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1、Programmable Logic ControllerA programmable logic controller (PLC) or programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or lighting fixtures. PLCs are used in many industries and
2、machines. Unlike general-purpose computers, the PLC is designed for multiple inputs and output 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
3、 memory. A PLC is an example of a real time system since output results must be produced in response to input conditions within a bounded time, otherwise unintended operation will result.1HistoryThe PLC was invented in response to the needs of the American automotive manufacturing industry. Programm
4、able logic controllers were initially adopted by the automotive industry where software revision replaced the re-wiring of hard-wired control panels when production models changed.Before the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles was accomplished using hun
5、dreds or thousands of relays, cam timers, and drum sequencers and dedicated closed-loop controllers. The process for updating such facilities for the yearly model change-over was very time consuming and expensive, as electricians needed to individually rewire each and every relay.In 1968 GM Hydramat
6、ic (the automatic transmission division of General Motors) issued a request for proposal for an electronic replacement for hard-wired relay systems. The winning proposal came from Bedford Associates of Bedford, Massachusetts. The first PLC, designated the 084 because it was Bedford Associates eighty
7、-fourth project, was the result. Bedford Associates started a new company dedicated to developing, manufacturing, selling, and servicing this new product: Modicon, which stood for MOdular DIgital CONtroller. One of the people who worked on that project was Dick Morley, who is considered to be the fa
8、ther of the PLC. The Modicon brand was sold in 1977 to Gould Electronics, and later acquired by German Company AEG and then by French Schneider Electric, the current owner.One of the very first 084 models built is now on display at Modicons headquarters in North Andover, Massachusetts. It was presen
9、ted to Modicon by GM, when the unit was retired after nearly twenty years of uninterrupted service. Modicon used the 84 moniker at the end of its product range until the 984 made its appearance.The automotive industry is still one of the largest users of PLCs.2DevelopmentEarly PLCs were designed to
10、replace relay logic systems. These PLCs were programmed in ladder logic, which strongly resembles a schematic diagram of relay logic. This program notation was chosen to reduce training demands for the existing technicians. Other early PLCs used a form of instruction list programming, based on a sta
11、ck-based logic solver.Modern PLCs can be programmed in a variety of ways, from ladder logic to more traditional programming languages such as BASIC and C. Another method is State Logic, a very high-level programming language designed to program PLCs based on state transition diagrams.Many early PLCs
12、 did not have accompanying programming terminals that were capable of graphical representation of the logic, and so the logic was instead represented as a series of logic expressions in some version of Boolean format, similar to Boolean algebra. As programming terminals evolved, it became more commo
13、n for ladder logic to be used, for the aforementioned reasons. Newer formats such as State Logic and Function Block (which is similar to the way logic is depicted when using digital integrated logic circuits) exist, but they are still not as popular as ladder logic. A primary reason for this is that
14、 PLCs solve the logic in a predictable and repeating sequence, and ladder logic allows the programmer (the person writing the logic) to see any issues with the timing of the logic sequence more easily than would be possible in other formats.3ProgrammingEarly PLCs, up to the mid-1980s, were programme
15、d using proprietary programming panels or special-purpose programming terminals, which often had dedicated function keys representing the various logical elements of PLC programs. Programs were stored on cassette tape cartridges. Facilities for printing and documentation were very minimal due to lac
16、k of memory capacity. The very oldest PLCs used non-volatile magnetic core memory.More recently, PLCs are usually programmed using special application software written for use on desktop computers, and connecting between the desktop computer and the PLC such as via Ethernet or RS-232 cabling. Such s
17、oftware allows entry and editing of the ladder style logic, and then may provide additional functionality to assist debugging and troubleshooting the software, for example by highlights portions of the logic to show current status during operation or via simulation. Finally, the software may allow u
