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Introduction to
Programmable Logic Controllers
(PLC's)
Industrial Control Systems
Fall 2006
Lecture – Introduction to PLC's
MME 486 – Fall 2006
1 of 47
The Need for PLCs
• Hardwired panels were very time consuming
to wire, debug and change
...
– Solid-state not mechanical
– Easy to modify input and output devices
– Easily programmed and maintained by plant
electricians
– Be able to function in an industrial environment
Lecture – Introduction to PLC's
MME 486 – Fall 2006
2 of 47
The First Programmable Logic
Controllers (PLCs)
• Introduced in the late 1960’s
• Developed to offer the same functionality as
the existing relay logic systems
• Programmable, reusable and reliable
–
–
–
–
Could withstand a harsh industrial environment
They had no hard drive, they had battery backup
Could start in seconds
Used Ladder Logic for programming
Lecture – Introduction to PLC's
MME 486 – Fall 2006
3 of 47
Programmable Logic Controller
• A programmable logic controller (PLC)
is a specialized computer used to
control machines and process
...
The program takes
the place of much of
the external wiring
that would be required
for control of a process
...
Since all the logic is
contained in the PLC’s
memory, there is no
chance of making a
logic wiring error
...
Lecture – Introduction to PLC's
MME 486 – Fall 2006
7 of 47
Advantages of a PLC Control System
More Flexibility:
Original equipment manufacturers (OEMs) can provide system updates
for a process by simply sending out a new program
...
End-users can modify
the program in the
field
...
The cost
savings using PLCs have been so significant that relay control is
becoming obsolete, except for power applications
...
Lecture – Introduction to PLC's
MME 486 – Fall 2006
9 of 47
Advantages of a PLC Control System
Communications Capability:
A PLC can communicate with other controllers or computer equipment
...
Lecture – Introduction to PLC's
MME 486 – Fall 2006
10 of 47
Advantages of a PLC Control System
Faster Response Time:
PLCs operate in real-time which means that an event taking place in the
field will result in an operation or output taking place
...
Lecture – Introduction to PLC's
MME 486 – Fall 2006
11 of 47
Advantages of a PLC Control System
Easier To Troubleshoot:
PLCs have resident diagnostic and override functions allowing users to
easily trace and correct software and hardware problems
...
• A closed architecture or proprietary system, is one
whose design makes it more difficult to connect devices
and programs made by other manufacturers
...
Lecture – Introduction to PLC's
MME 486 – Fall 2006
15 of 47
I/O Configurations
Fixed I/O
• Is typical of small PLCs
• Comes in one package, with
no separate removable units
...
• Lower in cost – but
lacks flexibility
...
This feature greatly increases
your options and the unit’s
flexibility
...
Lecture – Introduction to PLC's
MME 486 – Fall 2006
17 of 47
I/O Configurations
Modular I/O
When a module slides into the rack,
it makes an electrical connection with a series of
contacts - called the backplane
...
Lecture – Introduction to PLC's
MME 486 – Fall 2006
18 of 47
Power Supply
• Supplies DC power to other
modules that plug into the rack
...
• In small and micro PLC systems, the power supply
is also used to power field devices
...
• Consists of a microprocessor
for implementing the logic, and
controlling the communications
among the modules
...
Processor
Module
• The processor accepts input data from
various sensing devices, executes the
stored user program, and sends
appropriate output commands to
control devices
...
Lecture – Introduction to PLC's
MME 486 – Fall 2006
21 of 47
I/O Section
Input Module
• Forms the interface
by which input field
devices are connected
to the controller
...
Lecture – Introduction to PLC's
MME 486 – Fall 2006
22 of 47
I/O Section
Output Module
• Forms the interface
by which output field
devices are connected
to the controller
...
Lecture – Introduction to PLC's
MME 486 – Fall 2006
23 of 47
Programming Device
PC with appropriate
software
• 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 communicates with the PLC
processor via a serial or parallel data communications link
Lecture – Introduction to PLC's
MME 486 – Fall 2006
24 of 47
Programming Device
Hand-held unit
with display
• 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
Lecture – Introduction to PLC's
MME 486 – Fall 2006
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Programming Device
Hand-held unit
with display
• Hand-held units are often used on the factory floor
for troubleshooting, modifying programs, and
transferring programs to multiple machines
...
You give each input and output device
an address
...
The scan time, the
time required for one full
cycle, provides a measure
of the speed of response
of the PLC
Lecture – Introduction to PLC's
MME 486 – Fall 2006
36 of 47
PLC Operating Cycle
Each
The
can be though of as a set of normally
open contacts
can be considered to represent a coil that,
when energized, will close a set of contacts
...
Either of these conditions
provides a continuous path from left to right across the
rung that includes the coil
...
The change requires that the manual pushbutton
control should be permitted to operate at any pressure but
not unless the specified temperature setting has been
reached
...
If a relay system were used, it would require some
rewiring of the system, as shown, to achieve the
desired change
...
If a PLC is used, no rewiring is necessary!
The inputs and outputs are still the same
...
An instruction is a command that will cause a PLC
to perform a certain predetermined operation
...
Examine a bit for an ON condition
XIO (Examine OFF)
...
Turn ON a bit (non retentive)
OTL (Output Latch)
...
Unlatch a bit (retentive)
TOF (Timer Off-Delay)
...
Turn an output ON or OFF after its
rung has been ON for a preset time
interval
CTD (Count Down)
...
Use a software counter to count up
to a specified value
Lecture – Introduction to PLC's
MME 486 – Fall 2006
47 of 47