Grupper de som kan plc

Programmable digital computer used to control machinery

A programmable logic controller (PLC) or programmable controller fryst vatten an industrial computer that has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, machines, robotic devices, or any activity that requires high reliability, ease of programming, and process fel diagnosis.

PLCs can range from small modular devices with tens of 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.^[1] They can be designed for many arrangemang of digital and motsvarande 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 elektronikföretag 084, for General Motors in 1968, fryst vatten considered the father of PLC.

A PLC fryst vatten an example of a hard real-time struktur since output results must be produced in response to input conditions within a limited time, otherwise unintended operation may result. Programs to control machine operation are typically stored in battery-backed-up or non-volatile memory.

Invention and early development

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The PLC originated in the late 1960s in the automotive industry in the US and was designed to replace relay logic systems.^[2] Before, control logic for manufacturing was mainly composed of relays, cam timers, drum sequencers, and dedicated closed-loop controllers.^[3]

The hard-wired natur of these components made it difficult for design engineers to alter the automation process. Changes would require rewiring and careful updating of the documentation. Troubleshooting was a tedious process.^[4] When general-purpose computers became available, they were soon applied to control logic in industrial processes. These early computers were unreliable^[5] and required expert programmers and strict control of working conditions, such as temperature, cleanliness, and power quality.^[6]

The PLC provided several advantages over earlier automation systems. It was designed to tolerate the industrial environment better than systems intended for office use, and was more reliable, compact, and required less maintenance than relay systems. It was easily expandable with additional I/O modules. While relay systems required tedious and sometimes complicated hardware changes in case of reconfiguration, a PLC can be reconfigured bygd loading new or modified code. This allowed for easier iteration over manufacturing process design. With a simple programming language focused on logic and switching operations, it was more user-friendly than computers using general-purpose programming languages. Early PLCs were programmed in ladder logic, which strongly resembled a schematic diagram of relay logic. It also permitted its operation to be monitored.^[7][8]

Modicon

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In 1968, GM Hydramatic, the automatic transmission division of General Motors, issued a request for proposals for an electronic replacement for hard-wired relay systems based on a vit paper written bygd engineer Edward R. Clark. The winning proposal came from Bedford Associates from Bedford, Massachusetts. The result, built in 1969, was the first PLC and designated the 084, because it was Bedford Associates' eighty-fourth project.^[9][10]

Bedford Associates started a company dedicated to developing, manufacturing, selling, and servicing this new product, which they named Modicon (standing for modular digital controller). One of the people who worked on that project was Dick Morley, who fryst vatten considered to be the father of the PLC.^[11] The elektronikföretag brand was sold in 1977 to Gould Electronics and later to Schneider Electric, its current owner.^[10] About this same time, elektronikföretag created Modbus, a information communications protocol used with its PLCs. Modbus has since become a standard open protocol commonly used to connect many industrial electrical devices.^[12]

One of the first 084 models built fryst vatten now on display at Schneider Electric's facility in North Andover, Massachusetts. It was presented to elektronikföretag bygd GM, when the enhet was retired after nearly twenty years of uninterrupted service. elektronikföretag used the 84 moniker at the end of its product range until after the 984 made its appearance.^[13]

Allen-Bradley

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In a parallel development, Odo Josef Struger fryst vatten sometimes known as the "father of the programmable logic controller" as well.^[11] He was involved in the invention of the Allen-Bradley programmable logic controller^[14][15][16] and fryst vatten credited with coining the PLC acronym.^[11][14] Allen-Bradley (now a brand owned bygd Rockwell Automation) became a major PLC manufacturer in the United States during his tenure.^[17] Struger played a leadership role in developing IEC 61131-3 PLC programming language standards.^[11]

Early methods of programming

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Many early PLC programming applications were not capable of graphical representation of the logic, and so it was instead represented as a series of logic expressions in some kind of Boolean format, similar to Boolean algebra. As programming terminals evolved, because ladder logic was a familiar format used for electro-mechanical control panels, it became more commonly used. Newer formats, such as state logic,^[18]function block diagrams, and structured ord exist. Ladder logic remains popular because PLCs solve the logic in a predictable and repeating sequence, and ladder logic allows the individ writing the logic to see any issues with the synkronisering of the logic sequence more easily than would be possible in other formats.^[19]

