Uncertain State Space

Specify uncertain system in Simulink


The Uncertain State Space block lets you model parametric and dynamic uncertainty in Simulink®. The block accepts uncertain state space (uss) models or any model that can be converted to uss, such as umat, ureal, umargin, and ultidyn objects.


Uncertain system variable (uss)

Linear state-space model with uncertainty (uss object). Specify an uss object using one of the following:

  • Function or expression that evaluates to an uss object. For example:

    • ss(ureal('a',-5),5,1,1)

    • wt*input_unc, where input_unc is an ultidyn object and wt and input_unc are defined in the MATLAB® workspace.

  • Variable name, defined in the MATLAB workspace. For example, unc_sys, where you define unc_sys = ss(ureal('a',-5),5,1,1) in the workspace. This returns an uss object.

  • Model type that can be converted to an uss object. For example:

    • LTI models (tf, zpk and ss)

    • Uncertain matrix (umat)

    • Uncertain real parameters (ureal)

    • Uncertain dynamics (ultidyn).

When the block is in a model with synchronous state control (see the State Control block), you must specify a discrete-time model.

Uncertainty value (struct or [] to use nominal value)

Values of uncertain variables. The uss object that you enter in the Uncertain system variable (uss) field depends on uncertain variables (ureal or ultidyn object). Use this field to specify the values of these uncertain variables to use for simulation or linearization. Specify the value as one of the following:




Use nominal values.


Use user-defined values. For example, struct('a',1) specifies a value of 1 for the uncertain variable a.

Use ufind and usample to generate randomized values of uncertain variables for Monte Carlo simulation. For more information, see Simulate Uncertain Model at Sampled Parameter Values and Vary Uncertain Values Across Multiple Uncertain Blocks in the Robust Control Toolbox User's Guide.

Initial states (nominal dynamics)

If the nominal value of the uncertain state variable, unc_sys.NominalValue where unc_sys is the uncertain system variable specified in the Uncertain system variable field, has states, specify the initial condition for these states. The value defaults to zero.

Initial states (uncertain dynamics)

If the uncertain system contains some dynamic uncertainty (ultidyn), specify the initial state of these dynamics. The value defaults to zero.

Introduced in R2009b