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Control engineering has evolved over TIme. In the past humans were the mainmethod for controlling a system. More recently electricity has been used for control andearly electrical control was based on relays. These relays allow power to be switched onand off without a mechanical switch. It is common to use relays to make simple logicalcontrol decisions. The development of low cost computer has brought the most recent revoluTIon,the Programmable Logic Controller (PLC). The advent of the PLC began in the1970s, and has become the most common choice for manufacturing controls.PLCs have been gaining popularity on the factory floor and will probably remainpredominant for some TIme to come. Most of this is because of the advantages they offer.• Cost effecTIve for controlling complex systems.• Flexible and can be reapplied to control other systems quickly and easily.• Computational abilities allow more sophisticated control.• Trouble shooting aids make programming easier and reduce downtime.• Reliable components make these likely to operate for years before failure.