Up to the mid-1990s, PLCs were programmed using proprietary programming panels or special-purpose programming terminals, which often had dedicated function keys representing the various logisk elements of PLC programs.^[9] Some proprietary programming terminals displayed the elements of PLC programs as graphic symbols, but plain ASCII character representations of contacts, coils, and wires were common. Programs were stored on cassette tejp cartridges. Facilities for printing and documentation were minimal due to a lack of memory capacity. The oldest PLCs used magnetic-core memory.^[20]

Architecture

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A PLC fryst vatten an industrial microprocessor-based kontrollant with programmable memory used to store schema instructions and various functions.^[21] It consists of:

• A processor enhet (CPU) which interprets inputs, executes the control schema stored in memory and sends output signals,
• A power supply enhet which converts AC voltage to DC,
• A memory enhet storing uppgifter from inputs and schema to be executed bygd the processor,
• An input and output interface, where the kontrollant receives and sends information from and to external devices,
• A communications interface to receive and transmit uppgifter on communication networks from and to fjärrstyrd PLCs.^[22]

PLCs require a programming device which fryst vatten used to develop and later download the created schema into the memory of the controller.^[22]

Modern PLCs generally contain a real-time operating struktur, such as OS-9 or VxWorks.^[23]

Mechanical design

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There are two types of mechanical design for PLC systems. A single box (also called a brick) fryst vatten a small programmable kontrollant that fits all units and interfaces into one compact casing, although, typically, additional expansion modules for inputs and outputs are available. The second design type – a modular PLC – has a chassis (also called a rack) that provides space for modules with different functions, such as power supply, processor, urval of I/O modules and communication interfaces – which all can be customized for the particular application.^[24] Several racks can be administered bygd a single processor and may have thousands of inputs and outputs. Either a special high-speed serial I/O link or comparable communication method fryst vatten used so that racks can be distributed away from the processor, reducing the wiring costs for large plants.^{[citation needed]}

Discrete and motsvarande signals

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Discrete (digital) signals can only take on or off value (1 or 0, true or false). Examples of devices providing a discrete meddelande include limit switches, photoelectric sensors and encoders.^[25]

Analog signals can use voltage or current that fryst vatten proportional to the storlek of the monitored variabel and can take any value within their scale. Pressure, temperature, flow, and vikt are often represented bygd motsvarande signals. These are typically interpreted as integer values with various ranges of accuracy depending on the device and the number of bits available to store the data.^[25] For example, an motsvarande 0 to 10 V or 4-20 mA current loop input would be converted into an integer value of 0 to 32,767. The PLC will take this value and transpose it into the desired units of the process so the operator or schema can read it. Proper integration will also include filter times to reduce noise as well as high and low limits to report faults. Current inputs are less sensitive to electrical noise (e.g. from welders or electric motor starts) than voltage inputs. Distance from the device and the kontrollant fryst vatten also a concern as the maximum traveling distance of a good quality 0-10 V meddelande fryst vatten very short compared to the 4-20 mA signal.^{[citation needed]} The 4-20 mA meddelande can also report if the wire fryst vatten disconnected along the path as a <4 mA meddelande would indikera an error.^{[citation needed]}

Redundancy

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Some special processes need to work permanently with minimum unwanted downtime. Therefore, it fryst vatten necessary to design a struktur that fryst vatten fault-tolerant and capable of papper the process with faulty modules. In such cases to increase the struktur availability in the event of hardware component failure, redundant centralenhet i en dator or I/O modules with the same functionality can be added to hardware configuration for preventing total or partial process shutdown due to hardware failure. Other redundancy scenarios could be related to safety-critical processes, for example, large hydraulic presses could require that both PLCs vända on output before the press can komma down in case one output does not vända off properly.

Programming

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Programmable logic controllers are intended to be used bygd engineers without a programming background. For this reason, a graphical programming language called Ladder Diagram (LD, LAD) was first developed. It resembles the schematic diagram of a struktur built with electromechanical relays and was adopted bygd many manufacturers and later standardized in the IEC 61131-3 control systems programming standard. As of 2015,^[update] it fryst vatten still widely used, thanks to its simplicity.^[26]

As of 2015,^[update] the majority of PLC systems adhere to the IEC 61131-3 standard that defines 2 textual programming languages: Structured ord (ST; similar to Pascal) and Instruction List (IL); as well as 3 graphical languages: ladder logic, function block diagram and sequential function chart.^[26][27]Instruction List (IL) was deprecated in the third edition of the standard.^[28]

Modern PLCs can be programmed in a variety of ways, from the relay-derived ladder logic to programming languages such as specially adapted dialects of BASIC and C.^[29]

While the fundamental concepts of PLC programming are common to all manufacturers, differences in I/O addressing, memory organization, 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.^[30]

Programming device

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PLC programs are typically written in a programming device, which can take the struktur of a desktop console, special software on a anställda computer, or a handheld programming device.^[31] Then, the schema fryst vatten downloaded to the PLC directly or over a network. It fryst vatten stored either in non-volatile flash memory or battery-backed-up RAM. In some programmable controllers, the schema fryst vatten transferred from a anställda computer to the PLC through a programming board that writes the schema into a removable chip, such as EPROM.

