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A programmable logic controller (PLC) or programmable controller is an industrial digital computer which has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, or robotic devices, or any activity that requires high reliability control and ease of programming and process fault diagnosis.
PLCs were first developed in the automobile manufacturing industry to provide flexible, ruggedized and easily programmable controllers to replace hard-wired relays, timers and sequencers. Since then, they have been widely adopted as high-reliability automation controllers suitable for harsh environments. A PLC is an example of a 'hard' real-time system since output results must be produced in response to input conditions within a limited time, otherwise unintended operation will result.
- 3Programming
- 4Functionality
- 5PLC topics
Overview[edit]
PLC system in a rack, left-to-right: power supply unit (PSU), CPU, interface module (IM) and communication processor (CP).
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 a count of thousands of I/O, and which are often networked to other PLC and SCADA systems.
They can be designed for multiple arrangements of digital and analog I/O, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed-up or non-volatile memory.
It was from the automotive industry in the USA that the PLC was born. Before the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles was mainly composed of relays, cam timers, drum sequencers, and dedicated closed-loop controllers. Since these could number in the hundreds or even thousands, the process for updating such facilities for the yearly model change-over was very time consuming and expensive, as electricians needed to individually rewire the relays to change their operational characteristics.
When digital computers became available, being general-purpose programmable devices, they were soon applied to control sequential and combinatorial logic in industrial processes. However these early computers required specialist programmers and stringent operating environmental control for temperature, cleanliness, and power quality. To meet these challenges the PLC was developed with several key attributes. It would tolerate the shop-floor environment, it would support discrete (bit-form) input and output in an easily extensible manner, it would not require years of training to use, and it would permit its operation to be monitored. Since many industrial processes have timescales easily addressed by millisecond response times, modern (fast, small, reliable) electronics greatly facilitate building reliable controllers, and performance could be traded off for reliability.[1]
Invention and early development[edit]
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 white paper written by engineer Edward R. Clark. The winning proposal came from Bedford Associates of Bedford, Massachusetts. The first PLC, designated the 084 because it was Bedford Associates' eighty-fourth project, was the result.[2] 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 'father' of the PLC.[3] The Modicon brand was sold in 1977 to Gould Electronics, 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 Schneider Electric's facility in North Andover, Massachusetts. It was presented 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.
In a parallel development Odo Josef Struger is sometimes known as the 'father of the programmable logic controller' as well.[4][5] He was involved in the invention of the Allen-Bradley programmable logic controller (PLC) during 1958 to 1960.[6][7][8] Struger is credited with creating the PLC acronym.[4] Allen-Bradley (now a brand owned by Rockwell Automation), the manufacturer of the controller, became a major programmable logic controller device manufacturer in the United States during the tenure of Struger.[9]
Early PLCs were designed to 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 stack-based logic solver.
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. Another method is state logic, a very high-level programming language designed to program PLCs based on state transition diagrams. The majority of PLC systems today adhere to the IEC 61131/3 control systems programming standard that defines 5 languages: Ladder Diagram (LD), Structured Text (ST), Function Block Diagram (FBD), Instruction List (IL) and sequential function chart (SFC).
Many early PLCs 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 common for ladder logic to be used, for the aforementioned reasons and because it was a familiar format used for electro-mechanical control panels. 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 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.
Programming[edit]
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 program is stored in the PLC either in battery-backed-up RAM or some other non-volatile flash memory. Often, a single PLC can be programmed to replace thousands of relays.[10]
Early PLCs, up to the mid-1990s, were programmed using proprietary programming panels or special-purpose programming terminals, which often had dedicated function keys representing the various logical elements of PLC programs.[2] 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 tape cartridges. Facilities for printing and documentation were minimal due to lack of memory capacity. The oldest PLCs used non-volatilemagnetic core memory.
More recently, PLCs are programmed using application software on personal computers, which now represent the logic in graphic form instead of character symbols. The computer is connected to the PLC through USB, Ethernet, RS-232, RS-485, or RS-422 cabling. The programming software allows entry and editing of the ladder-style logic. In some software packages, it is also possible to view and edit the program in function block diagrams, sequence flow charts and structured text. Generally the software provides functions for debugging and troubleshooting the PLC software, 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 computer to the PLC through a programming board which writes the program into a removable chip such as an EPROM.
