MATLAB CONTROL SYSTEM TOOLBOX 9 Spécifications télécharger pdf (Page 38)

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101

2 LTI Models

2-20

By convention, the sample time of continuous-time models is Ts = 0.Setting

Ts = –1 leaves the sample time of a discrete-time model unspecified. For

example,

h = tf([1 –0.2],[1 0.3],–1)

produces

Transfer function:

z – 0.2

-------

z + 0.3

Sampling time: unspecified

Note Do not simply omit Ts in this case. This would make h a

continuous-time transfer function.

If you forget to specify the sample time when creating your model, you can still

set it to the correct value by reassigning the LTI property

Ts.See“Sample

Time” on page 2-33 for more information on setting this property.

Discrete-Time TF and ZPK Models

Youcanspecifydiscrete-timeTF andZPKmodelsusingtf andzpk as indicated

above. Alternatively, it is often convenient to specify such models by:

1 Defining the variable z as a particular discrete-time TF or ZPK model with

the appropriate sample time

2 Entering your TF or ZPK model directly as a rational expression in z.

This approach parallels the procedure for specifying continuous-time TF or

ZPK models using rational expressions. This procedure is described in “SISO

Transfer Function Models” on page 2-8 and “SISO Zero-Pole-Gain Models” on

page 2-12.

For example,

z = tf('z', 0.1);

H = (z+2)/(z^2 + 0.6*z + 0.9);

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