Programmable logic controllers are used to controlling or regulating machines and systems. We give an overview of the function, the advantages and the disadvantages.
A programmable logic controller, or PLC for short, is a component that is programmed and used to regulate or control a system or machine. PLCs are used in various industries, including hardness testing machines, injection moulding machines and fully automatic production systems.
Definition: what is a PLC?
The term “PLC” is derived from the English term “Programmable Logic Controller”, or PLC for short. The programmable logic controller is defined in accordance with EN 61131
“A PLC is a digitally operating electronic system for use in industrial environments with programmable memory for the internal storage of user-oriented control instructions for the implementation of specific functions such as logic control, sequence control, time, counting and arithmetic functions to be transferred through digital or analogue input – and output signals to control different types of machines and processes.
Specific functions such as sequence control can be implemented in PLCs so that both the input and output signals of processes or machines can be controlled in this way.
Historical digression: Programmable logic controller
When exactly the success story of programmable logic controllers began is not exactly known or is controversial. On the one hand, the origins of the PLC are dated to the end of the 1950s – because many authors see 1958 as a milestone for programmable logic controllers with the registration of the “Simatic” trademark, which has meanwhile become a synonym for PLC. Other authors date the beginning of PLC technology to the end of the 1960s and argue that the programmable logic controller was developed in the United States of America at that time.
The fact is: At that time, demands were being made by the automotive industry that a flexible replacement for conventional electromechanical controls was necessary. This, in turn, was attributed to the fact that traditional controls were no longer able to cope with the rapidly increasing degree of automation.
The developer of the PLC is generally considered to be Richard E. Morley, who in 1969 presented a Modicon, i.e. a logic system based on a semiconductor. A few years later, in 1974, the first PLC from Klaschka and Pilz came onto the market in Germany. In 1979, other manufacturers in Germany followed suit. Since then, numerous manufacturers have established themselves on the European market. While some of the 300 or so companies produce programmable logic controllers for almost all purposes, most manufacturers have specialized in very specific niches.
How a PLC works
If it is a simple PLC, it has inputs, outputs as well as an operating system, i.e. firmware, and an interface. The respective user program can be loaded via this interface. This also defines how the outputs are switched depending on the inputs.
The operating system also guarantees that the user program always has the current status of the encoders. Ultimately, the outputs are switched on the basis of the user program in such a way that the system or the machine works in an intended manner.
In order to connect the programmable logic controller to the system or machine, both actuators, which are generally connected to the outputs of the programmable logic controller, and sensors are used. Status displays are also used. Basically, the sensors are located at the PLC inputs, through which the programmable logic controller receives information about what is going on in the system or machine.
Examples of sensors are:
The following applies as actuators
Contactors for switching on electric motors
electric valves for compressed air or hydraulics
Drive control modules
With regard to a PLC, it should also be noted that it can be implemented in different ways. This means that it can be implemented as a single device, as a software emulation, as a PC plug-in card, etc. Often there are also modular solutions in which the PLC is assembled from several plug-in modules.
In the case of individual devices, they have different advantages in terms of miniaturization and are cheaper if they are to perform smaller automation tasks. If, on the other hand, it is a question of devices that have a modular structure, they have the characteristic advantages inherent in a modular system. In general, it is about problem-free expandability, enormous flexibility and cost savings due to the use of a large number of identical modules; these can also be produced in large numbers.
Application and practical examples for PLC
Due to the fact that PLCs make it possible to carry out repeatable processes just as easily as obtaining information, they are used in many different areas. In the steel industry, medical technology, the beverage industry, and actually in all industrial areas that run automatically, PLCs are also used. And not only in industrial plants, we also encounter PLCs every day, for example in traffic lights, elevators, sewage treatment plants or an escalator.
In the meantime, so-called soft PLCs also exist, whereby reliable hardware is just as essential as the installation of an operating system for them to function properly.
In principle, a programmable logic controller in software form must share the PC processor performance with the operating system, whereby in certain cases installed user software can be added to this.
The processor of the PC, therefore, has the task, on the one hand, to process the absolutely deterministic and real-time running PLC programs, on the other hand, additional requirements are placed on the processor of the PC by other software. This software usually has lower demands than the PLC – both in terms of deterministic timing and reliability.
The problem, however, is that the processor of the PC has to switch back and forth between the two application worlds, as this can have a negative effect on the performance of the soft PLC.
PLC manufacturers (selection)
Programmable logic controllers are produced by various manufacturers around the world, with Siemens being the company with the largest market shares worldwide. The largest PLC manufacturers around the world include:
Eaton (formerly Moeller)
The advantages of a PLC at a glance
The following shows the advantages compared to a connection-programmed controller (VPS).
Basically simplified changes
If a control is to be realized or implemented, this generally does not succeed immediately. As a rule, this is gradually perfected instead. The desired result is usually only achieved after various modifications. While the wiring must be changed at regular intervals with a connection-programmed controller, this is much easier with a programmable logic controller, because only the program has to be changed. This results in a significant reduction in the time required to modify the control. It is also possible to make late changes to the program.
Space requirements and material
In general, the implementation and evaluation of the counting and timing functions, the evaluation of different states and links are carried out internally in a programmable logic controller. In this way, the costs for the materials (for example timing relays and contactor relays) are saved, which would be incurred with a connection-programmed control.
Once a control program has been written, it can be used and copied as often as desired. This is different with a VPS because the control would have to be created separately in each case. In fact, a PLC is also much more cost-efficient when the control tasks are to be duplicated.
Document and comment options
If a PLC program is written, it is possible and also useful if helpful comments are also written for the respective instructions. In this way, existing programs can be understood much more easily and modified if necessary.
Time efficiency or time savings
Programmable logic controllers require less assembly work, less material and less wiring. For this reason, any projects can be implemented in a much shorter time frame. Editing in groups is also possible so that subcomponents can be created and combined later.
Remote diagnosis and maintenance
Another advantage of programmable logic controllers is that the respective program can also be controlled from another location without any problems. The same applies to the monitoring and modification of the program – all in all, a PLC is an extremely practical solution.
In addition, PLCs have numerous other advantages: They score with low energy consumption, different networking options with other IT systems and their very high reliability.
The disadvantages of a programmable logic controller
In principle, programmable logic controllers have only minor disadvantages. A disadvantage, however, is that a PLC is too expensive for systems that are not very complex or small.
Furthermore, there is a need for additional infrastructure and technology such as security sets, data carriers, programming devices, etc. In addition, well-trained and appropriately qualified personnel are required to implement programmable logic controllers.