Under the IEC 61131-3 standard, PLCs can be programmed using standards-based programming languages. The most commonly used programming language is Ladder diagram (LD) also known as Ladder logic. It uses Contact-Coil logic to make programs like an electrical control diagram. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers. 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 languages for programmable control systems: function block diagram (FBD), ladder diagram (LD), structured text (ST; similar to the Pascal programming language), instruction list (IL; similar to assembly language), and sequential function chart (SFC).[11] These techniques emphasize logical organization of operations.[10]
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.
Control example shown in ladder diagram[edit]
This is a programming example in ladder diagram which shows the control system. A ladder diagram is a method of drawing control circuits which pre-dates PLCs. The ladder diagram resembles the schematic diagram of a system built with electromechanical relays.
As an example, say a facility needs to store water in a tank. The water is drawn from the tank by another system, as needed, and our example system must manage the water level in the tank by controlling the valve that refills the tank. . Shown are:
- Two inputs (from the low and high level switches) represented by contacts of the float switches
- An output to the fill valve, labelled as the fill valve which it controls
- An 'internal' contact, representing the output signal to the fill valve which is created in the program.
- A logical control scheme created by the interconnection of these items in software
In ladder diagram, the contact symbols represent the state of bits in processor memory, which corresponds to the state of physical inputs to the system. If a discrete input is energized, the memory bit is a 1, and a 'normally open' contact controlled by that bit will pass a logic 'true' signal on to the next element of the ladder. Therefore, the contacts in the PLC program that 'read' or look at the physical switch contacts in this case must be 'opposite' or open in order to return a TRUE for the closed physical switches. Internal status bits, corresponding to the state of discrete outputs, are also available to the program.
In the example, the physical state of the float switch contacts must be considered when choosing 'normally open' or 'normally closed' symbols in the ladder diagram. The PLC has two discrete inputs from float switches (Low Level and High Level). Both float switches (normally closed) open their contacts when the water level in the tank is above the physical location of the switch.
When the water level is below both switches, the float switch physical contacts are both closed, and a true (logic 1) value is passed to the Fill Valve output. Water begins to fill the tank. The internal 'Fill Valve' contact latches the circuit so that even when the 'Low Level' contact opens (as the water passes the lower switch), the fill valve remains on. Since the High Level is also normally closed, water continues to flow as the water level remains between the two switch levels. Once the water level rises enough so that the 'High Level' switch is off (opened), the PLC will shut the inlet to stop the water from overflowing; this is an example of seal-in (latching) logic. The output is sealed in until a high level condition breaks the circuit. After that the fill valve remains off until the level drops so low that the Low Level switch is activated, and the process repeats again.
A complete program may contain thousands of rungs, evaluated in sequence. Typically the PLC processor will alternately scan all its inputs and update outputs, then evaluate the ladder logic; input changes during a program scan will not be effective until the next I/O update. A complete program scan may take only a few milliseconds, much faster than changes in the controlled process.
Programmable controllers vary in their capabilities for a 'rung' of a ladder diagram. Some only allow a single output bit. There are typically limits to the number of series contacts in line, and the number of branches that can be used. Each element of the rung is evaluated sequentially. If elements change their state during evaluation of a rung, hard-to-diagnose faults can be generated, although sometimes (as above) the technique is useful. Some implementations forced evaluation from left-to-right as displayed and did not allow reverse flow of a logic signal (in multi-branched rungs) to affect the output.
Functionality[edit]
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 data handling, storage, processing 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. Desktop computer controllers have not been generally accepted in heavy industry because the 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]
Basic functions[edit]
The most basic function of a programmable controller is to emulate the functions of electro-mechanical relays. Discrete inputs are given a unique address, and a PLC instruction can test if the input state is on or off. Just as a series of relay contacts perform a logical AND function, not allowing current to pass 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 logical OR. In an electro-mechanical relay wiring diagram, a group of contacts controlling one coil is called a 'rung' of a 'ladder diagram ', and this concept is 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 is usually no limit to the number of times an input, output or internal coil can be referenced in a PLC program.
