Controller Technical Information

The Controller unit operation simulates a feedback controller. You can specify a desired value for a flowsheet unit operation parameter or stream parameter (called the specification or specification variable) in the final converged solution. 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 control variable will be varied by the controller. It cannot be changed by any other unit operation.

The controller will observe the change of the specification variable. That variable cannot be fixed by any other unit operation. Instead, it must be allowed to change.

Each controller requires one specification variable and one control variable.

The "specification is met" or the "specification is satisfied" when PRO/II finds a converged solution for the simulation which matches the desired value for the specification variable.

Calculation Method

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

Placement of the Controller

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

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

A 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 controller will fail to find a value of the control variable that satisfies the desired value for the specification variable:

- The control variable reaches a maximum or minimum before the specification is satisfied.

- The maximum number of iterations is reached before the specification is satisfied.

- The specification variable does not change as the control variable is changed.

- The specification error does not improve after three iterations.

When the controller fails, by default the simulation will continue if the controller is contained in a flowsheet recycle loop or in a calculation recycle loop. However, you can request that the simulation calculations stop if the control variable reaches a maximum or minimum before the specification is satisfied.

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