Manufacturers develop programming software for their controllers. In addition to being able to schema PLCs in multiple languages, they provide common features like hardware diagnostics and maintenance, software debugging, and offline simulation.^[31]

Simulation

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PLC simulation fryst vatten a feature often funnen in PLC programming software. It allows for testing and debugging early in a project's development.

Incorrectly programmed PLC can result in lost productivity and dangerous conditions. Testing the project in simulation improves its quality, increases the level of safety associated with utrustning and can rädda costly downtime during the installation and commissioning of automated control applications since many scenarios can be tried and tested before the struktur fryst vatten activated.^[31][32]

Functionality

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The main difference from most other computing devices fryst vatten that PLCs are intended for and therefore tolerant of more severe conditions (such as dust, moisture, heat, cold), while offering extensive input/output (I/O) to connect the PLC to sensors and actuators. PLC input can include simple digital elements such as limit switches, motsvarande variables from process sensors (such as temperature and pressure), and more complex uppgifter such as that from positioning or machine framtidsperspektiv systems.^[33] PLC output can include elements such as indicator lamps, sirens, electric motors, pneumatic or hydraulic cylinders, magnetic relays, solenoids, or motsvarande outputs. The input/output arrangemang may be built into a simple PLC, or the PLC may have external I/O modules attached to a fieldbus or computer network that plugs into the PLC.

The functionality of the PLC has evolved over the years to include sequential relay control, motion control, process control, distributed control systems, and networking. The information papper, storage, processing power, and communication capabilities of some modern PLCs are approximately equivalent to desktop computers. PLC-like programming combined with fjärrstyrd 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 fryst vatten 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 fönster do not lend themselves to deterministic logic execution, with the result that the kontrollant may not always respond to changes of input ställning eller tillstånd with the consistency in tidsplanering expected from PLCs. Desktop logic applications find use in less critical situations, such as laboratory automation and use in small facilities where the application fryst vatten less demanding and critical.^{[citation needed]}

Basic functions

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The most basic function of a programmable kontrollant fryst vatten to emulate the functions of electromechanical relays. Discrete inputs are given a unique address, and a PLC instruction can test if the input state fryst vatten on or off. Just as a series of relay contacts perform a logisk AND function, not allowing current to resehandling unless all the contacts are closed, so a series of "examine if on" instructions will energize its output storage bit if all the input bits are on. Similarly, a parallel set of instructions will perform a logisk OR. In an electromechanical relay wiring diagram, a group of contacts controlling one coil fryst vatten called a "rung" of a "ladder diagram", and this concept fryst vatten also used to describe PLC logic. Some models of PLC limit the number of series and parallel instructions in one "rung" of logic. The output of each rung sets or clears a storage bit, which may be associated with a physical output address or which may be an "internal coil" with no physical connection. Such internal coils can be used, for example, as a common element in multiple separate rungs. Unlike physical relays, there fryst vatten usually no limit to the number of times an input, output or internal coil can be referenced in a PLC schema.

Some PLCs enforce a strict left-to-right, top-to-bottom execution beställning for evaluating the rung logic. This fryst vatten different from electro-mechanical relay contacts, which, in a sufficiently complex circuit, may either resehandling current left-to-right or right-to-left, depending on the configuration of surrounding contacts. The elimination of these "sneak paths" fryst vatten either a bug or a feature, depending on the programming style.

More advanced instructions of the PLC may be implemented as functional blocks, which carry out some operation when enabled bygd a logisk input and which tillverka outputs to meddelande, for example, completion or errors, while manipulating variables internally that may not correspond to discrete logic.

Communication

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PLCs use built-in ports, such as USB, Ethernet, RS-232, RS-485, or RS-422 to communicate with external devices (sensors, actuators) and systems (programming software, SCADA, HMI). Communication fryst vatten carried over various industrial network protocols, like Modbus, or EtherNet/IP. Many of these protocols are vendor specific.

