Detection and discrimination of instabilities in process control loops

• US 7,039,537 B2
• Filed: 08/19/2004
• Issued: 05/02/2006
• Est. Priority Date: 09/29/1997
• Status: Expired due to Fees

• Current Assignee: Fisher Controls International LLC (Emerson Electric Company)
• Original Assignee: Fisher Controls LLC
• Inventors: Junk, Kenneth W.
• Primary Examiner(s): Hoff, Marc S.
• Assistant Examiner(s): O MALLEY, MARY CATHERINE
• Application Number:
  US10/922,014
• Publication Number:
  US 20050021298A1
• Time in Patent Office: 621 Days
• Field of Search: 702/77, 702/33, 702/35, 702 41- 44, 702 57- 59, 702/138, 702/179, 702181-185, 700/9, 700/282, 731/68, 706/915
• 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 source of an instability within a process control loop comprising the steps of:

measuring a first signal and a second signal within the process control loop when the process control loop is connected on-line within a process control environment, the step of measuring including measuring a pressure signal as the first signal and a travel signal as the second signal;

storing the measured signals as signal data; and

performing an analysis on the stored signal data to determine the source of the instability within the process control loop, the analysis including determining a positive or a negative correlation between the first and the second signals and determining the source of the instability based on the determined correlation, and wherein the step of determining includes the step of identifying the source of the instability as friction when the pressure signal is positively correlated to the travel signal.

2. A method of determining the source of an instability within a process control loop comprising the steps of:

measuring one or more signals within the process control loop when the process control loop is connected on-line within a process control environment;

storing the one or more measured signals as signal data;

performing an analysis on the stored signal data to determine the source of the instability within the process control loop; and

wherein the step of measuring includes the steps of measuring a first signal and a second signal within the process control loop when the process control loop is connected on-line within the process control environment and wherein the step of performing an analysis includes the steps of identifying a lead/lag relationship between the first and the second signals and determining the source of the instability based on the determined lead/lag relationship.

3. The method of claim 2, wherein the step of measuring measures a pressure signal as the first signal and a travel signal as the second signal and wherein the step of determining includes the step of identifying the source of the instability as friction when the pressure signal leads the travel signal.

4. The method of claim 2, wherein the step of measuring measures a pressure signal as the first signal and a travel signal as the second signal and wherein the step of determining includes the step of identifying the source of the instability as an external force when the pressure signal lags the travel signal.

5. The method of claim 2, wherein the step of measuring measures a pressure signal as the first signal and a travel signal as the second signal and wherein the step of determining includes the step of identifying the source of the instability as an external force when the pressure signal is negatively correlated to the travel signal.

6. A method of determining the source of an instability within a process control loop comprising the steps of:

measuring one or more signals within the process control loop when the process control loop is connected on-line within a process control environment;

storing the one or more measured signals as signal data;

performing an analysis on the stored signal data to determine the source of the instability within the process control loop; and

wherein the step of performing an analysis includes the steps of determining a phase lag introduced by each of a number of elements within a process control loop and identifying the element that introduces the most phase lag in the process control loop.

7. A method of determining the source of an instability within a process control loop comprising the steps of:

measuring one or more signals within the process control loop when the process control loop is connected on-line within a process control environment;

storing the one or more measured signals as signal data;

performing an analysis on the stored signal data to determine the source of the instability within the process control loop; and

wherein the step of performing an analysis includes the steps of determining a phase lag introduced by each of a number of elements within a process control loop and ordering the elements according to the amount of phase lag each element introduces into the process control loop.

8. A method of determining the source of an instability within a process control loop comprising the steps of:

measuring one or more signals within the process control loop when the process control loop is connected on-line within a process control environment;

storing the one or more measured signals as signal data;

performing an analysis on the stored signal data to determine the source of the instability within the process control loop; and

wherein the step of performing an analysis includes the steps of determining a phase lag introduced by each of a number of elements within a process control loop and providing a cumulative phase lag chart indicting the detected phase lags.

9. A method of determining the source of an instability within a process control loop comprising the steps of:

measuring one or more signals within the process control loop when the process control loop is connected on-line within a process control environment;

storing the one or more measured signals as signal data;

performing an analysis on the stored signal data to determine the source of the instability within the process control loop; and

wherein the step of measuring includes the step of measuring two or more signals and the step of performing an analysis includes the steps of adding the phases of the two or more signals together to produce a summed phase signal and comparing the summed phase signal to a threshold.

