Modern industry uses many extremely complex technological processes, which in turn requires the use of very advanced and reliable control systems. Increasing efficiency and competitiveness cannot be achieved without the automation of many activities. In recent years, PLC programmable controllers have become one of the basic control systems.
The programmable logic controller PLC (Programmable Logic Controller) is the result of the rapid development of microprocessor technology in the 20th century and today it is one of the basic control elements of modern technological processes. The world's first PLC controller was created in 1969 in the United States and was Modicon 084. This device weighed as much as 46 kg and the program memory size was 4 kB. Today it's a ridiculous number, but back then it was impressive. Programmable controllers were first used in the automotive industry, but quickly gained recognition in other industrial sectors, replacing relay control systems and sequential control devices. In the 1990s, there was virtually no modern factory that didn't use programmable controllers PLC.
Programmable controllers – division
You can find a lot on the market PLC controllers various manufacturers (e.g. Eaton ES4P-221-DRXD1 easySafety), which offer various functionalities. Such devices can be classified using various criteria. One of them is the number of supported inputs/outputs (I/O - Input/Output): large ones with support for up to 4096 I/O (e.g. Eaton XC-CPU101-C256K-8DI-6DO), medium ones with support for up to 512 I/O and small ones with support for up to 128 I/O. Another division is made according to design features. Here we distinguish controllers without housing that are installed inside machines and devices, compact controllers with small dimensions and a simple housing (e.g. Eaton EC4P-222-MTAD1), modular controllers in the form of medium and large devices in which the user selects the necessary modules and installs them in special panels (e.g. Schneider TSX3705028DR1) and drivers integrated with the operator panel.
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Construction of a programmable controller
We have already mentioned that controllers may differ in functionality, parameters and housing, but their basic structure consists of the same elements:
- Charger – most often it is impulse and is characterized by high efficiency of the AC/DC converter and high power with small dimensions, it can work in difficult industrial conditions, it has overvoltage and overload protection. PLC controllers most often operate on a 24 V power supply, although some models can also be connected directly to the power grid. In compact controllers, the power supply is mounted inside the housing (e.g. Eaton EC4P-221-MTAX1) or can be a separate element in modular controllers.
- Central processing unit (CPU) – this is the main component of the controller. Its task is to manage the operation of the PLC, execute the control program, operate individual modules and communicate with other system elements. The heart of the CPU is the microprocessor. Its parameters determine the controller's operating speed, communication capabilities, memory size, number of supported I/O variables, etc. The type of memory that stores the control program also depends on the microprocessor: random access RAM (e.g. Schneider TSX3721101), EEPROM - permanent memory only for reading, programmed and erased electrically, and FLASH EPROM - in newer controllers, permanent memory, programmed and erased electrically.
- Digital input modules – basic controller input circuits convert current signals from external devices into digital signals. They can be powered by alternating or direct current.
- Digital output modules – output signals from the controller to the circuits of the controlled object. Coils of contactors and relays that control the operation of machines are usually connected here.
- Analog input modules – convert analog input signals into a digital form that is read by the processor. The most important element of these modules is the analog-to-digital converter.
- Analog output modules – they convert the numerical value from the controller's memory into continuous signals connecting the executing program with the controlled machine.
- Communication modules – data exchange interface between the controller and other devices (e.g. other controllers, actuators, HMI devices, etc.), which together create the control system. Newer generation communication modules are often equipped with their own processor that ensures correct data transmission.
- Special modules – they can be used to expand the functions of PLC controllers, e.g. temperature measurement modules, high-speed counters, PWM pulse generators, etc.
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How does a PLC work?
The programmable controller, of course, operates in a manner determined by the user program stored in the controller's memory. It is a sequence of instructions in the form of an algorithm that controls the operation of the machine. The control program is prepared using a computer or programmer and then saved in the controller's memory. Based on this program, the microprocessor performs appropriate actions.
Each PLC controller operates according to the following cycle repeated in a loop: controller initialization (testing the correctness of circuits and component configuration), reading input signals (reading the controller's physical inputs and saving them in memory), program execution (processing the control program instructions, generating the output state and saving them it in memory), saving output signals (taking the output state values and writing them to the controller's physical output ports), communication support (when network communication is used) and self-diagnostics (the controller checks the operation of components).
How to choose programmable controllers?
Since there are many different ones available on the market programmable controllers , you should select the appropriate model for the needs of a given control system. There are various aspects to consider here. First of all, you need to take into account the specificity of a given application and consider whether a local or distributed control system will be more beneficial. When choosing a PLC controller, you need to decide on the number and type of input/output points and determine whether the controller will also perform other tasks apart from control. It is also worth paying attention to the communication interfaces built into the PLC and their types (e.g. USB, Ethernet, etc.). The ease of use of the controller programming environment is also important (it is better to choose a PLC that supports the languages used by the company's engineers) and whether a given controller will be compatible with already used PLC models. The manufacturer's technical support is also important.
Or maybe a programmable relay?
In some cases, it may be a better choice programmable relay instead of a driver. It must be emphasized right away that the relay has a smaller number of available functions than the PLC, and has limited I/O expansion and communication capabilities. Nevertheless, a programmable relay has its advantages. First of all, it is easier to integrate with the system you are already using. Such a relay can usually be quickly programmed using the built-in keyboard and screen, and its price is lower than that of a controller. The relay is, for example, ideal for training and teaching new employees. A programmable relay can be successfully used in simple systems that have only a few dozen inputs/outputs, when we want to program a not very complex control program. Then the cost of such control will be lower than when using a PLC controller.
Programmable controller is currently an irreplaceable element in complex control systems that operate in smaller and larger industrial plants around the world. Thanks to PLC, you can flexibly program technological processes and automate them, optimizing production costs.