A Programmable Logic Controller (PLC) is a digital computer used to control electromechanical processes in factories. PLCs were introduced in the late 1960s to replace relay-based control systems. The first commercial PLC was developed by Modicon for General Motors. Later, as microprocessors became available, PLCs evolved to be more sophisticated. A PLC has components like a power supply, input/output modules, a processor, and a programming device to control inputs from sensors and outputs to devices. PLCs can operate in harsh industrial environments and use simple ladder logic programming. A Programmable Automation Controller (PAC) is similar but designed for more complex automation with greater flexibility, memory, and control
2. PLC
A Programmable Logic Controller (PLC) is a
digital computer used for automation of
electromechanical processes, such as control of
machinery on factory assembly lines, control of
amusement rides, or control of lighting fixtures.
3. PLC was introduced in late 1960’s to remove relay’s based
system.
First commercial & successful Programmable Logic
Controllers was designed and developed by Modicon as a
relay replacer for General Motors.
Earlier, it was a machine with thousands of electronic
parts.
Later ,in late 1970’s,the microprocessor became reality &
greatly enhanced the role of PLC permitting it to evolve
form simply relay to the sophisticated system as it is today.
4. 4
Major Components of a Common PLC
PROCESSOR
POWER
SUPPLY
I M
N O
P D
U U
T L
E
O M
U O
T D
P U
U L
T E
PROGRAMMING
DEVICE
From
SENSORS
Pushbuttons,
contacts,
limit switches,
etc.
To
OUTPUT
Solenoids,
contactors,
alarms
etc.
5. Major Components of PLC
POWER SUPPLY
Provides the voltage needed to run the primary PLC components
I/O MODULES
Provides signal conversion and isolation between the internal
logic level signals inside the PLC and the field’s high level signal.
PROCESSOR
Provides intelligence to command and govern the activities of
the entire PLC systems.
PROGRAMMING DEVICE
Used to enter the desired program that will determine the
sequence of operation and control of process equipment or
driven machine.
7. Programming a PLC
Ladder Logic
Functional block diagram
Structural text(similar to Pascal)
Instruction list(similar to assembly language)
8. Why PLC ?....
• The controller is designed in modular form, so that sub-
assemblies could be removed easily for replacement or
repair.
• A PLC can work in the harsh and extreme conditions of the
industry where other systems such as a Micro-Controller
cannot.
• The system becomes reusable.
• The method used to program the controller is simple, so
that it can be easily understood by plant personnel.
10. PAC
PAC stands for Programmable Automation Controller.
PLC is best suited for machine control but has limited
memory and a focus on discrete I/O with on/off
control.
On the other hand , a PAC is geared more toward
complex automation system architectures composed
of a number of PC-based software applications
11.
12. Features of PAC
Composed of a number of PC-based software
applications
Including HMI (human machine interface) functions
Architecture based on Ladder logic
Provide greater flexibility in programming
Larger memory capacity
13. Advantages of PACs over PLCs
More open architecture
Modular design
More analog I/O options
Better process control capabilities
More built in communication ports
Programming over USB ports
Tag based programming (one software package
capable of programming multiple models )