Basic Control System unit5

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INTRODUCTION TO PLC
INTRODUCTION TO PLC
OBJECTIVES
General Objective : To apply and integrate the concept of PLC
Specific Objectives : At the end of the unit you will be able to :
 Identify the PLC terminology
 Explain the PLC historical background
 Describe the functions of PLC
 Explain the advantages of PLC
 Draw the PLC block diagram including processor unit,
memory, the power supply unit, input and output
interface and the programming device
 Explain and draw the logic function LD, OR, AND, AND
NOT, LD NOT, OUT, TIM, CNT
UNIT5

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INTRODUCTION TO PLC
 Design and develop ladder program
5.0 EXPLANATION OF PLC
In today’s fast-moving, highly competitive industrial world, a company
must be flexible, cost-effective and efficient if it wishes to survive. In the
process and manufacturing industries, this has resulted in a great demand
for industrial control systems in order to streamline operations in terms of
speed, reliability and product output.
Control systems such as hard-wire relay, logic and computer systems,
can and do provide effective control of industrial processes and plant.
However, each of this systems has limitations or disadvantages that may
often be overcome through the use of a Programmable Logic Controller
(PLC).
What is a Programmable Logic Controller?
A PLC is a digital electronics system which uses a programmable
memory for implementing specific functions such as logic, sequencing,
timing, counting and arithmetic to control through analog or digital
input/output modules, various types of machines or processes.
INPUTINPUT
Do you have a motorcycle?
If the answer is yes, explain
to your students of the PLC
system which is used in the
traffic light.

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INTRODUCTION TO PLC
5.1 PLC TERMINOLOGY
In general, a control systems is a collection of electronic
devices and equipment which are in place to ensure the stability,
accuracy and smooth transition of a process or a manufacturing
activity. It takes any form and varies in scale of implementation, from a
power plant to a semiconductor machine. As a result of rapid
advancement of technology, complicated control tasks accomplished
with a highly automated control system, which may be in the form of
Programmable Controller (PLC) and possibly a host computer , etc.
Besides signal interfacing to the field device (such as operator
panel, motors, sensors, switches, solenoid valves and etc.),
capabilities in network communication enable a big scale
implementation and process coordination besides providing greater
flexibility in realizing distributed control system. Every single
component in a control system plays an important role regardless of
size. For instance, as shown in Figure 5.1 the PLC would not know
the happenings around it without any sensing devices. It is also
unable to activate any moving mechanism if there is no motor
installed. And if necessary, an area host computer has to be in place
to coordinate the activities in a specific area at the shop floor.
(Source :
Manual
OMRON (1999),
Beginner Guide
To PLC)

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INTRODUCTION TO PLC
Figure 5.1 : It could also be an application as small as single PLC
controlling a single or some output service
5.2 PLC HISTORICAL BACKGROUND
PLCs were used mainly in the motor industries in the early 1970s
where they replaced large relay panels. Not only do PLCs take up much
lesser space than the relay system, they are also more reliable in operation
over longer periods. One important point to take note is that PLCs are very
flexible in terms of modifying or changing the control sequences. It is now
possible to change or modify the control system without having to connect or
disconnect a single wire. It is simply done by changing the program
(software), using a program console or graphic programming panel attached
to the programmer logic controller.
Industry demands on PLCs are increasing rapidly and this has
encouraged manufacturers to develop whole families of microprocessors-
based systems having various level of performance. Available PLCs now
range from small self-contained units to sophisticated modular systems with
a range of add-on function modules for tasks such as analog input/output
and communications. This modular system allows the expansion or
upgrading of a control system with minimum costs and interruption.
In 1968, a group of engineers from General Motors developed the
concept of PLC with an initial specification. The PLC must be :
1) Easy to program and reprogram
2) Easy to maintain and repair
3) More reliable in an industrial environment
4) Smaller in size than its relay equivalent
5) Cost-competitive
PLCs highly desireable in
a wide variety of industrial-
plant and process-control
application.

