National Instruments 370753C-01 Manual De Usuario

[num,den] = numden(Sys)

The 

numden( )

 function returns the numerator and denominator 

polynomials comprising a single-input, single-output system in transfer 
function form. If the system is in state-space form, an internal conversion 
is performed to find the transfer function equivalent, but the format of the 
system variable itself remains unchanged. State-space systems used in 
conjunction with 

numden( )

 must be single-input, single-output. 

As noted in the 

 section, common roots in 

the numerator and denominator polynomials are not canceled.

Example 2-5 uses the state-space system from Example 2-2 to illustrate the 
use of 

numden( )

.

sys4=system([0,1;-0.75,0],[1,0]',[0,1],0,

{dt=0.5});

[num,den] = numden(sys4)?

num (a polynomial) =

-0.75

den (a polynomial) =

(z

2

 + 0.75)

Because 

num

 and 

den

 are polynomial objects and not a complete system, 

the discrete sampling time is not explicitly saved. You can use 

check( )

 

with the 

convert

 keyword to map the two internal representations to each 

other, as described in the 

 section. 

However, notice that z was used as the polynomial variable, indicating that 
these numerator and denominator polynomials were obtained from a 
discrete-time system. Had the system been continuous, s would have been 
used instead of z.

dt = period(Sys)

The 

period( )

 function extracts the sample period (in seconds) of a 

system. If the system is continuous, 

period( )

 will return zero. 

In Example 2-5, you found the numerator and denominator polynomials 
corresponding to the discrete state-space system. Example 2-6 combines