Systems and methods for limiting integral calculation components in PID controllers

• US 6,424,873 B1
• Filed: 12/30/1999
• Issued: 07/23/2002
• Est. Priority Date: 12/30/1999
• Status: Expired due to Term

• Current Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Original Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Inventors: Przybylski, Frank J.
• Primary Examiner(s): Black, Thomas
• Assistant Examiner(s): BARNES-BULLOCK, CRYSTAL JOY
• Application Number:
  US09/475,326
• Time in Patent Office: 936 Days
• Field of Search: 700/42, 700/41, 700/43, 700/8, 700/33, 318/610, 318/609
• US Class Current: 700/42
• 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 5/01 electric

Abstract

A control apparatus is disclosed that comprises a primary proportional, integral, differential (“PID”) controller capable of receiving a first setpoint and a first process variable and generating therefrom a second setpoint; and a secondary controller capable of receiving the second setpoint and a second process variable and generating therefrom an output control signal, wherein the primary PID controller is capable of receiving from the secondary controller a feedback signal 1) that indicates that a previous value of the second setpoint exceeds a limit associated with an output control signal of the secondary controller, and 2) that transfers a value of a signal from the secondary controller. The primary PID controller is then capable of limiting the contribution of the integral calculation component in a PID calculation that generates a new current value of the second setpoint. The integral calculation component may be excluded, included, or partially included in the PID calculation in order efficiently minimize the effect of undesirable erratic output signals.

Claims

1. A control apparatus for controlling a process, said control apparatus comprising:

a primary proportional, integral, differential (PID) controller capable of receiving a first setpoint and a first process variable and generating therefrom a second setpoint; and

a secondary controller capable of receiving said second setpoint and a second process variable and generating therefrom an output control signal, wherein said primary PID controller is capable of receiving from said secondary controller a feedback signal that indicates that a previous value of said second setpoint exceeds a limit associated with said output control signal and that transfers a value of a signal from said secondary controller and, in response to said feedback signal indication, said primary PID controller one of;

excludes an integral calculation component of a PID calculation performed by said primary PID controller in response to a determination that the current sum of a proportional calculation component and a derivative calculation component of said PID calculation exceeds said previous value of said second setpoint; and

includes a portion of said integral calculation component in response to a determination that the inclusion of said integral calculation component in said current PID calculation would otherwise cause said current value of said second setpoint to exceed said previous value of said second setpoint.

2. The control apparatus as set forth in claim 1 wherein said limit associated with said output control signal is an upper limit.

3. The control apparatus as set forth in claim 2 wherein said primary PID controller one of:

includes said integral calculation component of said PID calculation performed by said primary PID controller in response to a determination that the inclusion of said integral calculation component in said current PID calculation would not cause the current second setpoint of primary PID controller to exceed said previous value of said second setpoint; and

includes said integral calculation component of said PID calculation performed by said primary PID controller in response to a determination that a high said primary PID controller integral limit flag has not been set.

4. The control apparatus as set forth in claim 1 wherein said limit associated with said output control signal comprises an upper limit and a lower limit.

5. The control apparatus as set forth in claim 1 wherein said limit associated with said output control signal is a lower limit.

6. The control apparatus as set forth in claim 3 wherein said primary PID controller one of:

includes said integral calculation component of said PID calculation performed by said primary PID controller in response to a determination that the inclusion of said integral calculation component in said current PID calculation would not cause the current second setpoint of primary PID controller to be less than said previous value of said second setpoint; and

includes said integral calculation component of said PID calculation performed by said primary PID controller in response to a determination that a low integral limit flag has not been set.

7. A control apparatus as set forth in claim 1 wherein said limit associated with said output control signal is a variable integral limit, and wherein in response to said feedback signal indication, said primary PID controller one of:

excludes an integral calculation component of a PID calculation performed by said primary PID controller in response to a determination that the current sum of a proportional calculation component and a derivative calculation component of said PID calculation exceeds said previous value of said variable integral limit; and

includes a portion of said integral calculation component in response to a determination that the inclusion of said integral calculation component in said current PID calculation would otherwise cause said current value of said second setpoint to exceed said previous value of said second setpoint.

8. The control apparatus as set forth in claim 7 wherein said primary PID controller one of:

includes said integral calculation component of said PID calculation performed by said primary PID controller in response to a determination that the inclusion of said integral calculation component in said current PID calculation would not cause the current second setpoint of primary PID controller to exceed said previous value of said second setpoint when a low integral limit flag has been set;

includes said integral calculation component of said PID calculation performed by said primary PID controller in response to a determination that the inclusion of said integral calculation component in said current PID calculation would not cause the current second setpoint of primary PID controller to be less than said previous value of said second setpoint when a high integral limit flag has been set; and

includes said integral calculation component of said PID calculation performed by said primary PID controller in response to a determination that an integral limit flag has been set to a not limited status.

9. A process control system comprising at least one process controller, wherein said at least one process controller comprises:

a primary proportional, integral, differential (PID) controller capable of receiving a first setpoint and a first process variable and generating therefrom a second setpoint; and

a secondary controller capable of receiving said second setpoint and a second process variable and generating therefrom an output control signal, wherein said primary PID controller is capable of receiving from said secondary controller a feedback signal that indicates that a previous value of said second setpoint exceeds a limit associated with said output control signal and that transfers a value of a signal from said secondary controller and, in response to said feedback signal indication, said primary PID controller one of;

excludes an integral calculation component of a PID calculation performed by said primary PID controller in response to a determination that the current sum of a proportional calculation component and a derivative calculation component of said PID calculation exceeds said previous value of said second setpoint; and

includes a portion of said integral calculation component in response to a determination that the inclusion of said integral calculation component in said current PID calculation would otherwise cause said current value of said second setpoint to exceed said previous value of said second setpoint.

