Programmable logic controllers (PLCs) are modular, industrially hardened computers which perform control functions through modular input and output (I/O) modules. The modularity of PLC allows the user to combine generic I/O modules with a suitable controller to form a control system specific to his is most simply understood needs. The operation of a controller by envisioning that it is repeadly performs three steps:
1. Reads inputs from input modules.
2. Solved reprogrammed control logic.
3. Generates outputs to output modules base on the control logic solutions. Input and output devices of the process are connected to the PLC and the control program is entered into PLC memory.
In application, it controls through analog and digital inputs and outputs the varying load constant speed operation of an inductor motor. Also, the PLC continuously monitors the inputs and activates the outputs according to the control program. The PLC system is of modular type composed of specific hardware building blocks (module). Which plug directly into a proprietary bus: a Central Processor Unit (CPU), a power supply unit, input-output modules I/O and a program terminal. Such a modular approach has the advantage that the initial configuration can be expanded for other future applications such as multi machine systems or computer linking.