Basics of Programmable Logic Controllers (PLCs)

Digital computer used for automation of electromechanical processes.

Programmable Logic Controllers (PLCs) are a type of industrial digital computer that has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, robotic devices, or any activity that requires high reliability, ease of programming, and process fault diagnosis.

Introduction to Programmable Logic Controllers (PLCs)

PLCs were first developed in the automobile manufacturing industry to provide flexible, robust 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 takes input from field devices and based on the pre-programmed logic, provides output to operate other devices.

Understanding the Architecture of PLCs

The architecture of a PLC follows a layered approach that includes the processor (CPU), memory, power supply unit, I/O modules, and communication interfaces. The CPU executes the control program stored in the memory. The power supply unit provides the necessary power for the PLC to operate. The I/O modules provide an interface for the CPU to interact with the process. The communication interfaces allow the PLC to communicate with other PLCs or computer equipment.

Basics of PLC Programming

PLC programming is done using a language recognized by the PLC. Languages that are used can be divided into two categories: text-based and graphical. Text-based languages include Instruction List (IL) and Structured Text (ST). Graphical languages include Ladder Diagram (LD), Function Block Diagram (FBD), and Sequential Function Chart (SFC).

Different Types of PLCs and Their Applications

There are three types of PLCs: Unitary PLCs, Modular PLCs, and Rack PLCs. Unitary PLCs are compact and suitable for smaller control systems with up to a few dozen I/Os. Modular PLCs have separate modules for the CPU, power supply, and I/O modules, and are suitable for medium to large control systems. Rack PLCs are similar to modular PLCs but are designed for very large control systems.

PLCs find applications in various industries including automotive, pharmaceutical, food and beverage, oil and gas, and water treatment plants. They are used in conveyor systems, manufacturing, HVAC systems, and more.