Specifications

Design Controller Using the Design Tool
3-43
Constraint Softening Scenarios: 1 = None, 2 = Hard, 3 = Soft
Curve 1 is without output constraints, which is identical to curve 1 in CSTR Outputs
for Disturbance Scenarios 1 and 2. This controller allows the C
A
output to vary freely,
but the controlled temperature returns to its setpoint within 10 time units after the
disturbance happens.
Curve 2 shows the behavior with a hard upper limit at C
A
= 3. Once C
A
reaches this
bound, the controller must use its one manipulated variable (T
c
) to satisfy the constraint,
so it's no longer able to control the temperature.
Curve 3 shows the result for a soft constraint. The controller reaches a compromise
between the competing objectives: temperature control and constraint satisfaction. As
you'd expect, performance lies between the curve 1 and curve 2 extremes.
The numerical value of the relaxation band represents a relative tolerance for constraint
violations, not a strict limit (if it were the latter, it would be a hard constraint). If you
were to increase its relaxation band (currently set at 100), performance would move
toward Case 1, and vice versa.
If you have multiple constraints, you can harden or soften them simultaneously using
the slider at the bottom of the controller's constraint softening dialog box (see Constraint
Softening Dialog Box).