Specifications

Design Controller for Nonsquare Plant
3-59
More Manipulated Variables Than Outputs
In this situation, default Model Predictive Control Toolbox settings should provide offset-
free output-setpoint tracking, but the manipulated variables are likely to drift.
One way to avoid this is to use manipulated variable setpoints. If there are N
e
excess
manipulated variables and you hold N
e
of them at target values, the rest should not drift.
Rather, they will attain the values needed to eliminate output offset.
To define a manipulated variable setpoint:
1
Enter the setpoint value in the Nominal field in the signal properties view – see
Model Predictive Control Toolbox Design Tool's Signal Definition View.
2
Assign a nonzero input weight using the Weight entry on the controller's Weight
Tuning tab – see Controller Options — Weight Tuning Tab.
In step 2, the magnitude of the input weight determines the extent to which the
manipulated variable can deviate from its target during a transient. See “Input Weights”
on page 3-28 for more discussion and mathematical details.
You might want to allow such deviations temporarily in order to provide better output
setpoint tracking. In that case, use a relatively small input weight. If you want the
manipulated variable to stay near its target value at all times, increase its input weight.
Another way to avoid drift is to constrain one or more manipulated variables to a narrow
operating region. You can even hold an MV constant by setting its lower and upper
bounds to the same value (in which case its nominal value should also be set to this
value), or by setting both of its rate constraints to zero. To define constraints, use the
controller's Constraints tab (see “Defining Manipulated Variable Constraints” on page
3-32).