Some PLCs enforce a strict left-to-right, top-to-bottom execution order for evaluating the rung logic. This is different from electro-mechanical relay contacts, which in a sufficiently complex circuit may either pass current left-to-right or right-to-left, depending on the configuration of surrounding contacts. The elimination of these 'sneak paths' is either a bug or a feature, depending on programming style.
More advanced instructions of the PLC may be implemented as functional blocks, which carry out some operation when enabled by a logical input and which produce outputs to signal, for example, completion or errors, while manipulating variable internally that may not correspond to discrete logic.
Timers and counters[edit]
The main function of a timer is to keep an output on for a specific length of time. A good example of this is a garage light, where you want power to be cut off after 2 minutes so as to give someone time to go into the house. The three different types of timers that are commonly used are a Delay-OFF, a Delay-ON, and a Delay-ON-Retentive. A Delay-OFF timer activates immediately when turned on, counts down from a programmed time before cutting off, and is cleared when the enabling input is off. A Delay-ON timer is activated by input and starts accumulating time, counts up to a programmed time before cutting off, and is cleared when the enabling input is turned off. A Delay-ON-Retentive timer is activated by input and starts accumulating time, retains the accumulated value even if the (ladder-logic) rung goes false, and can be reset only by a RESET contact.
Counters are primarily used for counting items such as cans going into a box on an assembly line. This is important because once something is filled to its max the item needs to be moved on so something else can be filled. Many companies use counters in PLC's to count boxes, count how many feet of something is covered, or to count how many pallets are on a truck. There are three types of counters, Up counters, Down counters, and Up/Down counters. Up counters count up to the preset value, turn on the CTU (CounT Up output) when the preset value is reached, and are cleared upon receiving a reset. Down counters count down from a preset value, turns on the CTD (CounT Down output) when 0 is reached, and are cleared upon reset. Up/Down counters count up on CU, count down on CD, turn on CTUD (CounT Up/Down output) when the preset value is reached, and cleared on reset.[12]
Programmable logic relay (PLR)[edit]
In more recent years, small products called PLRs (programmable logic relays), and also by similar names, have become more common and accepted. These are 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 needed, 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 controllers. Most of these have 8 to 12 discrete inputs, 4 to 8 discrete 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 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 navigate and edit the logic. Most have a small plug for connecting 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 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 logics.
PLC topics[edit]
Features[edit]
Control panel with PLC (grey elements in the center). The unit consists of separate elements, from left to right; power supply, controller, relay units for in- and output
Control panel with a PLC user interface for thermal oxidizer regulation.
The main difference from most other computing devices is 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, analog variables from process sensors (such as temperature and pressure), and more complex data such as that from positioning or machine vision systems.[13] PLC output can include elements such as indicator lamps, sirens, 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 fieldbus or computer network that plugs into the PLC.
Scan time[edit]
A PLC program generally loops i.e. executes repeatedly, as long as the controlled system is running. At the start of each execution loop, the status of all physical inputs are copied to an area of memory, sometimes called the 'I/O Image Table', which is accessible to the processor. The program then runs from its first instruction rung down to the last rung. It takes some time for the processor of the PLC to evaluate all the rungs and update the I/O image table with the status of outputs.[14] Scan times of a few milliseconds may be encountered for small programs and fast processors, but for older processors and very large programs much longer scan times (on the order of 100 ms) may be encountered. Excessively long scan times may mean the response of the PLC to changing inputs or process conditions is too slow to be useful.
As PLCs became more advanced, methods were developed to change the sequence of ladder execution, and subroutines were implemented.[15] This enhanced programming could be used to save scan time for high-speed processes; for example, parts of the program used only for setting up the machine could be segregated from those parts required to operate at higher speed. Newer PLCs now have the option to run the logic program synchronously with the IO scanning. This means that IO is updated in the background and the logic reads and writes values as required during the logic scanning.
Special-purpose I/O modules may be used where the scan time of the PLC is too long to allow predictable performance. Precision timing 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 rotation 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 is done by a dedicated module that is unaffected by the speed of program execution on the PLC.
