Detection and discrimination of instabilities in process control loops

• US 6,804,618 B2
• Filed: 12/05/2001
• Issued: 10/12/2004
• Est. Priority Date: 09/29/1997
• Status: Expired due to Term

• Current Assignee: Fisher Controls International LLC (Emerson Electric Company)
• Original Assignee: Fisher Controls International LLC (Emerson Electric Company)
• Inventors: Junk, Kenneth W.
• Primary Examiner(s): Hoff, Marc S.
• Assistant Examiner(s): Raymond, Edward
• Application Number:
  US10/006,955
• Publication Number:
  US 20020040284A1
• Time in Patent Office: 1,042 Days
• Field of Search: 702/77, 702/33, 702/35, 702/41, 702/42, 702/43, 702/44, 702/57, 702/58, 702/59, 702/138, 702/179, 702/181, 702/182-185
• US Class Current: 702/77
• CPC Class Codes: G05B 23/0229 knowledge based, e.g. expert systems; genetic algorithms
• CPC Class Codes: G05B 5/01 electric

Abstract

A method and apparatus determines the presence of and source of instabilities, such as limit cycles, within a process control loop while the process control loop operates on-line within a process environment. The method and apparatus measures one or more signals within a process control loop when the process control loop is connected on-line within a process control environment, stores the measured signal as signal data and then performs one or more statistical analyses on the stored signal data to determine whether instabilities exist and, if so, whether the source of an instability is within a servo loop or outside of a servo loop and is due to friction, external forces or mechanical anomalies.

Claims

1. A method of determining the existence of an instability within a process control loop using a process control device comprising the steps of:

measuring one or more signals within the process control loop when the process control loop and the process control device are connected on-line and continuously in service under normal operating conditions within a process control environment;

storing the one or more measured signals as signal data; and

performing an analysis on the stored signal data to determine the existence of an instability within the process control loop.

2. The method of claim 1, wherein the step of measuring includes the step of measuring two signals and the step of performing an analysis includes the steps detecting the number of reversals in each of the two signals over a particular period of time and determining the difference in the number of reversals in the two signals over the particular period of time.

3. The method of claim 1, wherein the step of performing an analysis includes the step of performing a Fourier transform on the one or more signals and detecting changes in the spectrum of the one or more signals to determine the existence of an instability.

4. The method of claim 1, wherein the step of performing an analysis includes the step of using the Wiener-Khinchine relation to determine the existence of an instability.

5. The method of claim 4, wherein the step of using the Wiener-Khinchine relation includes the step or identifying changes in the spectrum of the one or more signals by calculating changes in the variance of the one or more signals.

6. The method or claim 1, wherein the step of measuring includes the step of measuring two signals and wherein the step of performing an analysis includes the step of determining the variance of the two signals, calculating the ratio of the variances of the two signals and comparing the ratio to a preset value in determine the existence of in instability.

7. The method of claim 6, wherein the step of measuring includes the steps of measuring a command signal and a travel signal.

8. The method of claim 6, wherein the step of calculating includes the step of calculating the ratio of the variances of the two signals recursively.

9. The method of claim 8, wherein the step of calculating the ratio of of the variance or the two signals recursively includes the step of using forgetting factors.

10. A system to be used in a process control environment to determine an existence of an instability within a process control loop, the system comprising:

a computer readable memory;

a first routine stored on the computer readable memory and adapted to be executed on a processor to collect and store data indicative of one or more signals associated with the process control loop and the process control device while operating on line and continuously in service under normal operating conditions within the process control environment; and

a second routine stored on the computer readable memory and adapted to be executed on a processor to perform a statistical analysis on the stored data to determine the existence of an instability within the process control loop.

11. The system of claim 10, wherein the second routine is adapted to use the data indicative of the one or more signals to determine a phase lag introduced by each of a number of elements into a process control loop and to identify the element that introduces the most phase lag into the process control loop.

12. The system of claim 10, wherein the second routine is adapted to use the data indicative of the one or more signals to determine a phase lag introduced by each of a number of elements within a process control loop and to order the elements according to the amount of phase lag each element introduces into the process control loop.

13. The system of claim 10, wherein, the second routine is adapted to use the data indicative of the one or mare signals to determine a phase lag introduced by each of a number of elements within a process control loop and to provide a cumulative phase lag chart indicting the detected phase lags.

14. The system of claim 10, wherein the first routine is adapted to collect and store data indicative of first and second signals and wherein the second routine is adapted to identify a lead/lag relationship between the first and the second signals and to determine the source of the instability based on the determined lead/lag relationship.

15. The system of claim 14, wherein the first signal is a pressure signal and the second signal is a travel signal and wherein the second routine is adapted to identify the source of the instability as fraction when the pressure signal leads the travel signal.

16. The system of claim 14, wherein the first signal is a pressure signal and the second signal is a travel signal and wherein the second routine is adapted to identity the source of the instability as an external force when the pressure signal lags the travel signal.

17. The system of claim 10, wherein the first routine is adapted to collect and store data indicative or first and second signals and wherein the second routine is adapted to identify a positive or a negative correlation between the first and the second signals and to determine the source of the instability based on the determined correlation.

18. The system of claim 17, wherein the first signal is a pressure signal and the second signal is a travel signal and wherein the second routine is adapted to identify the source of the instability as friction when the pressure signal is positively correlated to the travel signal.

19. The system of claim 17, wherein the first signal is a pressure signal and the second signal is a travel signal and wherein the second routine is adapted to identify the source of the instability as an external force when the pressure signal is negatively correlated to the travel signal.

20. The system of claim 10, wherein the first routine is adapted to collect and store data indicative of first and second signals and wherein the second routine is adapted to sum the phases of the first and second signals together to produce a summed phase signal and to compare the summed phase signal to a threshold to identify the source of the instability.

21. The system of claim 20, wherein the second routine is adapted to determine that the source of the instability is in a component between the first and second signals when the summed phase signal is approximately equal to −

180 degrees.

22. The system of claim 10, wherein the first routine is adapted to collect and store data pertaining to two signals and wherein the second routine is adapted to detect the number of reversals in each of the two signals over a particular period of time and to determine the difference in the number of reversals in the two signals over the particular period or time.

23. The system of claim 10, wherein the second routine is adapted to perform a Fourier transform on the one or more signals and to detect changes in the spectrum of the one or more signals to identify the existence of an instability.

24. The system of claim 10, wherein the second routine is adapted to use the Wiener-Khinchine relation to determine the existence of an instability.

25. The system of claim 24, wherein the second routine identifies changes in the spectrum of the one or more signals by calculating changes in the variance of the one or more signals.

26. The system of claim 10, wherein the first routine is adapted to collect and store data pertaining to two signals and wherein the second routine is adapted to determine the variance of the two signals calculate the ratio of the variances of the two signals and compare the ratio to a preset value to determine the existence of an instability.

27. The system of claim 26, wherein the second routine is adapted to calculate the ratio of the variances of the two signals recursively.

28. The system of claim 26, wherein the second routine is adapted to calculate the ratio of the variances recursively using forgetting factors.