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Automatic Control System
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-1-CHAPTER - 1
Introduction to Automatic Control Systems
1.1. INTRODUCTION:
The objective of this chapter is to familiarize the reader with the following subjects:
1. What a control system is.
2. Why control systems are important.
3. What the basic components of a control system are.
4. Types of control systems.
5. Why feedback is incorporated into most control systems.
6. Some examples of control system applications.
A basic control system is an arrangement of physical components connected or related in a manner to
command, direct or to regulate itself or another system. Any control system requires an input and the controlled
quantity becomes the output which follows the input. The difference between the input and the output is called
error. In a sentence control systems are in abundance in modern civilization.
In recent years, control systems have assumed an increasingly important role in the development and
advancement of modern civilization and technology. Practically every aspect of our day-to-day activities is
affected by some type of control systems. Control systems are found in abundance in all sectors of industry, such
as quality control of manufactured products, automatic assembly line, machine-tool control, space technology and
weapon systems, computer control, transportation systems, robotics, MicroElectroMechanical Systems(MEMS),
nanotechnology, and many others. Even the control of inventory and social and economic systems may be
approached from the theory of automatic control.
1.2. BASIC COMPONENTS OF A CONTROL SYSTEM:
The basic ingredients of a control system can be described by:
1. Objectives of control.
2. Control-system components.
3. Results or outputs.
The basic relationship among these three components is illustrated in fig1.2.1. In more technical terms,
the objectives can be identified with inputs, or actuating signals, u, and the results are also called outputs, or
controlled variables, y. In general, the objective of the control system is to control the outputs in some prescribed
manner by the inputs through the elements of the control system.
Objectives Results
Control System r
Figure 1.2.1 Basic components of a control system.
1.3. TYPES OF CONTROL SYSTEMS:
The control system can be divided into two groups namely open-loop and closed-loop control system.
1.3.1. Open-Loop Control System:
The open loop control system is also known as control system without feedback or non feedback control
systems. In open loop systems the control action is independent of the desired output. In this system the output is
not compared with the reference input.
The component of the open loop systems are controller and controlled process, as shown by the block
diagram of fig 1.3.1. An input signal or command r is applied to the controller, whose output acts as the actuating
signal u; the actuating signal then controls the controlled process so that the controlled variable y (desired output)
will perform according to some prescribed standards. In simple cases, the controller can be an amplifier,
mechanical linkage, filter, or other control elements, depending on the nature of the system. In more sophisticated
cases, the controller can be a computer such as a microprocessor. Because of the simplicity and economy of
open loop control systems, we find this type of system in many non critical applications.
, -2-
Reference input r Actuating signal u Controlled variable y
Controller Controlled
Process
Figure 1.3.1 Elements of an open-loop control system.
Examples of open-loop control systems:
1. Automatic washing machine is the example of the open loop systems. In the machine the operating time is
set manually. After the completion of set time the machine will stops, with the result we may or may not
get the desired (output) amount of cleanliness of washed cloths because there is no feedback is provided
to the machine for desired output.
2. Immersion rod is another example of open loop system. The rod heats the water but how much heating is
required is not sense by the rod because of no feedback to the rod.
3. A field control d.c motor is the example of open loop system.
4. An electric switch in which a man-made control system controls the flow of electricity. The apparatus or
person flipping the switch is not a part of this control system. Flipping the switch on or off may be
considered as the flow or non-flow of electricity. This becomes an open loop control system because the
control action is independent.
5. The operation of ordinary traffic signals which control traffic at roadway intersections is another example
of an open loop control system wherein all control signals are pre-set by timing mechanisms.
Advantages of open-loop control systems:
1. Open loop control systems are simple.
2. Open loop control systems are economical.
3. Less maintenance is required and not difficult.
4. Proper calibration is not a problem.
Disadvantages of open-loop control systems:
1. Open loop systems are inaccurate.
2. These are not reliable.
3. Optimization is not possible.
1.3.2. Closed-Loop Control System:
Closed loop control systems are also known as feedback control systems. In closed loop control systems the
control action is dependent on the desired output. If any system having one or more feedback paths forming a
closed loop system. In closed loop systems the output is compared with the reference input and error signal is
produced. The error signal is fed to the controller to reduce the error and desired out put is obtained. Fig 1.3.2
illustrates the block diagram of a basic closed-loop control system in which the control action (input) is dependent
upon the controlled variable (output). The output variable c(t) is compared with the reference input r(t).This
comparison is done by an element called error detector. The output of the error detector is the actuating signal
e(t).
Error
signal
Reference Amplifier Controller Controlled
Controlled
Input r(t) + Process
Output c(t)
Feedback
Figure 1.3.2 Block diagram of closed-loop system.
Examples of closed-loop control systems:
1. In a room we need to regulate the temperature & humidity for comfortable living. Air conditioners are
provided with thermostat. By measuring the actual room temperature & compared it with desired temperature, an
error signal is produced, the thermostat turns ON the compressor or OFF the compressor. The block diagram is
shown in fig 1.3.3,
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