18、ploading and downloading of the program between the computer and the PLC, for backup and restoration purposes. Alternately, specific devices known as programming boards are used to hard wire the logic into the controller by the use of a removable chip, such as an EEPROM, where the program is transfe
19、rred to the programming board from the workstation via serial or other bus logic.4FunctionalityThe functionality 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, process
20、ing power and communication capabilities of some modern PLCs are approximately equivalent to desktop computers. PLC-like programming combined with remote I/O hardware, allow a general-purpose desktop computer to overlap some PLCs in certain applications. Regarding the practicality of these desktop c
21、omputer based logic controllers, it is important to note that they have not been generally accepted in heavy industry because the desktop computers run on less stable operating systems than do PLCs, and because the desktop computer hardware is typically not designed to the same levels of tolerance t
22、o temperature, humidity, vibration, and longevity as the processors used in PLCs. In addition to the hardware limitations of desktop based logic, operating systems such as Windows do not lend themselves to deterministic logic execution, with the result that the logic may not always respond to change
23、s in logic state or input status with the extreme consistency in timing as is expected from PLCs. Still, such 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, because they
24、are generally much less expensive than PLCs.In more recent years, small products called PLRs (programmable logic relays), and also by similar names, have become more common and accepted. These are very much like PLCs, and are used in light industry where only a few points of I/O (i.e. a few signals
25、coming in from the real world and a few going out) are involved, and low cost is desired. These small devices are typically made in a common physical size and shape by several manufacturers, and branded by the makers of larger PLCs to fill out their low end product range. Popular names include PICO
26、Controller, NANO PLC, and other names implying very small controllers. Most of these have between 8 and 12 digital inputs, 4 and 8 digital outputs, and up to 2 analog inputs. Size is usually about 4 wide, 3 high, and 3 deep. Most such devices include a tiny postage stamp sized LCD screen for viewing
27、 simplified ladder logic (only a very small portion of the program being visible at a given time) and status of I/O points, and typically these screens are accompanied by a 4-way rocker push-button plus four more separate push-buttons, similar to the key buttons on a VCR remote control, and used to
28、navigate and edit the logic. Most have a small plug for connecting via RS-232 to a personal computer so that programmers can use simple Windows applications for programming instead of being forced to use the tiny LCD and push-button set for this purpose. Unlike regular PLCs that are usually modular
29、and greatly expandable, the PLRs are usually not modular or expandable, but their price can be two orders of magnitude less than a PLC and they still offer robust design and deterministic execution of the logic.5FeaturesThe main difference from other computers is that PLCs are armored for severe con
30、ditions (such as dust, moisture, heat, cold) and have the facility for extensive input/output (I/O) arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches, analog process variables (such as temperature and pressure), and the positions of complex positioning systems. S
31、ome use machine vision. On the actuator side, PLCs operate electric motors, pneumatic or hydraulic cylinders, magnetic relays, 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 computer network that plug
32、s into the PLC.6System styleA small PLC will have a fixed number of connections built in for inputs and outputs. Typically, expansions are available if the base model has insufficient I/O.Modular PLCs have a chassis (also called a rack) into which are placed modules with different functions. The pro
33、cessor and selection of I/O modules is customised for the particular application. Several racks can be administered by a single processor, and may have thousands of inputs and outputs. A special high speed serial I/O link is used so that racks can be distributed away from the processor, reducing the
34、 wiring costs for large plants.7PLC compared with other control systemsPLCs are well-adapted to a range of automation tasks. These are typically industrial processes in manufacturing where the cost of developing and maintaining the automation system is high relative to the total cost of the automati
