PROGRAMMABLE LOGIC CONTROLLER
(PLC)
MUNIRA MOHAMED NAZARI
PPK BIOPROSES
UniMAP



Introduction
PLC Size and Application
Basic PLC Programming
PLC in Agricultural Industry
PLC omponents
Advantages
PLC Vs Computer

A Programmable Logic Controller (PLC) is a solidstate device designed to perform logic functions
previously accomplished by electromechanical relays.

A Programmable Logic Controller (PLC) is an industrial
computer control system that continuously monitors the state
of input devices and makes decisions based upon a custom
program, to control the state of devices connected as outputs.




Almost any production line, machine function or process
can be automated using a PLC.
It is capable not only performing relay switching tasks,
but also counting, calculating, comparing and the
processing the analogue signals.
The speed and accuracy of the operation can be greatly
enhanced using this type of control system.
But the biggest benefit in using a PLC is the ability to
change and replicate the operation or process while
collecting and communicating vital information.
Basic PLC Operation
PRINCIPLES OF OPERATION
Example:
A mixer motor is to be used to automatically stir a vat when the
temperature and pressure reach preset values. Direct manual operation of
the motor is provided by means of a separate pushbutton station. The
process is monitored with temperature and pressure sensor switches that
close their respective contacts when conditions reach their preset values.

The use of PLCs and similar devices in the
agricultural industry is widespread and growing.

Example
◦
◦
◦
◦
◦
food processing
building environmental control
grain drying
aquaculture production
tractor and machinery systems

A PLC consists of following main parts:
A PLC can be divided into parts: the central processing unit (CPU), the
input/output (I/O) section, the power supply and the programming device.

The power supply supplies dc power to other modules that plug in rack
and other field devices.
 The processor (CPU) is the ‘brain’ of the PLC. It consists of a
microprocessor for implementing the logic and controlling the
communications among the modules.

Required memory for storing the results of the logical operation
performed by the microprocessor.
 The CPU is designed so that the user can enter the desired circuit in
ladder logic.
 The programming devices are used to enter the desired program into
the memory of the processor.
 This program is entered using relay ladder logic. The program
determines the sequence of operation and ultimate control of the
equipment or machinery.
There are two ways
incorporated into PLC:
in
which
I/O
is
1) Fixed I/O:
It’s small PLCs that come in one package with
no separate, removable units. The processor
and I/O are packed together and the I/O
terminals are available but cannot be
changed.
Advantage:
Lower cost, the number of available I/O
points is varies and usually can be expanded
by buying additional units of fixed I/O.
Disadvantage:
Lack of flexibility, limited with the quantities
and type of packaging. If any part in the unit
fails, the whole unit must be replaced.
2) Modular I/O:
 It’s divided by compartment into
separate modules can be plugged.
 Thus, increases the options and the
unit’s flexibility. The modules
available can be chose from
manufacturer and mix them as
desired.
 It’s consisting of a rack, power
supply, processor module (CPU),
input/output (I/O modules) and
operator interface for programming
and monitoring.

Offer several advantages over a conventional
relay:
Using the relay method for motor control
PLC ladder logic diagram
Relay method
PLC
Some important characteristics distinguish PLCs from general-purpose
computers.
 Unlike computers, the PLC is designed to operate in the industrial
environment with the wide range of ambient temperature and humidity. A
well designed PLC is not affected by the electrical noise inherent in most
industrial locations.
 Hardware and software of PLCs are designed for easy use by plant
electricians and technicians. Unlike the computer, the PLC is
programmed in relay ladder logic or other easily learned languages. The
PLC comes with its program language built into permanent memory,
whereas a personal computer requires a disk operating system (DOS).
PLC is limited by the language it comes with, unless it is a modular type
that enables to plug in a language module.
 PCs have large user memory (several megabytes), which enables you to
load and use software stored on disk. Any language capabilities can be
made via this software. PLCs are not design with this kind of flexibility
but are meant to be specialized computers for control, interfacing and
control with external devices.
 Computers are complex computing machines capable of executing
several programs or task simultaneously and in any order. Most PLC
executes a single program in an orderly and sequential fashion from first
to last instruction.
 PLCs have been designed for installation and maintenance by plant
electricians who are not required to be highly skilled computer
technicians. PLC has been designed for simplified trouble shooting
because they include indicators and written fault information. Easily
connected and replaced for modular interfaces with the field devices.
Major Size Categories
Types of PLC
The I/O Section

PLCs are divided into three major size categories:

Small:

Medium:

Large:
◦ It covers units up to 128 I/Os and memories up to 2K bytes. It
capable of providing simple to advanced levels of machine
control.
◦ It have up to 2048 I/Os and memories up to 32K bytes. Special
I/Os modules make medium PLCs adaptable to temperature,
pressure, flow, weight, position and any type of analog
function encountered in process control applications.
◦ The most sophisticated units of PLCs family. They have up to
16,000 I/Os and memories up to 2M bytes. It has almost
unlimited applications and can control individual production
processes or entire plants.

