Creating and Manipulating Models    

Extracting and Modifying Subsystems

Subsystems relate subsets of the inputs and outputs of a system. The transfer matrix of a subsystem is a submatrix of the system transfer matrix. For example, if sys is a system with two inputs, three outputs, and I/O relation

then gives the relation between the first input and third output.

Accordingly, use matrix-like subindexing to extract this subsystem.

The resulting subsystem SubSys is an LTI model of the same type as sys, with its sample time, time delay, I/O name, and I/O group property values inherited from sys.

For example, if sys has an input group named controls consisting of channels one, two, and three, then SubSys also has an input group named controls with the first channel of SubSys assigned to it.

If sys is a state-space model with matrices a, b, c, d, the subsystem sys(3,1) is a state-space model with data a, b(:,1), c(3,:), d(3,1). Note the following rules when extracting subystems:

You can use similar syntax to modify the LTI model sys. For example,

redefines the I/O relation between the first input and third output, provided NewSubSys is a SISO LTI model.

The following rules apply when modifying LTI models:

Other standard matrix subindexing extends to LTI objects as well. For example,

extracts the subsystem mapping the first two inputs to the third output.

selects the first input and all outputs, and

extracts a subsystem with the same inputs, but only the first and third outputs.

For example, consider the two-input/two-output transfer function

To extract the transfer function from the first input to the first output, type

Next reassign to and modify the second input channel of T by typing


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