Vary Uncertain Values Across Multiple Uncertain Blocks

This example shows how to simulate a Simulink® model containing multiple Uncertain State Space blocks. You can sample all the uncertain blocks at once using the uvars command. This approach is useful when your model contains large numbers of uncertain variables or Uncertain State Space blocks.

Uncertain Model

Open the model rctMultiUncertainModel.

mdl = "rctMultiUncertainModel";
open_system(mdl)

The model contains two Uncertain State Space blocks. The Unmodeled dynamics block is preconfigured to represent uncertain dynamics with a frequency-dependent weight of the form wt*input_unc.

input_unc = ultidyn('input_unc',[1 1]);
wt = makeweight(0.25,130,2.5);

The other uncertain block is configured to represent a first-order system with an uncertain pole location.

unc_sys = ss(ureal('a',-1,'Range',[-2 -.5]),1,5,0);

A step input feeds the uncertain system, and the MultiPlot Graph block shows the system.

Simulate Nominal Model

To simulate the model, Simulink must set the uncertain parameters in both of these blocks to specific, non-uncertain values. Use the Uncertainty value parameter to specify these values. In rctMultiUncertainModel, both blocks are preconfigured to use the workspace variable vals for that parameter. To simulate the model using the nominal values of all uncertain parameters, set vals = [].

vals = [];
sim(mdl);

Generate Random Sample of All Uncertain Parameters

The ufind command finds all uncertain parameters in all the Uncertain State Space blocks across the entire model, and returns a structure containing their names and values.

uvars = ufind(mdl)

uvars = 

  struct with fields:

            a: [1x1 ureal]
    input_unc: [1x1 ultidyn]

Use usample to generate a random sample of the uncertain parameters in uvars. Set vals to this sample value.

vals = usample(uvars)

vals = 

  struct with fields:

            a: -0.7779
    input_unc: [1x1 ss]

The Uncertainty value parameter in each Uncertain State Space block is already set to vals. When you simulate the model, for each block, Simulink uses the value in vals that corresponds to the uncertain parameters in that block.

sim(mdl)

Simulate Multiple Random Samples

To simulate the model at multiple random values, repeat the process of generating random values for vals inside a for loop. Each time, usample generates new values for the uncertain elements in the model, and the plot is updated with another step response.

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