Process control with unreliable communications

• US 7,620,460 B2
• Filed: 08/04/2006
• Issued: 11/17/2009
• Est. Priority Date: 10/25/2005
• Status: Active Grant

• Current Assignee: Fisher-Rosemount Systems Incorporated (Emerson Electric Company)
• Original Assignee: Fisher-Rosemount Systems Incorporated (Emerson Electric Company)
• Inventors: Chen, Deji, Blevins, Terrence L., McMillan, Gregory K., Nixon, Mark J.
• Primary Examiner(s): Jarrett; Ryan A
• Application Number:
  US11/499,013
• Publication Number:
  US 20070112905A1
• Time in Patent Office: 1,201 Days
• Field of Search: 700 29- 31, 700 40- 45
• US Class Current: 700/29
• CPC Class Codes: G05B 11/42 for obtaining a characteristic which is both proportional and time-dependent, e.g. P. I., P. I. D.
• CPC Class Codes: G05B 2219/31121 Fielddevice, field controller, interface connected to fieldbus
• CPC Class Codes: G05B 2219/34413 Add time stamp to command message

Abstract

Disclosed are methods and devices for controlling a process with a control signal. Iterations of a control routine are implemented to generate the control signal, and when an indication of a response to the control signal is unavailable, a feedback contribution to the control signal is maintained over one or more of the iterations of the control routine. The feedback contribution is then modified upon receiving the response indication, in which the feedback contribution is determined in accordance with an elapsed time between the received response indication and a previous communication of the response indication.

Claims

1. A method of controlling a process with a control signal, the method comprising:

implementing iterations of a control routine to generate the control signal;

when an indication of a response to the control signal is unavailable, maintaining a feedback contribution to the control signal over one or more of the iterations of the control routine; and

modifying the feedback contribution upon receiving the response indication;

wherein modifying the feedback contribution comprises determining an elapsed time between a new update of the response indication and a previous update of the response indication if a new update of the response indication has been received and modifying the feedback contribution using the elapsed time.

2. The method of claim 1, wherein the feedback contribution is a reset contribution.

3. The method of claim 1, wherein the feedback contribution is a derivative contribution.

4. The method of claim 1, wherein modifying the feedback contribution further comprises determining the elapsed time based on a difference between times of receipt for the new update of the response indication and the previous update of the response indication.

5. The method of claim 1, wherein modifying the feedback contribution further comprises determining the elapsed time based on time stamps associated with the new update of the response indication and the previous update of the response indication.

6. The method of claim 1, wherein the response indication is a measurement of a process parameter controlled by the control routine.

7. The method of claim 6, wherein the process parameter is a process variable controlled by a field device responsive to the control signal.

8. The method of claim 1, wherein:

the control routine is a proportional-integral-derivative (PID) control routine for controlling a process variable in accordance with a set point;

the feedback contribution is generated by one of the integral and derivative components of the PID control routine; and

,

the response indication comprises a measurement of either the process variable or a process parameter responsive to the control signal to control the process variable.

9. The method of claim 1, wherein the control routine implements a model predictive control (MPC) scheme such that the feedback contribution is based on a process model response.

10. The method of claim 1, further comprising monitoring a new value flag to determine when the indication of the response to the control signal is unavailable.

11. The method of claim 10, wherein maintaining the feedback contribution comprises calculating a reset contribution based on the new value flag.

12. The method of claim 11, wherein calculating the reset contribution comprises utilizing feedback from a downstream element to determine the reset contribution.

13. The method of claim 12, wherein calculating the reset contribution further comprises maintaining the reset contribution if a changed value of the feedback from the downstream element is not received when expected.

14. The method of claim 1, further comprising receiving the response indication via a wireless transmission.

15. The method of claim 1, further comprising transmitting the response indication to facilitate modifying the feedback contribution once the response indication is available.

16. The method of claim 1, wherein the iterations of the control routine are event-triggered.

17. The method of claim 1, wherein the iterations are periodic.

18. A device for controlling a process with a control signal, the device comprising:

a processor;

a communication interface to receive an indication of a response to the control signal; and

,

a computer-readable medium in which a control routine is stored for execution by the processor to generate the control signal;

wherein the control routine is configured to;

determine a contribution to the control signal based on the response indication;

maintain the control signal contribution over one or more iterations of the control routine when the response indication is not received by the communication interface;

determine an elapsed time between a new update of the response indication and a previous update of the response indication if a new update of the response indication has been received; and

update the control signal contribution with a feedback contribution using the elapsed time.

19. The device of claim 18, wherein the communications interface is configured to receive the response indication via a wireless transmission.

20. The device of claim 18, wherein the feedback contribution is a reset contribution.

21. The device of claim 18, wherein the feedback contribution is a derivative contribution.

22. The device of claim 18, wherein the control routine is configured to determine the elapsed time based on time stamps associated with the new update of the response indication and the previous update of the response indication.

23. The device of claim 18, wherein the control routine is configured to implement a model predictive control (MPC) scheme such that the feedback contribution is based on a process model response.

24. The device of claim 18, wherein the control routine is configured to monitor a new value flag to determine when the indication of the response to the control signal is unavailable.

25. The device of claim 24, wherein the control routine is configured to determine a reset contribution based on the new value flag and feedback from a downstream element.

26. A process control system for use in controlling a process based on a receipt of non-periodic updates of an indication of a response to a control signal of the process, the process control system comprising:

a computer readable medium; and

a control module routine stored on the computer readable medium to be executed by a processor, the control module routine when executed being responsive to the updates of the response indication to generate a control signal for controlling the process, the control module routine including;

a first routine that executes to determine a contribution to the control signal based on the updates of the response indication;

a second routine that executes to maintain the control signal contribution over one or more iterations of the control module routine when the response indication is not received;

a third routine that executes to determine an elapsed time between a new update of the response indication and a previous update of the response indication if a new update of the response indication has been received; and

a fourth routine that executes to update the control signal contribution with a feedback contribution using the elapsed time.