DECOUPLED FEEDFORWARD-FEEDBACK CONTROL SYSTEM

• US 3,758,762 A
• Filed: 07/10/1972
• Issued: 09/11/1973
• Est. Priority Date: 07/10/1972
• Status: Expired due to Term

• Current Assignee: Leeds & Northrup Co.
• Original Assignee: Bernard Littman, Charles W. Ross
• Inventors: Ross, Charles W., Littman, Bernard
• Primary Examiner(s): Botz, Eugene G.
• Application Number:
  US05/270,419
• Time in Patent Office: 428 Days
• Field of Search: 235/150.1
• US Class Current: 700/45
• CPC Class Codes: F01K 13/02 Controlling, e.g. stopping or starting
• CPC Class Codes: F23N 1/082 using electronic means
• CPC Class Codes: F23N 2223/34 with feedforward processing
• CPC Class Codes: G05B 13/042 in which a parameter or coefficient is automatically adjusted to optimise the performance
• CPC Class Codes: G05B 17/02 electric

Abstract

A control system combining feedforward and feedback control of a manipulated variable of a process is arranged to provide a modification of the error signal to which the feedback control responds so as to effect a decoupling of the feedforward and feedback control actions. The error signal is diminished by a signal of magnitude sufficient to minimize or reduce by a predetermined amount the response by the feedback control to action by the feedforward control as reflected in changes in the controlled variable. Provision is also made to avoid unnecessary feedback control in response to set point changes when feedforward action is provided from the set point changes.

Claims

1. In a process control system having means for providing both feedback and feedforward control action to minimize the deviation of a controlled variable from its set point, the improvement which comprises:

means for modifying the effective error signal to which the feedback control action responds so as to cancel those changes of the effective error signal which will be corrected by the anticipated effect of the feedforward control action.

2. In a process control system having means for providing both feedback and feedforward control action to minimize the deviation of a controlled variable from its set point, the improvement whih comprises:

means for modifying the effective error signal to which the feedback control action responds so that the modification is effective to cancel that part of the effective error signal which will be corrected by the anticipated effect on the process of the feedforward control action.

3. In a process control system having means for providing both feedback and feedforward control action to minimize the deviation of a controlled variable from its set point, the improvement which comprises:

means for modifying the effective error signal to which the feedback control action responds so as to prevent a change in the effective error signal when the controlled variable changes due to feedforward control action.

4. In a process control system having means for providing both feedback and feedforward control action to minimize the deviation of a controlled variable from its set point, the improvement which comprises:

means for modifying the effective error signal to which the feedback control action resPonds so as to prevent a change in the effective error signal in response to the feedforward portion of that signal to which the feedforward control action responds.

5. In a process control system having means for providing both feedback and feedforward control action to minimize the deviation of a controlled variable from its set point, the improvement which comprises:

means for modifying the effective error signal to which the feedback control action responds so as to change the effective error signal by an amount substantially equal and opposite to the change in the effective error signal anticipated as a result of the effect on the process of the feedforward control action.

6. In a process control system having means for providing both feedback and feedforward control action to minimize the deviation of a controtled variable from its set point, the improvement which comprises:

means for modifying the effective error signal to which the feedback control action responds so as to minimize the chance in the effective error signal in response to a change in set point, when that change in set point initiates feedforward control action.

7. In a process control system having means for providing both feedback and feedforward control action to minimize the deviation of a controlled variable from its set point, the improvement which comprises:

means for modifying the effective error signal to which the feedback control action responds so as to minimize the change in the effective error signal resulting from a change in the controlled variable due to feedforward control action from a change in process demand.

8. A process control system for modifying a manipulated variable of the process so as to reduce the deviation of the controlled variable from its set point comprising:

feedback control means responsive to the difference between the controlled variable and the set point to modify the manipulated variable until said controlled variable corresponds with said set point, feedforward control means responsive to changes in the value of a process quantity indicative of a change in the process demand and operable to modify the manipulated variable as a function of the magnitude of the change of said quantity, said modification being in direction to minimize the deviation of the controlled variable from the set point, and model means representing said process and operative to respond to said feedforward control of said manipulated variable to modify said difference between the controlled variable and the set point so as to modify the effective error signal to which said feedback control means is responsive to cancel that change in the effective error signal which will be corrected by the anticipated effect on the controlled variable of the action of the feedforward control means.

9. A process control system as set forth in claim 8 in which the function is proportional to the magnitude of the change.

