Integrated optimal model predictive control in a process control system

• US 6,721,609 B1
• Filed: 06/14/2000
• Issued: 04/13/2004
• Est. Priority Date: 06/14/2000
• 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., Nixon, Mark J., Wojsznis, Wilhelm K., Seeman, Richard C.
• Primary Examiner(s): PATEL, RAMESH B
• Application Number:
  US09/593,327
• Time in Patent Office: 1,399 Days
• Field of Search: 700/53, 700/54, 700/32, 700/28, 700/33, 700/44, 700/45, 700/29, 700/30, 700/31
• US Class Current: 700/28
• CPC Class Codes: G05B 13/0275 using fuzzy logic only
• CPC Class Codes: G05B 13/048 using a predictor

Abstract

A model predictive controller having a set of inputs adapted to receive process output signals and further having a set of outputs adapted to supply process input signals to process control devices is configured to perform integrated optimal model predictive control. The controller provides integrated optimal control using a feedback path that couples a selected output to an input having a setpoint that is equal to the constraint limit of the selected output. The controller maintains the selected output at its constraint limit until one of the outputs, other than the selected output, is going to reach or exceed the associated constraint limit for that output. When the output is going to reach or exceed the associated constraint limit for that output, the controller relaxes the setpoint until the output is no longer going to reach or exceed the associated constraint limit for that output.

Claims

1. A multi-input/multi-output logic device for use in controlling a process comprising:

a plurality of inputs adapted to receive process output signals;

a further input having an associated setpoint;

a multiplicity of outputs adapted to provide process control signals for controlling the process;

a feedback path that connects a selected one of the outputs to the further input;

a logic unit configured to produce the multiplicity of outputs using the plurality of inputs and the further input and adapted to adjust the selected output until the further input has reached the associated setpoint; and

wherein each of the multiplicity of outputs has an associated constraint limit, and wherein the logic unit comprises a prediction unit adapted to predict a future value for each of the multiplicity of outputs and a comparison unit adapted to compare the future value for each of the multiplicity of outputs to the constraint limit associated with that output to determine whether any one of the outputs, other than the selected output, is going to reach or exceed the associated constraint limit for that output.

2. The logic device of claim 1, wherein the logic unit is a model predictive controller.

3. The logic device of claim 1, wherein the logic unit is a fuzzy logic controller.

4. The logic device of claim 1, wherein the setpoint is associated with a constraint limit for the selected output.

5. The logic device of claim 1, wherein each of the multiplicity of outputs has an associated constraint limit, and wherein the logic unit further comprises a setpoint adjustment unit adapted to adjust the setpoint from a first setpoint setting to a second setpoint setting when one of the outputs, other than the selected output, is going to reach or exceed the associated constraint limit for that output.

6. The logic device of claim 5, wherein the first setpoint setting is a value that is equal to the constraint limit of the selected output and wherein the second setpoint setting is a value that is relaxed with respect to the constraint limit of the selected output.

7. The logic device of claim 5, wherein the setpoint adjustment unit is further adapted to adjust the setpoint from the second setpoint setting to the first setpoint setting when the one of the outputs is no longer going to reach or exceed the constraint limit associated with that output.

8. The logic device of claim 7, wherein the setpoint adjustment unit is further adapted to adjust the setpoint from the second setpoint setting to the first setpoint setting in a series of step-wise adjustments.

9. The logic device of claim 1, wherein the logic unit further comprises a setpoint adjustment unit adapted to adjust the setpoint from a first setpoint setting to a second setpoint setting when one of the outputs, other than the selected output, is going to reach or exceed the associated constraint limit for that output, wherein the first setpoint setting is a value that is equal to the constraint limit of the selected output and wherein the second setpoint setting is a value that is relaxed with respect to the constraint limit of the selected output.

10. The logic device of claim 9, wherein the setpoint adjustment unit is further adapted to continue adjusting the setpoint in a relaxed direction relative to the second setpoint setting when the one of the outputs, other than the selected output, is still going to reach or exceed the associated constraint limit for that output, when the setpoint is at the second setpoint setting.

11. The logic device of claim 9, wherein the setpoint adjustment unit is further adapted to adjust the setpoint from the second setpoint setting to the first setpoint setting when the one of the outputs, other than the selected output, is no longer going to reach or exceed the associated constraint limit for that output.

12. The logic device of claim 11, wherein the setpoint adjustment unit is further adapted to adjust the setpoint from the second setpoint setting to the first setpoint setting in a series of step-wise adjustments.

13. The logic device of claim 9, wherein the further input comprises a first further input, the associated setpoint comprises a first setpoint, the feedback path comprises a first feedback path and the selected output comprises a first selected output and wherein the logic device further comprises a second further input having a second setpoint, and a second feedback path that directly connects a second selected output to the second further input, wherein the setpoint adjustment unit is further adapted to change the second setpoint from a third setpoint setting to a fourth setpoint setting when the one of the outputs, other than the selected output, is going to reach or exceed the associated constraint limit for that output, wherein the third setpoint setting is the constraint limit of the second selected output and the fourth setpoint setting is relaxed with respect to the constraint limit of the second selected output.