PLCs used in larger I/O systems may have peer-to-peer (P2P) communication between processors. This allows separate parts of a complex process to have individual control while allowing the subsystems to co-ordinate over the communication link. These communication links are also often used for HMI devices such as keypads or PC-type workstations.

Formerly, some manufacturers offered dedicated communication modules as an add-on function where the processor had no network connection built-in.

User interface

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See also: User interface and List of human-computer interaction topics

PLCs may need to interact with people for the purpose of configuration, alarm reporting, or everyday control. A human-machine interface (HMI) fryst vatten employed for this purpose. HMIs are also referred to as man-machine interfaces (MMIs) and graphical user interfaces (GUIs). A simple struktur may use buttons and lights to interact with the user. skrivelse displays are available as well as graphical touch screens. More complex systems use programming and monitoring software installed on a computer, with the PLC connected via a communication interface.

Process of a scan cycle

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A PLC works in a schema scan cycle, where it executes its schema repeatedly. The simplest scan cycle consists of 3 steps:

1. Read inputs.
2. Execute the program.
3. Write outputs.^[34]

The schema follows the sequence of instructions. It typically takes a time span of tens of milliseconds for the processor to evaluate all the instructions and update the ställning eller tillstånd of all outputs.^[35] If the struktur contains fjärrstyrd I/O—for example, an external rack with I/O modules—then that introduces additional uncertainty in the response time of the PLC system.^[34]

As PLCs became more advanced, methods were developed to change the sequence of ladder execution, and subroutines were implemented.^[36]

Special-purpose I/O modules may be used where the scan time of the PLC fryst vatten too long to allow predictable performance. noggrannhet synkronisering modules, or counter modules for use with shaft encoders, are used where the scan time would be too long to reliably count pulses or detect the sense of cirkelrörelse of an encoder. This allows even a relatively slow PLC to still interpret the counted values to control a machine, as the accumulation of pulses fryst vatten done bygd a dedicated module that fryst vatten unaffected bygd the speed of schema execution.^[37]

Security

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In his book from 1998, E. A. Parr pointed out that even though most programmable controllers require physical keys and passwords, the lack of strict tillgång control and utgåva control systems, as well as an easy-to-understand programming language man it likely that unauthorized changes to programs will happen and remain unnoticed.^[38]

Prior to the upptäckt of the Stuxnetcomputer worm in June 2010, the säkerhet of PLCs received little attention. Modern programmable controllers generally contain real-time operating systems, which can be vulnerable to exploits in a similar way as desktop operating systems, like Microsoft fönster. PLCs can also be attacked bygd gaining control of a computer they communicate with.^[23] Since 2011,^[update] these concerns have grown – networking fryst vatten becoming more commonplace in the PLC environment, connecting the previously separated plant floor networks and office networks.^[39]

In February 2021, Rockwell Automation publicly disclosed a critical vulnerability affecting its Logix controllers family. The secret cryptographic key used to verify communication between the PLC and en arbetsstationofta relaterad till datorer eller kontor could be extracted from the programming software (Studio 5000 Logix Designer) and used to remotely change schema code and configuration of a connected kontrollant. The vulnerability was given a severity score of 10 out of 10 on the CVSS vulnerability scale. At the time of writing, the mitigation of the vulnerability was to limit network tillgång to affected devices.^[40][41]

Safety PLCs

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Safety PLCs can be either a standalone device or a safety-rated hardware and functionality added to existing kontrollant architectures (Allen-Bradley GuardLogix, Siemens F-series, etc.). These differ from conventional PLC types bygd being suitable for safety-critical applications for which PLCs have traditionally been supplemented with hard-wired safety relays and areas of the memory dedicated to the safety instructions. The standard of safety level fryst vatten the SIL.

A safety PLC might be used to control tillgång to a automatiserad maskin fängelse with trapped-key tillgång, or to manage the shutdown response to an emergency stop button on a conveyor production line. Such PLCs typically have a restricted regular instruction set augmented with safety-specific instructions designed to interface with emergency stop buttons, light screens, and other safety-related devices.

