Specifications
3 Designing Controllers Using the Design Tool GUI
3-58
Design Controller for Nonsquare Plant
A nonsquare plant has an unequal number of manipulated variables and output
variables. This is common in practice, and the Model Predictive Control Toolbox software
supports an excess of manipulated variables or outputs. In such cases you will usually
need to modify default toolbox settings.
This section covers the following topics:
• “More Outputs Than Manipulated Variables” on page 3-58
• “More Manipulated Variables Than Outputs” on page 3-59
More Outputs Than Manipulated Variables
When there are excess outputs, you can't hold each at a setpoint. You have the following
options:
• Enforce setpoints on all outputs, in which case all will deviate from their setpoints to
some extent.
• Specify that certain outputs need not be held at setpoints by setting their weights to
zero on the controller's Weight Tuning tab.
The initial test of the CSTR controller used option 1 (the default), which caused both
outputs to deviate from their setpoints (see Plant Outputs for T Setpoint Scenario with
Added Data Markers). You can adjust the offset in each output by changing the output
weights. Increasing an output weight decreases the offset in that output (at the expense
of increased offset in other outputs).
The modified CSTR controller used option 2 (see the discussion in “Weight Tuning”
on page 3-24). In general, if the application has N
e
more outputs than manipulated
variables, setting N
e
output weights to zero should allow the remaining outputs to
be held at setpoints (unless the manipulated variables are constrained). This was
the case for the modified CSTR controller (see Improved Setpoint Tracking for CSTR
Temperature).
Outputs that have been “sacrificed” by setting their weights to zero can still be useful. If
measured, they can help the controller to estimate the plant's state, thereby improving
its predictions. They can also be used as indicators, or as variables to be held within an
operating region defined by output constraints.