Process of a scan cycle[edit]
There are 5 main steps in a scan cycle:
- Reading inputs
- Executing the program
- Processing communication requests
- Executing CPU diagnostics
- Writing outputs
System scale[edit]
A 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 processor and selection of I/O modules are customized for the particular application. Several racks can be administered by a single processor, and may have thousands of inputs and outputs. Either a special high speed serial I/O link or comparable communication method is used so that racks can be distributed away from the processor, reducing the wiring costs for large plants. Options are also available to mount I/O points directly to the machine and utilize quick disconnecting cables to sensors and valves, saving time for wiring and replacing components.
User interface[edit]
PLCs may need to interact with people for the purpose of configuration, alarm reporting, or everyday control. A human-machine interface (HMI) is employed for this purpose. HMIs are also referred to as man-machine interfaces (MMIs) and graphical user interfaces (GUIs). 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 programming and monitoring software installed on a computer, with the PLC connected via a communication interface.
Communications[edit]
Many models of PLCs have built-in communications ports, using RS-232, RS-422, RS-485, or Ethernet. Various protocols are usually included. Many of these protocols are vendor specific.
Most modern PLCs can communicate over a network to some other system, such as a computer running a SCADA (Supervisory Control And Data Acquisition) system or web browser.
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.
Security[edit]
Prior to the discovery of the Stuxnetcomputer worm in June 2010, security of PLCs received little attention. Modern PLCs generally contain a real-time operating system such as OS-9 or VxWorks, and exploits for these systems exist much as they do for desktop computer operating systems such as Microsoft Windows. PLCs can also be attacked by gaining control of a computer they communicate with.[16]
Simulation[edit]
In order to properly understand the operation of a PLC, it is necessary to spend considerable time programming, testing, and debugging PLC programs. PLC systems are inherently expensive, and down-time is often very costly. In addition, if a PLC is programmed incorrectly it can result in lost productivity and dangerous conditions. PLC simulation software such as PLCLogix can save time in the design of automated control applications and can also increase the level of safety associated with equipment since many'what if' scenarios can be tried and tested before the system is activated.[17]
Redundancy[edit]
Some special processes need to work permanently with minimum unwanted down time. Therefore, it is necessary to design a system which is fault-tolerant and capable of handling the process with faulty modules. In such cases to increase the system availability in the event of hardware component failure, redundant CPU 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.
PLC compared with other control systems[edit]
PLC installed in a control panel
Control center with a PLC for a RTO.
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 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 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 applications 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 economical. This is 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.
For high volume or very simple fixed automation tasks, different techniques are used. For example, a cheap consumer dishwasher would be controlled by an electromechanical 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) 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.[18]
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 or precision controls may also require customized solutions; for example, aircraft flight 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 version of commercial operating systems adapted for process control needs.[18]
Programmable controllers are widely used in motion, positioning, or torque control. Some manufacturers produce motion control units to be integrated with PLC so that G-code (involving a CNC machine) can be used to instruct machine movements.[19][citation needed]
PLCs may include logic for single-variable feedback analog control loop, a proportional, integral, derivative (PID) controller. 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 distributed control system (DCS) would instead be used. As PLCs have become more powerful, the boundary between DCS and PLC applications has been blurred.
PLCs have similar functionality 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, 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.
In recent years 'safety' PLCs have started to become popular, either as standalone models or as functionality and safety-rated hardware added to existing controller architectures (Allen-Bradley Guardlogix, Siemens F-series etc.). These differ from conventional PLC types as being suitable for use in safety-critical applications for which PLCs have traditionally been supplemented with hard-wired safety relays. For example, a safety PLC might be used to control access to a robot cell with trapped-key access, or perhaps to manage the shutdown response to an emergency stop on a conveyor production line. Such PLCs typically have a restricted regular instruction set augmented with safety-specific instructions designed to interface with emergency stops, light screens, and so forth. The flexibility that such systems offer has resulted in rapid growth of demand for these controllers.
Discrete (digital) and analog signals[edit]
Discrete (digital) signals behave as binary switches, yielding simply an On or Off signal (1 or 0, True or False, respectively). Push buttons, limit switches, and photoelectric sensors are examples of devices providing a discrete signal. Discrete signals are sent using either voltage or current, where a specific 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 digital I/O.
