User`s guide

Extracting and Modifying Subsystems
3-5
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 f irst input and third o utp u t.
Accordingly, we use matrix-like subindexing to extract this sub system.
SubSys = sys(3,1)
The resulting subsystem SubSys is an LTI model of the same type as sys,with
its samp le time, time delay, I/O name, and I/O group property v alues inherited
from
sys.
For example,if
sys has an input group named controls consisting of channels
one,two,and three,then
SubSys also hasani nput group named controls with
the first channel of
SubSys assigned to it.
If
sys is a state-space model with ma trices a, b, c, d,thesubsystemsys(3,1)
is a state-space model with data a, b(:,1), c(3,:), d(3,1).
Note:
• In the expression sys(3,1), the first index selects the output channel while
the second index selects the input channel.
• When extracting a subsystem from a given state-space model, the resulting
state-space model may not be minimal. Use the command
sminreal to
eliminate unnecessary states in the subsystem.
You can use similar syntax to modify the LTI model
sys. For example,
sys(3,1) = NewSubSys
redefines the I/O relation between the first input and third output, provided
NewSubSys is a SISO LTI model.
yHu
=
H 31
,()
y
3
H3,1
()
u
1
=