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1、精选优质文档-倾情为你奉上Programmable logic controllerA programmable logic controller (PLC) or programmable controller is a used for of processes, such as control of machinery on factory , amusement rides, or lighting fixtures. PLCs are used in many industries and machines. Unlike general-purpose computers, the

2、 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 . A PLC is an example of a system since output results must

3、 be produced in response to input conditions within a bounded time, otherwise unintended operation will result.1.History The PLC was invented in response to the needs of the American automotive manufacturing industry. Programmable logic controllers were initially adopted by the automotive industry w

4、here 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 hundreds or thousands of , , and and dedicated closed-loop controllers. The

5、process for updating such facilities for the yearly model was very time consuming and expensive, as needed to individually rewire each and every relay. In 1968 GM Hydramatic (the automatic transmission division of ) issued a request for proposal for an electronic replacement for hard-wired relay sys

6、tems. The winning proposal came from Bedford Associates of . The first PLC, designated the 084 because it was Bedford Associates eighty-fourth project, was the result. Bedford Associates started a new company dedicated to developing, manufacturing, selling, and servicing this new product: Modicon, w

7、hich stood for MOdular DIgital CONtroller. One of the people who worked on that project was , who is considered to be the father of the PLC. The Modicon brand was sold in 1977 to , and later acquired by German Company and then by French , the current owner. One of the very first 084 models built is

8、now on display at Modicons headquarters in . It was presented to Modicon by , 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 large

9、st users of PLCs.2.Development Early PLCs were designed to replace relay logic systems. These PLCs were programmed in , 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

10、of programming, based on a stack-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 , a designed to program PLCs based on . Many early PLCs did not have accompanying programming te

11、rminals 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 common for ladder logic to be used, for the af

12、orementioned 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 PLCs solve the logic in a predictable an

13、d 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.2.1Programming Early PLCs, up to the mid-1980s, were programmed using proprietary programming panels

14、 or special-purpose programming , which often had dedicated function keys representing the various logical elements of PLC programs. Programs were stored on . Facilities for printing and documentation were very minimal due to lack of memory capacity. The very oldest PLCs used non-volatile . More rec

15、ently, PLCs are programmed using application software on personal computers. The computer is connected to the PLC through , , or cabling. The programming software allows entry and editing of the ladder-style logic. Generally the software provides functions for debugging and troubleshooting the PLC s

16、oftware, for example, by highlighting portions of the logic to show current status during operation or via simulation. The software will upload and download the PLC program, for backup and restoration purposes. In some models of programmable controller, the program is transferred from a personal com

17、puter to the PLC though a which writes the program into a removable chip such as an or .3.Functionality The functionality of the PLC has evolved over the years to include sequential relay control, motion control, , and . The data handling, storage, processing power and communication capabilities of

18、some modern PLCs are approximately equivalent to . 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 computer based logic controllers, it is important to note that

19、 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 to temperature, humidity, vibration, and longevity as the proc

20、essors 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 changes in logic state or input status with the extreme consistency

21、 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 are generally much less expensive than PLCs. In more recent y

22、ears, 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 coming in from the real world and a few going out) are invol

23、ved, 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 Controller, NANO PLC, and other names implying very small co

24、ntrollers. 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 simplified ladder logic (only a very small portion of the p

25、rogram 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 navigate and edit the logic. Most have a small plug for conn

26、ecting via RS-232 or RS-485 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 and greatly expandable, the PLRs are usually not m

27、odular or expandable, but their price can be two less than a PLC and they still offer robust design and deterministic execution of the logic.4.PLC Topics4.1.Features The main difference from other computers is that PLCs are armored for severe conditions (such as dust, moisture, heat, cold) and have

28、the facility for extensive (I/O) arrangements. These connect the PLC to and . PLCs read limit , analog process variables (such as temperature and pressure), and the positions of complex positioning systems. Some use . On the actuator side, PLCs operate , or cylinders, magnetic , , or analog outputs.

29、 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 plugs into the PLC.4.2System scale A small PLC will have a fixed number of connections built in for inputs and outputs. Typically, expansions are available if t

30、he base model has insufficient I/O.Modular PLCs have a chassis (also called a rack) into which are placed modules with different functions. The processor and selection of I/O modules is customised for the particular application. Several racks can be administered by a single processor, and may have t

31、housands 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 wiring costs for large plants.4.3User interface PLCs may need to interact with people for the purpose of configuration, alarm reporting or everyday cont

32、rol. A simple system may use buttons and lights to interact with the user. Text displays are available as well as graphical touch screens. More complex systems use a programming and monitoring software installed on a computer, with the PLC connected via a communication interface.4.4Communications PL

