Error compensation for a process fluid temperature transmitter

• US 6,356,191 B1
• Filed: 06/17/1999
• Issued: 03/12/2002
• Est. Priority Date: 06/17/1999
• Status: Expired due to Term

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Rotvold, Eric D., Kirkpatrick, William R.
• Primary Examiner(s): Crosland, Donnie L.
• Application Number:
  US09/335,212
• Time in Patent Office: 999 Days
• Field of Search: 340/501, 340/508, 340/509, 340/506, 340/510, 340/511, 340/588, 340/589, 702/53, 702/54, 702/64, 702/99.E, 702/130, 702/133, 737/19, 737/08, 374/100, 700/9-12
• US Class Current: 340/501
• CPC Class Codes: G01K 7/20 in a specially-adapted circuit, e.g. bridge circuit
• CPC Class Codes: G08C 19/02 in which the signal transmitted is magnitude of current or voltage G08C19/36, G08C19/38 take precedence

Abstract

A process fluid temperature transmitter includes circuitry that improves compensation of error due to the presence of one or more non-associated voltage sources in a resistance temperature device (RTD) circuit. A method of compensating for such error is also provided.

Claims

1. A process fluid temperature transmitter with improved error compensation, the transmitter comprising:

a loop communicator couplable to a process control loop and adapted to communicate over the process control loop;

a plurality of terminals couplable to a resistance temperature device to form a circuit including at least one non-associated voltage source having a non-associated voltage;

a current source operably coupled to the plurality of terminals and adapted to pass a measurement current through the plurality of terminals;

a voltage measurement device coupled to the plurality of terminals and disposed to provide a first voltage value indicative of a first voltage across the plurality of terminals including the non-associated voltage; and

a controller coupled to the voltage measurement device and the loop communicator, the controller adapted to provide a process temperature value to the loop communicator for communication over the process control loop, the process temperature value related to the first voltage value and substantially independent of the non-associated voltage of the at least one non-associated voltage source.

2. The transmitter of claim 1, wherein:

the current source is adapted to pass the measurement current through the circuit during a first state, and is adapted to pass substantially no current through the circuit during a second state;

the voltage measurement device is adapted to provide the first voltage value during the first state, and to provide an error value related to the non-associated voltage of the at least one non-associated voltage source during the second state; and

the controller is adapted to subtract the error value from the first voltage value to provide the process temperature value.

3. The transmitter of claim 2, wherein all circuitry within the transmitter is adapted for intrinsic safety compliance.

4. The transmitter of claim 2, wherein the at least one non-associated voltage source comprises a plurality of non-associated voltage sources.

5. The transmitter of claim 2, wherein the controller is coupled to the current source and is adapted to stop measurement current flow through the circuit and wherein the voltage measurement device is adapted to measure the error value while the measurement current is stopped.

6. The transmitter of claim 2, and further comprising a switch coupled to the controller and electrically interposed between the current source and the plurality of terminals to selectively inhibit measurement current flow in response to an inhibit signal from the controller.

7. The transmitter of claim 2, wherein the loop communicator is adapted to communicate in accordance with a protocol selected from the group consisting of HART®

, FOUNDATION™

Fieldbus, Profibus, DeviceNet, CAN, and Lonworks.

8. The transmitter of claim 2, and further comprising a resistance temperature device coupled to the plurality of terminals, the resistance temperature device having a resistance that varies with a temperature of the resistance temperature device.

9. The transmitter of claim 8, wherein the resistance temperature device is constructed from a material selected from the group consisting of nickel and platinum.

10. The transmitter of claim 1, and further comprising a power supply couplable to the process control loop and adapted to receive power from the process control loop to wholly power the transmitter with power received from the process control loop.

11. The transmitter of claim 10, wherein the loop communicator is adapted to communicate over a 4-20 mA process control loop.

12. The transmitter of claim 1, wherein the process control loop includes a plurality of conductors ranging between 2 conductors and 16 conductors, inclusive.

13. The transmitter of claim 1, wherein the measurement current is an alternating current, and wherein the circuit includes at least one capacitor adapted to substantially pass a signal indicative of the alternating measurement current.

14. The transmitter of claim 1, wherein:

the measurement current is an alternating current, and the voltage measurement device is adapted to provide a second voltage value indicative of current flowing through the circuit; and

wherein the first and second voltage values are indicative of current flow through the circuit in opposite directions, and the process temperature value is related to the first and second voltage values.

15. The transmitter of claim 14, wherein the process temperature value is related to an average of absolute values of the first and second voltage values.

16. The transmitter of claim 1, wherein the at least one non-associated voltage source is a dissimilar metal junction.

17. A process fluid temperature transmitter with improved error compensation, the transmitter comprising:

means coupleable to a process control loop for communicating over the process control loop;

means for coupling to a resistance temperature device to form a circuit having a plurality of terminals and at least one non-associated voltage source having a non-associated voltage;

means for passing a measurement current through the means for coupling;

means for providing a first voltage value indicative of a first voltage across the plurality of terminals; and

means for providing a process temperature value to the means for communicating, the process temperature value being related to the first voltage value and substantially independent of the non-associated voltage.

18. A method of measuring process fluid temperature with a process fluid temperature transmitter with improved error compensation, the method comprising:

measuring a voltage across a resistance temperature device in a resistance temperature device circuit while a known measurement current flows through the circuit;

measuring a non-associated voltage within the circuit while the current flows through the circuit generated by at least one non-associated voltage source; and

providing a transmitter output indicative substantially solely of a resistance of the resistance temperature device circuit, and substantially independent of the non-associated voltage.

19. Computer-readable media storing a computer program comprising:

first measurement instructions for obtaining a voltage value indicative of a voltage across a resistance temperature device in a resistance temperature device circuit while a known measurement current flows through the circuit;

second measurement instructions for obtaining a non-associated voltage within the circuit while the current flows through the circuit generated by at least one non-associated voltage source; and

output instructions for providing a transmitter output indicative substantially solely of a resistance of the resistance temperature device circuit, the output being substantially independent of the non-associated voltage.