14. The logic device of claim 13, wherein the first setpoint and the second setpoint each have a priority level and wherein the setpoint adjustment unit is further adapted to adjust the first setpoint and the second setpoint in an order according to the priority level of the first setpoint and the second setpoint.

15. The logic device of claim 13, wherein the setpoint adjustment unit is further adapted to adjust the first setpoint and the second setpoint simultaneously.

16. The logic device of claim 1, wherein the feedback path includes an amplifier.

17. The logic device of claim 16, wherein the amplifier has a unit gain.

18. A multi-input/multi-output logic device for use in controlling a process comprising:

a plurality of inputs adapted to receive process output signals;

a further input having an associated setpoint;

a multiplicity of outputs adapted to provide process control signals for controlling the process;

a feedback path that connects a selected one of the outputs to the further input;

a logic unit configured to produce the multiplicity of outputs using the plurality of inputs and the further input and adapted to use the feedback path to control the selected output to be equal to the setpoint; and

wherein each of the multiplicity of outputs has an associated constraint limit, and wherein the logic unit comprises a prediction unit adapted to predict a future value for each of the multiplicity of outputs and a comparison unit adapted to compare the future value for each of the multiplicity of outputs to the constraint limit associated with that output to determine whether any one of the outputs, other than the selected output, is going to reach or exceed the associated constraint limit for that output.

19. The logic device of claim 18, and wherein the logic unit comprises a setpoint adjustment unit adapted to adjust the setpoint when one of the outputs, other than the selected output, is going to reach or exceed the associated constraint limit for that output.

20. The logic device of claim 19, wherein the logic unit comprises a model predictive controller.

21. A multi-input/multi-output logic device for use in controlling a process comprising:

a plurality of inputs adapted to receive process output signals;

a further input having an associated setpoint;

a multiplicity of outputs adapted to provide process control signals for controlling the process, wherein each of the multiplicity of outputs has an associated constraint limit;

a feedback path that connects a selected one of the outputs to the further input;

a logic unit configured to produce the multiplicity of outputs using the plurality of inputs and the further input and including a setpoint adjustment unit adapted to change the setpoint when one of the outputs, other than the selected output, is going to reach or exceed the associated constraint limit for that output; and

wherein the logic unit comprises a prediction unit adapted to predict a future value for each of the multiplicity of outputs and a comparison unit adapted to compare the future value for each of the multiplicity of outputs to the constraint limit associated with that output to determine whether any one of the outputs, other than the selected output, is going to reach or exceed the associated constraint limit for that output.

22. A method for controlling a process using a multi-input/multi-output logic device having a plurality of inputs adapted to receive process output signals and a multiplicity of outputs adapted to provide process control signals for controlling the process, wherein each of the outputs has an associated constraint limit, the method comprising the steps of:

connecting a selected one of the outputs to a further input of the logic device;

setting a setpoint associated with the further input to a first setpoint setting;

adjusting the selected output until the further input has reached the first setpoint setting;

calculating a future value for each of the multiplicity of outputs other than the selected output;

comparing the future value calculated for each of the multiplicity of outputs, other than the selected output, to the associated constraint limit for that output to determine whether any one of the outputs, other than the selected output, is going to reach or exceed the associated constraint limit for that output; and

adjusting the setpoint from the first setpoint setting to a second setpoint setting when the step of comparing determines that one of the outputs, other than the selected output, is going to reach or exceed the associated constraint limit for that output.

23. The method of claim 22, wherein the first setpoint setting is equal to a constraint limit of the selected output, and wherein the second setpoint setting is equal to a value that is relaxed relative to the constraint limit of the selected output.

24. The method of claim 22, further including the step of adjusting the setpoint from the second setpoint setting to the first setpoint setting when the step of comparing determines that the one of the outputs is no longer going to reach or exceed the associated constraint limit for that output.

25. The method of claim 22, further including the step of further adjusting the setpoint in a direction that is relaxed relative to the second setpoint setting when the step of comparing determines that the one of the outputs, other than the selected output, is still going to reach or exceed the associated constraint limit for that output, when the setpoint is set to the second setpoint setting.

26. The method of claim 22, further including the step of calculating the multiplicity of outputs from the plurality of inputs and the further input using a model predictive control technique.

27. The method of claim 22, further including the step of calculating the multiplicity of outputs from the plurality of inputs and the further input using a fuzzy logic control technique.

28. The method of claim 22, wherein the further input comprises a first further input, the associated setpoint comprises a first setpoint, and the selected output comprises a first selected output, and wherein the method further comprises the steps of;

connecting a second selected one of the outputs to a second further input;

setting a second setpoint associated with the second further input to a third setpoint setting;

adjusting the second selected output until the second further input has reached the third setpoint setting;

adjusting the second setpoint from the third setpoint setting to a fourth setpoint setting when the step of comparing determines that the one of the outputs, other than the selected output, is going to reach or exceed the associated constraint limit for that output; and

adjusting the setpoint from the fourth setpoint setting to the third setpoint setting when the step of comparing determines that the one of the outputs, other than the selected output, is no longer going to reach or exceed the associated constraint limit for that output.

29. The method of claim 28, further comprising the steps of;

assigning each of the first setpoint and the second setpoint a priority level, and

performing the adjustments to each of the first setpoint and the second setpoint in an order according to the priority level assigned to the first setpoint and the second setpoint.