Resistive element diagnostics for process devices

• US 6,594,603 B1
• Filed: 09/30/1999
• Issued: 07/15/2003
• Est. Priority Date: 10/19/1998
• Status: Expired due to Term

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Eryurek, Evren, Esboldt, Steven R.
• Primary Examiner(s): SHAH, KAMINI S
• Application Number:
  US09/409,114
• Time in Patent Office: 1,384 Days
• Field of Search: 73/11.5, 73/15.7, 374/1, 374/172, 324/601, 324/718, 702/104, 702/99, 702/130, 702/133, 702/136, 700/282, 709/250
• US Class Current: 702/104
• CPC Class Codes: G01K 15/00 Testing or calibrating of thermometers
• CPC Class Codes: G07C 3/00 Registering or indicating the condition or the working of machines or other apparatus, other than vehicles
• CPC Class Codes: G08C 19/02 in which the signal transmitted is magnitude of current or voltage G08C19/36, G08C19/38 take precedence

Abstract

Diagnostic circuitry of a process device is used to detect degradation of a resistive element of the process device while the process device remains online and without an additional power source. The diagnostic circuitry includes a testing circuit and a processing system. The testing circuit is coupled to the resistive element and is configured to apply a test signal to the resistive element. The test signal heats the resistive element and causes the resistive element to generate a response signal. The processing system compares a time constant of the response signal to a corresponding reference to detect degradation of the resistive element.

Claims

1. Diagnostic circuitry for detecting degradation of a resistive element of a process device, the diagnostic circuitry comprising:

a test signal source coupleable to the resistive element and configured to apply a test signal to the resistive element, wherein the test signal heats the resistive element and generates a response signal;

a processing system configured to receive the response signal and to compare a time constant of the response signal to a corresponding reference, wherein a difference can be determined between the time constant and the corresponding reference.

2. The diagnostic circuitry of claim 1, wherein the resistive element is selected from the group consisting of a resistance temperature detector, a thermocouple, a strain gauge, and a coil.

3. The diagnostic circuitry of claim 1, further comprising a memory connected to the processing system configured to store the reference.

4. The diagnostic circuitry of claim 1, wherein the process device is wholly loop powered.

5. The diagnostic circuitry of claim 1, wherein the process device is powered by a 4-20 mA loop.

6. The diagnostic circuitry of claim 1, wherein the process device comprises a 2-wire device.

7. The diagnostic circuitry of claim 1, wherein the process device comprises a temperature transmitter.

8. The diagnostic circuitry of claim 1, wherein the diagnostic circuitry is intrinsically safe.

9. The diagnostic circuitry of claim 1, wherein the test signal source is a current source, and wherein the diagnostic circuitry further comprises:

a switch coupled to the current source wherein actuation of the switch causes the test signal to be applied to the resistive element; and

a voltage detector configured to detect the response signal generated by the resistive element in response to the test signal.

10. The diagnostic circuitry of claim 1, wherein the processing system is further configured to produce a diagnostic output as a function of the difference.

11. The diagnostic circuitry of claim 10, wherein the diagnostic output is indicative of a condition of the resistive element.

12. The diagnostic circuitry of claim 10, wherein the diagnostic output is indicative of a life expectancy of the resistive element.

13. The diagnostic circuitry of claim 10, wherein the diagnostic output is in accordance with a communication protocol selected from a group consisting of Highway Addressable Remote Transducer, Fieldbus, Profibus, and Ethernet protocols.

14. The diagnostic circuitry of claim 1, wherein the processing system comprises a neural network.

15. The diagnostic circuitry of claim 1, wherein:

the process device is a process variable transmitter configured to measure a value of a process variable using the resistive element and to produce a process variable output indicative of the value; and

the processing system is configured to adjust the process variable output as a function of the difference.

16. A method for detecting degradation of a resistive element of a process device, comprising:

applying a test signal to the resistive element;

analyzing a response signal generated by the resistive element in response to the test signal, wherein the response signal is related to a resistance of the restive element during application of the test signal;

determining a time constant of the response signal; and

comparing the time constant to a corresponding reference.

17. The method of claim 16, further comprising providing a diagnostic output as a function of a difference between the time constant and the corresponding reference.

18. The method of claim 16, wherein the diagnostic output is indicative of a condition of the resistive element.

19. The method of claim 16, wherein the diagnostic output is indicative of a life expectancy of the resistive element.

20. The method of claim 16, further comprising measuring an operating temperature of the resistive element.

21. The method of claim 16, wherein the process device comprises a 2-wire device.

22. An apparatus for detecting degradation of a resistive element of a process device, comprising:

means for applying a test signal to the resistive element;

means for analyzing a response signal generated by the resistive element in response to the test signal, wherein the response signal is related to a resistance of the restive element during application of the test signal;

means for determining a time constant of the response signal; and

means for comparing the time constant to a corresponding reference.

23. The apparatus of claim 22, wherein the process device comprises a 2-wire device.

24. The apparatus of claim 22, further comprising means for producing a diagnostic output as a function of a difference between the time constant and the corresponding reference.

25. The apparatus of claim 24, wherein the diagnostic output is indicative of a condition of the resistive element.

26. The apparatus of claim 24, wherein the diagnostic output is indicative of a life expectancy of the resistive element.

27. The apparatus of claim 22, further comprising:

means for establishing a difference between the time constant and the corresponding reference; and

means for correcting an output produced by the resistive element as a function of the difference.

28. A computer readable medium having stored instructions executable by a processor capable of diagnosing a resistive element of a process device, the instructions comprising:

instructions for reading a response signal generated by the resistive element in response to a test signal, wherein the response signal is related to a resistance of the restive element during application of the test signal;

instructions for determining a time constant of the response signal;

instructions for reading a corresponding reference; and

instructions for comparing the time constant to the reference.

29. The article of claim 28, wherein the process device comprises a 2-wire device.