Single-ended:
◦ It involves one PLC controlling one process. This would be a
stand-alone unit and would not be used for communicating
with other computers or PLCs.

Multitask:
◦ It usually calls for a medium size PLC and involves one PLC
controlling several processes. It can be a subsystem for
larger processes and communicating with a central PLC.

Control management:
◦ It involves one PLC controlling several others. It requires a
large PLC processor designed to communicate with other
PLCs and possibly with a computer. The control
management PLC supervises several PLCs by downloading
programs that tell the other PLCs what has to be done.
 The memory size of PLC ranges from 1K to 2M. The
amount of memory required depends on the
application.
 Factors affecting the memory size are include:
 Number of I/O points used
 Size of control program
 Data-collecting requirements
Future expansion

The input and output interface modules provide the equivalents of
eyes, ears and tongue to the brain of PLC/CPU.

The I/O section consists of an I/O rack and individual I/O modules

Input interface modules accept signals from the machine or process
devices and convert them into signals that can be used by the
controller.

Output interface modules convert controller signals into external
signals used to control the machine or process

A slot in the PLC can hold any type of I/O module.



The I/O system provides an interface between
the hardwire components in the field and the
CPU.
A chassis is a physical hardware assembly that
houses devices such as I/O modules, processor
modules and power supplies.
Chassis come in different sizes according to the
number of slots they contain. In general, they
can have 4,8,12, or 16 slots.

There are two type of rack:

1) Logical rack:
◦ It is an addressable unit
consisting of 128 input points
and 128 output points.
◦ A rack uses 8 words in the input
image table and 8 words in the
output image table.
◦ An I/O group is a word in the
output
image
tables
that
corresponding to the word in
the input image file.
◦ A rack can contain a maximum
of 8 I/O groups, numbered from
0 through 7

There are two type of rack:

2) Remote I/O rack:
◦ One benefit of a PLC system is the ability to locate the I/O modules near the field
devices to minimize the amount of wiring required.
◦ It located away from the processor module. To communicate with the processor, the
remote rack uses a special communications network.
◦ The remote racks are linked to the local rack through communication module using
fibre optic and coaxial cable.


The location of a module within a rack and the terminal number of
a module to which an input or output device is connected will
determine the device’s address.
This address is used by the processor to identify where the device
is located to monitor or control it.

Basic addressing elements include:

1.Type:
◦ The type determines if an input or output is being address.

2.Slot:
◦ The slot number is the physical location of the I/O module. This
maybe a combination of the rack number and the slot number
when using expansion racks.

3.Word and Bit:
◦ The word and bit are used to identify the actual terminal
connection in a particular I/O module.
Ladder Logic and Coil
Ladder Diagram
Branching
Program Scan
PLC Programming Language



PLCs are primarily programmed in ladder logic,
which is really just a symbolic representation of
an electrical circuit whose symbols were chosen
to be similar to schematic symbols of electrical
devices.
The main function of PLC program is to control
outputs based on the condition of inputs.
The symbols used in ladder logic programming
can be divided into two broad categories:
Contacts (inputs) and Coils (outputs)


Most inputs to a PLC are simple devices that are either
on (true) or off (false). These inputs are sensors and
switches that detect part presence, empty or full status,
and so on.
Contacts can be thought of as switches. The two basic
kinds of switches are normally open and normally
closed.

A normally open switch does not pass current until it
closed.

A normally closed switch allows current flow until it
closed.


Coils are output symbols. There are many
types of real-world output devices: motors,
lights, pumps, counters, timers and relays.
The PLC examines the contacts (inputs) in the
ladder and turns the coils (outputs) on or off,
depending on the condition of the inputs.



The basic ladder diagram looks similar to a ladder. It
has two uprights and the rungs that make up the PLC
ladder.
The left and right uprights represent power. If we
connect the left and right uprights through a load,
power can flow through the rung from the left
upright to the right upright.
The PLC then runs the ladder and monitors the input
continually and controls output. This called
scanning.