10. The process control system as set forth in claim 9 wherein said limit associated with said output control signal is an upper limit.

11. The process control system as set forth in claim 10 wherein said primary PID controller one of:

includes said integral calculation component of said PID calculation performed by said primary PID controller in response to a determination that the inclusion of said integral calculation component in said current PID calculation would not cause the current second setpoint of primary PID controller to exceed said previous value of said second setpoint; and

includes said integral calculation component of said PID calculation performed by said primary PID controller in response to a determination that a high integral limit flag has not been set.

12. The process control system as set forth in claim 9 wherein said limit associated with said output control signal comprises an upper limit and a lower limit.

13. The process control system as set forth in claim 9 wherein said limit associated with said output control signal is a lower limit.

14. The process control system as set forth in claim 13, wherein said primary PID controller one of:

includes said integral calculation component of said PID calculation performed by said primary PID controller in response to a determination that the inclusion of said integral calculation component in said current PID calculation would not cause the current second setpoint of primary PID controller to be less than said previous value of said second setpoint; and

includes said integral calculation component of said PID calculation performed by said primary PID controller in response to a determination that a low integral limit flag has not been set.

15. The process control system as set forth in claim 9 wherein said limit associated with said output control signal is a variable integral limit, and wherein in response to said feedback signal indication, said primary PID controller one of:

excludes an integral calculation component of a PID calculation performed by said primary PID controller in response to a determination that the current sum of a proportional calculation component and a derivative calculation component of said PID calculation exceeds said previous value of said variable integral limit; and

includes a portion of said integral calculation component in response to a determination that the inclusion of said integral calculation component in said current PID calculation would otherwise cause said current value of said second setpoint to exceed said previous value of said second setpoint.

16. The process control system as set forth in claim 15 wherein said primary PID controller one of:

includes said integral calculation component of said PID calculation performed by said primary PID controller in response to a determination that the inclusion of said integral calculation component in said current PID calculation would not cause the current second setpoint of primary PID controller to exceed said previous value of said second setpoint when a low integral limit flag has been set;

includes said integral calculation component of said PID calculation performed by said primary PID controller in response to a determination that the inclusion of said integral calculation component in said current PID calculation would not cause the current second setpoint of primary PID controller to be less than said previous value of said second setpoint when a high integral limit flag has been set; and

includes said integral calculation component of said PID calculation performed by said primary PID controller in response to a determination that an integral limit flag has been set to not limited.

17. In a control apparatus of the type comprising a primary proportional, integral, differential (PID) controller capable of receiving a first setpoint and a first process variable and generating therefrom a second setpoint, and a secondary controller capable of receiving said second setpoint and a second process variable and generating therefrom an output control signal,

wherein said primary PID controller is capable of receiving from said secondary controller a feedback signal that indicates that a previous value of said second setpoint exceeds a limit associated with said output control signal and that transfers a value of a signal from said secondary controller,

wherein said primary PID controller performs a PID calculation to determine the second setpoint that said primary PID controller sends to said secondary controller, and

wherein said PID calculation includes a proportional calculation component and an integral calculation component and a differential calculation component,

a method for limiting the contribution of said integral calculation component to said PID calculation when said secondary controller has determined that a previous second setpoint of said primary PID controller has exceeded a limit associated with said output control signal of said secondary controller, said method comprising the steps of:

excluding said integral calculation component of said PID calculation in response to a determination that the current sum of said proportional calculation component and said derivative calculation component of said PID calculation exceeds said previous value of said second setpoint; and

including a portion of said integral calculation component in response to a determination that the presence of said unmodified integral calculation component in said current PID calculation would cause said current value of said second setpoint to exceed said previous value of said second setpoint.

18. The method as set forth in claim 17 further comprising the steps of:

determining whether the inclusion of said integral calculation component in said PID calculation will cause the current value of said second setpoint to exceed said previous value of said second setpoint;

including said integral calculation component in said current PID calculation in response to a determination that the inclusion of said integral calculation component in said PID calculation will not cause the current value of said second setpoint to exceed said previous value of said second setpoint;

determining whether the sum of the proportional calculation component and the derivative calculation component is less than said previous second setpoint of said primary PID controller in response to a determination that the inclusion of said integral calculation component in said PID calculation will cause the current value of said second setpoint to exceed said previous value of said second setpoint; and

excluding said integral calculation component from said PID calculation in response to a determination that the current sum of said proportional calculation component and said derivative calculation component of said PID calculation is not less than said previous value of said second setpoint of said primary PID controller.

19. The method as set forth in claim 18 further comprising the steps of:

determining whether including said integral calculation component in said current PID calculation will cause the current second setpoint of said primary PID controller to exceed said previous value of said second setpoint of said primary PID controller in response to a determination that the current sum of said proportional calculation component and said derivative calculation component of said PID calculation is less than said previous value of said second setpoint of said primary PID controller;

including said integral calculation component in said current PID calculation in response to a determination the presence of said integral calculation component in said current PID calculation will not cause the current second setpoint of said primary PID controller to exceed said previous second setpoint of said primary PID controller; and

partially limiting the contribution of said integral calculation component to said current PID calculation in response to a determination that the inclusion of said integral calculation component in said current PID calculation will cause the current second setpoint of said primary PID controller to exceed said previous value of said second setpoint of said primary PID controller.

20. The method as set forth in claim 19 wherein the step of partially limiting the contribution of said integral calculation component to said current PID calculation comprises:

subtracting from said PID calculation any portion of said integral calculation that causes said current second setpoint of said primary PID controller to exceed said previous second setpoint of said primary PID controller.