ElmoMC SimplIQ Digital Servo Drives-Bell Getting Started - Manual (Page 90)

The SimplIQ for Steppers Getting Started & Tuning and Commissioning Guide
MAN-BELGS (Ver. 1.1)
90
B.4 Feedback Control
The system we consider here is shown in block diagram form in Figure 84, where
P denotes the controlled transfer function (the plant).
C(s) P(s)
u y r
Sensor
-
Figure 84: Block diagram of a simple feedback system
The input to the plant in Figure 84, is the output of the controller
( )
s C , whose
input is the difference of two signals:
The command input, r , which is an external input to the system. The external
command does not depend on the system’s output and the user has complete
control over it.
The feedback input, which is the measured output of the plant.
A system as in Figure 74, that is, without the feedback, is called an open-loop
system. A system whose input depends on its output is called a closed-loop
system or a feedback system – for example the feedback system described in
Figure 84.
B.4.1 Why Feedback is Required
Given a plant, P , its output, y is generated by its input, u and disturbance, d .
d Pu y
+ = (13)
Our goal is to achieve a desired output using the input. A desired output,
r
y , can
be achieved using the input
( )
d y P u
r r
− = −1 (14)
The synthesis of (14) does not use the measured plant output, y . This open-loop
solution may not be applicable because of the following reasons:
The plant model is not known exactly.
The plant, even if known, may not be stably invertible.
The disturbance,
d , is not known or partially known, thus as before,
r
u which
depends on d may not be accurate enough.
By embedding the plant in a feedback structure, it is possible to bypass these
difficulties and thus achieve desired outputs to a very high degree of accuracy.
However, use of feedback has two major drawbacks: