basic control system unit1

E3145/1/1INTRODUCTION TO CONTROL SYSTEM

INTRODUCTION TO CONTROL SYSTEM

OBJECTIVES

General Objective : To know and to apply the concept of control system.

Specific Objectives : At the end of the unit you will be able to :

Explain the basic concept of control system with general block diagram of control system.

State the definition and terminology of control system elements.

Classify and differentiate the two categories of control system : open-loop and closed-loop systems.

Draw the block diagram of electrical equipments based on control system types.

List the advantages and disadvantages of open-loop and closed-loop control systems.

UNIT1


1.0 THE BASIC CONCEPT OF CONTROL SYSTEM

Automatic control has played a vital role in the advance of engineering and science. In addition to its extreme importance in space-vehicle system, missile-guidance systems, air-craft-autopiloting systems, robotic systems and the likes, automatic control has become an important and integral part of modern manufacturing and industrial processes. For example, automatic control is essential in the numerical of machine tools in the manufacturing industries.

The controlled variable is the quantity or condition that is measured and controlled. The manipulated variable is the quantity or condition that is varied by the controller so as to affect the value of the controlled variable. Normally, the controlled variable is the output of the system. Control means measuring the value of the controlled variable of the system and applying the manipulated variable to the system to correct or limit deviation of the measured value from a desired value.

INPUTINPUT

Control system is a characteristic which influences our lifes today. We use this system in many fields. For example, in industries, domestic, medical, robotics, generation of electrical energy and so on.


In studying control engineering, we first need to define additional terms that are necessary to describe control systems, such as plants,

disturbances, processes, feedback control systems and etc. Then a description of closed-loop and open-loop control systems and their advantages and disadvantages will be given in the following sections.

1.1 IDENTIFYING THE DEFINITION AND TERMINOLOGY RELATED TO CONTROL SYSTEM

The various definitions of the system variables and components are as mentioned below:

1.1.1 Pneumatic Control Systems

The working medium in a pneumatic control systems uses a compressible fluid, such as air because it may be exhausted to the atmosphere at the end of the device’s work cycle, thus eliminating the need for return lines.

1.1.2 Hydraulic Control Systems

Hydraulic control systems is the study of incompressible liquids, and hydraulic devices use an incompressible fluid, such as oil, for their working medium. Liquid level systems consisting of storage tanks and connecting pipes are a class of hydraulic systems whose driving force is due to relative difference in the liquid heights in the tanks.

1.1.3 Reference Input

It is the actual signal input to the control system.

1.1.4 Process


Any operation to be controlled. For example, chemical, economic, and biological processes.

1.1.5 Feedback Element

It is the unit which provides the means for feeding back to the output quantity in order to compare it with the reference input.

1.1.6 Disturbances

A disturbance is a signal that tends to adversely affect the value of the output of a system. If the disturbance is generated within the system, it is called internal, while an external disturbance is generated outside the system and is an input.


Activity 1A

TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT INPUT…!

1.1 Define the elements given below :a) Controlled variableb) Manipulated variablec) Control

1.2Define the elements given below :

a) Reference inputb) Feedback elementc) Disturbancesd) Processe) Hydraulic Control Systemf) Pneumatic Control System


Feedback To Activity 1A

1.1 a) Controlled variable is the quantity or condition that is measured and controlled. Controlled variable is the output of the system.

b) Manipulated variable is the quantity or condition there is varied by the controller so as to affect the value of the controlled variable.

c) Control means measuring the value of the controlled variable of the system and applying the manipulated variable to the system to correct or limit deviation of the measured value from a desired value.

