Self-diagnostic process control loop for a process plant

• US 8,909,360 B2
• Filed: 10/18/2012
• Issued: 12/09/2014
• Est. Priority Date: 12/05/2005
• Status: Active Grant

• Current Assignee: Fisher-Rosemount Systems Incorporated (Emerson Electric Company)
• Original Assignee: Fisher-Rosemount Systems Incorporated (Emerson Electric Company)
• Inventors: Blevins, Terrence L., Wojsznis, Wilhelm K., McMillan, Gregory K., Wojsznis, Peter
• Primary Examiner(s): Hartman, Jr., Ronald
• Application Number:
  US13/655,044
• Publication Number:
  US 20130046396A1
• Time in Patent Office: 782 Days
• Field of Search: 700 28- 29, 700 31- 32, 700 54- 55, 702/183
• US Class Current: 700/54
• CPC Class Codes: G05B 23/0251 Abstraction hierarchy, e.g. "complex systems", i.e. system is divided in subsystems, subsystems are monitored and results are combined to decide on status of whole system

Abstract

A method of diagnosing an adaptive process control loop includes measuring process control loop signal data, generating a plurality of process control loop parameters from the process loop signal data and evaluating a condition of the adaptive process control loop from one or more of the plurality of process control loop parameters. The process control loop data is generated as a result of a normal operation of one or more process control devices within the adaptive process control loop when the adaptive process control loop is connected on-line within a process control environment. A self-diagnostic process control loop includes a diagnostic tool adapted to receive a diagnostic index pertaining to a process control loop parameter for a plurality of components of the process control loop and for the complete process control loop. Each diagnostic index is generated from signal data by a corresponding index computation tool. The diagnostic tool is further adapted to evaluate a condition of the process control loop from one or more of the diagnostic indices.

Claims

1. A method of diagnosing a process control loop comprising a plurality of process control loop devices, wherein the process control loop is controlled based on adaptive process loop parameters calculated from an adapted model of the process control loop for a plurality of parameters of the process control loop, the method comprising:

evaluating on a computer a performance of the process control loop;

evaluating on the computer a condition of a process control loop device within the process control loop;

evaluating on the computer a quality of the adapted model of the process control loop;

evaluating on the computer a stability of the process control loop;

generating on the computer a diagnostic index from each of the evaluations

evaluating on the computer a condition of the process control loop from one or more of the diagnostic indices; and

automatically adjusting on the computer a process variable filter based on evaluating on the computer a condition of the process control loop from one or more of the diagnostic indices.

2. The method of claim 1, wherein generating on the computer a diagnostic index from each of the evaluations comprises generating on the computer a variability index from evaluating on the computer a performance of the process control loop.

3. The method of claim 2, wherein generating on the computer a variability index from evaluating on the computer a performance of the process control loop comprises calculating on the computer the variability index as:

4. The method of claim 1, wherein generating on the computer a diagnostic index from each of the evaluations comprises generating on the computer a minimum standard deviation from evaluating a performance of the process control loop.

5. The method of claim 4, wherein generating on the computer a minimum standard deviation from evaluating on the computer a performance of the process control loop comprises calculating on the computer the minimum standard deviation as:

6. The method of claim 1, wherein generating on the computer a diagnostic index from each of the evaluations comprises generating on the computer a dead band of a process control loop device from evaluating on the computer a condition of a process control loop device within the process control loop.

7. The method of claim 6, wherein generating on the computer a dead band of a process control loop device from evaluating on the computer a condition of a process control loop device within the process control loop comprises calculating on the computer a dead band of a process control loop device as:

b=avg(max Δ

_i)

if

|Δ

OUT(t−

1)*Δ

BKCAL_IN(t)|>

0

wherein;

Δ

_i(t)=|OUT(t−

i)−

BKCAL_IN(t)|,

Δ

OUT(t−

1)=OUT(t−

i)−

OUT(t−

i−

1),

Δ

BKCAL_IN(t)=BKCAL_IN(t)−

BKCAL_IN(t−

1),

OUT(t)=a process control signal to the process control loop device,

BKCAL_IN(t)=a back calculation signal from a process control loop device,

i=a back calculation signal delay from a process control loop device.

8. The method of claim 6, wherein generating on the computer a dead band of a process control loop device from evaluating on the computer a condition of a process control loop device within the process control loop comprises calculating on the computer a dead band of a process control loop device as:

9. The method of claim 1, wherein generating on the computer a diagnostic index from each of the evaluations comprises generating on the computer a hysteresis of a process control loop device from evaluating on the computer a condition of a process control loop device within the process control loop.

10. The method of claim 9, wherein generating on the computer a hysteresis of a process control loop device from evaluating on the computer a condition of a process control loop device within the process control loop comprises calculating on the computer a hysteresis of a process control loop device as:

h=b+δ

=avg(max Δ

_i)

wherein;

b=a dead band of the process control loop device,

δ

=a resolution of the process control loop device,

i=back calculation signal delay in process control loop module scans of the adaptive process control loop,

max Δ

1=a maximum value of a back calculation signal.

11. The method of claim 9, wherein generating on the computer a hysteresis of a process control loop device from evaluating on the computer a condition of a process control loop device within the process control loop comprises calculating on the computer a hysteresis of a process control loop device as:

h=2Ampl(OUT)

wherein Ampl(OUT)=the oscillation amplitude on the process control output signal.

12. The method of claim 1, wherein generating on the computer a diagnostic index from each of the evaluations comprises generating on the computer a resolution of a process control loop device from evaluating on the computer a condition of a process control loop device within the process control loop.

13. The method of claim 12, wherein generating on the computer a resolution of a process control loop device from evaluating on the computer a condition of a process control loop device within the process control loop comprises calculating on the computer the resolution of a process control loop device as:

δ

=h−

b

wherein;

b=a dead band of the process control loop device,

h=a hysteresis of the process control loop device.

14. The method of claim 12, wherein generating on the computer a resolution of a process control loop device from evaluating on the computer a condition of a process control loop device within the process control loop comprises calculating on the computer the resolution of a process control loop device as:

15. The method of claim 12, wherein generating on the computer a resolution of a process control loop device from evaluating on the computer a condition of a process control loop device within the process control loop comprises calculating on the computer a resolution of a process control loop device from an estimation of an oscillation period of the process control loop, wherein the estimation is expressed as:

16. The method of claim 1, wherein generating on the computer a diagnostic index from each of the evaluations comprises generating on the computer an adapted model quality index from evaluating on the computer a quality of the adapted model of the process control loop.

17. The method of claim 16, wherein generating on the computer an adapted model quality index from evaluating on the computer a quality of the adapted model of the process control loop comprises calculating on the computer a squared error of the adapted model as:

18. The method of claim 1, wherein generating on the computer a diagnostic index from each of the evaluations comprises generating on the computer an oscillation index from evaluating on the computer a stability of the process control loop.

19. The method of claim 1, further comprising setting on the computer an alarm based on evaluating on the computer a condition of the process control loop from one or more of the diagnostic indices.

20. The method of claim 1, further comprising automatically adjusting on the computer an adapted model of the adaptive process control loop based on evaluating on the computer a condition of the process control loop from one or more of the diagnostic indices.

21. The method of claim 1, wherein automatically adjusting on the computer a process variable filter comprises calculating a process variable filter time constant as:

22. The method of claim 21, wherein automatically adjusting on the computer a process variable filter further comprises applying effective process variable filter values as:

τ

_f=0.8τ

_f old+0.2τ

_f new.