Analog signals are like volume controls, with a range of 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 voltage or current with a magnitude proportional to the value of the process signal. For example, an analog 0 to 10 V or 4-20 mA input would be converted into an integer value of 0 to 32767.
Current inputs are less sensitive to electrical noise (e.g. from welders or electric motor starts) than voltage inputs.
PLCs are at the forefront of manufacturing automation. An engineer working in a manufacturing environment will at least encounter some PLCs, if not use them on a regular basis. Electrical engineering students should have basic knowledge of PLCs because of their widespread use in industrial applications.[20][21][22]
See also[edit]
References[edit]
- ^E. A. Parr, Industrial Control Handbook, Industrial Press Inc., 1999 ISBN0-8311-3085-7
- ^ abM. A. Laughton, D. J. Warne (ed), Electrical Engineer's Reference book, 16th edition,Newnes, 2003 Chapter 16 Programmable Controller
- ^'The father of invention: Dick Morley looks back on the 40th anniversary of the PLC'. Manufacturing Automation. 12 September 2008.
- ^ ab'Leaders of the Pack'. Retrieved 2008-06-20.
- ^'Odo Struger, automation expert'. Retrieved 2008-06-20.
- ^'A-B PLC inventor, Dr. Odo Struger, dies'. Archived from the original on 2008-12-03. Retrieved 2008-06-20.
- ^Brier, Steven E. (1998-12-27). 'O. Struger, 67, A Pioneer In Automation'. The New York Times. Retrieved 2008-07-27.
Dr. Odo J. Struger, who invented the programmable logic controller, which makes possible modern factory automation, amusement park rides and lavish stage effects in Broadway productions, died on December 8 in Cleveland. He was 67.
- ^Anzovin, p. 100, item # 2189. Programmable logic controller was invented by the Austrian-born American engineer Odo J. Struger in 1958-60 at the Allen-Bradley company in Milwaukee, WI, USA. A programmable logic controller, or PLC, is a simple electronic device that allows precise numerical control of machinery. It is widely used to control everything from washing machines to roller coaster to automated manufacturing equipment.
- ^'A short history of Automation growth'. Retrieved 2008-06-20.
- ^ abW. Bolton, Programmable Logic Controllers, Fifth Edition, Newnes, 2009 ISBN978-1-85617-751-1, Chapter 1
- ^Keller, William L Jr. Grafcet, A Functional Chart for Sequential Processes, 14th Annual International Programmable Controllers Conference Proceedings, 1984, p. 71-96.
- ^Yanik, P. (2017, April 11). Timers and Counters. Cullowhee, NC, United States of America.
- ^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.
- ^Maher, Michael J. Real-Time Control and Communications. 18th Annual ESD/SMI International Programmable Controllers Conference Proceedings, 1989, p. 431-436.
- ^Kinner, Russell H., P.E. Designing Programmable Controller Application Programs Using More than One Designer. 14th Annual International Programmable Controllers Conference Proceedings, 1985, p. 97-110.
- ^Byres (May 2011). 'PLC Security Risk: Controller Operating Systems - Tofino Industrial Security Solution'. www.tofinosecurity.com.
- ^Lin, Sally; Huang, Xiong (9 August 2011). Advances in Computer Science, Environment, Ecoinformatics, and Education, Part III: International Conference, CSEE 2011, Wuhan, China, August 21-22, 2011. Proceedings. Springer Science & Business Media. p. 15. ISBN9783642233449 – via Google Books.
- ^ abGregory K. McMillan, Douglas M. Considine (ed), Process/Industrial Instruments and Controls Handbook Fifth Edition, McGraw-Hill, 1999 ISBN0-07-012582-1 Section 3 Controllers
- ^Vosough and Vosough (November 2011). 'PLC and its Applications'(PDF). International Journal of Multidisciplinary Sciences and Engineering. 2.
- ^Erickson, Kelvin T. (1996). 'Programmable logic controllers'. Institute of Electrical and Electronics Engineers.
- ^Iqbal, S. (2008). 'Programmable Logic Controllers (PLCs): Workhorse of Industrial Automation'. 68-69. IEEEP Journal: 27–31.
- ^Petruzella, Frank D. (2005). 'Programmable logic controllers'. Tata McGraw-Hill Education.
