MATLAB CONTROL SYSTEM TOOLBOX 9 Betriebsanweisung PDF herunterladen (Seite 107)

100

101

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.

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