Process control transmitter having an externally accessible DC circuit common

• US 6,504,489 B1
• Filed: 05/15/2000
• Issued: 01/07/2003
• Est. Priority Date: 05/15/2000
• Status: Expired due to Term

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Westfield, Brian L., Roper, Weston, Nelson, Richard L.
• Primary Examiner(s): Horabik, Michael
• Assistant Examiner(s): WONG, ALBERT KANG
• Application Number:
  US09/571,111
• Time in Patent Office: 967 Days
• Field of Search: 340/870.3, 340/870.37, 340/870.39, 340/870.16, 340/501, 340/531, 737/00, 737/06, 702/62, 702/57, 702/116, 702/104, 702/85
• US Class Current: 340/870.3
• CPC Class Codes: G08C 19/02 in which the signal transmitted is magnitude of current or voltage G08C19/36, G08C19/38 take precedence

Abstract

Disclosed is a process control transmitter having an externally accessible DC circuit common that eliminates the need to perform level shifting of signals communicated between the transmitter and external processing electronics. The process control transmitter includes first, second and third terminals which feedthrough a housing. Circuitry contained in the housing is coupled to the first, second and third terminals and is adapted to communicate information to external processing electronics through the second and third terminals using a digital signal that is regulated relative to a DC common that is coupled to the second terminal. External processing electronics can couple to the second and third terminals and interpret the digital signal without having to perform level-shifting adjustments.

Claims

1. A process control transmitter having an externally accessible DC common, comprising:

first, second and third externally accessible feedthrough terminals, wherein the first and second terminals are couplable to a process control loop and adapted to conduct a loop current I_T through the transmitter;

a series-shunt regulator having an input terminal coupled to the first terminal and a shunt current output terminal coupled to the second terminal, the series-shunt regulator conducting a load current I_L and controlling the loop current I_T by regulating a shunt current I_S out the shunt current output terminal; and

circuitry energized by the load current I_L and adapted to control the loop current I_T in response to a sensor signal and provide a digital signal to the third terminal that has a voltage that is regulated relative to a DC common of the circuitry that is coupled to the second terminal, whereby the digital signal is externally accessible between the second and third terminals.

2. The process control transmitter of claim 1, wherein the series-shunt regulator comprises:

a series regulator coupled to the input terminal and adapted to conduct the load current I_L and provide a first feedback output representative of the load current;

a shunt adapted to conduct the shunt current I_S to the shunt current output terminal and provide a second feedback output representative of the shunt current I_S, wherein the loop current I_T is substantially a summation of the load current I_L and the shunt current I_S; and

a shunt current regulator carrying the shunt current I_S and adapted to control the loop current I_T to a predetermined value as a function of the first and second feedback outputs.

3. The process control transmitter of claim 1, wherein the transmitter is completely powered by the process control loop.

4. The process control transmitter of claim 1, wherein the digital signal is in accordance with a digital communication protocol.

5. The process control transmitter of claim 1, wherein:

the circuitry includes a process variable output coupled to the shunt current regulator; and

the series-shunt regulator is further adapted to control the loop current as a function of the process variable output, whereby the predetermined value relates to the process variable output.

6. The process control transmitter of claim 1, wherein the circuitry is configured to communicate with externally located processing electronics over the process control loop, in accordance with a communication protocol, using the series-shunt regulator.

7. The process control transmitter of claim 6, wherein the communication protocol is a digital communication protocol.

8. The process control transmitter of claim 2, wherein the first and second feedback outputs relate to DC components of the load and shunt currents, respectively.

9. The process control transmitter of claim 2, wherein the first and second feedback outputs relate to AC and DC components of the load and shunt currents, respectively.

10. The process control transmitter of claim 1, further comprising at least one of a fourth and fifth terminal adapted to provide logic level switching for the transmitter, wherein the fourth and fifth terminals are externally accessible feedthrough terminals.