Further reading[edit]
- Daniel Kandray, Programmable Automation Technologies, Industrial Press, 2010 ISBN978-0-8311-3346-7, Chapter 8 Introduction to Programmable Logic Controllers
- Tom Mejer Antonsen, 2018 'PLC Controls with Structured Text (ST)', ISBN978-87-4300-241-3, ISBN978-87-4300-242-0
- Walker, Mark John (2012-09-08). The Programmable Logic Controller: its prehistory, emergence and application(PDF) (PhD thesis). Department of Communication and Systems Faculty of Mathematics, Computing and Technology: The Open University. Archived(PDF) from the original on 2018-06-20. Retrieved 2018-06-20.
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Digital Archive
Table of Contents
- 2 Architecture of Distributed Control System
- 3 Process Control Unit of DCS
- 5 Difference between SCADA and DCS (DCS vs SCADA)
What is Distributed Control System DCS?
In recent years, the use of smart devices and field buses makes distributed control system (DCS) to be prominent in large and complex industrial processes as compared to the former centralized control system. This distribution of control system architecture around the plant has led to produce more efficient ways to improve reliability of control, process quality and plant efficiency.
Nowadays, distributed control system has been found in many industrial fields such as chemical plants, oil and gas industries, food processing units, nuclear power plants, water management systems, automobile industries, etc.
What is DCS System?
A distributed control system (DCS) is a specially designed automated control system that consists of geographically distributed control elements over the plant or control area.
Engineering Control Systems Plc Pdf Online
It differs from the centralized control system wherein a single controller at central location handles the control function, but in DCS each process element or machine or group of machines is controlled by a dedicated controller. DCS consists of a large number of local controllers in various sections of plant control area and are connected via a high speed communication network.
In DCS control system, data acquisition and control functions are carried through a number of DCS controllers which are microprocessor based units distributed functionally and geographically over the plant and are situated near area where control or data gathering functions being performed as shown in the figure above. These controllers able to communicate among themselves and also with other controllers like supervisory terminals, operator terminals, historians, etc.
Distributed individual automatic controllers are connected to field devices such as sensors and actuators. These controllers ensure the sharing of gathered data to other hierarchal controllers via different field buses. Different field buses or standard communication protocols are used for establishing the communication between the controllers. Some of these include Profibus, HART, arc net, Modbus, etc.
DCS is most suited for large-scale processing or manufacturing plants wherein a large number of continuous control loops are to be monitored and controlled. The main advantage of dividing control tasks for distributed controllers is that if any part of DCS fails, the plant can continue to operate irrespective of failed section.
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Architecture of Distributed Control System
As the name suggests, DCS has three main qualities. The first one is the distribution of various control functions into relatively small sets of subsystems, which are of semiautonomous, and are interconnected through a high speed communication bus. Some of these functions include data acquisition, data presentation, process control, process supervision, reporting information, storing and retrieval of information.
The second attribute of DCS is the automation of manufacturing process by integrating advanced control strategies. And the third characteristic is the arranging the things as a system. DCS organizes the entire control structure as a single automation system where various subsystems are unified through a proper command structure and information flow.
These attributes of DCS can be observed in its architecture shown in the diagram below. The basic elements comprised in a DCS include engineering workstation, operating station or HMI, process control unit or local control unit, smart devices, and communication system.
Engineering Workstation:
It is the supervisory controller over the entire distributed control system. It can be a PC or any other computer that has dedicated engineering software (for example, control builder F engineering station in case of ABB freelance distributed control system).
This engineering station offers powerful configuration tools that allow the user to perform engineering functions such as creating new loops, creating various input and output points, modifying sequential and continuous control logic, configuring various distributed devices, preparing documentation for each input/output device, etc.
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Operating Station or HMI
This is used to operate, monitor and control plant parameters. It can be a PC or any other monitoring device that has a separate software tool on which operator can view process parameter values and accordingly to take control action. For instance, it is a DigiVis software tool that can run on a simple PC-environment in case ABB DCS.
Operating stations can be a single unit or multiple units where a single unit performs functions like parameter value display, trend display, alarming, etc. while multiple units or PCs performs individual functions such as some PCs display parameters, some for trend archives, some for data logging and acquiring, etc.