10. A process control system as set forth in claim 8 in which the function is the sum of a proportional and a rate response to said magnitude of the change in the process quantity.

11. A boiler-turbine control system for modifying the firing rate to the boiler so as to reduce the deviation of the throttle pressure from its set point comprising:

feedback control means responsive to the difference between the measured throttle pressure and its set point to modify the firing rate until said throttle pressure corresponds with said set point, feedforward control means responsive to changes in the value of a demand quantity and operable to modify the boiler firing rate as a function of the value of said demand quantity, said modification being in direction to minimize the deviation of the controlled variable from the set point, and model means representing at least a part of said process and operative to respond to said feedforward control of said firing rate to modify said difference between the throttle pressure and its set point sO as to modify the effective error signal to which said feedback control means is responsive so as to cancel that change in the effective error signal expected due to the effect on the throttle pressure of the action of the feedforward control means.

12. A boiler-turbine control system for modifying the firing rate to the boiler so as to reduce the deviation of the throttle pressure from its set point while changing the set point to maintain the desired output from a generator driven by the turbine comprising:

feedback control means responsive to the difference between the measured throttle pressure and its set point to modify the firing rate until said throttle pressure corresponds with said set point, feedforward control means responsive to changes in the value of a demand quantity and operable to modify the boiler firing rate in proportion to the rate of change of said set point in anticipation of the expected change in the value of the measured throttle pressure due to the change in said firing rate, said modification being in direction to minimize the deviation of the controlled variable from the set point, and model means representing said process and operative to respond to said feedforward control of said firing rate to modify said difference between the throttle pressure and its set point so as to modify the effective error signal to which said feedback control means is responsive so as to cancel that change in the effective error signal expected due to the effect on the throttle pressure of the action of the feedforward control means.

13. A process control system comprising:

a feedback controller operable to modify a manipulated variable of said process in response to an effective error signal to reduce said effective error signal to zero, a feedforward controller operable to modify said manipulated variable in response to change in a demand signal, and process model means responsive to the modification of the manipulated variable due to action of said feedforward controller for producing a signal for modifying said effective error signal so as to reduce the effect on said effective error signal of changes in the controlled variable resulting from action of the feedforward controller in response to changes in said demand signal.

14. A process control system comprising:

a feedback controller operable to modify a manipulated variable of said process in response to an effective error signal to reduce said effective error signal to zero, a feedforward controller operable to modify said manipulated variable in response to the change in a set point signal representative of the desired value for a controller variable, process model means responsive to the modification of the manipulated variable due to action of said feedforward controller for producing a first signal for modifying said effective error signal so as to reduce the effect on said effective error signal of changes in the controlled variable resulting from action of the feedforward controller in response to changes in said set point, and means for producing a second signal for modifying said effective error signal in accordance with said set point signal as modified by a factor representative of the proportional gain of the feedforward controller so as to reduce the effect on said effective error signal of changes in said set point.

15. The method of controlling a variable of a process to a set point comprising the steps of:

automatically modifying by feedback control action a manipulated variable of the process in direction and extent to reduce the effective error between the controlled variable and the set point to zero, automatically modifying by feedforward control action the manipulated variable of the process in accordance with the magnitude of the change of a process condition, and automatically modifying said effective error signal so as to prevent a change in the effective error signal due to feedforward control action.

16. The method of claim 15 in which the process condition is the set point.

17. The method of claim 15 in which the process condition is a demand signal.

18. The method of controlling a variable of a process to a set point comprising the steps of:

automatically modifying by feedback control action a manipulated variable of the process in direction and extent to reduce the effective error between the controlled variable and the set point to zero, automatically modifying by feedforward control action the manipulated variable of the process in accordance with a function of the change of a process condition, and automatically modifying said effective error signal so as to prevent in the effective error signal when the controlled variable changes due to feedforward control action.

19. The method of claim 18 in which the function is proportional to the magnitude of the change in the process condition.

20. The method of claim 18 in which the function is the sum of a proportional and a rate response to said magnitude of the change in the process condition.

21. The method of controlling a variable of a process to a set point comprising the steps of:

automatically modifying by feedback control action a manipulated variable of the process in direction and extent to reduce the effective error between the controlled variable and the set point to zero, automatically modifying by feedforward control action the manipulated variable of the process in accordance with a function of the change of a process condition, and automatically modifying said effective error signal so as to cancel a predetermined part of the change in the effective error signal in response to a change in the controlled variable due to feedforward control action.