Non-periodic control communications in wireless and other process control systems

• US 7,587,252 B2
• Filed: 10/25/2005
• Issued: 09/08/2009
• 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: Blevins, Terrence L., McMillan, Gregory K., Nixon, Mark J., Chen, Deji
• Primary Examiner(s): Jarrett; Ryan A
• Application Number:
  US11/258,676
• Publication Number:
  US 20070093918A1
• Time in Patent Office: 1,414 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 2219/33192 Radio link, wireless
• CPC Class Codes: G05B 2219/34306 Power down, energy saving
• CPC Class Codes: Y02P 70/10 Greenhouse gas [GHG] capture, material saving, heat recovery or other energy efficient measures, e.g. motor control, characterised by manufacturing processes, e.g. for rolling metal or metal working

Abstract

Disclosed is a controller having a processor and a control module adapted for periodic execution by the processor and configured to be responsive to a process variable to generate a control signal for a process. An iteration of the periodic execution of the control module involves implementation of a routine configured to generate a representation of a process response to the control signal. The routine is further configured to maintain the representation over multiple iterations of the periodic execution of the control module and until an update of the process variable is available. In some cases, the update of the process variable is made available via wireless transmission of the process signal. In those and other cases, the controller may be included within a process control system having a field device to transmit the process signal indicative of the process variable non-periodically based on whether the process variable has changed by more than a predetermined threshold. In some embodiments, the field device also transmits the process signal if a refresh time has been exceeded since a last transmission.

Claims

1. A controller for use in controlling a process based on a process signal indicative of a value of a process variable of the process, the controller comprising:

a processor; and

,

a control module for periodic execution by the processor responsive to the process signal to generate a control signal for controlling the process, wherein the process signal includes non-periodic updates of the value of the process variable;

wherein the control module includes a routine that, during an iteration of the periodic execution of the control module, executes to determine an elapsed time between a previous update of the value of the process variable and a new update of the value of the process variable if a new update value of the process variable has been received, and to generate a representation of a process response to the control signal using the control signal and the elapsed time between the previous update of the value of the process variable and the new update of the value of the process variable, wherein the control module generates the control signal using the representation of the process response, and wherein the routine is further configured to maintain the representation of the process response over one or more iterations of the periodic execution of the control module at which a new update of the value of the process variable has not been received.

2. The controller of claim 1, wherein the updates of the value of the process variable are made available via wireless transmission of the process signal.

3. The controller of claim 1, wherein the control module comprises a closed-loop control scheme that utilizes the process response representation to determine the control signal.

4. The controller of claim 1, wherein the routine comprises a positive feedback network to determine the process response representation based on past values of the control signal.

5. The controller of claim 1, wherein the routine implements a filter algorithm to determine the process response representation.

6. The controller of claim 1, wherein the routine is further configured for implementation based on non-periodic updates of the process variable.

7. The controller of claim 1, wherein the process response representation comprises a process variable response representation, and wherein the routine is configured to update the process variable response representation the first execution time that the new update of the value of the process variable is available.

8. The controller of claim 7, wherein the routine determines an expected response of the process based on a previous expected response of the process, the control signal, and the elapsed time between the previous update of the value of the process variable and the new update of the value of the process variable.

9. The controller of claim 8, wherein the routine determines an updated reset contribution based on the elapsed time between the previous update of the value of the process variable and the new update of the value of the process variable.

10. The controller of claim 9, wherein the expected process response comprises a model that includes process or measurement delay.

11. A method of controlling a process having a process variable, the method comprising:

generating a control signal for the process based on the process variable; and

detecting whether a new update of the process variable is available and if a new update of the process variable is available, determining an elapsed time period since the last update of the process variable;

wherein generating a control signal includes generating a representation of a process response to the control signal using the control signal and the elapsed time period, wherein the control signal is based on the new update of the process variable and the representation of the process response, and maintaining the process response representation over one or more iterations of generating the control signal until a further update of the process variable is detected as being available.

12. The method of claim 11, further comprising receiving a wireless transmission of a process signal indicative of the new update of the process variable.

13. The method of claim 11, wherein generating the control signal further comprises executing a closed-loop control scheme that utilizes the process response representation to determine the control signal.

14. The method of claim 11, wherein the process response representation comprises a process variable response representation, and wherein generating the control signal further comprises updating the process variable response representation the first time that the new update of the process variable is available.

15. The method of claim 14, wherein generating the control signal further comprises determining an expected response of the process based on the control signal, and the elapsed time period.

16. The method of claim 15, wherein generating the control signal further comprises determining an updated reset contribution based on the elapsed time period.

17. A process control system for use in controlling a process based on a receipt of non-periodic updates of a value of a process variable 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 non-periodic updates of the value of the process variable to generate a control signal for controlling the process, the control module routine including;

a first routine that executes to determine an elapsed time between a last update of the value of the process variable and a newly received update of the value of the process variable in response to the receipt of the newly received update of the value of the process variable,

a second routine that executes to generate a feedback control component using a previous control signal and the elapsed time between the last update of the value of the process variable and the newly received update value of the process variable, and

a third routine that executes to combine the feedback control component to an error component determined from a setpoint value for the process variable and the newly received update value of the process variable to generate the control signal.

18. The process control system of claim 17, wherein the second routine executes to hold the feedback control component constant when a new update value of the process variable has not been received since a last execution cycle of the control module routine.

19. The process control system of claim 17, wherein the second routine executes to determine the feedback control component as an integral control component.

20. The process control system of claim 17, wherein the second routine executes to determine the feedback control component as a function of a ratio of the elapsed time between the last update of the value of the process variable and the newly received update value of the process variable and a reset time.