Two wire transmitter for converting a varying signal from a remote reactance sensor to a DC current signal

• US 4,250,490 A
• Filed: 01/19/1979
• Issued: 02/10/1981
• Est. Priority Date: 01/19/1979
• Status: Expired due to Term

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Dahlke, Charles J.
• Primary Examiner(s): Robinson, Thomas A.
• Application Number:
  US06/004,764
• Time in Patent Office: 753 Days
• Field of Search: 179/78 R, 179/78 A, 179/81 R, 340/200, 340/210, 73/141 R, 324/60 R, 324/61 R
• US Class Current: 340/870.37
• CPC Class Codes: G08C 19/10 using variable capacitance

Abstract

A two wire current transmitter provides a current signal representative of a parameter measured by a variable reactance sensor which may be located in a hostile environment, remote from the signal conditioning and transmitter electronics through transmission circuitry from the electronics including a separate transformer winding energized by a pair of twisted cables to reduce the capacitive, inductive and resistive effects of long wires. The transformer circuitry provides power to the sensor which provides the signal conditioning and transmitter electronics a signal, representing the parameter measured, which controls the current control, to modify the total current through the same two wires used to carry power to the signal conditioning and transmitter electronics, so that the total current is representative of the measured parameter.

Claims

1. An improved two wire current transmitter having a pair of terminals for connection through two wires to an external DC power supply means serially connected to an external load, said transmitter including a DC signal conditioning circuit having an oscillator and current control means for controlling current at said terminals, a remote sensor for measuring a change in parameter to be sensed mounted in position spaced substantially from said DC signal conditioning circuit and including means providing a direct current output signal, and means for coupling said sensor to said DC signal conditioning circuit to provide the direct current output to said DC signal conditioning circuit, wherein the improvement comprises a first transformer secondary located proximate to said DC signal conditioning circuit and coupled to the output of said oscillator, and a second transformer located proximate to said sensor and coupled to said first transformer secondary for providing excitation from said oscillator to said sensor, and said sensor further being coupled by elongated lines to said current control means for providing the direct current output to control the current at said terminals as a function of the change of said parameter.

2. A two wire current transmitter having a pair of terminals for connection to an external DC power supply means serially connected to an external load through two wires, DC current source means coupled to a first of said terminals, oscillator means coupled to said DC current source, reactive sensor means responsive to a physical parameter located non-adjacent and coupled to said oscillator means, rectification means coupled to and located adjacent to said sensor for providing a DC signal, transmission means for coupling said oscillator means and said sensor means to provide excitation thereto, DC current control means coupled to said rectification means, said rectification means providing DC signals representing said physical parameter to said current control means, said current control means further being coupled to said DC current source means and to a second of said terminals to provide a DC current in two wires connected to said terminals responsive to said parameter, wherein the improvement comprises said transmission means having a transformer adjacent to said sensor means and non-adjacent to said oscillator means for coupling excitation signals from said oscillator means to said sensor means and for reducing excessive lead wire effects of capacitance, inductance and resistance which normally result from long transmission lines.

3. An improved two wire transmitter in accordance with claim 2 wherein said transmission means further comprises a second transformer secondary coupled with and adjacent to said oscillator.

4. An improved two wire transmitter in accordance with claim 2 wherein said transmission means further comprises means for direct coupling from said oscillator to said transformer.

5. A two wire 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 and a serially connected load, AC reactive impedance means which varies as a function of the condition to be measured, 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 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 amplifier means having input signal terminals and providing an output 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 comparison of the first and second DC signals and a DC signal representing at least a portion of the total direct current flowing through a resistance, 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, wherein the improvement comprises a transmission means coupled between said AC reactive impedance means and said oscillator means.

6. Apparatus according to claim 5 wherein the improvement further comprises a transformer adjacent to said AC reactive impedance and coupled thereto for providing for said sensor and said current rectification means and said transformer to be disposed non-adjacent to said oscillator.

7. Apparatus according to claim 6 wherein said transmission means further provides excitation from said oscillator to said sensor.

8. Apparatus according to claims 5, 6 or 7 wherein said transmission means provides reduced effects of capacitance, inductance, and resistance of said non-adjacent disposition of said sensor from said oscillator.

9. An improved two wire current transmitter of the type in which a pair of terminals is provided for connection on a first side to an external current supply means and a load, a variable reactance sensor located non-adjacent said transmitter, an oscillator coupled to a second side of said terminal and to said non-adjacent variable reactance sensor to provide excitation to said sensor, said transmitter including a current control to control current through said terminals, said sensor providing a DC signal representative of the measured parameter back to said current control of said transmitter to adjust the current through said terminals as a function of the measured parameter wherein the improvement comprises first means mounted with said transmitter and excited by said oscillator, a transformer having a primary winding mounted with said sensor, means for coupling said first means and said primary means for receiving power from said oscillator and for providing excitation to said non-adjacent sensor, means connected to said sensor to carry a signal from said sensor to said oscillator and said current control of said transmitter.

10. The transmitter of claim 9 wherein said first means includes a transformer having a secondary, said means for coupling comprising a twisted pair of wires coupling said secondary to said primary.

11. The transmitter of claim 10 wherein said sensor includes rectification means providing only direct current signals from said sensor to said oscillator and current control.

12. Apparatus according to claim 9 further wherein the first means further comprises a second transformer secondary.

13. Apparatus according to claim 9 wherein the first means further comprises direct coupling.

14. Apparatus according to claims 9, 10, or 11 wherein first means further comprises means for exciting an adjacent linearity network.