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PLCs were first developed in the automobile manufacturing industry to provide flexible, rugged and easily programmable controllers to replace hard-wired relay logic systems.
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.
PLC originated in the late s in the automotive industry in the US and were designed to replace relay logic systems. The hard-wired nature made it difficult for design engineers to alter the process.
Changes would require rewiring and careful updating of the documentation. If even one wire were out of place, or one relay failed, the whole system would become faulty.
Often technicians would spend hours troubleshooting by examining the schematics and comparing them to existing wiring.
These early computers were unreliable  and required specialist programmers and strict control of working conditions, such as temperature, cleanliness, and power quality.
The PLC was developed with several advantages over earlier designs. It tolerated the industrial environment better than computers and was more reliable, compact and required less maintenance than relay systems.
This allowed for easier iteration over manufacturing process design. With simple programming language focused on logic and switching operations, it was more user-friendly than computers using general-purpose programming languages.
It also permitted for its operation to be monitored. This program notation was chosen to reduce training demands for the existing technicians.
Other PLCs used a form of instruction list programming, based on a stack-based logic solver. In , 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.
The winning proposal came from Bedford Associates from Bedford, Massachusetts. The result was the first PLC—built in —designated the , because it was Bedford Associates' eighty-fourth project.
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 is considered to be the "father" of the PLC. One of the first 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 made its appearance. In a parallel development Odo Josef Struger is sometimes known as the "father of the programmable logic controller" as well.
Many early PLCs 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, it became more common for ladder logic to be used, 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 [ when?
A primary reason for this is that 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 .
Up to the mids, 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.
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 non-volatile magnetic core memory. A PLC is an industrial microprocessor-based controller with programmable memory used to store program instructions and various functions.
PLCs require programming device which is used to develop and later download the created program into the memory of the controller. There are two types of mechanical design for PLC systems.
A single box , or 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.
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 , photoelectric sensors and encoders.
For example, a controller might use 24 V DC input with values above 22 V DC representing o n , values below 2 V DC representing o ff , and intermediate values undefined.
Analog signals can use voltage or current that is proportional to the size of 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.
The PLC will take this value and transpose it into the desired units of the process so the operator or program can read it.
Proper integration will also include filter times to reduce noise as well as high and low limits to report faults. Distance from the device and the controller is also a concern as the maximum traveling distance of a good quality V signal is very short compared to the mA signal.
The mA signal can also report if the wire is disconnected along the path as it would return a 0 mA signal. Some special processes need to work permanently with minimum unwanted downtime.
Therefore, it is necessary to design a system that is fault-tolerant and capable of handling the process with faulty modules.
Other redundancy scenarios could be related to safety-critical processes, for example, large hydraulic presses could require that both PLCs turn on an output before the press can come down in case one output does not turn off properly.
Programmable logic controllers are intended to be used by engineers without a programming background. It resembles the schematic diagram of a system built with electromechanical relays and was adopted by many manufacturers and later standardized in the IEC control systems programming standard.
As of [update] , it is still widely used, thanks to its simplicity. Even within the same product line of a single manufacturer, different models may not be directly compatible.
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 programming device.
It is stored either in non-volatile flash memory or battery-backed-up RAM. In some programmable controllers, the program is transferred from a personal computer to the PLC through a programming board that writes the program into a removable chip, such as EPROM.
Manufacturers develop programming software for their controllers. 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.
A program written on a personal computer or uploaded from PLC using programming software can be easily copied and backed up on external storage.
It allows for testing and debugging early in 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 equipment and can save costly downtime during installation and commissioning of automated control applications since many scenarios can be tried and tested before the system is activated.
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.
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.
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.
The most basic function of a programmable controller is to emulate the functions of electromechanical 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 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.
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 variables internally that may not correspond to discrete logic.
Many of these protocols are vendor specific. 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.
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.
A simple system may use buttons and lights to interact with the user. Text displays are available as well as graphical touch screens.
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