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EJ501
PROGRAMMABLE
LOGIC
CONTROLLER (PLC)
AND AUTOMATION
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UNIT 3
INTRODUCTION TO PLC SYSTEM
Objective :
1.
Understand basic characteristics of PLC



2.
Understand building structure of PLC

3.
CPU, memory unit, display and indicator unit, input and output interface
Understand PLC hardware unit

4.
PLC terminologies
PLC function/ background
Type / advantages of PLC
Housing unit, programming unit, power supply unit, printer unit
Explain input and output device
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INTRODUCTION TO PLC SYSTEM
Introduction PLC (Programmable Logic Control)
A digital device (microprocessor based controller) that uses a programmable memory
to store instructions and to implement functions such as logic, sequencing, timing,
counting and arithmetic in order to control machines and processes and has been
specifically designed to make programming easy.
PLCs are similar to computers but have certain features which are specific to their use
as controllers:
i.
ii.
iii.
They are rugged and designed to withstand vibrations, temperature, humidity
and noise.
The interfacing for inputs and outputs is inside the controller.
There are easily programmed.
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INTRODUCTION TO PLC SYSTEM
Definition PLC
A digitally operated electronic apparatus which uses a
programmable memory for the internal storage of
instructions for implementing specific functions such as
logic, sequencing, timing, counting, and arithmetic to
control, through digital or analog input/output modules,
various types of machines or processes.
- National Electrical Manufacturers Association
(NEMA)
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INTRODUCTION TO PLC SYSTEM
Definition PLC
An electronic system working digitally for the use in industrial environment
with a programmable memory for the internal storage of user-orientated control
instructions for the implementation of specific functions, like, for example
logic operation control, process control, timer, counting-and arithmetic
functions, in order to control various machines and processes through digital or
analog input- , and output variables.
The logic programmable controls as well as the peripheral installations (PLCsystem) are designed so that they can be easily integrated in an industrial
control system and be implemented in all their desired functions.
- International Standard
IEC 61131-1
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INTRODUCTION TO PLC SYSTEM
PLC Background
The first Programmable Logic Controller (PLC) was developed by a group
of engineers at General Motors in 1968
The company was looking for an alternative to replace complex relay
control systems :
 Must be programmable (and reprogrammable)
 Designed to operate in an industrial environment
 Must accept 120 V ac signals from pushbuttons, switches
 Outputs must switch and continuously operate loads such as motors
and relays of two amps rating
Before the 1980’s, programmable controllers were called PCs.
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INTRODUCTION TO PLC SYSTEM
PLC Functions
PLC performs the functions of conventional relays, timers,
counters.
Capable of storing instructions, such as sequencing, timing,
counting, arithmetic, data manipulation, and communication,
to control industrial machines and processes.
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INTRODUCTION TO PLC SYSTEM
Type of PLC - compact
TECOMAT
TC600
Omron
Mitsubishi
Zelio
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INTRODUCTION TO PLC SYSTEM
Type of PLC - modular
Omron
Mitsubishi
Eaton
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INTRODUCTION TO PLC SYSTEM
Type of PLC – plug in card
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INTRODUCTION TO PLC SYSTEM
Advantages and Disadvantages of PLC
Advantages
Disadvantages
Many inputs and outputs, excellent for
controlling and monitoring many
processes
Do not use the full capabilities of the
microprocessor
Designed for industrial environments,
robust and reliable
Ladder logic programming style is
cumbersome and prone to mistakes
Reprogrammable
Only good for yes/no decisions
Modular
Do not handle continuous inputs and
outputs, not good for “direct control”
Ideally suited to supervisory control
Inflexible
(compared
microprocessors)
Easy to set up, good for FMS
environment
to
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INTRODUCTION TO PLC SYSTEM
Initial Specifications
• The new control system had to be price competitive with the use of relay
systems.
• The system had to be capable of sustaining an industrial environment.
• The input and output interfaces had to be easily replaceable.
• The controller had to be designed in modular form, so that
subassemblies could be removed easily for replacement or repair.
• The control system needed the capability to pass data collection to a
central system.
• The system had to be reusable.
• The method used to program the controller had to be simple, so that it
could be easily understood by plant personnel.
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Recent PLCs
