Specifications

Precedence and Property Inheritance
3-3
Precedence and Property Inheritance
You can apply operations to LTI models of different types. The resulting type
is then determined by the rules discussed in “Precedence Rules” on page 2-5.
For example, if
sys1 is a transfer function and sys2 is a state-space model,
then the result of their addition
sys = sys1 + sys2
is a state-space model, since state-space models have precedence over transfer
function models.
To supersede the precedence rules and force the result of an operation to be a
given type, for example, a transfer function (TF), you can either:
•Convert all operands to TF before performing the operation
•Convert the result to TF after performing the operation
Suppose, in the above example, you want to compute the transfer function of
sys.Youcaneitheruseaprioriconversion of the second operand
sys = sys1 + tf(sys2);
or a posteriori conversion of the result
sys = tf(sys1 + sys2)
Note These alternatives are not equivalent numerically; computations are
carried out on transfer functions in the first case, and on state-space models in
the second case.
Anotherissueispropertyinheritance,thatis,how the operandpropertyvalues
are passed on to the result of the operation. While inheritance is partly
operation-dependent, some general rules are summarized below:
•In operations combining discrete-time LTI models, all models must have
identical or unspecified (
sys.Ts = –1) sample times. Models resulting from
such operations inherit the specified sample time, if there is one.
•Most operations ignore the
Notes and Userdata properties.