively high turns ratio, with the smaller number of turns coupled to said element.

15. Apparatus as claimed in claim 9, wherein said electronic means at said one station comprises:

means to adjust the zero and span settings of said output measurement signal whereby the adjustments can be made remotely from said field measurement location.

16. Apparatus as claimed in claim 15, wherein said electronic means comprises frequency-to-current converter means to develop a d-c current corresponding to the frequency of vibration of said element;

said zero-setting means comprising means to combine with said d-c current a second adjustable d-c current to produce a net zero current at a given vibration frequency.

17. Apparatus as claimed in claim 16, wherein said electronic means further comprises an amplifier responsive to said combined d-c currents;

said span-adjusting means comprising means to alter the effective gain of said amplifier, whereby there is no interaction between the settings of said span-adjusting means and said zero-adjusting means.

18. Industrial process instrument apparatus for developing at an instrument station an electrical signal responsive to the value of a process condition at a field measurement location remote from said instrument station, said process condition being represented at said measurement location by a force proportional to the value of such condition;

said instrument apparatus comprising;

a relatively thin, elongate element at said measurement location, said element being formed of an electrically-conductive material dimensioned to provide lateral bending flexibility;

means for supporting said element in a tensioned state providing for resonant lateral vibration of said element, the tension force of said element being variable in accordance with changes in the value of said condition, thereby to alter correspondingly the resonant vibratory frequency of said element;

permanent magnet means for producing a magnetic field through said element transverse to the longitudinal axis thereof;

a two-wire transmission line with the leads at one thereof coupled respectively to longitudinally-separated portions of said elongate element to form a conductive path for a-c current passing through said transmission line and said elongate element in series;

said two-wire transmission line extending from said measurement location to said instrument station;

electronic means at said instrument station comprising an amplifier with a positive feedback circuit adapted to serve as an oscillator;

means at said instrument station coupling the transmission line to said oscillator feedback circuit, the resonant characteristics of said vibratory element being reflected through said transmission line at an impedance level to interact with said positive feedback circuit so as to establish oscillations of said oscillator and control the frequency thereof at the resonant vibratory frequency of said element with electrical energy from said oscillator being coupled through said transmission lines to said vibratory element to maintain said element vibrating at said resonant frequency; and

means for producing an output measurement signal at said instrument station corresponding to the oscillatory frequency of said oscillator.

19. Industrial process instrument apparatus for developing an electrical signal responsive to the value of a process condition at a field measurement location, said process condition being represented at said measurement location by a force proportional to the value of such condition;

said instrument apparatus comprising;

a relatively thin, elongate element at said measurement location, said element being formed of a material dimensioned to provide lateral bending flexibility;

means for supporting said element in a tensioned state providing for resonant lateral vibration of said element, the tension of said element being variable in accordance with changes in the value of said condition, thereby to alter correspondingly the resonant vibratory frequency of said element;

a first two-wire line coupled at one end thereof to said elongate element;

electronic means coupled to the other end of said first two-wire line and including wire-excitation means for producing oscillatory electrical energy to be supplied over said first two-wire line to said vibratory element and to produce an a-c signal corresponding to the frequency of vibration;

a second two-wire line coupled at one end thereof to said electronic means;

d-c power supply means coupled to the other end of said second two-wire line to produce a flow of d-c current therethrough to supply all of the electrical power to said electronic means;

said electronic means further including control means responsive to the frequency of said a-c signal for controllably altering said d-c current so that the magnitude thereof reflects the frequency of vibration of said element, thereby to represent said force applied to said element.

20. Apparatus as in claim 19, wherein said control means comprises means responsive to said a-c signal for developing a control signal in accordance with the frequency of said a-c signal;

bias means for setting the magnitude of said control signal at zero corresponding to a frequency of vibration of said element when said force is zero;

said control means further including signal-adjusting means connected to said second two-wire line and responsive to said control signal for setting the magnitude of said d-c current in correspondence to said control signal.

21. Apparatus as in claim 20, including means coupled to said signal-adjusting means to set the value of said d-c current at a predetermined magnitude when the value of said control signal is zero, to provide a "live zero" output signal for the instrument apparatus.

22. Apparatus as in claim 21, wherein said control signal is developed in the form of a d-c signal.

23. Transmitter apparatus for use in an industrial process instrumentation system to produce a measurement signal responsive to the vibration frequency of a vibrating-wire force-sensor which is tensioned in accordance with an unknown force, comprising:

terminal means to receive leads coupled to said vibrating-wire;

electronic means including wire-excitation means coupled to said terminals and supplied with electrical power to produce vibratory movement of said wire at a frequency in accordance with the tension of said wire;

said wire-excitation means comprising an oscillator the frequency of which is controlled to the vibratory frequency of said wire;

frequency-to-analog conversion means coupled to said oscillator for developing a d-c signal proportional to the frequency of oscillations;

function-generating means comprising multiplier means including frequency-to-analog conversion means and having two inputs, one of said inputs receiving a signal corresponding to said frequency of oscillation, the other input receiving said d-c signal, said multiplier means producing an output porportional to the product of the signal values applied to said inputs providing a non-linear transfer-function characteristic at least substantially matching the transfer characteristic between the force applied to said wire and its resonant vibratory frequency; and

means coupled to the output of said function-generating means for developing a measurement signal corresponding to the output of said multiplier means and having a characteristic which is effectively linear with respect to changes in the tension of said wire;

both of said frequency-to-analog conversion means being of the type comprising a controllable d-c current source means with means for developing uniform-sized increments of electrical energy and for releasing said energy at a rate proportional to frequency.