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INTRODUCTION TO PLC
5.2.1 Developments of PLC
YEARS DESCRIPTION
1968 The concept of PLC was developed
1969 A hardware CPU controller was used with logic
instructions. It comes with 1K of memory and 128 I/O
points
1974 Using several processors within a PLC with add-on
timers and counters functions. Arithmetic operations
are also included. This PLC has 12K of memory and
1024 I/O points
1976 Remote I/O systems were introduced.
1977 Microprocessor-based PLC was introduced
1980 Intelligent I/O modules were developed
1983 Small low-cost PLCs were introduced
1985 onwards With networking capabilities
Table 5.1 PLC historical background
(Source : Manual SMC International Training (1999), Introduction to PLC Controller)

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INTRODUCTION TO PLC
Activity 5A
TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE
NEXT INPUT…!
PART 1
Questions 1 to 5 have four options: A, B, C or D. Choose the correct
answer from the options given.
5.1 The term PLC stands for :
A Personal Logic Computer
B Programmable Local Computer
C Personal Logic Controller
D Programmable Logic Controller
5.2 Which of the following is most likely to be the voltage level used
internally in a PLC, excluding the voltage levels that might occur during
conditioning in output/input channels:
A 5V
B 24V
C 110V
D 240V
5.3 The cycle time of a PLC is the time it takes to:
A Read an input signal
B Read all the input signals
C Check all the input signals against the program
D Read all the inputs, run the program and update all outputs

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INTRODUCTION TO PLC
PART 2
5.4 What is a Programmable Logic Controller (PLC)

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INTRODUCTION TO PLC
Feedback To Activity 5A
PART 1
5.1 D
5.2 A
5.3 D
PART 2
5.4 A PLC is a digital electronics system which uses a
programmable memory for implementing specific functions
such as logic, sequencing, timing, counting and arithmetic to
control through analog or digital input/output modules, various
types of machines or processes.

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INTRODUCTION TO PLC
5.3 FUNCTIONS OF PLC
PLCs are used in many “real world” applications. If there are
industries present, chance are good that there is a PLC present. If you are
involved in machining, packaging, material handling, automated assembly or
countless other industries you are probably already using them. If you are
not, you are wasting money and time. Almost any application that needs
some type of electrical control has a need for a PLC.
For example, let’s assume that when a switch turns on we want to turn
a solenoid on for 5 seconds and then turn it off regardless of how long the
switch is on for. We can do this with a simple external timer. But what if the
process included 10 switches and solenoids? We would need 10 external
timers. What if the process also needed to count how many times the
switches individually turned on? Thus we need a lot of external counters.
INPUTINPUT
PLCs are made to operate
for long periods of time in
adverse industrial
environment. For example,
temperature, humidity etc.

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INTRODUCTION TO PLC
As you can see the more bigger process you need, we have a PLC.
We can simply program the PLC to count its inputs and turn the solenoids for
the specified time.
There are so many applications where PLCs are used in the various
industries. Below is the list of applications :
i. conveyor system
ii. swimming pool
iii. traffic light system
iv. lift control system
v. food processing
vi. pick and place robot control
vii. packaging machine
viii. security control system
ix. car manufacturing plant
x. printing industries
xi. air condition control
xii. cement manufacturing
5.4 ADVANTAGES OF PLC
The following are the major advantages that can be distinguishably
realized :
i. The wiring of the system usually reduces by 80% compared
to conventional relay control system.
ii. The power consumption is greatly reduced as PLC consumes
much less power
iii. The PLC self diagnostic functions enable easy and fast
trouble shooting of the system

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INTRODUCTION TO PLC
iv. Modification of control sequence or application can easily be
done by programming through the console or computer
software without changing of I/O wiring, if no additional Input
or Output devices are required.
v. In PLC system spare parts for relay and hardware timers are
greatly reduced as compared to conventional control panel.
vi. The machine cycle time is improved tremendously due to the
speed of PLC operation is a matter of milliseconds. Thus,
productivity increases.
vii. It cost much less compared to conventional system in
situations when the number of I/Os is very large and control
functions are complex.
viii. The reliability of the PLC is higher than the mechanical relays
and timers.
ix. An immediate print out of the PLC program can be done in
minutes. Therefore, hardcopy of documentation can be easily
maintained.

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INTRODUCTION TO PLC
Activity 5B
NEXT INPUT…!
5.5 State five (5) applications of PLC.
5.6 State the five (5) advantages of PLC.

