UNIT IV: INDUSTRIAL CONTROLLERS

Industrial Control
System (ICS)
Industrial Control System (ICS) is a collective term used to describe
different types of control systems and related instrumentation, which
include the devices, systems, networks, and controls used to operate
and/or automate industrial processes.

• Discrete controllers

• Distributed control systems

• SCADA systems

• Programmable logic controllers

PLC Block Diagram
How Does a PLC work?
PLC Structure
 Types
Compact PLC
• The number of inputs and outputs are fixed. Because
I/O capabilities are decided by the manufacturer but
not by the user

Modular PLC
• The number of input and output are not fixed. Inputs
and outputs can be added to the modular
PLC systems by the user.
Scan cycle of PLC

Diagnostic Input Scan

Write Programme
Output Scan
 Scan Time

Scan Time refers to the amount of Time that CPU takes to

execute the Ladder Program, Read Input, Update Output Status
and Support Communication. Therefore, the larger the program
is, the longer the Scan Time will be.
Factor Affecting Scan
Time
Speed of
Processor

No. of O/P
Scan No. of I/P
Time

Length of
Programme
 Input Modules
Input modules interface directly to devices such as switches and

temperature sensors.

Input modules convert many different types of electrical signals such

as 120VAC, 24VDC, or 4-20mA, to signals which the controller can

understand.
Output Modules

Output modules take a signal from a PLC and convert it to a signal that a field

device needs to operate. Since there are different types of output devices, there is a

wide variety of output cards available, including both digital and analog cards.
TYPES

Discrete I/O

Analog I/O
Digital/Analog Modules
• Digital input modules adapt digital signals e.g. from proximity sensors.

• Digital output modules convert the internal signal level of PLC into digital
process signals e.g. relays.

• Analog input modules adapt analog process signals e.g. from transducers.

• Analog output modules convert internal digital values of the PLC to analog
process signals e.g. temperature controller.
 Digital and Analog
Digital modules use only a single bit to represent the state of the device. For

example, a switch is either open or closed. Therefore, the bit is either a 0 (switch

is open) or a 1 (switch is closed).

Analog modules use words to represent the state of a device. An analog signal

represents a value.. For example, the temperature could be 5, 9, 20, 100, etc

degrees. Analog modules use a value, such as 52, rather than a 0 or 1 to represent

the state of the device.

Inputs and Outputs

OUTPUTS

INPUTS MOTOR

CONTACTOR
LAMP

PUSHBUTTONS
PLC
What are Inputs?
• Switches and Pushbuttons
• Sensing Devices
• Limit Switches
• Photoelectric Sensors
• Proximity Sensors
• Condition Sensors
• Pressure Switches
• Level Switches
• Temperature Switches
• Vacuum Switches
• Float Switches
• Encoders
What are Outputs?

• Valves
• Motor Starters
• Solenoids
• Actuators
• Control Relays
• Horns & Alarms
• Stack Lights
• Fans
• Counter
• Pumps
• Printers
 Chassis and Backplane
All PLCs need some method of communicating between the

controller, I/O and communications modules. Here are three ways used to

accomplish this communications between the various components that make up

the PLC system.

 Power Supply
A power supply is needed to provide power to the PLC and any other

modules. Power supplies come in various forms:

• Power supply modules that fit into one of the slots in a chassis

•External power supplies that mount to the outside of a chassis

•Stand alone power supplies that connect to the PLC or I/O through a power cable

•Embedded power supplies that come as part of the PLC block.

Programming Software
Software that runs on a PC is required to configure and program PLCs

Different products may require different programming software.

Software allows programs to be written in several different languages.

Types of Programme Memory

Program memory

Programmable Non-programmable
(Read-write memory)

Non-alterable
Alterable
ROM /
PROM

UV erasable Electrically erasable

EPROM / REPROM EEPROM / EAPROM

Semiconductor RAM Semiconductor

EEPROM / EAPROM
PLC Requirements
List of items required when
working with PLCs:

1. Programming Terminal - laptop or

desktop PC.
2. PLC Software. PLC manufacturers have
their own specific software and license
key.
3. Communication cable for connection
from Laptop to PLC.
4. Backup copy of the ladder program (on
diskette, CDROM, hard disk, flash
memory). If none, upload it from the
PLC.
5. Documentation- (PLC manual, Software
manual, drawings, ladder program
printout, and Seq. of Operations manual.)
PLC Control Panel
PLC Relay
Simple PLC
network digital inputs

analog inputs / outputs

digital outputs
PLC Advantages
•Handles much more
complicated systems.
•Less and simple wiring.
•Increased Reliability.
•More Flexibility.
•Lower Cost .
•Faster Response.
•Easier to troubleshoot.
•Remote control capability.
•Communication Capability.
PLC Disadvantages

•In contrast to microcontroller systems that have what is called an

open architecture, most PLCs manufacturers offer only closed
architectures for their products .
•PLC devices are proprietary, which means that parts and software
from one manufacturer can't easily be used in combination with
parts of another manufacturer, which limits the design and cost
options.
•PLC were Designed for Relay Logic Ladder and have Difficulty
with some Smart Devices.
•To maximize PLC performance and Flexibility, a number of
Optional Modules must be added
PLC Applications
•Originally hardwired arrays of
relays were used to control the
operation of heavy machines
that contain motors and other
high power devices.
•PLCs were originally used to
substitute the switching relay
networks used in industrial
applications, but now they can
also be used implement other
tasks such as timing, delaying
counting, calculating,
comparing and processing of
analog signals.
 PLC Size
1. SMALL - It covers units with up to 128 I/O’s and memories up to 2
Kbytes.
- These PLC’s are capable of providing simple to advance levels or
machine controls.

2. MEDIUM- Have up to 2048 I/O’s and memories up to 32 Kbytes.

3. LARGE - The most sophisticated units of the PLC family. They have
up to 8192 I/O’s and memories up to 750 Kbytes.
- Can control individual production processes or entire plant.
Leading Brands of PLC
AMERICAN:

Allen Bradley

Gould Modicon

Texas Instruments

General Electric

Westinghouse

Cutter Hammer

Square D

EUROPEAN:

Siemens

Klockner &
Contacts and Coils in PLC Ladder Logic
• The most elementary objects in Ladder Diagram programming are
contacts and coils, intended to mimic the contacts and coils of
electromechanical relays.
• Contacts and coils are discrete programming elements, dealing with
Boolean (1 and 0; on and off; true and false) variable states.
• Each contact in a Ladder Diagram PLC program represents the reading of a
single bit in memory, while each coil represents the writing of a single bit
in memory.
• Discrete input signals to the PLC from real-world switches are read by a
Ladder Diagram program by contacts referenced to those input channels.
• In legacy PLC systems, each discrete input channel has a specific address
which must be applied to the contact(s) within that program.
• In modern PLC systems, each discrete input channel has a tag name
created by the programmer which is applied to the contact(s) within the
program.
Ladder Diagram Symbols