PROCESS TEMPERATURE TRANSMITTER WITH IMPROVED TEMPERATURE CALCULATION

• US 20100177800A1
• Filed: 01/07/2010
• Published: 07/15/2010
• Est. Priority Date: 01/09/2009
• Status: Active Grant

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Engelstad, Loren M., Rud, Jason H.
• Granted Patent: US 8,408,787 B2
• Time in Patent Office: Days
• US Class Current: 374/45
• CPC Class Codes: G01K 15/00 Testing or calibrating of thermometers
• CPC Class Codes: G01K 15/007 Testing
• CPC Class Codes: G01K 7/20 in a specially-adapted circuit, e.g. bridge circuit

Abstract

In a process temperature transmitter, a method for providing a process temperature output is described. The method includes providing a measurement current through a circuit including a reference resistor and a resistance temperature device (RTD). A first voltage across the reference resistor is measured while the measurement current flows through it. A first voltage across the RTD is also measured while the measurement current flows through it. A first resistance of the RTD is calculated based on the measured voltage across the reference resistor and the measured first voltage. A first process temperature output is provided based on the first resistance of the RTD. A second voltage is subsequently measured across the RTD while a measurement current flows through it. A second voltage across the reference resistor is then estimated based on the measured second voltage across the RTD and the measured first voltage across the reference resistor. A second resistance across the RTD is calculated based on the measured second voltage and the estimated second voltage. A second process temperature output is provided based on the second resistance.

Claims

1. In a process temperature transmitter, a method for providing a process temperature output, the method comprising:

providing a measurement current through a circuit including a reference resistor and a resistance temperature device (RTD);

measuring a first voltage across the reference resistor while the measurement current flows therethrough;

measuring a first voltage across the RTD while the measurement current flows therethrough;

calculating a first resistance of the RTD based on the measured voltage across the reference resistor and the measured first voltage;

providing a first process temperature output based on the first resistance of the RTD;

measuring a second voltage across the RTD while a measurement current flows therethrough, the second measurement occurring after measurement of the first voltage;

estimating a second voltage across the reference resistor based on the measured second voltage across the RTD and the measured first voltage across the reference resistor;

calculating a second resistance across the RTD based on the measured second voltage and the estimated second voltage; and

providing a second process temperature output based on the second resistance.

2. The method of claim 1, wherein the measuring steps are performed using an analog-to-digital converter of the process temperature transmitter, and wherein the calculations are performed using a processor of the process temperature transmitter.

3. The method of claim 1, wherein the process temperature output is provided over a process communication loop.

4. The method of claim 1, wherein estimating the second voltage is also based on a correction factor.

5. The method of claim 4, wherein the correction factor is based on a ratio of (a difference between a sensor voltage at a full scale input and a sensor voltage at zero input) and (a difference between a reference voltage at a full scale input and a reference voltage at zero input).

6. The method of claim 5, wherein the correction factor is calculated during a transmitter characterization process.

7. The method of claim 5, wherein the correction factor is calculated while the transmitter is coupled to a process.

8. The method of claim 7, wherein a change in the correction factor over time can be detected in order to apply a correction to voltage measured across the reference resistor.

9. The method of claim 7, wherein a change in correction factor over time is detected and used to provide diagnostic information.

10. The method of claim 9, wherein the diagnostic information is related to an influence of line frequency.

11. The method of claim 9, wherein the diagnostic information is related to electromagnetic interference.

12. The method of claim 1, wherein and further comprising:

measuring a subsequent voltage across the RTD while a measurement current flows therethrough, the subsequent measurement occurring after measurement of the second voltage;

estimating a subsequent voltage across the reference resistor based on the measured subsequent voltage and the measured first voltage across the reference resistor;

calculating a subsequent resistance across the RTD based on the measured subsequent voltage and the estimated subsequent voltage across the reference resistor; and

providing a subsequent process temperature output based on the subsequent resistance.

13. The method of claim 12, wherein the measurement of subsequent voltage, estimation of subsequent voltage, calculation of subsequent resistance and the provision of subsequent process temperature are repeated until a feedback condition is determined.

14. The method of claim 13, wherein the feedback condition is based on a number of cycles in which voltage across the RTD is measured.

15. The method of claim 13, wherein the feedback condition is based on the process temperature output.

16. In a process temperature transmitter, a method for providing a process temperature output, the method comprising:

providing a measurement current through a circuit including a reference resistor and a resistance temperature device (RTD);

measuring a voltage across the RTD while the measurement current passes therethrough;

estimating a reference voltage based on a previous reference voltage and the voltage measured across the RTD;

calculating a resistance of the RTD based on the estimated reference voltage and the measured voltage; and

providing a process temperature output based on the resistance of the RTD.

17. The method of claim 16, wherein the method is repeated until a feedback condition is determined.

18. The method of claim 17, wherein the feedback condition is based on a number of cycles in which voltage across the RTD is measured.

19. The method of claim 17, wherein the feedback condition is based on the process fluid temperature output.

20. A process temperature transmitter comprising:

a loop communicator configured to couple to a process communication loop and to communicate in accordance with a process protocol;

measurement circuitry configured to couple to a resistance temperature device having a resistance that varies with temperature and operably coupled to a reference resistor; and

a controller coupled to the measurement circuitry and the loop communicator, the controller being configured to provide at least one process temperature output based on a current resistance temperature device resistance and a previous reference resistor resistance.