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INTRODUCTION TO PLC
Feedback To Activity 5B
5.5 a) Conveyor system
b) Swimming pool
c) Traffic light system
d) Lift control system
e) Food processing
5.6
(i) The wiring of the system usually reduces by 80%
compared to conventional relay control system.
(ii) The power consumption is greatly reduced as PLC
consumes much less power
(iii) The PLC self diagnostic functions enable easy and fast
(iv) Modification of control sequence or application can
easily be done by programming through the console or
computer software without changing of I/O wiring, if no
additional Input or Output devices are required.
(v) In PLC system spare parts for relay and hardware timers
are greatly reduced as compared to conventional control
panel.
(vi) The machine cycle time is improved tremendously due to
the speed of PLC operation is a matter of milliseconds.
Thus, productivity increases.

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INTRODUCTION TO PLC
(vii) It cost much less compared to conventional system in
situations when the number of I/Os is very large and
control functions are complex.
(viii) The reliability of the PLC is higher than the mechanical
relays and timers.
5.5 PLC BLOCK DIAGRAM
Typically a PLC system has five basic components. These are
the processor unit, memory, the power supply unit, input/output
interface section and the programming device. Figure 5.2 shows the
basic arrangement.
INPUTINPUT

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INTRODUCTION TO PLC
Figure 5.2 Block Diagram of PLC
(Source : Manual OMRON (1999), Beginner Guide To PLC)
1. The processor unit or Central Processing Unit (CPU) is the
unit containing the microprocessor. This unit interprets the input
signals and carries out the control actions, according to the
program stored in its memory and communicating the decisions
as action signals to the outputs.
2. The power supply unit is needed to convert the mains a.c voltage
to the low d.c voltage (5V) necessary for the processor and the
circuits in the input and output interface modules.
3. The programming device is used to enter the required program
into the memory of the processor. The program is developed in
the device and then transferred to the memory unit of the PLC.
4. The memory unit is where the program is stored, that is to be
used for the control actions to be exercised by the
microprocessor. Ladder program, Timer and Counter Values
are stored in the user memory. Depending on user’s need,
various types of memory are available for choice : Read-Only
Memory (ROM), Random Access Memory (RAM), Erasable
Programmable Read Only Memory (EPROM) and Electrically
Erasable Programmable Read-Only Memory (EEPROM).
5. The input and output sections are where the processor receives
information from external devices and communicates
information to external devices
In addition, PLCs
consists of three
functional areas :
processing, memory
and input/output.

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INTRODUCTION TO PLC
5.6 PLC BASIC INSTRUCTIONS
A very commonly used method of programming PLCs is based on the
use of the ladder diagrams. Writing a program is then equivalent to drawing a
switching circuit. The ladder diagram consists of two vertical lines
representing the power rails. Circuits are connected as horizontal lines, i.e
the rungs of the ladder, between these two verticals.
In drawing a ladder diagram, certain conventions are adopted:
1. The vertical lines of the diagram represent the power rails between which
circuits are connected.
2. Each rung on the ladder defines one operation in the control process.
3. A ladder diagram is read from left to right and from top to bottom, Figure
5.3 shows the scanning motion employed by the PLC. The top rung is
read from left to right. Then the second rung down is read from left to
The most important from
any PLC programming
language is that it is
easily understood and
used in a control system.

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INTRODUCTION TO PLC
right and so on. When the PLC is in its run mode, it goes through the
entire ladder program to the end, the end rung of program being clearly
denoted, and then promptly resumes at the start. This procedure of going
through all the rungs of the program is termed a cycle.
Figure 5.3 The rung ladder
4. Each rung must start with an input or inputs and must end with at least
one output. The term input is used for a control action, such as closing
the contacts of a switch, used as an input to the PLC. The term output is
used for a device connected to the output of a PLC, e.g. a motor.
5. Electrical devices are shown in their normal condition. Thus a switch
which is normally open until some object closes it, is shown as open on
the ladder diagram. A switch that is normally closed is shown closed.
6. A particular device can appear in more than one rung of a ladder. For
example, we might have a relay which switches on one or more devices.
The same letters and/or numbers are used to label the device in each
situation.

