Specifications

3 Designing Controllers Using the Design Tool GUI
3-32
Results for T Setpoint 3 are very similar to those shown in Improved Setpoint Tracking
for CSTR Temperature and Plant Inputs for Modified Rate Weight, where a penalty rate
weight reduced the move sizes. If rate weights and blocking achieve the same ends, why
does the toolbox provide both features? One difference not evident in this simple problem
is that blocking applies to all the manipulated variables in your application, but each
rate weight affects one only.
Note To obtain the dashed lines shown for T Setpoint 2, activate the plot window
and select Property Editor from the View menu. The Property Editor appears at the
bottom of the window. Then select the curve you want to edit. The Property Editor lets
you change the line type, thickness, color, and symbol type. Select the axis labels to see
additional options.
By default, the toolbox plots each scenario on the same plot. If you recalculate a revised
scenario, it replots that result but doesn't change any others.
If you don't want to see a particular scenario, right-click the plot and use the Responses
menu option to hide it. (You can also close the plot window and recalculate selected
responses in a fresh window.)
Defining Manipulated Variable Constraints
Physical devices have limited ranges and rates of change. For example, the CSTR model's
coolant might be restricted to a 20 degree range (from –10 to 10) and its maximum rate
of change might be ±4 degrees per control interval. If these are true physical restrictions,
it's good practice to include them in the controller design. Otherwise the controller might
attempt an unrealistic adjustment.
To compare constrained and unconstrained performance for the CSTR example, select the
Blocking 1 controller in the tree, and rename it Unconstrained.
Select its Model and Horizons tab and turn off (clear) its Blocking option. Increase
Control horizon to 3, and reduce Control Interval to 0.25.
Delete the Blocking 2 and Blocking 3 controllers. (Click Yes or OK to dismiss the
resulting warning messages.)
Copy the Unconstrained controller. Name this copy MVconstraints, and select its
Constraints tab. Then enter the manipulated variable constraints shown in Entering
CSTR Manipulated Variable Constraints.