What is a PLC? Definition, Basics, and Function

Programmable Logic Controller What is a PLC? Definition, Basics, and Function

2024-11-15 From Katharina Juschkat 5 min Reading Time

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Programmable Logic Controllers (PLCs) are used to control or regulate machines and systems. Here, we provide an overview of their functionality, advantages, and disadvantages.

A Programmable Logic Controller (PLC) is a component that is programmed and used to control or regulate a system or machine. PLCs are utilized across various industries and can be found in applications such as hardness testing machines, injection molding machines, and fully automated production systems.

Definition: What is a PLC?

The term "PLC" stands for Programmable Logic Controller. According to EN 61131, a PLC is defined as follows:

Definition PLC

"A PLC is a digitally operating electronic system for use in industrial environments. It has a programmable memory to store user-oriented control instructions for implementing specific functions such as logic, sequence control, timing, counting, and arithmetic to control machines or processes via digital or analog input and output signals."
(EN 61131, Part 1)

Specific functions, such as sequence control, can be implemented in PLCs, allowing them to manage the input and output signals of machines and processes.

A Historical Perspective: The Origin of PLCs

The exact start of the PLC's success story is debated. While some trace its origins to the 1950s—marked by the 1958 registration of "Simatic" as a trademark, which became synonymous with PLCs—others cite the late 1960s in the United States as the beginning of PLC technology.
The need for PLCs emerged from the automotive industry's demand for a flexible replacement for traditional electromechanical controllers, which were inadequate for rapidly advancing automation.

Richard E. Morley is widely recognized as the inventor of the PLC. In 1969, he introduced the Modicon, a logic system based on semiconductors. A few years later, in 1974, the first PLCs from Klaschka and Pilz were launched in Germany. By 1979, other manufacturers in Germany followed suit. Since then, numerous manufacturers have established themselves in the European market. While some of the approximately 300 companies produce PLCs for almost all applications, most have specialized in specific niches.

How does a PLC work?

A basic PLC typically includes inputs, outputs, an operating system (firmware), and an interface. The user program can be loaded via this interface, and it defines how the outputs are activated based on the inputs.
The operating system ensures that the user program always has up-to-date information on the state of the sensors. Ultimately, the outputs are controlled by the user program in such a way that the system or machine operates as intended.

To connect the programmable logic controller (PLC) to a system or machine, both actuators (usually connected to the PLC outputs) and sensors are used. Additionally, status indicators are employed. Sensors are generally connected to the PLC inputs, allowing the PLC to receive information about the processes occurring in the system or machine.

Examples of sensors include:

Light barriers
Limit switches
Push buttons
Incremental encoders
Level sensors
Temperature sensors

Examples of actuators include:

Contactors for switching on electric motors
Electric valves for pneumatic or hydraulic systems
Drive control modules

It is also worth noting that a PLC can be implemented in various ways. It can be designed as a standalone device, a software emulation, a PC plug-in card, and so on. Modular solutions are also common, where the PLC is assembled from multiple plug-in modules.Standalone devices offer distinct advantages in terms of miniaturization and are more cost-effective for smaller automation tasks. In contrast, modular devices provide the characteristic benefits of a modular or construction kit system. These advantages include easy expandability, high flexibility, and cost savings through the use of a large number of identical modules, which can also be produced in large quantities.

Application and Practical Examples of PLCs

Because PLCs make it possible to execute repeatable processes as easily as obtaining information, they are used in many different fields. PLCs are employed in the steel industry, medical technology, the beverage industry, and virtually all industrial sectors that operate automatically. Beyond industrial plants, PLCs also play a role in everyday life, such as in traffic lights, elevators, wastewater treatment plants, or escalators.

Soft PLCs

Nowadays, so-called soft PLCs also exist. For these to function flawlessly, reliable hardware is just as essential as the installation of an operating system.

A programmable logic controller in software form must fundamentally share the PC’s processing power with the operating system, and in some cases, additional user-installed software may also be involved.
The PC’s processor is therefore tasked with two main responsibilities: executing the PLC programs, which must run deterministically and in real time, and handling the demands of other software. This additional software generally has lower requirements than the PLC—regarding deterministic timing as well as reliability.
However, the challenge lies in the processor having to switch between these two application domains, which can negatively impact the performance of the soft PLC.

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Manufacturers of PLCs (Selection)

Programmable logic controllers are produced by various manufacturers worldwide, with Siemens holding the largest global market share.
Some of the leading PLC manufacturers around the globe include:

Siemens
Rockwell Automation
Mitsubishi Electric
Schneider Electric
Omron
GE Fanuc
Eaton (formerly Moeller)
Hitachi
Fuji Electric

Advantages of a PLC at a Glance

The following outlines the advantages of a programmable logic controller (PLC) compared to a relay-based control system (RCS).

Simplified Modifications
Implementing or designing a control system rarely achieves perfection on the first attempt. Typically, it undergoes iterative improvements before the desired result is reached. With an RCS, rewiring is required frequently during modifications. In contrast, a PLC simplifies this process significantly since only the program needs to be changed. This reduces the time required for adjustments substantially and allows for late-stage program changes.

Space and Material SavingsIn a PLC, the implementation and evaluation of counting and timing functions, the assessment of various states, and logical operations are performed internally. This eliminates the need for materials like time relays and auxiliary contactors, which would otherwise be necessary with an RCS.

Program DuplicationOnce a control program is written, it can be used and replicated as often as needed. With an RCS, a new control system would need to be constructed separately each time. This makes a PLC significantly more cost-efficient when control tasks need to be replicated.

Documentation and Commenting OptionsWhen writing a PLC program, it is both possible and advisable to include helpful comments alongside the respective instructions. This makes it much easier to understand and modify existing programs later on.

Time EfficiencyPLCs require less assembly work, fewer materials, and minimal wiring effort. Consequently, projects can be realized in a much shorter time frame. Additionally, PLC programming can be divided among groups, enabling the creation of modular components that are later integrated into the overall system.

Remote Diagnostics and Maintenance
Another significant advantage of programmable logic controllers (PLCs) is the ability to control the program from a remote location without any issues. The same applies to monitoring and modifying the program. In summary, PLCs offer a highly practical solution for remote management.

Additional Advantages
Moreover, PLCs offer numerous additional advantages. They stand out for their low energy consumption, varied networking capabilities with other IT systems, and their exceptionally high reliability.

The disadvantages of a programmable logic controller

In general, programmable logic controllers have few disadvantages. However, one downside is that for less complex or smaller systems, a PLC can be too expensive.
Additionally, there is a need for extra infrastructure and technology, such as fuse sets, data carriers, programming devices, etc. Furthermore, the implementation of PLCs requires well-trained and appropriately qualified personnel.