1. ACOE347 – Data Acquisition and Automation Systems
Introduction to Programmable Logic Controllers (PLCs)
Automation Systems
What is a PLC?
Hardware components of a PLC
Memory structure of a PLC
PLC Modes of Operation
Wiring Diagram
Digital Inputs/Outputs
ACOE347
Introduction to PLCs

2. What is an Automation System?
Industrial Automation:
A system that controls automatically the operation of equipment such as machines or production processes in factories
ACOE347
Introduction to PLCs

3. What is an Automation System?
Home Automation:
A system that automatically controls the operation of home equipment and appliances such as lights, gates, doors, alarm systems etc
ACOE347
Introduction to PLCs

4. Industrial Automation Components
Controlled System:
Mechanical Components (conveyor belts, arms, rotary discs, etc)
Actuators (motors, solenoid valves, relays, heating elements, etc)
Sensors (tactile switches, opto-switches, proximity switches, etc)
Controller: A unit that reads the state of the sensors and activate the actuators according to the control program
Control Panel: Interface between the operator and the system.
Buttons, switches, lamps, displays, touch screens etc
ACOE347
Introduction to PLCs

5. Manually Controlled Systems
ACOE347
Introduction to PLCs

6. Custom made and programmed controllers
ACOE347
Introduction to PLCs

7. Industrial Computers or PCs
ACOE347
Introduction to PLCs

8. Programmable Logic Controllers (PLC)
A Programmable Logic Controller (PLC) is
a microprocessor based control system,
designed for use in an industrial environment,
programmed to sense, activate and control industrial equipment.
ACOE347
Introduction to PLCs

9. Main Units of a PLC?
The components that make a PLC work can be divided into three core areas.
The power supply and rack
The central processing unit (CPU)
The input/output (I/O) section
PLCs are “modular” because the rack can accept many different types of I/O modules that simply slide into the rack and plug in.
ACOE347
Introduction to PLCs

10. The Power Supply and Rack
The rack is the component that holds everything together. 
The rack has a backplane at the rear which allows the cards to communicate with the CPU. 
The power supply plugs into the rack as well and supplies to other modules that plug into the rack.  The most popular power supplies work with 120/240 VAC or 24 VDC sources.
ACOE347
Introduction to PLCs

11. Hardware components of a PLC
ACOE347
Introduction to PLCs

12. Memory structure of a PLC
ACOE347
Introduction to PLCs

13. PLC Mode of Operation A PLC operates in one of two modes:
Program Mode: Download the user’s program from the programming device to the memory of the PLC.
Run Mode: Execute the user’s program using the scan-based execution model, in three phases:
Read the state of the inputs and store them in the RAM referred to as the input image memory..
Execute the user’s program and update the state of the outputs in the RAM (output image memory).
Copy the states of the outputs from the RAM to the physical output devices.
The time needed to complete one scan cycle depends on the length and complexity of the user’s program.
PLC manufacturers specify the maximum number of program steps (say 1000) and the average scan time (few ms).
ACOE347
Introduction to PLCs

14. A Typical PLC from Allen-Bradley
ACOE347
Introduction to PLCs

15. DC Voltage Digital Input
The optoisolator isolates the internal components of the PLC and the external input devices.
Diode D1 provides protection during switching from inductive devices.
When the switch is closed, current flows through the status LED and the opto-isolator, giving a logic 1 to the input port.
ACOE347
Introduction to PLCs

16. Connecting DC Proximity Switches
ACOE347
Introduction to PLCs

17. AC Voltage Digital Input
AC input devices switch On and Off the mains voltage.
The mains voltage must be reduced using a voltage divider.
The mains voltage must be converted to DC using a rectifier and a smoothing circuit.
An AC proximity switch switches On and Off the mains voltage by triggering (firing) a triac. The proximity sensor controls the gate current of the triac.
ACOE347
Introduction to PLCs

18. Digital Output (Relay and Transistor)
Transistor outputs provide higher switching speeds with more switching life.
Transistor outputs can only be used with DC loads and are polarity sensitive. A diode is usually used for reverse polarity protection.
Another diode is usually used to protect the output transistor when switching inductive loads.
Most PLCs have relay outputs.
Relay outputs have the advantage that they can control DC as well as AC loads at relatively high currents.
The disadvantages of relays are due to their mechanical contacts.
ACOE347
Introduction to PLCs

19. Digital Output Using Triacs
Triac outputs are used to avoid the problems due to the mechanical contacts of relays.
An opto-diac is used to fire the triac.
An RC circuit is usually used to avoid problems due to the phase difference between the voltage and the current when driving inductive loads.
ACOE347
Introduction to PLCs

20. Typical Wiring Diagram
ACOE347
Introduction to PLCs