Introduction to PLC


A programmable logic controller (PLC) is a special form of microprocessor-based controller that uses a programmable memory to store instructions and to implement functions such as logic, sequencing, timing, counting and arithmetic in order to control machines and processes. As PLC stands for programmable logic controller, It is a control device built similar to a computer that can be programmed to use in automation of electro-mechanical processes such as control of machinery in factory assembly lines, oil refineries, waste water treatment plants, pharmaceutical manufacturing plants, cement plants, paper & pulp industries etc.

PLCs have the great advantage that the same basic controller can be used with a wide range of control systems. To modify a control system
and the rules that are to be used, all that is necessary is for an operator to key in a different set of instructions. There is no need to rewire. The result is a flexible, cost effective, system which can be used with control systems which vary quite widely in their nature and complexity.
PLCs are similar to computers but whereas computers are optimised for calculation and display tasks, PLCs are optimised for control tasks and the industrial environment.

Input devices

The term sensor is used for an input device that provides a usable output in response to a specified physical input. For example, a thermocouple is a sensor which converts a temperature difference into an electrical output. The term transducer is generally used for a device that converts a signal from one form to a different physical form. Thus sensors are often transducers, but also other devices can be transducers, e.g. a motor which converts an electrical input into rotation.

Output devices

The output ports of a PLC are of the relay type or optoisolator with transistor or triac types depending on the devices connected to them which are to be switched on or off. Generally, the digital signal from an output channel of a PLC is used to control an actuator which in turn controls some process. The term actuator is used for the device which transforms the electrical signal into some more powerful action which then results in the control of the process.