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Programmable Logic Controllers (PLCs): Application Field and Working Principles

The development of programmable logic controllers (PLCs) is an important part of modern engineering and automation. PLCs are widely used and sought-after solutions for the supervisory control and data acquisition (SCADA) of many industrial processes. They provide a reliable and fast way to drive and control devices as desired in various applications. But what are PLCs, exactly? There exist various types of PLCs on the market with different sizes, features, and integrations, so it is important to know their different application fields and understand their working principles in order to properly select the most suitable controller.

Defining a PLC

At the outset, a PLC is a device that uses computer-like programming logic to automate and support processes, both biological and technical. It is best to first consider the associated hardware components. A typical PLC includes on-board input and output modules (or I/O), a processor, memory, and at least one interface port (usually RS232, RS485, or Ethernet). The I/O modules serve to connect external sensors and control systems to the processor, while the interface port allows for further data exchange with external systems. This way, PLCs have the ability to control physical coordinate motion, on/off values, pulse-width modulated values, and PID-regulated values.

Application Field

Now that we have discussed the hardware part, the next step is to consider the application fields for PLCs. In general, PLCs are commonly used in robotic control, motion control, factory automation, process control, and safety systems.

Robotic control is one of the most popular application fields, and a PLC is often used in the “teach units” that motion and robotic control systems use to set up sequences and programs. PLCs are also widely used for motion control machines and in the automation of the production line. Factory automation is an area of great growth, and PLCs are great for monitoring and optimizing production, and for handling processes with minimal human intervention. In addition, PLCs are widely used in process control for biotechnological and pharmaceutical processes.

PLCs have also improved safety systems in factories by their remote monitoring capability. They can be adjusted as needed to respond to rapidly changing conditions while monitoring crucial safety bounds to ensure the safe operation of working machines.

Working Principle

PLCs execute programs written in native programming languages and seek to solve the same problems as discrete control systems, such as logic control, sequential control, motion control, and process control, only in a more efficient way. They are faster and are designed to be easily expandable.

When the power supplied to a PLC is turned on, the CPU runs a set of instructions that control processes within connected systems. These instructions, also called the “run-mode code”, allow the PLC to control the devices as defined by the program. The program is stored in memory and is automatically executed when power is turned on. The program can be tested and modified with the help of a personal computer or other device connected to the PLC.

Furthermore, PLCs can integrate different kinds of inputs and output signals from external devices. The logic instructions defined by the programmer can be used to control how these signals are processed, depending on the application. For example, if an input signal represents a high pressure reading, then instructions can be implemented such that the PLC turns an output off in response.

Conclusion

In conclusion, PLCs are crucial elements for supervising and controlling a variety of processes in modern engineering and automation. With so many different types of PLCs on the market, understanding their application fields, as well as their working principles and components, is necessary to ensure end users make a suitable selection for their specific requirement.

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