1、Programmable logic controller A programmable logic controller (PLC) or programmable controller is a digital computer used for automation of industrial processes, such as control of machinery on factory assembly lines. Unlike general-purpose computers, the PLC is designed for multiple inputs and outp
2、ut arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed or non-volatile memory. A PLC is an example of a real time system since output results must be produced in
3、response to input conditions within a bounded time, otherwise unintended operation will result. PLC and Programmable Logic Controller are registered trademarks of the Allen-Bradley Company, although they are widely used generically. Features The main difference from other computers is that PLC are a
4、rmored for severe condition (dust, moisture, heat, cold, etc) and have the facility for extensive input/output (I/O) arrangements. These connect the PLC to sensors and actuators. PLC read limit switches, analog process variables (such as temperature and pressure), and the positions of complex positi
5、oning systems. Some even use machine vision. On the actuator side, P L C operates electric motors, pneumatic or hydraulic cylinders, magnetic relays or solenoids, or analog outputs. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a c
6、omputer network that plugs into the PLC. P L C was invented as replacements for automated systems that would use hundreds or thousands of relays, cam timers, and drum sequencers. Often, a single PLC can be programmed to replace thousands of relays. Programmable controllers were initially adopted by
7、the automotive manufacturing industry, where software revision replaced the re-wiring of hard-wired control panels when production models changed. Many of the earliest PLC expressed all decision making logic in simple ladder logic which appeared similar to electrical schematic diagrams. The electric
8、ians were quite able to trace out circuit problems with schematic diagrams using ladder logic. This program notation was chosen to reduce training demands for the existing technicians. Other early PLC used a form of instruction list programming, based on a stack-based logic solver. The functionality
9、 of the PLC has evolved over the years to include sequential relay control, motion control, process control, distributed control systems and networking. The data handling, storage, processing power and communication capabilities of some modern PLC are approximately equivalent to desktop computers. P
10、LC-like programming combined with remote I/O hardware, allow a general-purpose desktop computer to overlap some PLC in certain applications. Under the IEC 61131-3 standard, PLC can be programmed using standards-based programming languages. A graphical programming notation called Sequential Function
11、Charts is available on certain programmable controllers. 60s in last century, because of American car industry need to carry on large-scale technique reformation and equipments to renew, from traditionally control equip to carry on a control after the electric appliances, not only the physical volum
12、e be huge, breakdown rate, gentle bad, ineffective live, consume ability, and adjust to try difficulty, the credibility is also bad.1968The year American G.M. company puts forward using conceive of new generation controller, consider from the customers angle and that company lifted to request at 10:
13、00 to the new generation controller and provided an explicit development target for each archduke department. Next year, from the American DEC (numerical equipments company) developed to succeed first the first set programmable logic controller PDP-14.About in the meantime the American MODICON compa
14、ny also develops 084 controllers. They publish, caused focus attention of whole world, other companies and Western Europe in the United States, Japanese the etc. industry be flourishing nation, also study to develop a product for is similar to one after another. The PLC absorbs micro-electronics tec
15、hnique and the calculator technical last word, so develop to quickly automate the whole automation of production line from the single machine very, is go to produce of the whole factory automation, from gentle manufacturing system, industrial robot arrive the large dispersion type control system and
16、 the PLC all undertakes to emphasize an important person color. The PLC technique represented nowadays electricity procedure to control of earliest enter level. Equip (like intelligence appearance, numeral turn handbill device, intelligence of the liquid press to compatibly move a valve set etc.) wi
17、th various unit automation through a PLC and the spot total line, calculator network system, constitute car and factory automate of complete system. Usually adopt in the in proper order control realm of industrial control before the PLC invent such as take a reading machine after the electric applia
18、nces, drum type switch, paper etc. machine, electricity type the spare part is to control a component, particularly is control after the electric appliances, become the spare part that use most extensively in switch control system in long lost the manufacturing the process the control the realm. But
19、, along with the development of industry modernization, production scale more and more big, labor rate of production and product quality of the request is continuously raising, for control system of the credibility also put forward higher request, originally possessed after electric appliances contr
20、ol system already unwell should need, investigate its reason BE: The action is slow-moving； The life span is short and the credibility be bad； The physical volume is big, consume an electricity have another； Design a manufacturing a period long, procedure modification time-consuming； Fall through to
21、 have a conversation with calculator. Expect behind till 60s in last century, although the small scaled calculator has been gradually perfect and apply realm is also continuously extending, the small scaled calculator used for switch control system, obviously exist the circumstance of Malaysia pull
22、a small car again, this is because the characteristics of small scaled calculator decide: The plait distance woven distance complications, requested to have higher level personnel and operate a personnel； Need the kit not- standard exterior connects and conditionally once requested to the environmen
23、t and the spot； Function excess, the machine resources can not well make use of； Build price high. Need and possibility, urge people to look for new exit, the PLC emerge with the tide of the times. It is apply in the car industry in the United States first, the PLC that is at this time uses solid(in
24、tegration) electric circuit to replace electric circuit after the electric appliances logic and use saving machine electric circuit in of saving digital(procedure) replace after the cloth line of electric appliances system, rule a logic relation with the procedure； Use the solid I/ O electric circui
25、t to examine the signal of button and limit switch, carry out organization by the control electrical engineering and the other for an exportation. The PLC system that is at this time has already started have as follows some characteristics: The environment adaptability is stronger, can be use in a c
26、ar the spot； There is higher credibility and diagnosis ability, maintain easily； Basic can adapt different manufacturing process need, the gentle degree more and greatly raised, want only the procedure in the change system namely convertibility a control logic, but didnt need to be reform or didnt r
27、eplace control hardware. Since 1976, the microprocessor starts lead into the PLC realm, consumedly strengthen a PLC function and make PLC from in brief replace after the electric appliances electric circuit, but develop to equip into the control which enters first. The PLC has to collect and handles
28、 a great deal of data, completion mathematics operation and correspond by letter with other intelligence spare parts nowadays of ability, and have the forerunners person machine to the words means(if keyboard, CRT and speech have a conversation), in recent years because of the spot emergence and rel
29、ated standard establishment of total line principle, and the quick development of product, the PLC become the spot total line of an importance constitute part, extended PLC applied realm further. Because the PLC raised function with gentle degree in the meantime, make its application quickly increas
30、e, and make widely available many other long-lost spare parts manufacture industry realm. Later on again expand to and criticize quantity to produce with continuous production line relevant of industrial realm. Along with the CIMS (calculator integration manufacturing system) development, the PLC is
31、 still used for factory by people to correspond by letter a network at present in, become a calculator with other intelligence controllers and calculator system together comprehensive control system in of importance constitute part, especially unit class and work station class. From 1969 the first s
32、et P L C publish up to now and the programmable controller about experienced three stages: First stage: The PLC capacity of the development is smaller, I/O the point be small at 120:00.The customer saves area capacity or so scan the speed as 2050 ms K B in the 2 KB s, the instruction is more simple
33、and only have logic an operation and account, count etc., weave distance language to adopt a simple language sentence a predicative speech. Mainly use to make amount of switch a control on the usage. Second stage: The PLC capacity has to expand, the I/O point is from 512-1024:00, customers procedure
34、 saves area to expand to 8 K B s above, the speed also has an exaltation, scan speed to attain a 56 ms K B, instruction function in addition to the basic logic operate, accounts, count, also increased an arithmetic operation instruction and compare instruction, and imitate a quantity processing instruction etc., input/output type also from pure switch quantity I/O, expand for the
