National Instruments 370753C-01 Manual De Usuario

© National Instruments Corporation

The open-loop systems analyzed in Chapter 4, 

, generally 

perform in a satisfactory manner only if the system model is very accurate 
and there are no external disturbances. These conditions usually are not 
met. Feedback presents an effective way to control the output of a system. 
The functions in this chapter address the problem of suitably controlling 
an open-loop plant through output feedback. They are most often applied 
to single-input, single-output (SISO) systems. With the exception of

 

rlocus( )

 and 

bode( )

, these functions also can be used with 

multi-input, multi-output (MIMO) systems.

The key principle of feedback is that the output of a system be fed back, 
compared to a reference or “desired” output value, and then the error 
between the two terms used to correct the system’s output so that it matches 
the reference. The basic diagram of a feedback control system is shown in 
Figure 5-1. 

Figure 5-1.  Feedback Control System Block Diagram

The output of the open-loop system is KH(s); the output of the closed-loop 
system shown in Figure 5-1 is given by:

U(s)

Y(s)

R(s)

K

+

–

H(s)

G(s)

G

cl

(s)

( )

( )

-----------

( )

1 KH s

( )

+

-------------------------

=