Simulate Uncertain Model at Sampled Parameter Values

Uncertain Model

The simple model rctUncertainModel contains an Uncertain State Space block with a step input. The step response signal feeds a MultiPlot Graph block.

mdl = "rctUncertainModel";
open_system(mdl)

By default, the Uncertain State Space block is configured to simulate the uncertain model ss(ureal('a',-5),5,1,1), which is a uss model with one uncertain parameter. For this example, create a model of a mass-spring damper system with an uncertain spring constant and damping constant.

m = 3;
c = ureal('c',1,'Percentage',20);
k = ureal('k',2,'Percentage',30);
usys = tf(1,[m c k])

Uncertain continuous-time state-space model with 1 outputs, 1 inputs, 2 states.
The model uncertainty consists of the following blocks:
  c: Uncertain real, nominal = 1, variability = [-20,20]%, 1 occurrences
  k: Uncertain real, nominal = 2, variability = [-30,30]%, 1 occurrences

Type "usys.NominalValue" to see the nominal value and "usys.Uncertainty" to interact with the uncertain elements.

To simulate this system, in the block parameters, enter usys for the Uncertain system variable parameter.

Alternatively, set the parameter value at the command line.

ublk = strcat(mdl,"/Uncertain State Space");
set_param(ublk,"USystem","usys");

Simulate Nominal Model

To simulate the model, Simulink must set the uncertain parameters in usys to specific, non-uncertain values. Use the Uncertainty value parameter to specify these values. By default, this parameter is set to [], which causes Simulink to use the nominal values of all uncertain parameters.

Simulate the model. The MultiPlot Graph block generates a plot of the nominal model response to the step input signal.

sim(mdl);

Simulate Specified Samples

To simulate the uncertain model with the uncertain parameters set to values other than the nominal values, set the Uncertainty value parameter to a structure whose fields are the uncertain elements of the uss model. For instance, create a structure samps that sets the damping constant to 1.2 and the spring constant to 1.7.

samps = struct('c',1.2,'k',1.7);

Set the Uncertainty value parameter to samps, and simulate the model. The MultiPlot Graph block adds this new system response to the same axis as the previous response.

set_param(ublk,"UValue","samps");
sim(mdl);

Simulate Random Samples

You can use the usample command to generate samples of usys at random values of the uncertain parameters. The command uvars = ufind(mdl) generates a structure containing all the uncertain parameters in the model.

uvars = ufind(mdl);

usample takes random samples of these parameters and returns a structure you can use for the Uncertainty value parameter. Set Uncertainty value to usample(uvars), and simulate the model.

set_param(ublk,"UValue","usample(uvars)");
sim(mdl);

The step response of the randomly sampled instance of usys is added to the MultiPlot Graph block. Simulate the model ten more times. Each time, usample generates new values for c and k, and the plot is updated with another step response.

for i=1:10
    sim(mdl);
end