Introduction to PLCs L1

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Introduction to ProgrammableLogic Controllers (PLC)

Composed byBrian Hernndez

December 2, 2007


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Introduction

Bedford Associates, founded by Richard Morleyintroduced the first Programmable LogicController in 1968

PLC was known as the Modular DigitalController from which the MODICON companyderived its name

Programmable Logic Controllers weredeveloped to provide a replacement for largerelay based control panels

These systems were inflexible requiring major rewiring or replacement whenever the controlsequence was to be changed


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Introduction

These systems were inflexible requiring major rewiring or replacement whenever the controlsequence was to be changed

Maintainable by technicians and electricalpersonnel

To support this the programming language of Ladder Logic was developed

Ladder Logic is based on the relay and contactsymbols technicians were used to through wiringdiagrams of electrical control panels


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Topics for PLCs

What is a Programmable Logic Controller (PLC)? Advantages Disadvantages Real world Relay Replacement Example of a Motor Control

How to program PLCs? Ladder diagrams Examples Signal Processing


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What is a PLC?

A special computer for logic controls

using ladder logic programming.


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What is a PLC?

A digital computer used for automation of industrial processes

PLC is designed for multiple inputs and outputarrangements

Programs to control machine operation PLC is an example of a real time system Armored for severe conditions compared to PCs Relay logic replacement Process Inputs and Outputs


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Advantages of PLCs

Easy to maintain Easy to program

Reliable in industrial environment Compact Long lasting Scalable


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Disadvantages of PLCs

Handling large amount of Data Displaying Data Handling Databases Complex data handling


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PLC a Real World RelayReplacement

Real world Parts: Switch

Bell Relay

Switch controlling ACcircuit or bell

Relay


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PLC a Real World RelayReplacement

Relay CoilSymbol

Relay Normallyopen Contact

Symbol

PLC InputSymbol

PLC OutputSymbol


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Example of a Motor Control

START STOP

output

START STOP

output

EQUIVALENT DIAGRAMS

NormallyOpen Contact

NormallyClosed Contact


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How to program PLCs?

Using a specialized Ladder Logic Software Allen-Bradley uses RsLogix 500 software to program

SLC, Micrologix and so on

Omron uses Cx Programmer software for itsSYSMAC PLCs

Other manufacturers has their own software


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Ladder Logic

Ladder logic is a method of drawing electricallogic schematics

A graphical language popular for PLCsControllers

Invented to describe logic made from relays Language resemble ladders, with two vertical

"rails" and a series of horizontal "rungs" between

them Rule-based language, rather than a procedurallanguage

Executed sequentially by software, in a loop


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Examples

Relay Logic

Ladder Logic


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Examples

Switch turns on a solenoid for 5 seconds andthen turn it off regardless of how long theswitch is on for Use an external timer

What happens if decided to add 2 or moretimers and counters? Will need a lot of external components and rewiring

Using a PLC simply program your timers andcounters


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PLC Signal Process

Inputs : describe the status of the processto the controller (external)

States : discrete modes the controller canbe in (internal)

State transitions : functions of the current

state and the inputs Outputs : actions initiated by the controller

based on the current state (external)


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PLC Signal Process

Through inputs the PLC monitors sensors either digital or analog Photo eye Proximity sensor Pressure switch

The output is to control processes either digital

or analog Motor Control a valve Light


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Conclusion

PLCs are replacement of Relay based systems Ladder Logic is develop to program this

specialize devices using graphical symbols

Easy to maintain and program but does nothandle complex data manipulation

Each manufacture has its own custom softwareto program PLCs

Inputs are external signal from processes for thePLC to monitor. Ex: sensors Outputs are internal signal from PLC to control a

particular process. Ex: motors


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References

Amunrud, A., (2002). Programmable Logic Controller. Retrieved on December 2,2007, from: http://www.coe.montana.edu/ee/courses/ee/ee367/pdffiles/aamunrud.pdf

PLCs.net, (2007). Learn PLCs. Retrieved on December 2, 2007, from: http://www.plcs.net/

SearchEng, (2007). Introduction to PLC's . Retrieved on December 2, 2007, from: http://www.searcheng.co.uk/articles/plc/intro.html Turg, L.T., (2006). Programmable Logic Controller ME 314 - Lecture 1 . PowerPoint

Presentation. Wikipedia, (2007). Programmable Logic Controller . Retrieved on December 2, 2007,

from: http://en.wikipedia.org/wiki/Programmable_logic_controller
http://www.coe.montana.edu/ee/courses/ee/ee367/pdffiles/aamunrud.pdfhttp://www.plcs.net/http://www.plcs.net/http://www.searcheng.co.uk/articles/plc/intro.htmlhttp://en.wikipedia.org/wiki/Programmable_logic_controllerhttp://en.wikipedia.org/wiki/Programmable_logic_controllerhttp://en.wikipedia.org/wiki/Programmable_logic_controllerhttp://www.searcheng.co.uk/articles/plc/intro.htmlhttp://www.searcheng.co.uk/articles/plc/intro.htmlhttp://www.plcs.net/http://www.plcs.net/http://www.plcs.net/http://www.coe.montana.edu/ee/courses/ee/ee367/pdffiles/aamunrud.pdf

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Introduction to PLCs L1