Model predictive controller

• US 5,740,033 A
• Filed: 10/13/1992
• Issued: 04/14/1998
• Est. Priority Date: 10/13/1992
• Status: Expired due to Term

• Current Assignee: The Dow Chemical Company (Dow, Inc.)
• Original Assignee: The Dow Chemical Company (Dow, Inc.)
• Inventors: McCroskey, Patrick S., Steckler, David K., Wassick, John M., McDonough, John J.
• Primary Examiner(s): Elmore, Reba I.
• Assistant Examiner(s): OAKES, BRIAN
• Application Number:
  US07/959,629
• Time in Patent Office: 2,009 Days
• Field of Search: 364/149, 364/150, 364/151, 364/152, 364/153-165, 364/148
• US Class Current: 700/29
• CPC Class Codes: G05B 13/048 using a predictor

Abstract

A model predictive controller for a process control system which includes a real-time executive sequencer and an interactive modeler. The interactive modeler includes both a process model and an independent disturbance model. The process model represents the dynamic behavior of the physical process, while the disturbance model represents current and future deviations from the process model. The interactive modeler estimates current process states from the process model and input data received from the executive sequencer. The executive sequencer then projects a set of future process parameter values, which are sought to be controlled, over a predetermined control horizon. The interactive modeler then solves a set of equations as to how the physical process will react to control changes in order to determine an optimized set of control changes. As a result, the process control system will be able to accurately track a predetermined set-point profile in the most effective and cost efficient manner.

Claims

1. In a process control system having input means for receiving signals indicative of measured process parameters, and output means for transmitting command signals which include at least one manipulated parameter value that is used to govern physical process equipment, a model predictive controller, comprising:

interactive modeling means for estimating the current state of at least one unmeasured process variable and for determining the value of at least one said manipulated parameter value for said process equipment at selected intervals, said interactive modeling means having both a process model which represents the dynamic behavior of the physical process equipment being controlled and an independent disturbance model which represents current deviations from said process model and future deviations over a predetermined prediction horizon, said process model including at least one nonlinear function which is specifically related to the physical process being controlled;

real-time executive means for directing the operation of said interactive modeling means, said real-time executive means having means for periodically transferring said signals indicative of measured process parameters to said interactive modeling means and means for transferring said manipulated parameter value from said interactive modeling means to said output means; and

means for testing the validity of said signals which are indicative of measured process parameters, the current estimated state of said unmeasured process variable, and the value of said manipulated parameter value.

2. The invention according to claim 1, wherein said real-time executive means includes means for projecting a desired set of future manipulated parameter values over said prediction horizon for transmittal to said interactive modeling means.

3. The invention according to claim 2, wherein said interactive modeling means includes means for solving a set of first order differential equations to derive how the physical process equipment will react to proposed changes to the value of said manipulated parameter value, and means for determining an optimized set of current and future manipulated parameter values.

4. The invention according to claim 3, wherein said set of future manipulated parameter values includes at least one set-point which enables said process control system to track a predetermined set-point profile.

5. The invention according to claim 3, wherein said real-time executive means causes said interactive modeling means to determine said optimized set of manipulated parameter values at a first interval rate and said real-time executive means further includes sequencing means for causing said interactive modeling means to estimate the current state of said measured process variable at a second interval rate which is greater than the first interval rate at which said real-time executive means causes said interactive modeling means to determine said optimized set of manipulated parameter values.

6. The invention according to claim 2, wherein said real-time executive means includes a real-time database of physical data received from said signals indicative of measured process parameters.

7. The invention according to claim 6, wherein the records stored in said real-time database are stored in the form of a set of linked lists.

8. The invention according to claim 7, wherein said real-time database includes a list of set-point corners for projecting future set-points, with each of said set-point corners including a floating point value and a step in time where said floating point value applies.

9. The invention according to claim 1, wherein said disturbance model has variable predictor means for estimating said future deviations as a function of both past and current deviations from said process model.

10. The invention according to claim 9, wherein said disturbance model is a step response variable disturbance model for predicting future disturbances which is independent of said process model.

