What is a Programmable Logic Controller (PLC)?

Many years before the PLC was invented, machine controllers used combinational circuits, like a combination of many relays, timers or any other devices, mechanical controller like cam shafts and drums, including electronic circuits to make a sequence process (logic control system). With this old sequential controller, the circuit looks very crowded and usually it’s hard to find the malfunctioning part, or to even modify the sequence, also requires more time and cost. The old sequential controller was implemented only in small scale or medium scale systems and was specific to a system or machine. Because in large scale systems, it is hard to test the ability and the efficiency of the entire system. This system also cannot be implemented in the process that needs a high speed control. So that’s why the PLC was invented. Many new industries have 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 equipment, 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 PLC 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 an important role 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 normally-open contacts and normally-close contacts, and can be used optionally within the PLC.

AoteWell Group, was established in 1998 as a distributor of Industrial automation and Commercial Electrical and Electronic Control, Automation and Motion Products, Repair and Engineering Services.

SIEMENS PLC:Simatic S7 ,Simatic S5

ALLEN BRADLEY PLC:SLC500, ControlLogix ,MicroLogix, CompactLogix


SCHNEIDER PLC:Modicon Quantum