TELEMETERING TRANSDUCER CIRCUITRY FOR CONVERTING A CAPACITANCE SIGNAL TO A DC CURRENT SIGNAL

• US 3,646,538 A
• Filed: 10/27/1969
• Issued: 02/29/1972
• Est. Priority Date: 10/27/1969
• Status: Expired due to Term

• Current Assignee: Roger L. Frick
• Original Assignee: Roger L. Frick
• Inventors: Frick, Roger L.
• Primary Examiner(s): Pitts, Harold I.
• Application Number:
  US04/869,608
• Time in Patent Office: 855 Days
• Field of Search: 340/200,177
• US Class Current: 340/870.37
• CPC Class Codes: G08C 19/10 using variable capacitance

Abstract

The circuitry including the variable capacitance sensor is excited by an external DC power source. A portion of the direct current power is converted to a high-frequency alternating current which is in circuit with the capacitance sensor and individual rectifying means for each of the capacitances. The rectified currents from the capacitances are compared to a reference current and the resulting signal together with a measure of all the other direct currents used by the circuit provide a signal to control the total current supplied to the circuitry. The total current then bears a prescribed and dependent relation to the capacitance of the sensor. The rectifying means form separate rectifying loops each of which is driven by an AC excitation winding. These rectifying loops are used in a special circuit configuration with a capacitance pressure sensor to provide a highly linear output signal as a function of differential pressure.

Claims

1. A two-wire current transmitter for providing a direct current signal proportional to a condition to be measured comprising a pair of excitation terminals for connection to direct current supply means, an AC reactive impedance means which varies as a function of the condition to be measured, an oscillator means energized by current drawn through the excitation terminals and having its output coupled to the AC impedance means for applying an alternating current therethrough, current rectification means in circuit with the oscillator and AC impedance means to provide a first DC voltage signal representative of the magnitude of the AC impedance, a reference DC means which provides a second DC voltage signal independent of the variable AC impedance, a current control amplifier means having input signal terminals and providing an output control signal in response to a signal at its input terminals and which is energized by current drawn through the excitation terminals, current control means connected between the excitation terminals and controlling the current drawn therethrough in response to the control signal, a first network means, including means summing the total direct current drawn through the excitation terminals, for arithmetic comparison of the first and second voltage signals and the total direct current, and means connecting the network means to the amplifier input signal terminals so that a control signal is produced which adjusts the current through the control means to provide a prescribed relation between total direct current and the condition to be measured.

2. The transmitter of claim 1 wherein the AC impedance means is a first capacitor which varies as a function of the condition to be measured.

3. The transmitter of claim 2 further comprising a second capacitor, AC coupling means for applying an alternating current through the second capacitor from the output of the oscillator, separate current rectification means in circuit with the oscillator and second capacitor to provide a third DC voltage signal representative of the magnitude of the second capacitor, reference means providing a fourth voltage signal, a second network means for arithmetic comparison of the third and fourth voltage signals, an oscillator control amplifier energized by current drawn through the excitation terminals and having its input coupled to the third and fourth voltage signals so that the amplifier output signal is a function thereof, and means coupling the output of the oscillator control amplifier to the oscillator so that the oscillator output is controlled as a function of the third and fourth voltage signals.

4. The transmitter of claim 3 further comprising a current source providing a substantially constant value of current therethrough and being connected between one of the pair of excitation terminals and at least one of the current and oscillator control amplifiers.

5. The transmitter of claim 4 wherein the current source comprises a transistor, a control resistor in series with the transistor emitter and a current terminal so that substantially all the current delivered by the current source flows through the control resistor, and a Zener diode connected between the current terminal and transistor base to effect a substantially fixed voltage drop across the control resistor.

6. Capacitance measuring circuitry for comparing a first variable capacitor to a second capacitor comprising first and second repetitive pulse excitation means each series connected with the first capacitor, and third and fourth repetitive pulse excitation means each series connected with the second capacitor, the repetitive pulse excitation means having positive and negative pulses, the first and third excitation means providing charge currents to substantially fully charge the first and second capacitors respectively to a first polarity during each positive pulse and the second and fourth excitation means providing charge currents to substantially fully charge the first and second capacitors respectively to a second opposite polarity during each negative pulse, DC coupling means providing unidirectional current paths between the first, second, third and fourth excitation means so that the respective charge currents may be separately measured or combined in a prescribed manner to give a resultant unidirectional current signal which is a function of the magnitude of the first and second capacitors and of the frequency and amplitude of the excitation means.

