What is a Programmable Logic Controller (PLC)?
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Many-many years before PLC was infented, machine controllers using a combinational circuits, like combine of many relays, timers or any other devices, mechanical controller like cams shaft and drums, include of electronics circuits to made a sequence process (logic control system). With this old sequensial controller, the circuit looks very crowded and usually it is hard to find the malfunction part, or even to modify the sequence, besides that it needs more time and cost too. The old sequensial controller also implemented only in small scale or medium scale system and specific to a system or machine, because in large scale system, it is hard to test the ability and the efficiency of the entire system. This system also cannot implemented in the process that needs a high speed control. So thats why the PLC was infented. Many new industries now a day already implemented PLC systems to control their machines.
PLC is operated by the instruction input from the input devices (like push button switch, selector switch, digital switch, etc. provided on the operation panel, or sensors input from the limit switch, proximity switch, etc.) used to detect the operation condition of the equiptment, and serve to control the driving loads, output devices (such as solenoid valve, motor, electromagnetic clutch, etc. and indication loads such as pilot lamp, digital indicator, etc.). The transmission of output signal against these input signals is determined by the contents of program to be provided to the PLC. The light loads such as small type solenoid valve, pilot valve, pilot lamp, etc. can be directly driven by the PLC, however, the heavy loads such as 3-phase motor, large capacity solenoid valve, etc. need to be driven through the contactor or the intermediate relay. Such contactor, intermediate relay, power breaker, etc. are installed in the control panel together with the PLC. The PLC will play the important roles as a small type, high-reliability and flexible brain when designing the automated product machining, assembling, transfer, inspection, packing, etc.
The PLC is composed of electronic circuits with a micro-computer (micro-controller) centered, however, it can be equivalently regarded as an integrated body of ordinary relay, timer, counter, etc. The input relay built in the PLC is driven by the external switch through the input terminal. The output relay built in the PLC is provided with various internal contacts in addition to the external output contact. Besides, it is incorporated with various types of elements such as timer, counter, auxiliary relay, state, coil and function block, etc. In addition, these elements are provided with many electrically normaly-open contacts and normally-close contacts, and can be used optionally with in the PLC.