Page 37 - Modern Control Systems

Section 1.3 Examples of Control Systems 11
The desired course is compared with a measurement of the actual course in order to
generate a measure of the error, as shown in Figure 1.9(b). This measurement is ob-
tained by visual and tactile (body movement) feedback, as provided by the feel of
the steering wheel by the hand (sensor). This feedback system is a familiar version
of the steering control system in an ocean liner or the flight controls in a large air-
plane. A typical dircction-of-travel response is shown in Figure 1.9(c).
A basic, manually controlled closed-loop system for regulating the level of fluid
in a tank is shown in Figure 1.10. The input is a reference level of fluid that the op-
erator is instructed to maintain. (This reference is memorized by the operator.) The
power amplifier is the operator, and the sensor is visual. The operator compares the
actual level with the desired level and opens or closes the valve (actuator), adjusting
the fluid flow out. to maintain the desired level.
Other familiar control systems have the same basic elements as the system
shown in Figure 1.3. A refrigerator has a temperature setting or desired temperature,
a thermostat to measure the actual temperature and the error, and a compressor
motor for power amplification. Other examples in the home are the oven, furnace,
and water heater. In industry, there are many examples, including speed controls;
process temperature and pressure controls; and position, thickness, composition, and
quality controls [14,17,18].
In its modern usage, automation can be defined as a technology that uses pro-
grammed commands to operate a given process, combined with feedback of infor-
mation to determine that the commands have been properly executed. Automation
is often used for processes that were previously operated by humans. When auto-
mated, the process can operate without human assistance or interference. In fact,
most automated systems are capable of performing their functions with greater ac-
curacy and precision, and in less time, than humans are able to do. A semiautomated
process is one that incorporates both humans and robots. For instance, many auto-
mobile assembly line operations require cooperation between a human operator
and an intelligent robot.
Feedback control systems are used extensively in industrial applications. Thou-
sands of industrial and laboratory robots are currently in use. Manipulators can pick
up objects weighing hundreds of pounds and position them with an accuracy of one-
tenth of an inch or better [28]. Automatic handling equipment for home, school,
and industry is particularly useful for hazardous, repetitious, dull, or simple tasks.

Fluid input

FIGURE 1.10
A manual control
system for
regulating the level
of fluid in a tank by
adjusting fne output
valve. The operator
views the level of
fluid through a port
in the side of the
tank. Valve ' Fluid output