Specifications

3 Designing Controllers Using the Design Tool GUI
3-48
Test Controller Robustness
It's good practice to test your controller's sensitivity to prediction errors. Classical phase
and gain margins are one way to quantify robustness for a SISO application. Robust
Control Toolbox™ software provides sophisticated approaches for MIMO systems. It
can also be helpful to run simulations. The following example illustrates the simulation
approach.
Plant Model Perturbation
Create a perturbed version of a model.
For this example, use the nonlinear CSTR model discussed in “CSTR Model”.
CSTRp = CSTR;
CSTRp.a=[-0.0303 -0.0113
-0.0569 -0.1836];
CSTRp.b=[-0.0857 0.0191
0.1393 0.4241];
This creates a copy of CSTR called CSTRp, then replaces the state space A and B matrices
with perturbed versions (compare to the originals defined in “State-Space Models” on
page 2-10). Use the following command to compare the two step responses:
step(CSTR, CSTRp)
Observe the difference in the responses (not shown).
Select Plant models in the tree. Click the Import button and import the CSTRp model.
Simulation Tests
Delete all controllers except the first one in the tree. If necessary, make its settings
identical to Unconstrained (see “Defining Manipulated Variable Constraints” on page
3-32).
Delete all scenarios except the first, naming that Accurate Model. Define its properties
as shown in Robustness Test, Accurate Plant Model Scenario. The scenario begins
with a step change in the temperature setpoint, followed 25 time units later by a step
disturbance in the reactant entering the CSTR.