1.2 a) Reference InputIt is the actual signal input to the control system.

b) Feedback ElementIt is the unit which provides the means for feeding back to the output quantity in order to compare it with the reference input.

c) Disturbances


A disturbance is a signal that tends to adversely affect thevalue of the output of a system. If the disturbance is generatedwithin the system, it is called internal , while an externaldisturbance is generated outside the system and is an input.

d) ProcessAny operation to be controlled. For example, chemical, economic, and biological processes.

e) Hydraulic Control Systems Hydraulic control systems is the studied of incompressible liquids, and hydraulic device use an incompressible fluid, such as oil, for their working medium. Liquid level systems consisting of storage tanks and connecting pipes are a class of hydraulic systems whose driving force is due to relative difference in the liquid heights in the tanks.

f) Pneumatic Control SystemsThe working medium in a pneumatic control systems using a compressible fluid, such as air because it may be exhausted to the atmosphere at the end of the device’s work cycle, thus eliminating the need for return lines.


1.2 EXPLANATION OF CONTROL SYSTEM TYPES

A control system may consists of a number of components. In order to show the functions performed by each component, in control engineering, we commonly use a diagram called the block diagram.

A block diagram of a system is a pictorial representation of the functions performed by each component and of the flow of signals. In a block diagram all system variables are linked to each other through functional blocks. The functional block or simply block is a symbol for the mathematical operation on the input signal to the block that produces the output.

Figure 1.1 shows an element of the block diagram. Such arrows are referred to as signals.

INPUTINPUT

Two types of control systems: a) open-loop systemb) closed-loop system

TransferFunction

G(S)


Figure 1.1 Element of a block diagram(Source : Katsuhiko Ogata (1990), Modern Control Engineering)

The advantages of the block diagram representation of a system lies in the fact that it is easy to form the overall block diagram for the entire system by merely connecting the blocks of the components according to the signal flow and that it is possible to evaluate the contribution of each component to the overall performance of the system.

Summing PointReferring to Figure 1.2, a circle with a cross is the symbol that

indicates a summing operation. The plus or minus sign at each arrowhead indicates whether that signal is to be added or subtracted.

Figure 1.2 Summing point(Source : Katsuhiko Ogata (1990), Modern Control Engineering)

Branch pointA branch point is a point from which the signal from a block goes

concurrently to other blocks or summing points.

Control systems are classified into two general categories: ☺ open-loop system☺ closed-loop system


The distinction is determined by the control action, which is that quantity responsible for activating the system to produce the output.

1.2.1 Open-loop Control System

An open-loop control system is one in which the control action is independent of the output. Figure 1.3 shows the block diagram of an open-loop control system (basic system) and Figure 1.4 shows the block diagram of an open-loop control system (automobile driving system).

Figure 1.3 An open-loop control systems (basic system)(Source : S.P. Eugene Xarier & Joseph Cyril Babu.J (1999), Principles of Control System)

Controller

Reference

Input

Process

Variable

Output

Accelarator pedal, links and carburettor

Signal

Force

Input (command)

Engine VehicleVariable

Speed

Output (controlled)


Figure 1.4 An open-loop control systems (Automobile driving system)(Source : S.P. Eugene Xarier & Joseph Cyril Babu.J (1999), Principles of Control System)

1.2.2 Closed-loop Control System

Closed-loop control systems are more commonly called feedback control systems. Feedback is the characteristic of closed-loop control systems which distinguishes them from open-loop systems. It is the property of closed-loop control systems which permits the output to be compared with the input of the system so that the appropriate control action may be formed as a function of the output and input. In general, feedback is said to exist in a system when closed sequence of cause-and-effect relation exists between system variables.

Figure 1.5 General block diagram of an automatic control system(Source : Katsuhiko Ogata (1990), Modern Control Engineering)

Reference

Input Output

Controlled

Error detector

Controller

Controller Process

Feedback path elements


1.2.3 Comparison between open-loop and closed-loop control systems

Open-loop Control System

The important features of open-loop control systems are :i. Their ability to perform accurately is determined by their

calibration, which simply implies, to establish the input-output relation to obtain a desired system accuracy.

ii. They are not generally troubled with problems of instability.

Closed-loop Control System

The important features of feedback are :i. Reduced effects of nonlinearities and distortionii. Increased accuracyiii. Increased bandwidthiv. Reduced sensitivity of the ratio of the output to input to

variations in system characteristics.v. Tendency towards oscillation or instability.


