User`s guide

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 wa nt to compute the transfer function of
sys.Youcaneitheruseaprioriconversion of the second operand
sys = sys1 + tf(sys2);
or aposterioriconversion 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.
Anotherissueis propertyinheritance, thatis, howthe operandproperty values
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 resulti ng from
such operations inherit the specified sample time, if there is one.
• Most operations ignore the
Notes and Userdata properti es.