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Electrical & Computer Engineering Dr. D. J. Jackson Lecture 1-
Programmable Logic
Controllers
Introduction to PLCs
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 1-
Outline
• Introduction
• Definition and History of the PLC
• Example PLCs
• Overall PLC System
• PLC Advantages and Disadvantages
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 1-
Definition And History Of The PLC
- A PLC is a user-friendly, microprocessor-
based specialized computer that carries out
control functions of many types and levels of
complexity.
- Its purpose is to monitor crucial process
parameters and adjust process operations
accordingly.
- Used extensively because the PLC
- Is easy to set up and program
- Behaves predictably
- Ruggedized
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 1-
Definition And History Of The PLC
- It can be programmed (to a degree),
controlled, and operated by a person
unskilled in operating (programming)
computers.
- Essentially, a PLC's operator draws the lines
and devices of ladder diagrams with a
keyboard/mouse onto a display screen.
- The resulting ladder diagram is converted
into computer machine language and run as
a program.
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 1-
Components in a PLC system
• CPU module, containing the processor and
memory
• Input and output modules, to allow the PLC
to read sensors and control actuators
- A wide variety of types are available
• Power supply for the PLC, and often sensors
and low power actuators connected to I/O
modules
• A rack or bus so the PLC can exchange data
with I/O modules
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 1-
PLC in a automated system
Power Supply
CPU
Module (PLC)
Digital Input Module
Digital Output Module
Analog Input Module
Analog Output Module
Programming unit
Network
Digital sensors (limit switches, proximity sensors)
Digital actuators (pneumatic valves, Indicator lamps)
Analog sensors (temperature, pressure sensors)
Analog actuators (motors, etc.)
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 1-
Additional PLC components
• A programming unit is necessary to create,
edit and download a user program to the PLC
• Additional components can include:
- Network interfaces : to allow PLCs to function in a networked environment
- Communication adapters for remote I/O devices: so I/O devices do not have to be physically close to the CPU module
- Operator interface devices: allow monitoring and/or data entry by operators
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 1-
Definition And History Of The PLC
- The PLC takes the place of much of the external wiring required for control of a process. - http://www.as-uk.co.uk/projectupgrade.html
- The PLC will operate any system that has output devices that go on and off (known as discrete, or digital, outputs).
- It can also operate any system with variable (analog) outputs.
- The PLC can be operated on the input side by on-off devices (discrete, or digital) or by variable (analog) input devices.
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 1-
PLC Advantages
• Flexibility
- In the past, each different electronically controlled production machine required its own controller; 15 machines might require 15 different controllers.
- Now it is possible to use just one model of a PLC to run any one of the 15 machines.
- Furthermore, you would probably need fewer than 15 controllers, because one PLC can easily run many machines.
- Each of the 15 machines under PLC control would have its own distinct program (or a portion of one running program).
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 1-
PLC Advantages
• Implementing Changes and Correcting
Errors
- With a wired relay-type panel, any program alterations require time for rewiring of panels and devices.
- When a PLC program circuit or sequence design change is made, the PLC program can be changed from a keyboard sequence in a matter of minutes.
- No rewiring is required for a PLC-controlled system.
- Also, if a programming error has to be corrected in a PLC control ladder diagram, a change can be typed in quickly.
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 1-
PLC Advantages
• Large Quantities of Contacts
- The PLC has a large number of contacts for each coil available in its programming.
- Suppose that a panel-wired relay has four contacts and all are in use when a design change requiring three more contacts is made. - Time would have to be taken to procure and install a new relay or relay contact block.
- Using a PLC, however, only three more contacts would be typed in. - Contacts are now a “software” component
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 1-
PLC Advantages
• Lower Cost
- Increased technology makes it possible to condense more functions into smaller and less expensive packages.
- Now you can purchase a PLC with numerous relays, timers, and counters, a sequencer, and other functions for a few hundred dollars.
• Pilot Running
- A PLC programmed circuit can be evaluated in the lab. The program can be typed in, tested, observed, and modified if needed, saving valuable factory time.
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 1-
PLC Advantages
• Documentation
- An immediate printout of the true PLC circuit is available in minutes, if required.
- There is no need to look for the blueprint of the circuit in remote files.
- The PLC prints out the actual circuit in operation at a given moment.
- Often, the file prints for relay panels are not properly kept up to date. A PLC printout is the circuit at the present time; no wire tracing is needed for verification.
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 1-
PLC Disadvantages
- Fixed Program Applications
- Some applications are single-function applications. It does not pay to use a PLC that includes multiple programming capabilities if they are not needed.
- Their operational sequence is seldom or never changed, so the reprogramming available with the PLC would not be necessary.
- Fail-Safe Operation
- In relay systems, the stop button electrically disconnects the circuit; if the power fails, the system stops.
- This, of course, can be programmed into the PLC; however, in some PLC programs, you may have to apply an input voltage to cause a device to stop. These systems may not be fail-safe.
