0195136047.pdf

16.3 DIGITAL CONTROL SYSTEMS 807

• No drift


• Less effect due to noise and disturbance.

Many physical systems have inherent sampling, and hence their behavior can be described by
sampled data or digital models. Even the dynamics of the social, economic, and biological systems
can be modeled by sampled-data system models.
The block diagram of Figure 16.3.3 is a functional representation of a type of digital control
system, in whichGandHserve the same function as in any feedback system. Note that the
error signal is sampled and a digital processor is used. The controller in this system is the digital
processor whose output, reconverted to an analog signal, becomes the excitation for the blockG.
As usual,Gis the subsystem that provides the output to be controlled. A central computer which
controls several functions could be used as a digital processor; or a microprocessor (special-
purpose computer) designed for the particular control function may also be used as a digital
processor. Large, high-speed computers with their speed, memory, and computational ability,
as well as programmability, are utilized for central control in large automated manufacturing
facilities. The control process may be summarized as follows:

1. The computer is programmed to indicate the sequence of operations required. Data are
   fed continuously from various monitoring stations on the progress of the process.


2. By comparing the actual and desired performance, the computer generates a new set of
   instructions to correct for the deviations.


3. The new set of instructions are usually converted to an analog signal, which in turn forms
   the excitation applied to the machines that actually do the manufacturing.

Examples of Discrete-Data and Digital Control Systems

A fewsimplifiedexamples are presented to illustrate some of the essential components of the
control systems.

SINGLE-AXISAUTOPILOTCONTROLSYSTEM OF ANAIRCRAFT ORMISSILE

Figure 16.3.4(a) shows the block diagram of a simplified single-axis (pitch, yaw, or roll) autopilot
control system with digital data. The objective of the control is to make the position of the airframe
follow the command signal. The rate loop included here helps to improve system stability. Figure
16.3.4(b) illustrates a digital autopilot control system in which the position and rate information
are obtained by digital transducers. Sample-and-hold devices are shown on the block diagram.
While a sampler essentially samples an analog signal at some uniform sampling period, the hold

−

+

Sampler G

H

RE Sampled-data C

error signal

A/D

converter

D/A

converter

Analog

signal

Numeric

signal

Digital

processor

Figure 16.3.3Block diagram of a type of digital control system.