Every system or machine has a controller, depending on the type of technology used, controller can be divided into pneumatic, hydraulic, electrical and electronic controllers. Frequently we use combination of different technologies. Furthermore, differentiation is made between hard wired programmable and programmable logic controller. The first is used primarily in cases, where any reprogramming by the user is out of the question and job size warrants the development of a special controller. Typical application for such controllers can be found in automatic washing machine, video cameras, and cars.
However, if the job size does not warrant the development of special controllers or if the user is to have the facility of making simple or independent program changes, or of setting timers and counters, then the use of universal controllers, where the program is written to an electronic memory, is the preferred option. The PLC represents such a universal controller. It can be used for different applications and via the program installed in its memory , provides the user with a simple means of changing , extending and optimising control processes.
PLC are widely used in Industries due to following reasons. .
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Cost of PLC automation is less and PLC is very versatile
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PLC can be commissioned and installed easily Programming of PLC is quite simple. Ladder programming is flexible
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They are not hard wired control. They can be programmed and reprogrammed to accommodate frequent changes in program
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Monitoring of on line work process is easy, therefore trouble shooting and maintenance of PLC is not a difficult task
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They can be classified as low cost automation devices
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They can be used in harsh environment where humidity and temperature are high. Their working is not effected by vibration and shock
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They can be used to execute complex mathematical algorithms, servomotor control, Stepping control, axis control, self – diagnosis, on line monitoring, condition monitoring, system trouble shooting, communicating to other PLCs, data acquisition, Networking, storage and report generation
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PLC are most suitable for low cost automation, where frequent changes to the control requirement would be expected during their operational life like in Batch type of production systems.