1.3 EXAMPLES OF APPLICATION CONTROL SYSTEMS TYPES ON ELECTRICAL EQUIPMENTS

1.3.1 Open-loop control system

1. Washing machineSoaking, washing, and rinsing in the washer operate on a time basis. The machine does not measure the output signal, that is, the cleanliness of the clothes.

2. Electric coffeemakerOne possible input for automatic electric coffeemaker is the amount of coffee used. Most coffeemakers have a dial which can be set for weak, medium or strong coffee. This setting usually regulates a timing mechanism. The brewing-time is therefore another possible input. The output of any coffeemaker can be chosen as coffee strength.

Examples of application of control system on electrical equipments : washing machine, toaster, coffeemaker, refrigerator, room lamp and etc.


1.3.2 Closed-loop System

1. Automatic toasterAssume that each heating element supplies the same amount of heat to both sides of the bread, and toast quality can be determined by its colour. The toaster is initially calibrated for a desired toast quality by means of the colour adjustment knob. This setting never needs readjustment unless the toast quality criterion changes. When the switch is closed, the bread is toasted until the colour detector sees the desired colour. Then the switch is automatically opened by means of the feedback linkage, which may be electrical or mechanical.

2. Automatic RefrigeratorThe input is the reference temperature and the output is the actual refrigerator temperature.


Activity 1B

TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT INPUT…!

1.3Many closed-loop and open-loop control system may be found in homes. Identify the electrical equipment below and describe them of the system is open-loop or closed-loop control systems.a. Electrical ovenb. Blenderc. Electric automatic kettled. Irone. Lamp

1.4State the differences between open-loop and closed-loop control systems.

1.5 Give the two general categories of control system.


Feedback To Activity 1B

1.3 a. Closed-loop control systemb. Open-loop control systemc. Closed-loop control systemd. Closed-loop control systeme. Open-loop control system

1.4

Open-loop control system Closed-loop control system

i. Their ability to perform accurately is determined by their calibration, which simply implies, to establish the input-output relation to obtain a desired system accuracy.

ii. They are not generally troubled with problems of instability.

i. Reduced effects of nonlinearities and distortion

ii. Increased accuracy

iii. Increased bandwidth

iv. Reduced sensitivity of the ratio of the output to input to variations in system characteristics.

v. Tendency towards oscillation or instability.


1.5 a) Open-loop systemb) Closed-loop system

KEY FACTS

1. Control system are classified into two general categories : open-loop and closed-loop systems.

2. In control engineering, to show the function performed by each component, we commonly use a diagram called the block diagram.


SELF-ASSESSMENT

You are approaching success. Try all the questions in this self-assessment section and check your answers with those given in the Feedback on Self-Assessment given on the next page. If you face any problems, discuss it with your lecturer. Good luck.

Q1-1 Name the major parts of a closed-loop control system.

Q1-2 (a) Draw the block diagram of open-loop and closed-loop control

systems.

(b) List the advantages of the block diagram.

(c ) Draw the general block diagram of an automatic control system.


Feedback To Self-Assessment

Have you tried the questions????? If “YES”, check your answers now.

Q1-1 i. Reference inputii. Controlleriii. Processiv. Outputv. Feedback element

Q1-2

(a) i. Open-loop control system

Controller

Reference

Input

Process

Variable

Output


ii. Closed loop control system

(b) The advantages of the block diagram representation of a system lies in the fact that it is easy to form the overall block diagram for the entire system by merely connecting the blocks of the components according to the signal flow and that it is possible to evaluate the contribution of each component to the overall performance of the system.

Reference

Input Output

Controlled

Error detector

Controller

Controller Process



(c) General block diagram of an automatic control system

Reference

Input Output

Controlled

Error detector

Controller

Controller Process


basic control system unit1

Education

control system value

control system introduction

control system feedback

control system activity

control system types

inputinput control system

categories of control

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