35、on, and where changes to the system would be expected during its operational life. PLCs contain input and output devices compatible with industrial pilot devices and controls; little electrical design is required, and the design problem centers on expressing the desired sequence of operations. PLC a
36、pplications are typically highly customized systems so the cost of a packaged PLC is low compared to the cost of a specific custom-built controller design. On the other hand, in the case of mass-produced goods, customized control systems are economic due to the lower cost of the components, which ca
37、n be optimally chosen instead of a generic solution, and where the non-recurring engineering charges are spread over thousands or millions of units.For high volume or very simple fixed automation tasks, different techniques are used. For example, a consumer dishwasher would be controlled by an elect
38、romechanical cam timer costing only a few dollars in production quantities.A microcontroller-based design would be appropriate where hundreds or thousands of units will be produced and so the development cost (design of power supplies, input/output hardware and necessary testing and certification) c
39、an be spread over many sales, and where the end-user would not need to alter the control. Automotive applications are an example; millions of units are built each year, and very few end-users alter the programming of these controllers. However, some specialty vehicles such as transit busses economic
40、ally use PLCs instead of custom-designed controls, because the volumes are low and the development cost would be uneconomic.Very complex process control, such as used in the chemical industry, may require algorithms and performance beyond the capability of even high-performance PLCs. Very high-speed
41、 or precision controls may also require customized solutions; for example, aircraft flight controls.Programmable controllers are widely used in motion control, positioning control and torque control. Some manufacturers produce motion control units to be integrated with PLC so that G-code (involving
42、a CNC machine) can be used to instruct machine movements.PLCs may include logic for single-variable feedback analog control loop, a proportional, integral, derivative or PID controller. A PID loop could be used to control the temperature of a manufacturing process, for example. Historically PLCs wer
43、e usually configured with only a few analog control loops; where processes required hundreds or thousands of loops, a distributed control system (DCS) would instead be used. As PLCs have become more powerful, the boundary between DCS and PLC applications has become less distinct.PLCs have similar fu
44、nctionality as Remote Terminal Units. An RTU, however, usually does not support control algorithms or control loops. As hardware rapidly becomes more powerful and cheaper, RTUs, PLCs and DCSs are increasingly beginning to overlap in responsibilities, and many vendors sell RTUs with PLC-like features
45、 and vice versa. The industry has standardized on the IEC 61131-3 functional block language for creating programs to run on RTUs and PLCs, although nearly all vendors also offer proprietary alternatives and associated development environments.可编程控制器可编程逻辑控制器（PLC）或可编程控制器是一种数字化的计算机，机电流程自动化应用，如机械控制的工厂流水
46、线，机动游戏，或照明装置。可编程控制器可用于许多工业和机器。不同于通用计算机，PLC是专为多输入和输出的安排，扩展温度范围，免疫电气噪声，振动和阻力和影响。方案，以控制机床操作通常是在电池供电或非易失性内存中存储。可编程控制器是一个实时系统的例子，因为产生输出结果必须输入条件响应时间范围内，否则将导致错误操作。1历史发明的PLC回应美国汽车制造业的需要。可编程逻辑控制器，最初通过的汽车工业调整软件取代了重新硬有线控制面板布线时，改变生产模式。 前PLC，控制测序，汽车制造和安全联锁逻辑实现使用数百或数千继电器，凸轮计时器和鼓音序器和专用的闭环控制器。在每年更新的模型等设施交接是非常耗时和昂贵的
47、过程，需要单独电工ReWire的每一个接力。 1968年，通用汽车Hydramatic（即通用汽车自动变速器分公司）发出了一个硬有线中继系统的电子替代方案的要求。获奖的建议来自贝德福德的贝德福德，马萨诸塞州协会。第一个可编程控制器，指定的084，因为它是贝德福德Associates的第八十四项目，也是结果。贝德福德协会开始一个新的公司，致力于开发，生产，销售和服务这一新产品：莫迪康，其中模块化数字控制器站着。谁的人在该项目的工作之一是迪克莫利，谁是被认为是“父亲的临立会”。该莫迪康品牌是在1977年出售给古尔德电子，后来由德国AEG公司收购公司和法国施耐德电气公司，目前的所有者，然后。 在建的
48、第一个084模式之一，现已在莫迪康在北安多佛，马萨诸塞州总部展出。它是由通用汽车介绍，以莫迪康在部队退役后，近20年的不间断服务。莫迪康使用在其产品范围的结束的84个名字，直到984于是出现。 汽车行业仍是PLC的最大用户之一。 2发展早期的PLC的目的是取代继电器逻辑系统。 PLC的程序，这些都在“阶梯逻辑”，它类似于一个强烈的继电器逻辑示意图。这一方案被选中符号，以减少对现有技术人员的培训需求。其他早期的PLC用编程的指令表的形式，在一个基于堆栈的逻辑解算器的基础。 现代PLC的编程可以在各种不同的方式，从梯子逻辑如BASIC和C的另一种方法，以更传统的编程语言是国家的逻辑，一个非常高层次
49、的编程语言PLC的设计方案基于状态转换图。 许多早期的PLC没有人陪同的编程终端的图形表示的逻辑能力，所以逻辑，而不是表现为一个逻辑布尔表达式中的一些格式，类似于布尔代数版本系列。作为编程终端的演变，变得更常见的梯形逻辑的使用，为上述原因。如国家和逻辑功能块（新格式，类似的描述方法的逻辑是在使用数字集成逻辑电路）存在，但他们仍然不如梯形逻辑受欢迎。一个原因是PLC的解决可预见和重复序列的逻辑，梯形逻辑允许程序员（写作的人的逻辑）来查看与时间序列的逻辑更容易比任何问题都可以在其他的主要原因格式。3程序设计早期的PLC，到80年代中期，都是用专有的编程板或特殊用途的编程终端，其中往往有专门的功能键代表不同的PLC程序逻辑元素。方案存储在盒式磁带。打印和文件的设施被很小的，由于内存容量不足。很古老的PLC使用非挥发性磁芯存储器。 最近，PLC的编程通常是使用特殊的应用软件在台式电脑上使用书面和桌面计算机之间的连接，并通过可编程控制器，如以太网或RS - 232电缆。这种软件允许进
