This article explains explicitly, the processes involved in automating industries using Programmable Logic Controller (PLC) and the real life or business applications of plc and also how to program a system that mixes two different liquids in a tank.
 automating industries using PLC 

Article Content

•    Introduction
•    Background
•    PLC definition
•    PLC manufacturers
•    PLC Programming software
•    How it works
•    Advantages & Disadvantages of PLC
•    Application of PLC
•    Ladder logic
•    Conclusions and Recommendations


The Programmable Logic Controller (PLC) has revolutionized the automation industry. Today PLCs can be found in everything from factory equipment to Automobiles, but prior to 1968 the programmable controller didn’t even exist. Instead what existed was a unique set of challenges that needed a solution.


•    Before the days of the PLC the only way to control machinery was through the use of relays.

•    For example, to control an electric motor, a relay could be attached between the power source and the motor. A control could be enforced when the motor is getting power by either energizing or de-energizing the relay.

•    There could be several motors in one factory that need to be controlled, so what do you do? You add lots of relays.

•    So factories started to amass electrical cabinets full of power relays.

•    To effect any changes, electricians would have go through the herculean task of  rewiring each relays ”Five hours to find it and five minutes to fix it”.

•    In 1968 the Hydramatic Division of the General Motors Corporation specified the design criteria for the first programmable controller.



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GM Hydra-Matic Mandate

The GM Hydra-Matic Mandate for PLC are as follows:
•    Less wiring/User friendly
•    Cost effective
•    Extensive I/O capability
•    Robust and able to withstand extreme operating conditions
•    Extraordinary processing time
The diagram below shows the GM Hydra-Matic Mandate for PLC.
The diagram shows the GM Hydra-Matic Mandate for PLC

PLC Pioneers

•    Dick Morley’s company Bedford Associates in Massachusetts proposed something called a Modular Digital Controller (MODICON).
•    The MODICON 084 brought the world’s first PLC into commercial production.
The picture below shows some PLC Pioneers posing with the MODICON 084.
The picture shows some PLC Pioneers posing with the MODICON 084

What is a PLC?

A Programmable Logic Controller (PLC) is an electronic device with a Programmable memory, multiple input and output terminals that is used to logically control various industrial machines and processes.
Below is the picture of a Programmable Logic Controller (PLC).
The diagram shows the picture of a Programmable Logic Controller (PLC)

Major Components of a PLC

–    Provides the voltage needed to run the primary PLC components.

–    Provides signal conversion and  isolation between the internal logic-level signals inside the PLC and the field’s high level signal.

–    Provides intelligence to command and govern the activities of the entire PLC systems.

–    Used to enter the desired program that will determine the sequence of operation and control of process equipment or driven machine.
The diagram below shows the block diagram for connecting the components of a PLC
The diagram shows the block diagram for connecting the components of a PLC

Leading Brands of PLC


1.    Allen Bradley
2.    Modicon
3.    Texas Instruments
4.    General Electric


1.    Siemens
2.    Festo  
3.    Telemechanique


1.    Toshiba
2.    Omron
3.    Mitsubishi

PLC Programming Software

•    Allen-Bradley – Rockwell RSLogix500/5000
•    Modicon -   Modsoft
•    Omron – CX-ONE
The picture of Omron – CX-ONE
•    Siemens– Simatic Step 7
Siemens– Simatic Step 7

 How does the PLC Software Work?

•    Self scan: Testing of its own hardware and software for faults.

•    Input scan: A PLC scan cycle begins with the CPU reading the status    of its inputs.

•    Logic scan: The application program is executed using the status of the inputs.

•    Output scan: The cycle ends by updating the outputs.
The diagram below illustrates better how the PLC software works.
The picture illustrates how the PLC software works

Concept of Ladder logic

The basic Concept of Ladder logic
Power flows through these contacts when they are closed. The normally open (XIC) is true when the input or output status bit controlling the contact is 1. The normally closed (XIO) is true when the input or output status bit controlling the contact is 0.

On Delay Timers (TON)

On Delay Timers

EN: This is the timer Enable bit that comes on once the timer is energised
DN: This is the timer Done bit that only comes on once the time is done counting
TT: This is the Timer Timing bit which comes on and stays on as long as the timer is counting and goes off once the timer is done counting.


Coils 5symbol
Coils represent relays that are energized when power flows to them. When a coil is energized it causes a corresponding output to turn on by changing the state of the status bit controlling the output to 1


AND Operation – Rung


OR Operation – Rung

In the rung above, it can be seen that either input A or B is to be true (1), or both are true, then the output C is true (1).


NOT Operation – Rung
In the rung above, it can be seen that if input A is  be true (1), then the output C is true (0) or when A is (0), output C is 1.

Tank Used to Mix Two Liquids

Operating principle of an automated Tank Used to Mix Two Liquids
A tank is used to mix two liquids. The control circuit operates as follows:

•    When the start button is pressed, solenoids A and B   energize. This permits the two liquids to begin filling the tank.

•    When the tank is filled, the float switch trips. This de-energizes solenoids A and B and starts the motor used to mix the liquids together.

•    The motor is permitted to run for one minute. After one minute has elapsed, the motor turns off and solenoid C energizes to drain the tank.

•    When the tank is empty, the float switch de-energizes solenoid C.

•    A stop button can be used to stop the process at any point.

•    Once the circuit has been energized it will continue to operate until it is manually stopped.

A Sample Program for the Mixing of Two Liquids

The picture below illustrates how to program the automated mixing of two liquids in a tank.
A Sample Program for the Mixing of Two Liquids

Advantages of Programmable Logic Controller (PLC)

•    Smaller in size
•    Cost Effective
•    Ease of maintenance/ troubleshooting
•    Rugged and reliable
•    Easier and faster to make changes
•    High Speed of Operation
•    Diagnosis can be centrally available

Disadvantage of PLC

•    PLC devices are proprietary

Real Life or Business Applications of PLC

•    Oil & Gas processing Plants
•    Factory/Process automation
•    Warehouse Automation
•    Traffic management controls
•    Power systems
•    Water treatment systems

Conclusions & Recommendations

As industrial automation evolves, student’s exposure to modern technologies will translate to increased opportunities in different industries, therefore it is expedient that our universities put in place a compulsory scheme on Factory Automation that ensures every engineering graduate gets a feel of what automation is all about before graduation. Lagos state has already started this by implementing a FESTO Authorized and Certified Training (FACT) Centre.

By: Iroakazi Chidiebere
Company: PZ Cussons Nigeria Plc

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