24. Transmitter apparatus for use in an industrial process instrumentation system to produce a measurement signal responsive to the vibration frequency of a vibrating-wire force-sensor which is tensioned in accordance with an unknown force, comprising:

terminal means to receive leads coupled to said vibrating-wire;

electronic means including wire-excitation means coupled to said terminals and supplied with electrical power to produce vibratory movement of said wire at a frequency in accordance with the tension of said wire;

said wire-excitation means comprising an oscillator the frequency of which is controlled to the vibratory frequency of said wire;

said electronic means further including means to produce a first signal proportional to said frequency of vibration;

function-generating means coupled to said first signal producing means to produce a second signal responsive to a multiple function of said frequency of vibration providing a non-linear transfer-function characteristic at least substantially matching the transfer characteristic between the force applied to said wire and its resonant vibratory frequency;

said function generating means comprising multiplier means having two inputs each receiving a signal corresponding to said frequency of oscillations, said multiplier means producing an output proportional to the product of the signal values applied to said inputs;

first means to direct to one of said inputs a signal having at least a component proportional to frequency;

second means to direct to the other of said inputs a signal which is proportional to frequency, whereby the output of said multiplier means represents a squared function of frequency;

third means to combine with the output of said multiplier a signal component which is directly proportional to frequency; and

means coupled to the output of said function-generating means for developing a measurement signal corresponding to said second signal and having a characteristic which is effectively linear with respect to changes in the tension of said wire.

25. Apparatus as claimed in claim 24, wherein said first means comprises a frequency-to-analog conversion means.

26. Apparatus as claimed in claim 25, wherein said second means comprises a frequency-to-analog conversion means like said first means.

27. Transmitter apparatus for use in an industrial process instrumentation system to produce a measurement signal responsive to the vibration frequency of a vibrating-wire force-sensor which is tensioned in accordance with an unknown force, comprising:

terminal means to receive leads coupled to said vibrating-wire;

electronic means including wire-excitation means coupled to said terminal means and supplied with electrical power to produce vibratory movement of said wire at a frequency in accordance with the tension of said wire;

said electronic means further including means to produce an alternating signal proportional to said frequency of vibration;

first multiplier means comprising first input means to receive a d-c signal and second input means coupled to said electronic means to receive an alternating output signal derived from said alternating signal, said first multiplier means including switch means operable by said alternating output signal and responsive to said d-c signal to produce a first analog output signal proportional to the product of said d-c signal and the frequency of said alternating output signal;

power supply means supplying a fixed magnitude d-c signal to said first input means of said first multiplier means whereby said first analog output signal is proportional to the wire vibration frequency;

second multiplier means comprising third input means coupled to the output of said first multiplier means to receive a d-c signal derived from said first analog output signal, and fourth input means coupled to said electronic means to receive an alternating output signal derived from said alternating signal, said second multiplier means including switch means operable by the received alternating output signal and responsive to the d-c signal supplied to said third input means to produce a second analog output signal proportional to the product of the d-c signal and the frequency of the alternating output signal supplied to said second multiplier means, said second analog output signal having a magnitude proportional to the square of the frequency of wire vibration; and

output means coupled to said second multiplier means to produce a measurement signal responsive to said second analog output signal.

28. Apparatus as claimed in claim 27, including zero-setting means to provide as a component of said first analog output signal a signal of adjustable magnitude permitting adjustment of the zero setting of the instrument.

29. Apparatus as claimed in claim 28, including span-setting means to adjust the magnitude of the d-c signal supplied to said second multiplier means, relative to the magnitude of said first analog output signal, thereby to alter the span of the instrument without causing interaction with the zero-setting.

30. Apparatus as claimed in claim 29, wherein said span-setting means comprises an adjustable gain amplifier receiving said first analog output signal at its input, and producing at its output the d-c signal supplied to said second multiplier means.

31. Apparatus as claimed in claim 27, including means to direct to said output means a signal component proportional to said first analog output signal to produce a corresponding component in said measurement signal, whereby the measurement signal includes components proportional both to the square of said frequency and to a linear function of the frequency.

32. Apparatus as claimed in claim 31, including means to direct to said output means a signal component of fixed but preselected magnitude to produce a corresponding component in said measurement signal.