Modern Control: Technology Components

CHAPTER

1
OBJECTIVES
After studying this chapter, you should be able to: • Distinguish between open-loop and closed-loop control systems. • Understand control system block diagrams. • Explain transfer functions. • Differentiate between analog and digital control systems. • Know how process control systems work. • Know how servomechanisms work.

Introduction to Control SystemsINTRODUCTION
A control system is a collection of components working together under the direction of some machine intelligence. In most cases, electronic circuits provide the intelligence, and electromechanical components such as sensors and motors provide the interface to the physical world. A good example is the modern automobile. Various sensors supply the on-board computer with information about theengine’s condition. The computer then calculates the precise amount of fuel to be injected into the engine and adjusts the ignition timing. The mechanical parts of the system include the engine, transmission, wheels, and so on. To design, diagnose, or repair these sophisticated systems, you must understand the electronics, the mechanics, and control system principles. In days past, so-calledautomatic machines or processes were controlled either by analog electronic circuits, or circuits using switches, relays, and timers. Since the advent of the inexpensive microprocessor, more and more devices and systems are being redesigned to incorporate a microprocessor controller. Examples include copying machines, soft-drink machines, robots, and industrial process controllers. Many of these machinesare taking advantage of the increased processing power that comes with the microprocessor and, as a consequence, are becoming more sophisticated and are including new features. Taking again the modern automobile as an example, the original motivation for the on-board computer was to replace the mechanical and vacuum-driven
1

2

CHAPTER 1

subsystems used in the distributor andcarburetor. Once a computer was in the design, however, making the system more sophisticated was relatively easy—for example, selfadjusting fuel/air ratio for changes in altitude. Also, features such as computer-assisted engine diagnostics could be had without much additional cost. This trend toward computerized control will no doubt continue in the future.

1.1 CONTROL SYSTEMS
Introduction andBackground
In a modern control system, electronic intelligence controls some physical process. Control systems are the “automatic” in such things as automatic pilot and automatic washer. Because the machine itself is making the routine decisions, the human operator is freed to do other things. In many cases, machine intelligence is better than direct human control because it can react faster or slower(keep track of long-term slow changes), respond more precisely, and maintain an accurate log of the system’s performance. Control systems can be classified in several ways. A regulator system automatically maintains a parameter at (or near) a specified value. An example of this is a homeheating system maintaining a set temperature despite changing outside conditions. A follow-up system causes anoutput to follow a set path that has been specified in advance. An example is an industrial robot moving parts from place to place. An event control system controls a sequential series of events. An example is a washing machine cycling through a series of programmed steps. Natural control systems have existed since the beginning of life. Consider how the human body regulates temperature. If the bodyneeds to heat itself, food calories are converted to produce heat; on the other hand, evaporation causes cooling. Because evaporation is less effective (especially in humid climates), it is not surprising that our body temperature (98.6°F) was set near the high end of Earth’s temperature spectrum (to reduce demand on the cooling system). If temperature sensors in the body notice a drop in...