33、Cs have built in communications ports, usually 9-pin , but optionally or . , or is usually included as one of the . Other options include various such as or . Other communications protocols that may be used are listed in the . Most modern PLCs can communicate over a network to some other system, suc

34、h as a computer running a (Supervisory Control And Data Acquisition) system or web browser. PLCs used in larger I/O systems may have (P2P) communication between processors. This allows separate parts of a complex process to have individual control while allowing the subsystems to co-ordinate over th

35、e communication link. These communication links are also often used for devices such as keypads or -type workstations.4.5Programming PLC programs are typically written in a special application on a personal computer, then downloaded by a direct-connection cable or over a network to the PLC. The prog

36、ram is stored in the PLC either in battery-backed-up or some other non-volatile . Often, a single PLC can be programmed to replace thousands of . Under the standard, PLCs can be programmed using standards-based programming languages. A graphical programming notation called is available on certain pr

37、ogrammable controllers. Initially most PLCs utilized Ladder Logic Diagram Programming, a model which emulated electromechanical control panel devices (such as the contact and coils of relays) which PLCs replaced. This model remains common today. IEC 61131-3 currently defines five programming languag

38、es for programmable control systems: FBD (), LD (), ST (, similar to the ), IL (, similar to ) and SFC (). These techniques emphasize logical organization of operations. While the fundamental concepts of PLC programming are common to all manufacturers, differences in I/O addressing, memory organizat

39、ion and instruction sets mean that PLC programs are never perfectly interchangeable between different makers. Even within the same product line of a single manufacturer, different models may not be directly compatible.5.PLC compared with other control systems PLCs are well-adapted to a range of task

40、s. 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 automation, and where changes to the system would be expected during its operational life. PLCs contain input and output devices c

41、ompatible with industrial pilot devices and controls; little electrical design is required, and the design problem centers on expressing the desired sequence of operations. PLC applications are typically highly customized systems so the cost of a packaged PLC is low compared to the cost of a specifi

42、c 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 can be optimally chosen instead of a generic solution, and where the non-recurring engineering charges are spread over thous

43、ands or millions of units. For high volume or very simple fixed automation tasks, different techniques are used. For example, a consumer would be controlled by an electromechanical costing only a few dollars in production quantities. A -based design would be appropriate where hundreds or thousands o

44、f units will be produced and so the development cost (design of power supplies, input/output hardware and necessary testing and certification) can 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 buil

45、t each year, and very few end-users alter the programming of these controllers. However, some specialty vehicles such as transit busses economically use PLCs instead of custom-designed controls, because the volumes are low and the development cost would be uneconomic. Very complex process control, s

46、uch as used in the chemical industry, may require algorithms and performance beyond the capability of even high-performance PLCs. Very high-speed or precision controls may also require customized solutions; for example, aircraft flight controls.Programmable controllers are widely used in motion cont

47、rol, positioning control and torque control. Some manufacturers produce motion control units to be integrated with PLC so that (involving a machine) can be used to instruct machine movements.PLCs may include logic for single-variable feedback analog control loop, a proportional, integral, derivative

48、 or . A PID loop could be used to control the temperature of a manufacturing process, for example. Historically PLCs were usually configured with only a few analog control loops; where processes required hundreds or thousands of loops, a (DCS) would instead be used. As PLCs have become more powerful

49、, the boundary between DCS and PLC applications has become less distinct. PLCs have similar functionality as . An RTU, however, usually does not support control algorithms or control loops. As hardware rapidly becomes more powerful and cheaper, , PLCs and are increasingly beginning to overlap in res

50、ponsibilities, and many vendors sell RTUs with PLC-like features and vice versa. The industry has standardized on the functional block language for creating programs to run on RTUs and PLCs, although nearly all vendors also offer proprietary alternatives and associated development environments.6.Dig

51、ital and analog signals Digital or discrete signals behave as binary switches, yielding simply an On or Off signal (1 or 0, True or False, respectively). Push buttons, limit switches, and are examples of devices providing a discrete signal. Discrete signals are sent using either or , where a specifi

52、c range is designated as On and another as Off. For example, a PLC might use 24 V DC I/O, with values above 22 V DC representing On, values below 2VDC representing Off, and intermediate values undefined. Initially, PLCs had only discrete I/O. Analog signals are like volume controls, with a range of

53、values between zero and full-scale. These are typically interpreted as integer values (counts) by the PLC, with various ranges of accuracy depending on the device and the number of bits available to store the data. As PLCs typically use 16-bit signed binary processors, the integer values are limited between -32,768 and +32,767. Pressure, temperature, flow, and weight are often represented by analog signals. Analog signals can use or with a magnitude proportional to the value of the process signal. For example, an analog 0 - 10 V input or would be into an integer value of 0 - 32767.专心-专注-专业