• Changes include both hardware (physical components)
and software (control program) upgrades
• Faster scan times are being achieved using new, advanced
microprocessor and electronic technology.
• Small, low-cost PLCs (Figure 1), capable of replacing four
to ten relays, now have more power than their predecessor,
the simple relay replacer.
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INTRODUCTION TO PLC SYSTEM
• A personal computer (PC) is the most commonly used programming device
• The software allows users to create, edit, document, store and troubleshoot
programs
• The personal computer communications with the PLC processor via a serial or
parallel data communications link
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INTRODUCTION TO PLC SYSTEM
• Hand-held programming devices are sometimes used to program small PLCs
• They are compact, inexpensive, and easy to use, but are not able to display as much
logic on screen as a computer monitor
• It is often used on the factory floor for troubleshooting, modifying programs, and
transferring programs to multiple machines
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Basic PLC Structure
Basic internal structure
of PLC consists
essentially of :
-processing unit (CPU)
-memory
-input/output units
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INTRODUCTION TO PLC SYSTEM
Hardware of the PLC
Hardware of the PLC are only
consist of several components;
such as Power Supply Unit,
CPU Unit, Memory Unit and
I/O Unit.
At the small PLC; Power
Supply, CPU, Memory and I/O
stay in the same Unit but at the
big PLC; Power Supply, CPU,
Memory and I/O in the separate
Unit.
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Power Supply
The Power Supply is used to give power to all the parts. The most PLC works with power
24 VDC or 220 VAC. The big PLC has the separate power supply and the small PLC has
power supply which is one. The Power Supply is also provided by the battery back up,
when happened the failure of power, automatically battery will replace the power supply to
supply the CPU, so that the program of memory user do not lose.
Memory Unit
The Memory Unit has function to save the data and program will be used by PLC. This
Memory is divisible into two type that is ROM and RAM. ROM contains the data like
facility of the logic program, facility to edit the program, facility to monitor the program,
facility for the communications, etc. The Data will be save permanently and will not lose
though the power supply is off. While RAM contains the data of user program, like ladder
diagram, memory data, I/O status, etc. The Data can be writed and read. RAM has the
character is not permanent, if the power suplply is off the data will lose. To avoid the
mentioned, so the power supply is provided by the battery back up to supply if the especial
power supply is off.
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CPU Unit (Central Processing Unit)
Without seeing to type of PLC ( small, medium,
big) processors and memory always in the same
unit. This unit is referred as the CPU. The CPU
is the brain of a PLC. The CPU is a 8 bit, 16 bit
or 32 bit microcontroller. The CPU has function
to handle communications with the external
apparatus, interconnectivity between the parts
of PLC, executes the program, managements
the memory, observing or perceiving the input
signal and gives the output signal ( as according
to the program or process run). The CPU is also
provided by the indicator lamp as indicator of
the happening of mistake and damage.
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I/O Unit
The I/O Unit is the interface unit has function to convert the input signal and output
signal so that the CPU can communicate with the external apparatus like the switch,
stepping motor, solenoid, sensor, etc. The I/O Unit has two type; Discrete I/O and
Analog I/O.
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Omron PLC
LG PLC
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Input Devices
PLCs can receive input from either automated or manual devices. The PLC
could receive data from the user via a pushbutton switch, keyboard, or similar
device. Automated input could come from a variety of devices: microswitches,
timers, encoders, photosensors, and so on. Some devices, like the Limit Switch
shown below, turn ON or OFF when the equipment actually makes contact
with it. Other devices, like the Photoelectric Switch and Proximity Switch
shown below, use other means, such as light or inductance, in order to get
information about the equipment being monitored.
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Output Devices
A PLC can output to a myriad of devices for use in automated control. Almost
anything that you can think of could be controlled (perhaps indirectly) by a PLC.
Some of the most common devices are motors, Solenoids, Servomotors, Stepping
Motors, valves, switches, indicator lights, buzzers, and alarms. Some of these output
devices; such as the motors, Solenoids, Servomotors, Stepping Motors, and valves;
affect the controlled system directly. Others; such as the indicator lights, buzzers,
and alarms; provide output to notify personnel.
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