Diagnostics for resistive elements of process devices

• US 6,754,601 B1
• Filed: 09/30/1999
• Issued: 06/22/2004
• Est. Priority Date: 11/07/1996
• Status: Expired due to Term

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Rome, Gregory H., Esboldt, Steven R., Eryurek, Evren
• Primary Examiner(s): Barlow, John
• Assistant Examiner(s): VO, HIEN XUAN
• Application Number:
  US09/409,098
• Time in Patent Office: 1,727 Days
• Field of Search: 702/104, 702/130, 73/11.5, 73/15.7, 324/537-551, 324/649
• 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, without the use of an additional power source. In addition, once degradation of the resistive element is detected, the diagnostic circuitry can be compensated for automatically. 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 change in 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 that relates to a resistance of the resistive element;

a processing system configured to measure a change in the response signal between two points of the response signal that are temporally spaced apart, and to establish a difference between the change in the response signal and a 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 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 to heat the resistive element and to generate a response signal that is related to a resistance of the resistive element;

measuring a change in the response signal between two points of the response signal that are temporally spaced apart; and

comparing the change in the response signal to a corresponding reference to establish a difference that is indicative of degradation of the resistive elements.

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

18. The method of claim 17, wherein the corresponding reference relates to the operating temperature.

19. The method of claim 16, wherein the process device is a two-wire device.

20. The method of claim 16, wherein the reference is a change between two points along a reference response signal.

21. The method of claim 16, wherein each of the two points represents a voltage drop across the resistive element.

22. The method of claim 16, wherein one of the two points represents an initial voltage drop and the other of the two points represents a final voltage drop.

23. The method of claim 16, wherein the test signal is applied for a selected period of time.

24. The method of claim 16, further comprising measuring a first operating temperature defined as the operating temperature of the resistive element before applying the test signal, and measuring a second operating temperature defined as the operating temperature of the resistive element after the test signal is applied and comparing the first operating temperature second operating temperature.

25. The method of claim 16, further comprising producing a diagnostic output as a function of the difference.

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

27. The method of claim 25, wherein the diagnostic output is indicative of a life-expectancy of the resistive element.

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

means for applying a test signal to the resistive element to heat the resistive element and to generate a response signal that is related to a resistance of the resistive element;

means for measuring a change in the response signal between two points along the response signal; and

means for comparing the change in the response signal to a corresponding reference to establish a difference that is indicative of degradation of the resistive element.

29. The apparatus of claim 28, further comprising:

means for establishing a difference between the change in the response signal and the corresponding reference; and

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

30. 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 resistive element;

instructions for reading a change between two points along the response signal;

instructions for reading a corresponding reference; and

instructions for comparing the change to the reference to establish a difference that is indicative of degradation of the resistive element.