Programmable logic controller

Virtual PLCs

In recent years, the introduction of virtual PLCs has expanded the scope of programmable logic controllers. Virtual PLCs are software-based controllers that simulate the functions of traditional PLCs but are executed on general-purpose hardware, offering a more cost-effective and flexible alternative. These controllers enable automation systems to be managed without the need for dedicated hardware, making them ideal for applications requiring simulation, remote control, or cloud-based systems.^[9]

Modicon

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 from Bedford, Massachusetts. The result, built in 1969, was the first PLC and designated the 084, because it was Bedford Associates' eighty-fourth project.^[10][11]

Bedford Associates started a company, Modicon, Inc.,^[12] 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 is considered to be the father of the PLC.^[13] The Modicon brand was sold in 1977 to Gould Electronics and later to Schneider Electric, its current owner.^[11] About this same time, Modicon created Modbus, a data communications protocol to be used with its PLCs. Modbus has since become a standard open protocol commonly used to connect many industrial electrical devices.^[14]

One of the first Modicon 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 like Modicon Micro 84 and Modicon TSX CSY 84 until after the 984 made its appearance.^[15]

Allen-Bradley

In a parallel development, Odo Josef Struger is sometimes known as the "father of the programmable logic controller" as well.^[13] He was involved in the invention of the Allen-Bradley programmable logic controller.^[16][17][18] Prior to the release of the IBM "Personal Computer" in 1981, these devices were commonly known simply as "Programmable Controllers", or "PC's. But rapid expansion of the personal computer industry in the early 1980's, the abbreviation "PC" was rapidly and universally accepted as a reference to "Personal Computers". Odo was later credited with coining the "PLC" acronym^[13][16] to distinguish these two technologies, And the inclusion of the word "Logic" was an easy choice as "Ladder Logic" had already been accepted as the name of the programming language. Allen-Bradley (now a brand owned by Rockwell Automation) went on to become the dominant PLC manufacturer in the United States during his tenure.^[19] Struger played a leadership role in developing IEC 61131-3 PLC programming language standards.^[13] Allen-Bradley's PLC-5 family, preceded by the 1774-PLC (released in the 1980's, was the first modular design found in modern-day systems.^[20]

While Modicon was the most popular of the first full featured PLC's through the early 1980's, Allen Bradley overtook them in the mid-1980's and grew to own the largest market share of PLC hardware and software in North America.^[21]

Early methods of programming

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,^[22] function block diagrams, and structured text exist. Ladder logic remains popular because PLCs solve the logic in a predictable and repeating sequence, and ladder logic allows 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.^[23]

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 logical elements of PLC programs.^[10] 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 a lack of memory capacity. The oldest PLCs used magnetic-core memory.^{[citation needed]}

Mechanical design

There are two types of mechanical design for PLC systems. A single box (also called a brick) is a small programmable controller 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, selection of I/O modules and communication interfaces – which all can be customized for the particular application.^[27] 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.^{[citation needed]}

Discrete and analog signals

Discrete (digital) signals can only take on or off value (1 or 0, true or false). Examples of devices providing a discrete signal include limit switches and photoelectric sensors.^[28]

Analog signals can use voltage or current that is analogous to the monitored variable and can take any value within their scale. Pressure, temperature, flow, and weight are often represented by analog 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.^[28] For example, an analog 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 translate it into the desired units of the process so the operator or program can read it.

Redundancy

Some special processes need to work permanently with minimum unwanted downtime. Therefore, it is necessary to design a system that is fault tolerant. 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 a hardware configuration to prevent a 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 two PLCs turn on an output before the press can come down in case one PLC does not behave properly.

Programming device

Manufacturers develop programming software for their PLCs. In addition to being able to program PLCs in multiple languages, they provide common features like hardware diagnostics and maintenance, software debugging, and offline simulation.^[34]

PLC programs are typically written in a programming device, which can take the form of a desktop console, special software on a personal computer, or a handheld device.^[34] The program is then downloaded to the PLC through a cable connection or over a network. It is stored either in non-volatile flash memory or battery-backed-up RAM on the PLC. In some PLCs, the program is transferred from the programming device using a programming board that writes the program into a removable chip, such as EPROM, that is then inserted into the PLC.

Simulation

An incorrectly programmed PLC can result in lost productivity and dangerous conditions for programmed equipment. PLC simulation is a feature often found in PLC programming software. It allows for testing and debugging early in a project's development. Testing the project in simulation improves its quality, increases the level of safety associated with equipment and can save time during the installation and commissioning of automated control applications since many scenarios can be tried and tested before the system is activated.^[34][35]

Basic functions

The most basic function of a programmable logic controller is to emulate the functions of electromechanical relays. Within the PLC programming environment, 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 performs 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 electromechanical 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.^[a] The output of each rung sets or clears a storage bit, which may be associated with a discrete output 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.

More advanced instructions of the PLC may be implemented as functional blocks, which carry out some operation, such as manipulating an internal variable, when enabled by a logical input and which produce outputs to signal, for example, completion or errors.

Communication

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 tools, SCADA and other user interfaces). Communication is carried over various industrial network protocols, like Modbus, or EtherNet/IP. Many of these protocols are vendor specific. Formerly, some manufacturers offered dedicated communication modules as an add-on function where the processor had no network connection built in.

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 coordinate over the communication link.

User interface

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.

PLC chip or embedded controller

These are for small machines and systems with low or medium volume. They can execute PLC languages such as ladder logic, function block diagram and sequential function chart. They are similar to traditional PLCs, but their small size allows developers to design them into custom printed circuit boards like a microcontroller, without computer programming knowledge, but with a language that is easy to use, modify and maintain. Architecturally, they sit between the classic PLC or micro-PLC and microcontrollers.^{[citation needed]}

Microcontrollers

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 controllers, because the volumes are low and the development cost would be uneconomical.^[44]

Single-board computers

Very complex process control, such as that 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.^[44]

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

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Programmable logic relays (PLRs) or smart relays are similar to PLCs and are used in light industries where only a few points of I/O 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 their low-end product range. Most of these have 8 to 12 discrete inputs, 4 to 8 discrete outputs, and up to 2 analog inputs. 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 push-buttons used to navigate and edit the logic. Most have an RS-232 or RS-485 port for connecting to a PC as an alternate means of programming. Unlike PLCs, PLRs are usually not modular or expandable.

A variant of PLCs, used in remote locations, is the remote terminal unit or RTU. An RTU is typically a low power, ruggedized PLC whose key function is to manage the communications links between the site and the central control system (typically SCADA) or in some modern systems, "The Cloud". Unlike factory automation using wired communication protocols such as Ethernet, communications links to remote sites are often radio-based and are less reliable. To account for the reduced reliability, RTU will buffer messages or switch to alternate communications paths. When buffering messages, the RTU will timestamp each message so that a full history of site events can be reconstructed. RTUs, being PLCs, have a wide range of I/O and are fully programmable, typically with languages from the IEC 61131-3 standard that is common to many PLCs, RTUs and DCSs. In remote locations, it is common to use an RTU as a gateway for a PLC, where the PLC is performing all site control and the RTU is managing communications, time-stamping events and monitoring ancillary equipment. 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.

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