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Introduction to Programmable

Logic Controllersand Ladder Logic

Welcome

The purpose of this Presentation is to give you a very simple look Programmable Logic Controllers and its programming using Ladder Logic.

There is no need for you to be familiar in any way with this field. The only requirement is that you

enter with an open mind.

To begin with..

What is

automation?

You may have come across the word “automated” very usually!

Automation is the use of machines, control systems and information technologies to monitor and control the production.

In other words, an automated system is any system that require minimum or no human intervention.

Automation has made our life extremely simple.

Earlier, processes were carried out by operator, hence were much prone to human errors.

So we can imply that with Industrial control, factory automation, and PLC (if are done well), we can save a lot of time, materials, energy, and money.

Where do we begin?

Let’s recall the story of an explorer in Africa who asked the native tribesman,

How does one eat a huge animal like an elephant?"

The tribesman looked at the explorer in astonishment and replied, "We eat it just like

everything else, one bite at a time."

Simple as it is, industrial control is comprised of many smaller circuits. By exploring and learning simple smaller circuits, you can learn to build a complete automated system.

Programmable logic controllers

Lets Start With…

What is

a small computer with a built-in operating system which monitor inputs and other variable values, make decisions based on a stored program, and control outputs to automate a process or machine.

Programmable logic controllersWhat is

Well, that was simple isn’t it ?

Lets put it in another way..

A PLC is similar to a computer which is designed to be programmed once, and run repeatedly as needed.

Elements of PLC

Let’s have al look at..

Input ModulesSenses the input signal, isolates it and converts it to DC level.

Central Processor Unit (CPU)

microprocessor system that contains the system memory and its decision making unit very similar to that of a computer’s CPU.


Output Modulereceives the convert control signals from the CPU and them converts them into digital or analog values that can be used to control various output devices.




Programming Device Personal computer or a Laptop

And Communication Cable

Output Modulereceives the convert control signals from the CPU and them converts them into digital or analog values that can be used to control various output devices.




PLC scanWhat is a

Reads Inputs

Execute Programs

Diagnostics & Communication

Updates Outputs

PLC scan

PLC scan

Programming PLC

The first and still most popular programming language used with PLC is Ladder Logic Programming (LAD).

They are called "ladder" diagrams because they resemble a ladder,

with two vertical rails (supply power) and as many "rungs"

(horizontal lines).

The first and still most popular programming language used with PLC is Ladder Logic Programming (LAD).

Basic Ladder Logic Symbols

Normally Open (NO) contactPasses power (on) when coil driving the contact is on.

Contacts

Normally Closed (NC) contactPasses power (on) when coil driving the contact is off.


Normally Open (NO) contactPasses power (on) when coil driving the contact is on.

Contacts

Output or CoilWhen power flows through the Coils via contacts they are energized.


Schematic of ladder diagram

Selecting a PLC

Criteria

• Number of logical inputs and outputs.• Memory• Number of special I/O modules• Scan Time• Communications• Software

Functional Operation

AND OperationOR Operation

NOT Operation

Lets have a look at their truth table and Ladder Logic diagram

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AND Operation

Both inputs S4 and S5 must be true (1) in order for the output L3 to be true (1).

Ladder diagram

OR Operation

Ladder diagram

Either input S1 or S2 is true (1), or both are true, then the output L2 is true (1).

37

NOT Operation

If input S1 is be true (1), then the output L1 is true (0) or when A is (0), output C is 1.

Ladder diagram

Programming Functions

TimersCounters

Timers

On-Delay Timer (TON)Off-Delay Timer (TOF)Retentive On-Delay Timer (TONR)

Counters

Count UP Counter (CTU)Count DOWN Counter (CDU)Count UP/DOWN Counter (CTUD)

Programming Examples

Lets start with simple examples…


D = (A AND B) OR C


Triggering the Relay Coil

Ladder Diagram for Triggering the Relay Coil

Learn the Basics of Programming Logic Controller & Ladder Logic by Downloading this E-book

This E-book contains

Programmable logic controllersPLC ScanBasic Ladder Logic SymbolsFunctional Operation Programming Functions Programming To PLC (Practical Examples)Illustration of PLC Scan

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For Further Reading

Introduction to Programmable Logic Controller and Ladder Logic