Uncertain Model Interconnections

Feedback Around an Uncertain Plant

It is possible to form interconnections of uss objects. A common example is to form the feedback interconnection of a given controller with an uncertain plant.

First create the uncertain plant. Start with two uncertain real parameters.

gamma = ureal('gamma',4); 
tau = ureal('tau',.5,'Percentage',30);

Next, create an unmodeled dynamics element, delta, and a first-order weighting function, whose DC value is 0.2, high-frequency gain is 10, and whose crossover frequency is 8 rad/sec.

delta = ultidyn('delta',[1 1],'SampleStateDimension',5); 
W = makeweight(0.2,6,6);

Finally, create the uncertain plant consisting of the uncertain parameters and the unmodeled dynamics.

P = tf(gamma,[tau 1])*(1+W*delta);

You can create an integral controller based on nominal plant parameters. Nominally the closed-loop system will have damping ratio of 0.707 and time constant of 2*tau.

KI = 1/(2*tau.Nominal*gamma.Nominal); 
C = tf(KI,[1 0]);

Create the uncertain closed-loop system using the feedback command.

CLP = feedback(P*C,1);

Plot samples of the open-loop and closed-loop step responses. As expected the integral controller reduces the variability in the low frequency response.

subplot(2,1,1); 
stepplot(P,5) 
subplot(2,1,2); 
stepplot(CLP,5)

MATLAB figure

Basic Model Interconnections

All the model arithmetic and model-interconnection commands of Control System Toolbox™ software work with uncertain models. These include:

For more information about model interconnections, see Model Interconnection.