0195136047.pdf

806 BASIC CONTROL SYSTEMS

(a)

(b) (c)

−

+

Sampler

Input

r(t)

Error signal

e(t)

e(t)

Data

hold

(filter)

Discrete data

e*(t)

e*(t)

p

2 T

0

T 3 T 4 T

t t

Output

c(t)

Controlled

process

Figure 16.3.1(a)Typical closed-loop sampled-data control system.(b)Continuous-data input to sampler

of (a).(c)Discrete-data output of sampler of (a).

samplings. Incorporating sampling into a control system has several advantages, including that

of time sharing of expensive equipment among various control channels.

The purpose of the filter located between the sampler and the controlled process is for

smoothing, because most controlled processes are normally designed to receive analog signals.

Figure 16.3.2 illustrates a typical digital control system, in which the signal at one or more

points of the system is expressed in a numerical code for digital-computer or digital-transducer

processing in the system. Because of the digitally coded (such as binary-coded) signals in some

parts of the system, it becomes necessary to employ digital-to-analog (D/A) and analog-to-digital

(A/D) converters. In spite of the basic differences between the structures and components of a

sampled-data and a digital control system, from an analytical standpoint both types of systems

are treated by the same analytical tools.

Sampled data and digital control offer several advantages over analog systems:

• More compact and lightweight


• Improved sensitivity


• Better reliability, speed, and accuracy


• More flexibility and versatility (in programming)


• Lower cost


• More rugged in construction

Digital

coded

input Digital Output

computer

Controlled

Digital-to-analog converter process

Analog-to-digital converter

Figure 16.3.2Typical digital control system.