Programmable Logic Controllers
The modern solution for the problem of how to provide discrete-state control is to use a computer-based device called a programmable controller (PC) or programmable logic controller (PLC).
A PLC (i.e. Programmable Logic Controller) is a device that was invented to replace the necessary sequential relay circuits for machine control. The PLC works by looking at its inputs and depending upon their state, turning on/off its outputs. The user enters a program, usually via software, that gives the desired results.
PLCs are used in many “real world” applications. If there is industry present, chances are good that there is a PLC present. If you are involved in machining, packaging, material handling, automated assembly or countless other industries you are probably already using them. If you are not, you are wasting money and time. Almost any application that needs some type of electrical control has a need for a PLC.
Hard-wired system and PLC
Any industrial process control involves the use of coils, relay contacts, solenoids, timers and the like. Prior to the introduction of the programmable logic controllers, the control circuits were permanently wired according the particular process control required for every industrial process there was one control panel with several input and output devices or components. They were all soldered or wired on permanent or semi permanent modules and were suitable for that process control only. In short this control circuitry was called a hard-wired control system.
It is not flexible to accept new control jobs. If modification was required, the machines had to be stopped for several hours and also the space in the control panel would not be sufficient to accommodate the new relays added.
A PLC also contains relays, coils, contacts etc., but they are not hard – wired or soldered permanently. Their terminals are always ready for varieties of connecting patterns. So PLC control is very flexible, it can be modified in short time by a revised program. Fault finding (trouble – shooting) is easy and less time consuming, because the possible faults can be located even at the programming stage and program feeding stage. PLC control console can easily be used for different process control which is not at all possible in a hard – wired control system.
PLC advantages over relays
Compared to conventional ordinary relay controls, a programmable logic controller enjoys the following benefits or advantages.
The wiring of a PLC system usually reduces by 80 percent compared to conventional relay control system. So, cabinet size is small leading to less floor space requirements.
The power consumption is greatly reduced as PLC consumes much less power.
Self diagnostic functions enable easy and fast trouble shooting of the system.
A single panel can be used in multiple applicable by merely changing the program.
Spare parts for relays and timers are greatly reduced
PLC speed of operation is in the milliseconds range; hence machine cycle time is improved tremendously. Thus productivity is increased.
Costs much less particularly where there are many I / O devices and the control functions are complex.
Very reliable operation
A print out of the PLC program can be obtained in a short time. Therefore hardcopy of documentation can be easily maintained.
Can be installed easily.
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