Programmable logic controller: Difference between revisions

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A '''programmable logic controller''' ('''PLC''') or '''programmable controller''' is an industrial [[computer]] that has been [[ruggedized]] and adapted for the control of manufacturing processes, such as [[assembly line]]s, machines, [[robotic]] devices, or any activity that requires high reliability, ease of programming, and process fault diagnosis.

PLCs can range from small modular devices with tens of [[Input/output|inputs and outputs]] (I/O)]], in a housing integral with the processor, to large rack-mounted modular devices with thousands of I/O, and which are often networked to other PLC and [[SCADA]] systems.<ref>Tubbs, Stephen Phillip. ''Programmable Logic Controller (PLC) Tutorial, Siemens Simatic S7-1200.'' Publicis MCD Werbeagentur GmbH; 3rd ed., 2018.</ref> They can be designed for many arrangements of digital and analog I/O, extended temperature ranges, immunity to [[electrical noise]], and resistance to vibration and impact.

PLCs were first developed in the automobile manufacturing industry to provide flexible, rugged and easily programmable controllers to replace hard-wired [[relay logic]] systems. [[Dick Morley]], who invented the first PLC, the Modicon 084, for [[General Motors]] in 1968, is considered the father of PLC.

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===Simulation===

An incorrectly programmed PLC can result in lost productivity and dangerous conditions for programmed equipment. PLC simulation is a feature often found in PLC programming software. It allows for testing and [[debugging]] early in a project's development. Testing the project in simulation improves its quality, increases the level of safety associated with equipment and can save time during the installation and commissioning of automated control applications since many scenarios can be tried and tested before the system is activated.<ref name=":7" /><ref>{{cite book |last1=Lin |first1=Sally |url=https://books.google.com/books?id=CHYlTBxqrM8C&pg=PA553 |title=Advances in Computer Science, Environment, Ecoinformatics, and Education, Part III: International Conference, CSEE 2011, Wuhan, China, August 21-22, 2011. Proceedings |last2=Huang |first2=Xiong |date=9 August 2011 |publisher=Springer Science & Business Media |isbn=9783642233449 |pages=15 |via=Google Books }}</ref><!--[[User:Kvng/RTH]]-->

==Functionality==

[[File:Siemens Simatic S7-416-3.jpg|thumb|upright|PLC system in a rack, left-to-right: power supply unit (PSUPS), CPU, interface module (IM) and communication processor (CP)]]

[[File:PLC Control Panel.png|thumb|upright|Control panel with PLC (gray elements in the center). The unit consists of separate elements, from left to right: [[power supply]], controller, [[relay]] units for input and output.]]

The main difference fromcompared to most other computing devices is that PLCs are intended for and therefore tolerant of more severe environmental conditions (such as dust, moisture, heat, cold), while offering extensive [[input/output]] (I/O) to connect the PLC to [[sensor]]s and [[actuator]]s. PLC input can include simple digital elements such as [[limit switch]]es, analog variables from process sensors (such as temperature and pressure), and more complex data such as that from positioning or [[machine vision]] systems.<ref>Harms, Toni M. & Kinner, Russell H. P.E., ''Enhancing PLC Performance with Vision Systems''. 18th Annual ESD/HMI International Programmable Controllers Conference Proceedings, 1989, p. 387-399.</ref> PLC output can include elements such as indicator lamps, sirens, [[electric motor]]s, [[pneumatic]] or [[hydraulic]] cylinders, magnetic [[relay]]s, [[solenoid]]s, or analog outputs. The input/output arrangements may be built into a simple PLC, or the PLC may have external [[I/O module]]smodules attached to a fieldbus or computer network that plugs into the PLC.<!--[[User:Kvng/RTH]]-->

The functionality of the PLC has evolved over the years to include sequential relay control, motion control, [[process control]], [[distributed control system]]s, and [[computer network|networking]]. The data handling, storage, processing power, and communication capabilities of some modern PLCs are approximately equivalent to [[desktop computer]]s. PLC-like programming combined with remote I/O hardware, allows a general-purpose desktop computer to overlap some PLCs in certain applications. Desktop computer controllers have not been generally accepted in heavy industry because desktop computers run on less stable operating systems than PLCs, and because the desktop computer hardware is typically not designed to the same levels of tolerance to temperature, humidity, vibration, and longevity as the processors used in PLCs. Operating systems such as Windows do not lend themselves to deterministic logic execution, with the result that the controller may not always respond to changes of input status with the consistency in timing expected from PLCs. Desktop logic applications find use in less critical situations, such as laboratory automation and use in small facilities where the application is less demanding and critical.{{citation needed|date=November 2014}}