How does DMCplus work? DMCplus是如何工作的？（2）

Figure7: Develop a plan of control moves to drive the error to zero

图7：制定一控制计划驱动误差为零

Figure8: Controlled variable actions for Complex Fractionator

图8：复杂精馏塔被控变量动作

Figure9: Manipulated Variable move plans for Complex Fractionator

图9：复杂精馏塔操作变量动作计划

The figure above shows the development of a detailed control action for a one MV,one CV system.The desired effect of the control action is defined by the mirror image of the CV Prediction about the CV Steady-State Target (CV Set Point).

If control action could be found that had exactly the desired effect, the error would be exactly canceled out. The CV would go immediately to the steady-state target and remain there across the future time horizon.

Since we know the effect needed from the control action, and since the model of the process describes the effect on a dependent variable of a move in an independent variable, it is reasonable to assume that the model can be used in planning the future MV control moves.

The figure above also shows the control action planned for the MV. Note that the CVplot displays a "CV Prediction with Control Moves". This curve is theresult of adding the effect of the calculated control action to the CV Prediction (based only on past moves in the independent variables).

The two figures above show a snap shot of a controller solution for the Complex Fractionator. These figures will be used to demonstrate the key features of the DMCplus controller.

Each box contains information on one CV or MV. The y-axis is in engineering units ofthe CV or MV, while the x-axis is time. The left side of each box represents current time. Notice that the future moves are calculated approximately halfway across the time to reach steady state.

Also notice that the right hand side of each box is beyond the steady-state time,referred to as the "controller time horizon".This extension of time is required to allow the entire effect of the future most move to be seen. So the"controller time horizon" is equal to the steady-state time plus the time of the future most move.

Each of the three CVs has a prediction denoted by the dotted line. These predictions are based on the past history of the five independent variables and the model shown previously in this section (Figure 4).

Thesepredictions represent where the three controlled variables are predicted to goin the absence of any control action.

Also note that all of the MVs and CVs have upper and lower operating limits. These limits define an acceptable operating region. The predicted steady-state values of the CVs and the current values of the MVs define the current steady-state operating point.

Using this point, economic information, and the acceptable operating region defined by the operating limits, the Steady-State solver calculates the optimal steady-state operating point. This appears as the steady-state target on the right side of each box. There is a steady-state target for every MV and CV inthe controller.

The next step is to develop a detailed plan of control action. This plan can beseen in Figure 9, where a series of future moves has been calculated for each MV. These moves extend about half way across the steady-state time, and are required to reach the MV steady-state target.

These moves are calculated by minimizing the errors for all three CVs between the Predictions and the CV steady-state targets. Figure 8 shows a "Prediction with Control Moves" for each CV. This curve represents how that CV is predicted to respond in the future,based on the control action shown in Figure 9.

Notice that all MVs and CVs are predicted to end up at their respective steady-state targets, calculated by the Steady-State solver. This is no accident; if the MVs are required to end up at their steady-state target, the CVs must end up at their targets also.

A final point to make is that Figures 8 and 9 represent a single calculation of the controller at one control interval only. The first move of the 14 calculated in each MV is sent to the regulatory control system, and the rest of the moves are thrown away.

If,at the next control cycle, no disturbances have entered the system and model mismatch is negligible, the solution will be very similar to the remainder of the solution from the previous cycle.

However,if a major disturbance has entered the system, the optimal steady-state operating point will change, as will the predictions themselves (reflecting the disturbance). This will require an immediate change in control strategy, so the control action in this scenario will not resemble the solution from the previous control cycle.

It is for this reason that all the moves except for the first one are thrown away.It is still essential that these future moves be calculated, since the size ofthe first control move will be affected by projected MV constraint violations in the future.In other words, a particular MV might have to be moved more now in order to prevent a future limit violation. This would not be known if the entire trajectory of future moves was not calculated,subject to the operating limits.

2015.9.11

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