Model predictive control apparatus

• US 5,347,446 A
• Filed: 10/08/1992
• Issued: 09/13/1994
• Est. Priority Date: 02/08/1991
• Status: Expired due to Term

• Current Assignee: Kabushiki Kaisha Toshiba (Toshiba Corporation)
• Original Assignee: Kabushiki Kaisha Toshiba (Toshiba Corporation)
• Inventors: Iino, Yutaka, Ohya, Junko
• Primary Examiner(s): Smith, Jerry
• Assistant Examiner(s): TRAMMELL, JAMES P
• Application Number:
  US07/938,256
• Time in Patent Office: 705 Days
• Field of Search: 364/148-158, 364/159, 364/165, 364/164
• US Class Current: 700/29
• CPC Class Codes: G05B 13/048 using a predictor

Abstract

A model predictive control apparatus constructed in consideration of limit conditions of the process and also suitable for a multi-input/output system and an input device suitable for such a control apparatus are disclosed.

The model predictive control apparatus includes an transformation unit for transforming the cost function and the limit condition to a conditional expression only relating to the manipulated variable. A suitable cost function parameter adjustment unit in which robustness is taken into consideration is provided. Alternatively, by designation of a response time constant, a weight corresponding thereto is calculated or determined to carry out evaluation of stability margin.

In addition, the input device is adapted to carry out, through a picture on screen, setting or alteration of a predictive model or control parameters, etc. to be inputted, thus to suitably conduct setting while confirming it on the screen.

Claims

1. A model predictive control apparatus comprising:

prediction means in which a model approximating a dynamic characteristic of a controlled system having a plurality of manipulated variables and controlled variables is used to determine a predictive equation of future values of the controlled variables; and

arithmetic means adapted to transform limit conditions of a control condition to manipulated variables, and to calculate a manipulated variable to minimize an cost function in a quadratic form relating to a difference between a future reference value and a control variable, and a manipulated variable, set on the basis of said predictive equation while satisfying said limit conditions, thus to give the determined manipulated variable to said controlled system.

2. A model predictive control apparatus as set forth in claim 1, which further comprises:

an input device for respectively inputting limit conditions, parameters of the cost function, and controlled variable reference values relating to future values of controlled variables and manipulated variables of the controlled system,

said arithmetic means comprising;

cost function transformation means for transforming an cost function in a quadratic form having, as variables, a predictive controlled variable based on said predictive equation, a deviation from a reference value of said controlled variable future value, and a change in the manipulated variable (manipulated variable future value) to be given to said controlled system, thus to set an cost function with respect to a quadratic programming;

limit condition transformation means for transforming a limit condition inputted from said input device on the basis of said predictive equation to set a limit condition with respect to a quadratic programming; and

optimal manipulated variable calculation means for sequentially calculating, by using a quadratic programming, future values of manipulated variables to minimize the cost function set by said cost function transformation means and the limit condition set by said limit condition transformation means.

3. A model predictive control apparatus as set forth in claim 2,

wherein said prediction means is adapted to determine a predictive equation indicating a future value of a controlled variable y by a past controlled variable y and a past manipulated variable change rate Δ

u,

wherein said limit condition transformation means is such that upper and lower limit equations relating to a controlled variable y, its change rate Δ

y, a manipulated variable u, and its change rate Au are used as limit conditions to transform all these conditions to an inequality limit condition relating to Δ

u,

wherein said cost function transformation means is adapted to transform the cost function to a function of Δ

u, and

wherein said optimal manipulated variable calculation means is adapted to calculate an optimal manipulated variable change rate Δ

u by using a quadratic programming.

4. A model predictive control apparatus as set forth in claim 2, wherein said arithmetic means further includes cost function adjustment means for adjusting an inputted cost function to give it to said cost function transformation means.

5. An input device for inputting control parameters of the model predictive control apparatus as set forth in claim 1,

said input device comprising:

screen display means for visually displaying various control information on the screen;

future reference characteristic input means for inputting a new future reference value from the content displayed on said screen display means;

limit condition input means for inputting a new limit condition corresponding to said reference characteristic from the content displayed on said screen display means;

simulation arithmetic means in which said new future reference value and said new limit condition are used to repeatedly perform said model predictive control operation with respect to a dynamic characteristic model of the controlled system to calculate a predictive controlled variable response indicating a response characteristic of said predictive controlled variable;

simulation display means for graphically displaying said new future reference value, said new limit condition, and said predictive controlled response on said screen display means; and

setting means responsive to a setting command generated after graphic display to set said new future reference value and said new limit condition at said model predictive control apparatus.

6. An input device for a model predictive control apparatus as set forth in claim 5, wherein said future reference characteristic input means allows said screen display means to display thereon a plurality of transfer functions stored in advance to input, as a new future reference value, a response curve that an operator has selected.

7. An input device for a model predictive control apparatus as set forth in claim 5, wherein said future reference characteristic input means is adapted to calculate a curve passing through a plurality of coordinates designated by an operator on said screen to thereby input a new future reference value.

8. An input device for a model predictive control apparatus as set forth in claim 5, wherein said limit condition input means is adapted to allow said screen display means to display thereon a limit condition set in advance in correspondence with a plurality of transfer functions stored in advance and a transfer function that an operator has selected to input it as a new limit condition.

9. An input device for a model predictive control apparatus as set forth in claim 7, wherein said limit condition input means is adapted to calculate a reference curve passing through a plurality of coordinates designated by an operator to thereby input it as a new control condition.

10. An input device for a model predictive control apparatus as set forth in claim 8, wherein said limit condition input means is adapted to implement a predetermined operation to said new future reference value to thereby input it as a new control condition.