11. In a process control system having input means for receiving signals indicative of measured process parameters, processor means for making process control decisions, and output means for transmitting command signals from said processor means which include at least one manipulated parameter value that is used to govern physical process equipment, a model predictive controller, comprising:

interactive modeling means for estimating the current state of at least one unmeasured process variable and for determining the value of at least one manipulated parameter value, said interactive modeling means having both a process model which represents the dynamic behavior of the physical process equipment being controlled and a disturbance model which represents current deviations from said process model and future deviations over a prediction horizon, said disturbance model having variable predictor means for estimating said future deviations as a function of both past and current deviations from said process model; and

real-time executive means for coordinating the operations of said interactive modeling means with said processor means and for projecting a set of future controlled parameter values over said prediction horizon in cooperation with said interactive modeling means and said processor means;

wherein said real-time executive means includes a time-recorded database of measured data received from said processor means, and data interface means for periodically transmitting said measured data to said interactive modeling means with data identifiers employed by said interactive modeling means, said data identifiers of said interactive modeling means being different than the data identifiers used by said processor means.

12. The invention according to claim 11, wherein said interactive modeling means includes means for periodically determining an optimized set of current and future manipulated parameter values.

13. The invention according to claim 12, wherein said real-time executive means causes said interactive modeling means to determine said optimized set of manipulated parameter values at a first interval rate and said real-time executive means includes sequencing means for causing said interactive modeling means to estimate the current state of said unmeasured parameter at a second interval rate which is greater than the first interval rate at which said real-time executive means causes said interactive modeling means to determine said optimized set of manipulated parameter values.

14. A process control system, comprising:

input means for receiving signals indicative of measured process parameters;

output means for transmitting command signals which include at least one manipulated parameter value that is used to govern physical process equipment;

processor means for making independent process control decisions which determine manipulated parameter values in response to said input signals;

interactive modeling means for periodically estimating the current state of at least one unmeasured process variable and for determining the proposed value of at least one manipulated parameter, said interactive modeling means having both a process model which represents the dynamic behavior of the physical process being controlled and a disturbance model which represents current deviations from said process model and future deviations over a prediction horizon; and

real-time executive means for coordinating the operation of said interactive modeling means with said processor means and for testing the validity of said signals indicative of measured process parameters, the current state of each said unmeasured process variable, and each said manipulated parameter value proposed by said interactive modeling means;

said processor means having means for determining whether to implement at least one manipulated parameter value that has been validated by said executive means.

15. The invention according to claim 14, wherein said disturbance model has variable predictor means for estimating said future deviations as a function of both past and current deviations from said process model.

16. The invention according to claim 15, wherein said executive means includes means for projecting a set of future controlled parameter values over said prediction horizon in cooperation with said interactive modeling means and said processor means.

17. The invention according to claim 16, wherein said interactive modeling means includes means for periodically determining a set of current and future manipulated parameter values which will minimize deviations from said set of desired controlled parameter values.

18. The invention according to claim 15, wherein said disturbance model is a step response variable disturbance model for predicting future disturbances which is independent of said process model.

19. A method of controlling a process, comprising the steps of:

receiving input signals which represent measured parameter values from a plurality of sensors;

periodically transmitting a time-recorded set of said measured parameter values to an interactive modeler which includes a nonlinear process model, which is derived from physical relationships for the process, and an independent disturbance model;

estimating the current state of at least one unmeasured process parameter in said interactive modeler from said process model;

projecting a set of desired controlled parameter values over a predetermined control horizon;

periodically estimating how said process will react to proposed changes to the value of at least one manipulated parameter over said control horizon, and determining a set of current and future manipulated parameter values which will minimize deviations from said set of desired controlled parameter values;

testing the validity of said input signals, said current state of each estimated unmeasured process parameter, and each current said manipulated parameter value; and

causing a process control device to implement only manipulated parameter values which are determined to be valid.

20. In a process control system having input means for receiving signals indicative of measured process parameters, and output means for transmitting command signals which include at least one manipulated parameter value that is used to govern physical process equipment, a model predictive controller, comprising:

interactive modeling means for estimating the current state of at least one unmeasured process variable and for determining the value of at least one said manipulated parameter value for said process equipment at selected intervals, said interactive modeling means having both a process model which represents the dynamic behavior of the physical process equipment being controlled and an independent, evolving and step-response variable disturbance model which represents current deviations from said process model and future deviations over a predetermined prediction horizon, said process model including at least one nonlinear function which is specifically related to the physical process equipment being controlled, and said disturbance model using a window of past disturbance to predict future disturbances;

real-time executive means for directing the operation of said interactive modeling means, said real-time executive means having means for periodically transferring some of said signals indicative of measured process parameters to said interactive modeling means and means for transferring said manipulated parameter value from said interactive modeling means to said output means; and

means for testing the validity of said signals which are indicative of measured process parameters, the current estimated state of said unmeasured process variable, and the value of said manipulated parameter value.