The amount of time it takes for the PLC to go through the ladder logic
each time is called the scan time.
Scan time varies from PLC to PLC. Even a low PLC scan time is in
milliseconds. The longer the ladder logic is, the longer the scan time is.
Conceptual view of a PLC system. The real-world inputs are attached to an
input module (left side of the figure). Outputs are attached to an output
module (right side of the figure). The centre of the figure shows the logic
that the CPU must evaluate by looking at the inputs and then turning on
outputs based on the logic. In this case, if input 0 (a normally open
switch) is closed, output 0 (the doorbell) turns on.




Each time the PLC scans the doorbell ladder, it checks the state
of the input switch before it enters the ladder (time 1).
While in the ladder, the PLC then decides whether it needs to
change the state of any outputs (evaluation during time 2).
After the PLC finishes evaluating the logic (time 2), it turns on
or off any outputs based on the evaluation (time 3).
The PLC then returns to the top of ladder, checks the inputs
again, and repeats the entire process. The total of three stages
makes up scan time.

Normally Closed Contacts
◦ The normally closed will pass power until it
is activated. A normally closed contact in a
ladder diagram passes power while the
real-world input associated with it is off.
◦ A home security system is an example of
the use of normally closed logic. Assume
that the security system was intended to
monitor the two entrance doors to a house.
◦ One way to wire the house would be to wire
one normally open switch from each door
to the alarm, just like a doorbell switch.
◦ Then if a door opened, the system closes
the switch and sounds the alarm.



Often it is desirable to turn on an output for more than one
condition.
For example, in house, the doorbell should sound under
two conditions: the front button in pushed or the rear door
is pushed. The ladder, called branch
Two paths (or conditions) can turn on the doorbell. (this
can also be called a parallel condition or logical OR
condition)

Two type of basic scan
pattern:
◦ Horizontal scan:
Processor examines input
and output instructions from
the first command, top left in
the program, horizontally,
rung by rung.
◦ Vertical scan:
Processor examines input
and output instructions from
the top left command entered
in
the
ladder
program,
vertically, column by column
and page by page.
Scan process and scan cycle
Horizontal and vertical scanning

Refers to the method by which
user communicates information
to the PLC.

There are three most common
languages:
◦ Ladder diagram language: the most
common used by PLC language.
◦ Boolean language: The statements
refers to the basic AND, OR and NOT
logic gate function.
◦ Function chart system: It is a method
of programming a control system that
uses a more structured approach.




Function chart programming used
function blocks (steps and
transition units).
It is a pictorial representation or a
special type of flow chart of a
sequential control process.
Allow the description of the
process to become the actual
control program and aids in
understanding the system and
localizing problems.
A block of logic can be
programmed as a module and
transition logic ensures that only
appropriate software modules will
operate at any given time.




Simpler programming, faster scan time, enhanced
maintainability and ease of future enhancements.
The overall program may be fairly complex but
individually simple, divide the program into several
steps or stages instead of creating long ladder program.
Processors understand which parts of the program are
active and scan only the active steps, thus reducing scan
time.
Reducing interlocks programming time, the processor
does not even scan those parts of the program that are
inactive, makes control much easier.



The form of programming commonly used with
PLCs.
Involves each program task being specified as
through a rung of a ladder.
Involves writing a program in a similar manner
to drawing a switching circuit.
Figure 1: (a), (b) Alternative ways of drawing an electric circuit, (c)
comparable rung in a ladder program


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.
Figure 2: Ladder program

The sequence followed by a PLC when carrying out
a program can be summarized as:
◦ Scan the inputs associated with one rung of the ladder
program.
◦ Solve the logic operation involving those input.
◦ Set/reset the outputs for that rung.
◦ Move on to the next rung and repeat operations 1,2,3.
◦ Move on to the next rung and repeat operations 1,2,3.
◦ Move on to the next rung and repeat operations 1,2,3.
◦ And so on until the end of the program with each rung of
the ladder program scanned in turn. The PLC then goes
back to the beginning of the program and start again.

Obtained by combinations of switches.
AND
OR
NOR
NAND
EXCLUSIVE-OR (XOR)

Truth table.
Inputs
A
B
C
Output
0
0
0
0
0
0
1
0
0
1
0
0
0
1
1
0
1
0
0
0
1
0
1
1
1
1
0
1
1
1
1
1
OR logic
situation
Shop Door System
Shop open switch
Customer
approaching
sensor
Solenoid output
Off
Off
Off
Off
On
Off
On
Off
Off
On
On
On
AND
logic
situation
Example
PLC is used to controlling lubricating oil being dispensed from a tank.
This is possible by using two sensors, one near the bottom (low level)
and one near the top (high level), as shown in the picture below. Both of
our inputs will be normally open fiber-optic level sensors. When they
are NOT immersed in liquid they will be ON. When they are immersed in
liquid they will be OFF.