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INTRODUCTION TO PLC
7. The inputs and outputs are all identified by their addresses, the notation
used depending on the PLC manufacturer. This is the address of the
input or output in the memory of the PLC. The OMRON series of PLCs
precedes input elements by an 00 and output elements by an 01 and
uses the following numbers :
Inputs 00000 - 00011 (12 possible inputs)
Outputs 01000 - 01007 ( 8 possible outputs)
Meanwhile the Mitsubishi F series of PLCs precedes input elements by an
X and output elements by a Y and uses the following numbers :
Inputs X400 - 407 , 410 - 413 , 500 - 507, 510 - 513
(24 possible inputs)
Outputs Y430 - 437, 530 - 537
(16 possible outputs)
Ladder Diagrams’ Explanation
Ladder diagram uses standard symbols to represent the circuit
components and functions found in a control system.
Ladder symbols
Input, normally-open contact
Input, normally-closed contact
Inputs in series connections

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INTRODUCTION TO PLC
Inputs in parallel connections
Outputs device
Figure 5.4 : Ladder Symbols
Logic Instructions (Mnemonic)
The instruction set consists of logic instructions also known as
mnemonics that represent the actions to be performed within a given PLC.
Each program instruction consists of two parts : a mnemonic operation
component also known as opcode and an address or data component that
identifies a particular element (e.g output) within the PLC. Table 5.2 below is
an example of a mnemonic code.
Address Instructions Data
00000 LD 00000
00001 OUT 01000
00002 END(01)
Table 5.2 Mnemonic Code

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INTRODUCTION TO PLC
Here the instruction refers to output (01) number 0.
Input/Output (I/O) Assignment
These instructions are used to program logic circuits in ladder diagram
form, by assigning all physical inputs and outputs with an operand (address)
suitable to the PLC being used. The I/O assignment used differ between
manufacturers but certain common terms exist.
Logic Instructions and Graphic Programming
Logic instructions are used as the basic programming language for
PLCs. Although logic instructions are easy to earn and use, it can be very
time – consuming to check and relate a large coded program to the actual
circuit function. Furthermore, logic instruction tends to vary between different
types of PLC. A factory or plant may use a range of different PLCs, in which
confusion can result over differences in the instruction sets.
Name/Mnemonic Symbol Description
LOAD
(LD)
Creates a normally open
condition as the first
condition off the bus bar. All
instruction lines begin with
either LOAD or LOAD NOT.
LOAD NOT
(LD NOT)
Creates a normally closed
condition as the first
condition off the bus bar. All
instruction lines begin with
either LOAD or LOAD NOT.

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INTRODUCTION TO PLC
AND
(AND)
Combines a normally open
condition in series with a
previous condition.
AND NOT
(AND NOT)
Combines a normally
closed condition in series
with a previous condition.
OR
(OR)
Combines a normally open
condition in parallel with a
previous condition.
OR NOT
(OR NOT)
Combines a normally
closed condition in parallel
with a previous condition.
AND LOAD
(AND LOAD)
Combines two group of
conditions in series. These
groups are called blocks.
OR LOAD
(OR LOAD)
Combines two parallel
groups of conditions. These
group are called block.
OUTPUT
(OUT)
Specifies an output bit that
is to be turned ON for an
ON execution condition and
OFF for an OFF condition.
OUTPUT NOT
(OUT NOT)
Specifies an output bit that
is to be turned OFF for an
ON execution and ON for
an OFF condition.

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INTRODUCTION TO PLC
TIMER
(TIM)
Creates a 0-1 s
decrementing timer that
starts from the set value
(SV) when the execution
condition turns ON.
COUNTER
(CNT)
Counts down the number of
times the input conditions
turns ON. Each time the
input condition turns ON,
the present value (PV) is
reduced by 1 and when the
count reaches 0, the
Completion Flag (accessed
through the counter
number) turns ON.
NO OPERATION
(NOP) None
Does nothing. Can be
inserted into a program
when modification are
made that would otherwise
change program address.
Table 5.3 Basic Instructions
5.7 DESIGN AND WRITE PROGRAM LADDER
Examples of Logic Function
1. AND function
ADDRESS INSTRUCTIONS DATA
01000
00000 00001 00002
TI
M
CP
CNT
R

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INTRODUCTION TO PLC
00000 LD 00000
00001 AND 00001
00002 AND 00002
00003 OUT 01000
2. OR function
00000 LD 00001
00001 OR 00002
00002 OR 00003
00003 OUT 01001
3. LOAD and LOAD NOT function
00000 LD 00000
00001 OUT 01000
00002 LD NOT 00001
00003 OUT 01001
4. AND and AND NOT function
00000 LD 00000
00001 AND NOT 00001
00002 AND 00002
00003 OUT 01002
00001
01001
00002
00003
01000
01001
00000
00001
01002
00000 00001 00002