10. The method of claim 9, wherein the step of performing an analysis includes the step of determining that the source of the instability is between the two signals when the summed phase signal is approximately equal to −

180 degrees.

11. A method of determining the source of an instability within a process control loop comprising the steps of:

measuring one or more signals within the process control loop when the process control loop is connected on-line within a process control environment;

storing the one or more measured signals as signal data;

performing an analysis on the stored signal data to determine the source of the instability within the process control loop; and

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.

12. A system for determining a source of an instability within a process control loop when the process control loop is connected on-line within a process environment, the system comprising:

a sensor that measures a first signal within the process control loop when the process control loop is connected on-line within the process environment;

a memory that stores the measured first signal as signal data;

a processor adapted to perform an analysis on the stored signal data to determine the source of the instability;

a second sensor that measures a second signal within the process control loop when the process control loop is connected on-line within the process environment; and

and wherein the processor is adapted to identify the source of the instability based on a lead/lag relationship between the first signal and the second signal.

13. The system of claim 12, wherein the memory stores the measured second signal as secondary signal data and the processor is adapted to perform the analysis on the stored signal data and the stored secondary signal data to determine the source of the instability.

14. The system of claim 12 wherein the first sensor is a pressure sensor that produces a pressure signal as the first signal and the second sensor is a travel sensor that produces a travel signal as the second signal and wherein the processor is adapted to identify the source of the instability as friction when the pressure signal leads the travel signal.

15. The system of claim 12 wherein the first sensor is a pressure sensor that produces a pressure signal as the first signal and the second sensor is a travel sensor that produces a travel signal as the second signal and wherein the processor is adapted to identify the source of the instability as an external force when the pressure signal lags the travel signal.

16. The system of claim 12 wherein the processor is adapted to identify a lead/lag relationship between the first signal and the second signal and to determine the source of the instability based on the determined lead/lag relationship.

17. The system of claim 12, wherein the processor is adapted to identify a positive or a negative correlation between the first signal and the second signal and to determine the source of the instability based on the determined correlation.

18. The system of claim 17, wherein the first sensor is a pressure sensor that produces a pressure signal as the first signal and the second sensor is a travel sensor that produces a travel signal as the second signal and wherein the processor 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 sensor is a pressure sensor that produces a pressure signal as the first signal and the second sensor is a travel sensor that produces a travel signal as the second signal and wherein the processor 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. A system for determining a source of an instability within a process control loop when the process control loop is connected on-line within a process environment, the system comprising:

a sensor that measures a first signal within the process control loop when the process control loop is connected on-line within the process environment;

a memory that stores the measured first signal as signal data;

a processor adapted to perform an analysis on the stored signal data to determine the source of the instability;

a second sensor that measures a second signal within a process control loop when the process control loop is connected on-line within the process environment and wherein the memory stores the measured second signal as secondary signal data and the processor is adapted to perform the analysis on the stored signal data and the stored secondary signal data to determine the source of the instability; and

wherein the processor is adapted to use at least the first and second signals to determine a phase lag introduced by each of a number of elements into a process control loop; and

further wherein based on the amount of phase lag introduced by each of the number of elements into the process control loop the processor is adapted to order the elements according to the amount of phase lag or to identify the element that introduces the most phase lag into the process control loop.

21. The system of claim 20, wherein the processor is adapted to provide a cumulative phase lag chart indicating the detected phase lags.

22. A system for determining a source of an instability within a process control loop when the process control loop is connected on-line within a process environment, the system comprising:

a first sensor that measures a first signal within the process control loop when the process control loop is connected on-line within the process environment;

a second sensor that measures a second signal within a process control loop when the process control loop is connected on-line within the process environment

a memory that stores the measured first signals as signal data and stores the secondary signals as secondary signal data; and

a processor adapted to perform an analysis on the stored signal data and the secondary signal data to determine the source of the instability;

wherein the processor 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.

23. The system of claim 22, wherein the processor 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.

24. A system for determining a source of an instability within a process control loop when the process control loop is connected on-line within a process environment, the system comprising:

a first sensor that measures a first signal within the process control loop when the process control loop is connected on-line within the process environment;

a second sensor that measures a second signal within a process control loop when the process control loop is connected on-line within the process environment

a memory that stores the measured first signals as signal data and stores the secondary signals as secondary signal data; and

a processor adapted to perform an analysis on the stored signal data and the secondary signal data to determine the source of the instability;

wherein the processor 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 of time.