11. An input device for a model predictive control apparatus as set forth in claim 1,

said input device comprising:

cost function memory means for storing an cost function serving as an operation index of said process equipment with at least a difference between said predictive controlled variable and said future reference value and said manipulated variable being as a parameter;

screen display means for displaying said cost function on the screen;

cost function input means adapted to update said cost function by parameters of said cost function inputted through a picture on the screen to form a new cost function;

simulation arithmetic means for repeatedly performing said predictive control operation to calculate a group of values of said new cost function;

simulation display command means for allowing said screen display means to graphically display thereon the values of said new cost function; and

setting means responsive to a setting command generated after graphic display to set said new cost function at said model predictive control apparatus.

12. An input device for a model predictive control apparatus as set forth in claim 11, wherein a control cost function indicating control performance with respect to said process equipment is used as said cost function.

13. An input device for a model predictive control apparatus as set forth in claim 11, wherein an economical cost function indicating an operation cost of said process equipment is used as said cost function.

14. A model predictive control apparatus as set forth in claim 1, wherein said apparatus further comprises:

an input device for inputting control specification parameters;

reading means for reading said control specification parameters input through said input device;

calculating means for calculating optimal values of proposed parameters in a quadratic cost function, including a control horizon, weighting factors, and parameters of a pole assignment polynomial of a closed loop, on the basis of a model of controlled system and control specification parameters;

display means for visually displaying the parameters on a screen; and

transfer means for transferring the proposed parameters to said display means and said arithmetic means.

15. The model predictive control apparatus as set forth in claim 14, wherein said reading means receives a first robust performance parameter relating to a threshold value through said input device;

and wherein said calculating means further comprises;

means for obtaining sensitivity factors of changes in future controlled variables with respect to increments in the manipulated variables,

means for forming a sensitivity matrix by arranging the sensitivity factors,

means for obtaining singular values of the sensitivity matrix, and

means for determining the control horizon by comparing the singular values with said first robust performance parameter, a determined control horizon being displayed by said display means and transferred to said arithmetic means.

16. The model predictive control apparatus as set forth in claim 14, wherein said reading means receives a second robust performance parameter relating to a threshold value of stability margin through said input device;

and wherein said calculating means further comprises;

means for varying the weighting factors and the parameters of the pole assignment polynomial of a closed loop step by step with constant increments,

means for obtaining a value of a sensitivity function,

means for comparing an obtained sensitivity function value with said second robust parameter to judge whether the maximum singular value of the sensitivity function satisfies a predetermined requirement, and

means for repeating said varying of said weighting factors and parameters until said requirement is satisfied, determined weighting factors or parameters of the pole assignment polynomial of a closed loop being displayed by said display means and transferred to said arithmetic means.

17. The model predictive control apparatus as set forth in claim 14, wherein said reading means receives a third robust performance parameter relating to robust stability margin through said input device;

and wherein said calculating means further comprises;

means for varying the weighting factors and the parameters of the pole assignment polynomial of a closed loop step by step with constant increments,

means for obtaining a value of a complementary sensitivity function,

means for comparing an obtained value of the complementary sensitivity function with said third robust parameter to judge whether the maximum singular value of the complementary sensitivity function satisfies a predetermined requirement, and

means for repeating said varying of said weighting factors and parameters until said requirement is satisfied, determined weighting factors or parameters of the pole assignment polynomial of a closed loop being displayed by said display means and transferred to said arithmetic means.

18. A model predictive control apparatus comprising:

prediction means in which a model approximating a dynamic characteristic of a controlled system is used to determine a predictive equation of future values of controlled variables;

arithmetic means for calculating or determining a manipulated variable to minimize an cost function in a quadratic form relating to a difference between a future reference value and a controlled variable, and a manipulated variable set on the basis of said predictive equation while satisfying a limit condition to give the manipulated variable thus calculated to said controlled system;

response time constant setting means for setting a response time constant indicating a rise time at which said controlled system should be operated;

weighting factor parameter calculation means for calculating a weighting factor including said response time constant at its exponential part, the value thereof increasing with lapse of time, and

cost function setting means for constructing a new cost function in which the calculated weighting coefficient is built in to set it as said cost function.

19. A model predictive control apparatus as set forth in claim 18, which further comprises:

arithmetic means for calculating a stability margin parameter indicating a stability with respect to a change in the controlled system of the model predictive control system where said new cost function is set;

simulation means for recording the value of said stability margin parameter corresponding to an instantaneous value of said response time constant while changing the value of the response time constant set at said response time constant setting means; and

display means for visually displaying the relationship between the value of the recorded response time constant and the value of the stability margin parameter.

20. A model predictive control apparatus as set forth in claim 18, which further comprises:

arithmetic means for calculating a stability margin parameter indicating a stability with respect to a change of the controlled system of the model predictive control system where said new cost function is set;

stability margin parameter setting means for storing the designated stability margin parameter; and

response time constant adjustment means for changing the value of the response time constant held by said response time constant setting means until the value of said calculated stability margin parameter becomes in correspondence with the value of the stability margin parameter stored in said stability margin parameter setting means.

21. A model predictive control apparatus as set forth in claim 18, wherein said new cost function is an cost function expressed below, obtained by multiplying the term relating to a controlled variable y(k+j), a reference value y*(k+j), a manipulated variable increment Δ

u(k+j), and polynomial D(Z^-1) for determining a closed loop pole assignment by weighting factor ρ

=exp(-Δ

/Tr) where Δ

is a control period, and Tr is a response time constant;

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