According to the plant control device of this invention, based on a modified target value candidate of a control output, a reference governor uses a prediction model in which a closed loop system including a plant and a feedback controller are modeled to sequentially calculate, across a finite prediction horizon, a predicted value of state quantities of a plant including a specific state quantity on which a constraint is imposed. At such time, if a predicted value of a specific state quantity relating to a certain modified target value candidate conflicts with a constraint, the reference governor excludes the modified target value candidate from candidates for a final modified target value. Thus, the computational load required to modify a target value of a control output is decreased while ensuring the satisfiability of a constraint.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A plant control device, comprising: a feedback controller that determines a control input of a plant by feedback control so that a control output of the plant approaches a target value, and a reference governor that modifies the target value that is provided to the feedback controller, wherein: the reference governor is configured to execute: prediction model calculation processing for sequentially calculating, across a finite prediction horizon, predicted values of state quantities of the plant that include a specific state quantity on which a constraint is imposed, using a prediction model in which a closed loop system including the plant and the feedback controller is modelled, based on a modified target value candidate of the control output, evaluation function calculation processing for calculating an evaluation value of the modified target value candidate using a previously defined evaluation function based on a calculation result obtained by the prediction model calculation processing, and modified target value determination processing for executing the prediction model calculation processing and the evaluation function calculation processing with respect to a plurality of modified target value candidates, and determining a final modified target value based on respective evaluation values of the plurality of modified target value candidates; and in a case where a predicted value of the specific state quantity that is predicted by prediction model calculation processing that relates to a certain modified target value candidate conflicts with a constraint, the reference governor excludes the certain modified target value candidate from candidates for the final modified target value. 2. The plant control device according to claim 1, wherein, in a case where a predicted value of the specific state quantity conflicts with a constraint during performance of prediction model calculation processing relating to a certain modified target value candidate, the reference governor cancels remaining calculations of the prediction model calculation processing relating to the certain modified target value candidate. 3. The plant control device according to claim 2, wherein: in the prediction model calculation processing, the reference governor calculates a predicted value of the state quantity discretely at a prediction cycle that is previously set, and in a case where a predicted value of the specific state quantity conflicts with a constraint at a discrete time point during a period from an initial discrete time point until a final discrete time point in prediction model calculation processing relating to a certain modified target value candidate, the reference governor cancels calculations of a predicted value of the state quantity at remaining discrete time points. 4. The plant control device according to claim 3, wherein the reference governor changes a threshold value for determining whether or not a predicted value of the specific state quantity conflicts with a constraint to a stricter value as discrete time points relating to the prediction model calculation processing proceed. 5. The plant control device according to claim 3, wherein: in the evaluation function calculation processing, the reference governor uses an evaluation function that gives a progressively favorable evaluation value as a difference between a predicted value of the control output at respective discrete time points calculated by the prediction model calculation processing and an original target value of the control output decreases; and in the modified target value determination processing, the reference governor determines a modified target value candidate for which the evaluation value is a most favorable value to be the final modified target value. 6. The plant control device according to claim 1, wherein: in the modified target value determination processing, the reference governor updates the modified target value candidate in accordance with a previously defined updating rule; and according to the updating rule, a next modified target value candidate is determined by means of a combination of a direction of a change in an evaluation value of a current modified target value candidate relative to an evaluation value of a previous modified target value candidate and a direction of a change in the current modified target value candidate relative to the previous modified target value candidate. 7. The plant control device according to claim 6, wherein, in the modified target value determination processing, if the evaluation value of the current modified target value candidate is a more favorable value than the evaluation value of the previous modified target value candidate, the reference governor provisionally determines the current modified target value candidate to be a final modified target value, and if the evaluation value of the current modified target value candidate is not a more favorable value than the evaluation value of the previous modified target value candidate, the reference governor maintains a final modified target value that is provisionally determined at a previous time as it is. 8. The plant control device according to claim 6, wherein in a case where remaining calculations are cancelled during performance of the prediction model calculation processing due to a conflict with a constraint, in the modified target value determination processing the reference governor increases a number of update operations for the modified target value candidate in accordance with an amount of a decrease in a computational load that accompanies cancellation of the calculations.