Editing Automation (section)

====Programmable Logic Controllers====
Industrial automation incorporates programmable logic controllers in the manufacturing process. [[Programmable logic controller]]s (PLCs) use a processing system which allows for variation of controls of inputs and outputs using simple programming. PLCs make use of programmable memory, storing instructions and functions like logic, sequencing, timing, counting, etc. Using a logic-based language, a PLC can receive a variety of inputs and return a variety of logical outputs, the input devices being sensors and output devices being motors, valves, etc. PLCs are similar to computers, however, while computers are optimized for calculations, PLCs are optimized for control tasks and use in industrial environments. They are built so that only basic logic-based programming knowledge is needed and to handle vibrations, high temperatures, humidity, and noise. The greatest advantage PLCs offer is their flexibility. With the same basic controllers, a PLC can operate a range of different control systems. PLCs make it unnecessary to rewire a system to change the control system. This flexibility leads to a cost-effective system for complex and varied control systems.<ref>{{cite book|last1=Bolten|first1=William|title=Programmable Logic Controllers|date=2009|edition=5th|page=3}}</ref>

PLCs can range from small "building brick" devices with tens of I/O in a housing integral with the processor, to large rack-mounted modular devices with a count of thousands of I/O, and which are often networked to other PLC and [[SCADA]] systems.

They can be designed for multiple arrangements of digital and analog [[Input/output stream|inputs and outputs]] (I/O), extended temperature ranges, immunity to [[noise (electronics)|electrical noise]], and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed-up or [[non-volatile memory]].

It was from the automotive industry in the United States that the PLC was born. Before the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles was mainly composed of [[relay]]s, [[cam timer]]s, [[drum sequencer (controller)|drum sequencer]]s, and dedicated closed-loop controllers. Since these could number in the hundreds or even thousands, the process for updating such facilities for the yearly model [[changeover|change-over]] was very time-consuming and expensive, as [[electrician]]s needed to individually rewire the relays to change their operational characteristics.

When digital computers became available, being general-purpose programmable devices, they were soon applied to control sequential and combinatorial logic in industrial processes. However, these early computers required specialist programmers and stringent operating environmental control for temperature, cleanliness, and power quality. To meet these challenges, the PLC was developed with several key attributes. It would tolerate the shop-floor environment, it would support discrete (bit-form) input and output in an easily extensible manner, it would not require years of training to use, and it would permit its operation to be monitored. Since many industrial processes have timescales easily addressed by millisecond response times, modern (fast, small, reliable) electronics greatly facilitate building reliable controllers, and performance could be traded off for reliability.<ref name=Parr00>E. A. Parr, ''Industrial Control Handbook'', Industrial Press Inc., 1999 {{ISBN|0-8311-3085-7}}</ref>