Process Control Unit of DCS
It is also called as a local control unit, distribution controller, or process station. A distributed control system can consists of one or more process stations that can be extended with different types of I/O units. These controllers consist of a powerful CPU module, field bus or communication module with extended field bus capability and either direct or remote connected I/Os.
The field devices like sensors and actuators are connected to I/O modules of this unit. Some field devices can be directly connected to field bus (such as Profibus) without any I/O module, which can be termed as smart field devices.
These units acquire the information from various sensors via input module, analyze and process it based on the control logic implemented and sends the output signals via output modules to have control on actuators and relays.
In case of <rel “nofollow” a href=”http://new.abb.com/control-systems/essential-automation/freelance/controller/ac800f” target=”_blank”>ABB DCS, AC800F module (consider, for instance) acts as process station, which is responsible for acquiring and controlling the data from the process. This unit consists of a power supply along with CPU section, Ethernet section, Profibus section and remote communication interface unit for I/Os interfacing as shown in the figure where first module is AC 800F unit and other one is remote I/O (also called as communication interface module).
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Communication System
The communication medium plays a major role in the entire distributed control system. It interconnects the engineering station, operating station, process station and smart devices with one another. It carries the information from one station to another. The common communication protocols used in DCS include Ethernet, Profibus, Foundation Field Bus, DeviceNet, Modbus, etc.
It is not mandatory to use one protocol for entire DCS, some levels can use one network whereas some levels use different network. For instance, consider that field devices, distributed I/Os and process station are interconnected with Profibus while the communication among engineering station, HMI and process station carried though Ethernet as shown in the figure below.
The major advantage of DCS is the redundancy of some or all levels of the control area. Most of the cases critical processes are installed with redundant controllers and redundant communication networks such that problem in main processing line should not affect the monitoring and control functions because of the redundant processing section.
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Smart or Intelligent Devices
The intelligent field devices and field bus technology are advanced features of DCS technology that replaces traditional I/O subsystems (I/O modules). These smart devices embed the intelligence required for simple sensing and control techniques into the primary sensing and actuating devices. And hence it replaces the need for a DCS controller to perform routine sensing and control process.
These field devices can be directly connected to field bus so that sourcing of multiple measurements to the next higher level control station is possible via digital transmission line by eliminating extraneous hardware such as local I/O modules and controllers.
Working & Operation of DCS System
The operation of DCS goes like this; Sensors senses the process information and send it to the local I/O modules, to which actuators are also connected so as to control the process parameters. The information or data from these remote modules is gathered to the process control unit via field bus. If smart field devices are used, the sensed information directly transferred to process control unit via field bus.
The collected information is further processed, analyzed and produces the output results based on the control logic implemented in the controller. The results or control actions are then carried to the actuator devices via field bus. The DCS configuring, commissioning and control logic implementation are carried at the engineering station as mentioned earlier. The operator able to view and send control actions manually at operation stations.
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Difference between SCADA and DCS (DCS vs SCADA)
Although both DCS and SCADA are monitoring and control mechanisms in industrial installations, they have different goals. There exist some commonality between DCS and SCADA in terms of hardware and its components, however, there are certain requirements by the end applications that separates a robust and cost-effective DCS from the viable SCADA system. Some of the differences between DCS and SCADA are listed below.
- DCS is process oriented, whereas SCADA is data-gathering oriented. DCS emphasizes more on control of the process and it also consists of supervisory control level. And as a part of doing so, it presents the information to the operator. On the other hand, SCADA concentrates more on acquisition process data and presenting it to the operators and control centre.
- In DCS, data acquisition and control modules or controllers are usually located within a more confined area and the communication between various distributed control units carried via a local area network. SCADA generally covers larger geographical areas that use different communication systems which are generally less reliable than a local area network.
- DCS employs a closed loop control at process control station and at remote terminal units. But in case of SCADA there is no such closed loop control.
- DCS is process state driven where it scans the process in regular basis and displays the results to the operator, even on demand. On the other hand, SCADA is event driven where it does not scan the process sequentially, but it waits for an event that cause process parameter to trigger certain actions. Hence, DCS does not keep a database of process parameter values as it always in connection with its data source, whereas SCADA maintains a database to log the parameter values which can be further retrieved for operator display and this makes the SCADA to present the last recorded values if the base station unable to get the new values from a remote location.