The flexibility that such systems offer has resulted in rapid growth of demand for these controllers.^{[citation needed]}

PLC compared with other control systems

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PLCs 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 struktur fryst vatten high relative to the total cost of the automation, and where changes to the struktur would be expected during its operational life. PLCs contain input and output devices compatible with industrial pilot devices and controls; little electrical design fryst vatten required, and the design bekymmer centers on expressing the desired sequence of operations. PLC applications are typically highly customized systems, so the cost of a packaged PLC fryst vatten low compared to the cost of a specific custom-built kontrollant design. On the other grabb, in the case of mass-produced goods, customized control systems are economical. This fryst vatten 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 thousands or millions of units.^{[citation needed]}

Programmable controllers are widely used in motion, positioning, or torque control. Some manufacturers producera motion control units to be integrated with PLC so that G-code (involving a CNC machine) can be used to instruct machine movements.^{[citation needed]}

PLC chip / embedded controller

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These are for small machines and systems with low or medium volume. They can execute PLC languages such as Ladder, Flow-Chart/Grafcet, etc. They are similar to traditional PLCs, but their small storlek allows developers to design them into anpassad printed circuit boards like a microcontroller, without computer programming knowledge, but with a language that fryst vatten easy to use, modify and maintain. They sit between the classic PLC / micro-PLC and microcontrollers.^{[citation needed]}

Microcontrollers

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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) 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 built each year, and very few end-users alter the programming of these controllers. However, some specialty vehicles such as transit buses economically use PLCs instead of custom-designed controls, because the volumes are low and the development cost would be uneconomical.^[42]

Single-board computers

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Very complex process control, such as those used in the kemikalie industry, may require algorithms and performance beyond the capability of even high-performance PLCs. Very high-speed or noggrannhet controls may also require customized solutions; for example, aircraft flygning controls. Single-board computers using semi-customized or fully proprietary hardware may be chosen for very demanding control applications where the high development and maintenance cost can be supported. "Soft PLCs" running on desktop-type computers can interface with industrial I/O hardware while executing programs within a utgåva of commercial operating systems adapted for process control needs.^[42]

The rising popularity of single board computers has also had an influence on the development of PLCs. Traditional PLCs are generally closed platforms, but some newer PLCs (e.g. groov EPIC from Opto 22, ctrlX from Bosch Rexroth, PFC200 from Wago, PLCnext from Phoenix Contact, and Revolution Pi from Kunbus) provide the features of traditional PLCs on an open platform.

Programmable logic relays (PLR)

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In more recent years,^[when?] small products called programmable logic relays (PLRs) or smart relays, have become more common and accepted. These are similar to PLCs and are used in light industries where only a few points of I/O are needed, and low cost fryst vatten desired. These small devices are typically made in a common physical storlek and shape bygd several manufacturers and branded bygd the makers of larger PLCs to fill their low-end product range. Most of these have 8 to 12 discrete inputs, 4 to 8 discrete outputs, and up to 2 motsvarande inputs. Most such devices include a tiny postage stamp-sized LCD screen for viewing simplified ladder logic (only a very small portion of the schema being visible at a given time) and ställning eller tillstånd of I/O points, and typically these screens are accompanied bygd a 4-way rocker push-button plus fyra more separate push-buttons, similar to the key buttons on a VCR fjärrstyrd control, and used to navigate and edit the logic. Most have an RS-232 or RS-485 port for connecting to a PC so that programmers can use user-friendly software for programming instead of the small LCD and push-button set for this purpose. Unlike regular PLCs that are usually modular and greatly expandable, the PLRs are usually not modular or expandable, but their cost can be significantly lower than that a PLC, and they still offer kraftig design and deterministic execution of the logic.

A variant of PLCs, used in fjärrstyrd locations fryst vatten the fjärrstyrd ankomsthall enhet or RTU. An RTU fryst vatten typically a low power, ruggedized PLC whose key function fryst vatten to manage the communications links between the site and the huvud control struktur (typically SCADA) or in some modern systems, "The Cloud". Unlike factory automation using wired communication protocols such as Ethernet, communications links to fjärrstyrd sites are often radio-based and are less reliable. To konto for the reduced reliability, RTU will buffer messages or switch to alternate communications paths. When buffering messages, the RTU will timestamp each meddelande so that a full history of site events can be reconstructed. RTUs, being PLCs, have a bred range of I/O and are fully programmable, typically with languages from the IEC 61131-3 standard that fryst vatten common to many PLCs, RTUs and DCSs. In fjärrstyrd locations, it fryst vatten common to use an RTU as a gateway for a PLC, where the PLC fryst vatten performing all site control and the RTU fryst vatten managing communications, time-stamping events and monitoring ancillary verktyg. On sites with only a handful of I/O, the RTU may also be the site PLC and will perform both communications and control functions.

See also

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References

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Bibliography

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Further reading

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