11. A process control transmitter comprising:

first, second and third externally accessible feedthrough terminals, wherein the first and second terminals are couplable to a process control loop and adapted to conduct a loop current I_T through the transmitter;

a base module including;

a series-shunt regulator having an input terminal coupled to the first terminal and a shunt current output terminal coupled to the second terminal, the series-shunt regulator conducting a load current I_L and controlling the loop current I_T by regulating a shunt current I_S out the shunt current output terminal; and

circuitry energized by the load current I_L and adapted to receive a sensor signal and provide a digital signal to the third terminal that has a voltage that is regulated relative to a DC common of the circuitry that is coupled to the second terminal, whereby the digital signal is externally accessible between the second and third terminals.

12. The process control transmitter of claim 11, wherein the series-shunt regulator comprises:

a series regulator coupled to the input terminal and adapted to conduct the load current I_L and provide a first feedback output representative of the load current;

a shunt adapted to conduct the shunt current I_S to the shunt current output terminal and provide a second feedback output representative of the shunt current I_S, wherein the loop current I_T is substantially a summation of the load current I_L and the shunt current I_S; and

a shunt current regulator carrying the shunt current I_S and adapted to control the loop current I_T to a predetermined value as a function of the first and second feedback outputs.

13. The transmitter of claim 11, further comprising an expansion module couplable to the first, second, and third terminals, whereby the expansion module communicates with the circuitry of the base module through the second and third terminals.

14. The transmitter of claim 13, wherein the expansion module provides at least one feature selected from a group consisting of calculating mass flow rate and expanding communication capabilities.

15. The transmitter of claim 13, wherein the expansion module communicates with the base module through the second and third terminals in accordance with a digital communication protocol.

16. The transmitter of claim 11, wherein the third terminal is adapted to power and communicate information to, a liquid crystal display (LCD).

17. The process control transmitter of claim 11, wherein the transmitter is completely powered by the process control loop.

18. The process control transmitter of claim 11, wherein:

the circuitry includes a process variable output coupled to the shunt current regulator; and

the series-shunt regulator is further adapted to control the loop current as a function of the process variable output, whereby the predetermined value relates to the process variable output.

19. The process control transmitter of claim 11, wherein the circuitry is configured to communicate with externally located processing electronics over the process control loop, in accordance with a communication protocol, using the series-shunt regulator.

20. The process control transmitter of claim 19, wherein the communication protocol is a digital communication protocol.

21. The process control transmitter of claim 12, wherein the first and second feedback outputs relate to DC components of the load and shunt currents, respectively.

22. The process control transmitter of claim 12, wherein the first and second feedback outputs relate to AC and DC components of the load and shunt currents, respectively.

23. The process control transmitter of claim 11, further comprising at least one of a fourth and fifth terminal adapted to provide logic level switching for the transmitter, wherein the fourth and fifth terminals are externally accessible feedthrough terminals.

24. A method of manufacturing a process control transmitter, comprising:

forming first, second and third terminals which feedthrough a housing, the first and second terminals being couplable to a process control loop and adapted to conduct a loop current I_T through the transmitter and the third terminal;

installing a series-shunt regulator in the housing having an input terminal coupled to the first terminal and a shunt current output terminal coupled to the second terminal, the series-shunt regulator conducting a load current I_L and controlling the loop current I_T by regulating a shunt current I_S out the shunt current output terminal; and

installing circuitry in the housing that is energized by the load current I_L and adapted to receive a sensor signal and provide a digital signal to the third terminal that has a voltage that is regulated relative to a DC common of the circuitry that is coupled to the second terminal, whereby the digital signal is externally accessible between the second and third terminals.

25. The method of claim 24, including powering the transmitter through the process control loop.

26. The method of claim 24, wherein the digital signal is in accordance with a digital communication protocol.

27. The method of claim 24, wherein the external processing electronics includes one of a liquid crystal display and an expansion module.