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INTRODUCTION TO PLC
5. OR and OR NOT function
00000 LD NOT 00000
00001 OR NOT 00001
00002 OR 00002
00003 OUT 01003
6. AND and OR function
00000 LD 00000
00001 AND 00001
00002 OR 00002
00003 AND 00003
00004 OUT 01004
7. END function
00000 LD 00000
00001 AND NOT 00001
00002 OUT 01005
00003 END (01)
01003
00000
00001
00002
00000 00001 00003
00002
01004
01005
END
00000 00001

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INTRODUCTION TO PLC
Activity 5C
NEXT INPUT…!
PART 1
5.7 Decide whether each of these statements is True (T) or False (F).
Figure shows a ladder diagram rung for which:
(i) The input contacts are normally open
(ii) There is an output when there is an input to the
contacts
A (i) T (ii) T
B (i) T (ii) F
C (i) F (ii) T
D (i) F (ii) T
PART 2
5.8 Draw a block diagram showing in very general terms the main units in
a PLC.
5.9 State five (5) advantages of PLC.
5.10 What is the address of the input or output in the memory of the PLC
manufacturer below :
(i) OMRON series
(ii) MITSUBISHI F series

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INTRODUCTION TO PLC
5.11 Draw the ladder symbols to represent the circuit components and
function in a control system such as :
(i) Input, normally -open contact
(ii) Input, normally – closed contact
(iii) Inputs in series connections
(iv) Inputs in parallel connections
(v) Output devices

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INTRODUCTION TO PLC
Feedback To Activity 5C
PART 1
5.7 A
PART 2
5.8 Block diagram of PLC

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INTRODUCTION TO PLC
5.9 i. The wiring of the system usually reduces by 80%
compared to conventional relay control system.
ii. The power consumption is greatly reduced as PLC
consume much less power
iii. The PLC self diagnostic functions enable easy and fast
iv. Modification of control sequence or application can
easily be done by programming through the console or
computer software without changing of I/O wiring, if no
additional Input or Output devices are required.
v. In PLC system spare parts for relay and hardware
timers are greatly reduced as compared to conventional
control panel.
5.10 (i) OMRON series
Inputs 00000 - 00011 (12 possible inputs)
Outputs 01000 - 01007 ( 8 possible outputs)
(ii) MITSUBISHI F series
Inputs X400 - 407 , 410 - 413 , 500 - 507, 510 - 513
(24 possible inputs)
Outputs Y430 - 437, 530 - 537
(16 possible outputs)

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INTRODUCTION TO PLC
5.11 Ladder symbols
a) Input, normally-open contact
b) Input, normally-closed contact
c) Inputs in series connections
d) Inputs in parallel connections
e) Outputs device

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INTRODUCTION TO PLC
KEY FACTS
1. PLC stands for PROGRAMMABLE LOGIC CONTROL.
2. PLC system has five basic components : processor unit, memory, the
power supply unit, input/output interface section and the programming
device.

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INTRODUCTION TO PLC
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.
Q5-1 What is the characteristic of a PLC ?
Q5-2 Why do we need a PLC ?
Q5-3
(a) Draw the logic instructions below:
(i) AND NOT
(ii) TIM
(iii) OUTPUT
(iv) OR
(v) OR NOT
(vi) LOAD NOT
(b) Draw the ladder rungs to represent: Two switches which are
normally open and both have to be closed for a motor to operate.
(c) Convert the ladder diagrams given below to its equivalent
mnemonic codes.
01000
00000 00001 00002 00003
00004
00005

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INTRODUCTION TO PLC
Feedback To Self-Assessment
Have you tried the questions????? If “YES”, check your answers now.
Q5-1 The PLC must be :
a) Easy to program and reprogram
b) Easy to maintain and repair
c) More reliable in an industrial environment
d) Smaller in size than its relay equivalent
e) Cost-competitive
Q5-2 PLC are used in bigger processes because we can simply
program the PLC to count its inputs and turn the solenoids for the
specified time.
Q5-3
(a)
i.
ii.
iii.
TI
M

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INTRODUCTION TO PLC
iv.
v.
vi.
vii.
(b)

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INTRODUCTION TO PLC
(c)
Address Instruction Data
00000 LD 01000
00001 AND NOT 01001
00002 LD 00002
00003 AND 00003
00004 OR 00004
00005 OR 00005
00006 AND LD ---
00007 OUT 01000
00008 END(01)

Basic Control System unit5

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