7. The circuit of claim 6 wherein the first, second, third and fourth excitation means are driven from a common oscillator, means for controlling the product of oscillator amplitude and frequency comprising a direct current oscillator control signal, a first voltage reference, means for summing currents from the first and second capacitors to provide a first voltage signal proportional to their added magnitude of capacitance, means to compare the first voltage signal to the voltage reference and to adjust the level of the control signal so that the oscillator is controlled to provide a product of amplitude and frequency as a function of the first and second capacitors and the voltage reference.

8. The circuit of claim 7 wherein each of the first, second, third and fourth excitation means comprises a separate secondary transformer winding in series circuit with a separate current rectifier.

9. The circuit of claim 7 wherein the oscillator output is controlled to be directly proportional to the voltage reference and inversely proportional to the sum of the first and second capacitors, and further comprising second means to provide a second voltage signal proportional to a subtraction of currents from the first and second capacitors respectively so that the second voltage signal is directly proportional to the difference in magnitude of the first and second capacitors and is inversely proportional to the sum of magnitude of the first and second capacitors.

10. The circuit of claim 9 further comprising a third capacitor, fifth excitation means providing direct currents proportional to the third capacitor and means to combine a direct current proportional to the third capacitor with the currents proportional to the first and second capacitors to provide a modified first voltage signal which is proportional to the magnitude of the first capacitor plus the second capacitor minus the third capacitor.

11. The circuit of claim 10 wherein the oscillator and the first and second means are energized solely from direct current delivered over two-wire transmission means, and further comprising a current control stage for controlling the total current delivered over the two-wire transmission means, network means to provide a third voltage signal proportional to the total current, comparison means for comparing the second and third voltage signals and providing a fourth signal proportional to the difference thereof, and means coupling the fourth signal to the current control stage for providing a control of the total current as a function of the third voltage signal.

12. Measurement apparatus providing a direct current signal through a source and load proportional to a condition to be measured comprising a pair of excitation terminals for connection to a direct current source and load, capacitive impedance means which varies as a function of the condition to be measuRed, first means energized by current drawn through the excitation terminals coupled to the capacitive impedance means for applying an alternating current therethrough, second means in circuit with the first means and capacitive impedance means to provide a first DC voltage signal representative of the magnitude of the capacitive impedance, a reference DC means energized through said excitation terminals which provides a second DC voltage signal independent of the variable capacitive impedance, a current control amplifier means energized through said excitation terminals having input signal terminals and providing an output control signal at a level responsive to signals at its input signal terminals, current control means connected in circuit with the excitation terminals and connected to said current control amplifier, said current control means controlling the current through said excitation terminals in response to said control signal, a first network means including means summing the total direct current through the excitation terminals for arithmetic comparison of the first and second DC voltage signals and the total direct current, and means connecting the network means to the current control amplifier means so that a control signal is produced by said current control amplifier means which adjusts the current through the current control means to provide a prescribed relation between total direct current through said excitation terminals and the condition to be measured.

13. A remote sensing system controlling a direct current through a pair of terminals series connected to a load and power source including current supply circuit means powered by said source through said terminals, control means including a voltage reference coupled to said current supply to provide a known DC voltage source, a variable AC impedance sensor responsive to a parameter, excitation means coupled to and powered from said current supply circuit means for providing AC excitation to said variable AC impedance, rectifying means coupled to the output of said AC impedance to provide a direct current signal proportional to said AC impedance, a current control means connected across said terminals to control total current through said terminals, said current control means including amplifier means having a signal input means and having an output means regulating current flow through said current control means, and network means coupling the output of said rectifier means and said current control means to the input means of said amplifier means whereby a change in the signal from said rectifying means causes a change in amplifier output and thereby a change in current through said current control means and said terminals, said change in current being effective through said network to rebalance the signals at the input means of said amplifier means.

14. A two-wire current transmitter for providing a direct current signal proportional to a condition to be measured comprising a pair of excitation terminals for connection to direct current supply means, an AC reactive impedance means which varies as a function of the condition to be measured, a repetitive pulse excitation means energized by current drawn through the excitation terminals and having its output coupled to the AC impedance means for applying an alternating current therethrough, current rectification means in circuit with the repetitive pulse excitation means and AC impedance means to provide a first DC signal representative of the magnitude of the AC impedance, a reference DC means which provides a second DC signal independent of the variable AC impedance, a current control means having input signal terminals and providing an output control signal in response to a signal at its input terminals and which is energized by current drawn through the excitation terminals, said current control means controlling the current drawn therethrough in response to the control signal, a first network means, iNcluding means summing the total direct current drawn through the excitation terminals, for arithmetic comparison of the first and second DC signals and the total direct current, and means connecting the network means to the current control means so that a control signal is produced which adjusts the current through the current control means to provide a prescribed relation between total direct current and the condition to be measured.