Chapter 2 Linear System Representation
© National Instruments Corporation 2-7 Xmath Control Design Module
The system( ) function can create both the transfer-function and
state-space forms of the system object. It requires four compatibly-sized
matrices to create a state-space system, or a pair of polynomials to create
a transfer function.
You can use optional keywords to store additional information about your
system. Assigning dt to a positive scalar value indicates that the system is
discrete, with a sampling period equal to that value. If dt is not specified,
the system is continuous, with a sampling period defaulting to zero.
Because information indicating whether the system is continuous or
discrete is encapsulated within the system object itself, Xmath does not
have separate functions for discrete- and continuous-time system analysis.
Systems can be recognized by Xmath’s functions as discrete or continuous
using the check( ) function and handled accordingly. For more
information, refer to the Using check( ) with System Objects section.
The capability to assign a discrete sample rate does not actually discretize
a continuous-time system, however. For information on discretizing a
system, refer to the Discretizing a System section.
A shortcut for creating state-space systems with an all-zero D matrix is
to use a null-matrix specifier ([]) for the D matrix instead of entering an
appropriately sized zero matrix. This will automatically set the D matrix to
be a zero matrix with row size equal to the row size of C, and column size
equal to the column size of B.
In addition, descriptive names for the inputs and outputs of a system can
be specified as vectors of string names and assigned to the inputNames,
outputNames, and stateNames keywords. stateNames is valid only
when used in conjunction with a state-space system, as is the keyword X0,
which can be used to set a vector of initial values for the states.
When you have created a system, you can modify it by changing the values
of any of the keywords discussed in this section by calling system( ) with
the appropriate keyword setting.
Examples 2-1 and 2-2 illustrate how system( ) can be called to create a
transfer function and state-space system, respectively. system( ) also can
be used to change the attributes of an existing system.
Note In Example 2-3, the [] notation indicates that the D matrix should be an
appropriately sized (in this case, scalar) zero matrix.