- In terms of applications, DCS is used for installations within a confined area, like a single plant or factory and for a complex control processes. Some of the application areas of DCS include chemical plants, power generating stations, pharmaceutical manufacturing, oil and gas industries, etc. On the other hand SCADA is used for much larger geographical locations such as water management systems, power transmission and distribution control, transport applications and small manufacturing and process industries.
In spite of these major differences, the modern DCS and SCADA systems come with common standard facilities while dealing process plant automation. However, the choice between DCS and SCADA depends on its client and end application requirement. But if the client choice between these two, by gaining equal requirement from the process, DCS is the economical choice as it help to reduce the cost and offer better control.
DCS Systems from Different Vendors
Some of the available DCS systems include
- ABB- Freelance 800F and 800 xA
- Yokogawa- Centum CS 3000 and 1000
- Honeywell-TDC 3000
- Emerson- Delta V Digital Automation
- Siemens- Simatic PCS 7
- Allen- Bradley- NetLinx
Dear engineers and readers, we hope the furnished data is informative for you. If you are working with DCS or any other automation technologies or you might have known something about DCS, please do not hesitate to share your experience with DCS in the comment section below as it would helpful for readers. We would like to hear from you. As it would helpful for readers. We would like to hear from you.
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DIFFERENCE BETWEEN DCS ,PLC AND SCADA
SCADA (Supervisory contol and Data Acquisition) is essentially a computer based (usually PC nowadays) system for displaying field data. The difference between it and HMI (Human Machine Inerface) software aren’t significant.Most SCADA systems are tag based, and a lot of HMI software is set up to read registers in PLCs directly. SCADA software like Wonderwareor the iFixinclude graphics displays, trending, alarms, data logging, and recipes.They will work with PLC or DCS systems. I like to refer to most SCADA systems as “leg savers” because they collect and display the data without making a guy run around with a clip board. Most SCADA software is capable of control logic and recipe control, but these functions are often unused. SCADA is a software. Used to monitor,control and acquire data from field devices even from remote locations. its I/p & O/p are represented in images.Each object is defined using name
The SCADA has to get its field process data from field devices, generally either a DCS or PLC based system. It often includes communications over serial link, but many use ethernet, radio telemtery, and leased line or dial-up phone lines. In the last couple of years remote I/O is also used for getting field data into the SCADA screen .
DCS has its roots in the process industry, where control was generally based on independent single loop PID controllers. DCS was a communications link to get the analog process data, and in some cases tuning info and status info, back to a central computer. The control logic resided in the controllers, and any programming was in proprietary and often obscure languages. The computer display evolved into today’s SCADA systems. The communications was almost always serial, and usually employed proprietary protocols. Bristol Babcock, Fisher Porter, and Foxboro were big names in this kind of product.
The original PLCs were relay replacers, communications was limited, and analog data functions were expensive and complicated. Over the years the ability of PLCs to handle math functions, analog data, and communcations has mushroomed, and now equal or exceed those of a DCS. DCS and PLC systems are becoming indistinguishable. Most DCS’s now have open protocols, and are physically starting to look like a PLC (Bristol Babcock’s Control Wave for example).In large plants the DCS is king because most owners want a single source of hardware support and service, and this mentality naturally denies the PLC a foothold. Package vendors are no longer required to provide PLC for their system. Everything is connected to the DCS. At the same time PLCs began to include DCS type functions like PID and multiple processors (Allen Bradley’s ControlLogix for example). PLCs are gaining market share in the process industries, but a lot of engineers still stick with DCS because of unique industry-specific functions. PLCs are generally, but not always, less expensive.
PLC is is a solid state device. It controls the output of the process through the program given in Ladder diagrams.I/p & O/p are represented in NO,NC and coil contacts.Each component involved are defined using Address. A program is written which relates the inputs to the outputs. The output can activate motors solenoid valves etc. The program could be reasonably complicated. PLCs have their roots in manufacturing, where most control was done through relay logic and based on discrete status info from limit switches etc.
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