A programmable logic controller (PLC), or programmable controller, is an industrial digital computer that has been strengthened and adapted for the control of manufacturing processes, such as assembly lines or robotic devices, or any activity requiring high reliability control and ease of programming and the diagnosis of process failures.
They were first developed in the automotive industry to provide flexible, robust and easily programmable controllers to replace relays and hard-wire timers. Since then, they have been widely adopted as high reliability automation controllers suitable for hostile environments. A PLC is an example of a "hard" real-time system since the output results must occur in response to input conditions within a limited time, otherwise an unintended operation will result.
PLCs can range from small "building brick" devices with dozens of inputs and outputs (I / O), in an integral casing with the processor, to rackmount modular devices with thousands of I / O and are often networked to other PLC and SCADA systems.
They can be designed for multiple analog and digital I / O configurations, extended temperature ranges, immunity to electrical noise and resistance to vibration and impact. Programs to control the operation of the machine are usually stored in battery backup or non-volatile memory.
He was from the automotive industry in the USA. that the PLC was born. Prior to the PLC, security control, sequencing and safety interlock logic for automobile manufacturing consisted mainly of relays, cam timers, battery sequenators and dedicated closed circuit controllers. Since these could be hundreds or even thousands, the process to upgrade such facilities for the annual model change was very time-consuming and costly, as electricians needed to individually redo the relays to change their operating characteristics.
When digital computers became programmable devices of general use, they were soon applied to control sequential and combinatorial logic in industrial processes. However, these early equipment required specialized programmers and strict environmental control for temperature, cleanliness and energy quality. To solve these challenges the PLC was developed with several key attributes. It would tolerate the plant environment, support discrete inputs and outputs (bit-form) in an easily extensible way, require no years of training to use, and allow control of its operation. Since many industrial processes have response times easily addressed in millisecond response times, modern electronics (fast, small, reliable) greatly facilitate the creation of reliable controllers and the performance can be exchanged for reliability.