Multivariable Controller - Technical Information

The Multivariable Controller (MVC) unit operation is an expanded form of the Controller. It simulates two or more feedback process controllers in PRO/II. You can specify desired values for an unlimited number of flowsheet unit operation or stream parameters (called specifications or specification variables) in the final converged solution. For each specification, another flowsheet unit or stream parameter (called the control variable) will be varied by PRO/II in order to match the desired value for the specification variable. The number of variables must equal the number of specifications.

The control variables will be varied by the Multivariable Controller. They cannot be changed by any other unit operation.

The Multivariable Controller will observe the change of the specification variables. Those variables cannot be fixed by any other unit operation. Instead, they must be allowed to change.

The "specifications are met" or the "specifications are satisfied" when PRO/II finds a converged solution for the simulation which matches the desired values for all the specification variables.

Calculation Method

The Multivariable Controller uses an iterative search technique to find the value of each control variable necessary to satisfy the desired values for the specification variables. The specification variables should change continuously and monotonically as the control variables are changed. If any specification variable changes discontinuously, exhibits maxima or minima, or does not change as the control variable changes, then the MVC is likely to experience convergence problems.

Placement of the Multivariable Controller

The Multivariable Controller usually will be placed in the calculation sequence after the unit operation where the specification variable is calculated. The MVC should return the calculations to the first unit operation that is affected by changes to any control variable. PRO/II determines the next unit operation to be calculated by default. However, you may override this default if desired.

If the Multivariable Controller is placed inside a flowsheet recycle loop, PRO/II will calculate the MVC as part of the recycle loop. However, the user may change the calculation sequence to force the Multivariable Controller to be calculated after the flowsheet recycle loop converges.

The Multivariable Controller always creates another calculation recycle loop. Therefore it is possible to increase the calculation time of the simulation significantly. Also it is necessary to be sure that inner recycle loops have tighter tolerances than outer recycle loops.

Reasons for Failure

Under the conditions listed below, the Multivariable Controller will fail to find values of all control variables which satisfy all the specification variables:

Any control variable reaches a maximum or minimum before all the specifications are satisfied.
The maximum number of iterations is reached before all specifications are satisfied.
One or more specification variables do not change as the control variables are changed.
The specification error in any variable does not improve after three iterations.

When the Multivariable Controller fails, by default the simulation will stop if any control variable exceeds its maximum or minimum limits. However, you can request that the simulation calculations continue if any control variable reaches a limiting value before all the specifications are satisfied. In this case, the limiting value for that control variable will be used in the calculations.

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