PLC's: A Comprehensive Guide

PLC stands for Programmable Logic Controller.

A Programmable Logic Controller (PLC) is a type of digital computer used to control electromechanical processes in industries such as manufacturing, oil and gas, water treatment, power plants, and others. It uses programmable memory for processing and controlling inputs/outputs (I/O) in real-time based on the stored program. It replaces relay-based and hard-wired control systems.

A PLC works by continuously monitoring inputs, executing a stored program, and controlling outputs based on the program's logic. The stored program, usually written in a high-level programming language, can be updated as needed to adapt to changing process conditions. Inputs to the PLC can include signals from sensors and switches, and outputs can control actuators and drives to regulate a process.

The PLC uses internal memory to store the program and process data, and it communicates with other devices through networks or serial communication interfaces. The PLC continuously scans the inputs, updates the internal memory, and executes the stored program to control the outputs, forming a closed-loop control system.

A PLC typically consists of the following components:

1. Processor Unit:
The central processing unit (CPU) that performs the calculations and logical operations.

2. Input/Output (I/O) Modules:
Connections to the sensors and actuators of the process being controlled.

3. Power Supply:
Provides electrical power to the PLC and its components.

4. Memory:
Stores the program, data, and parameters for the PLC.

5. Programming Terminal or PC:
Used to program the PLC and monitor its operation.

6. Communication Interface:
Enables the PLC to communicate with other devices, such as HMIs, remote I/O, and other PLCs.

7. Optional Add-On Modules:
Such as analog input/output modules, specialty I/O modules, and communication modules.

These components work together to allow the PLC to continuously monitor inputs, execute the stored program, and control outputs in real-time, based on the program's logic.

Programmable Logic Controllers (PLCs) are widely used in industries to control and automate processes. To effectively control these processes, PLCs must be programmed using a suitable programming method. The various methods for programming a PLC, include ladder logic, functional block programming, sequential function charts, and structured text.

Tip: The company that provides the PLC hardware often also offers software for programming it. There will be no missing features if you choose a different vendor. However, this approach is evolving and may change in the future.

The two primary programming methods of a Programmable Logic Controller (PLC) are graphical form and textual language.

1. Graphical form:
This method uses a graphical representation of the control logic, such as ladder logic diagrams, to program the PLC. This approach is intuitive and easy to understand, especially for those familiar with electrical control diagrams.

2. Textual language:
This method uses a textual language, such as structured text or sequential function charts, to program the PLC. This approach offers more flexibility and advanced programming capabilities compared to graphical form, and is suitable for complex control logic and algorithms.

Both programming methods have their advantages and disadvantages, and the choice of method depends on the specific requirements of the application and the experience and preferences of the programmer.

To effectively control these processes, PLCs must be programmed using a suitable programming method. In this article, we will discuss the various methods for programming a PLC, including ladder logic, functional block programming, sequential function charts, and structured text. Understanding these methods will help you select the most appropriate programming technique for your PLC-based control system.

These include:

Structured Text (ST) is a high-level textual programming language used to program Programmable Logic Controllers (PLCs). It is used to describe the control logic of a PLC in a structured and organized manner.

Structured text is similar to the Pascal programming language and uses statements, variables, functions, and control structures such as if-then-else, case, and loop statements to define the control logic.

This programming method is suitable for complex control logic, algorithmic programming, and mathematical calculations. It offers more advanced programming capabilities compared to graphical programming methods such as ladder logic and is often preferred by experienced programmers for its flexibility and readability.

Tip: Structured Text (ST) programming language is that it provides a high level of control and readability, making it well-suited for complex control tasks and programming.

Ladder Logic (LD) is a programming language used to program Programmable Logic Controllers (PLCs) in the industrial control systems. It is based on the idea of electrical ladder diagrams, used in control systems, and is used to create control logic for automated systems.

The programming language uses graphical symbols to represent contacts, coils, timers, and other control elements, and represents the control logic in a way that is easily understandable to engineers and technicians familiar with electrical control systems.

Ladder Logic is a simple, intuitive, and widely-used language that provides a high level of control over automated processes, making it a popular choice for PLC programming.

Tip: Ladder Logic is often considered the simplest programming language for programming a Programmable Logic Controller (PLC).

Function Block Diagram (FBD) is a graphical programming language used to program Programmable Logic Controllers (PLCs) in industrial control systems. It uses blocks, or symbols, to represent specific functions, such as logical operations, mathematical operations, and timing functions.

These blocks are connected with lines to create a flowchart-style diagram, which represents the control logic of the system. FBD is designed to be easy to understand, and it provides a higher level of abstraction than other PLC programming languages, such as Ladder Logic.

This makes it well-suited for complex control tasks and for use by engineers and technicians who are not familiar with electrical control systems. FBD is widely used in industry and is considered to be a simple and effective way to program PLCs.

Tip: Function Block Diagram (FBD) is that it provides a high level of abstraction and simplicity, making it a good choice for engineers and technicians who are not familiar with electrical control systems.

One example of an easy-to-learn PLC is the Siemens S7-1200 series. This PLC is designed for small to medium-sized control applications and is known for its user-friendly interface, simple programming software, and intuitive programming languages such as Ladder Logic and Function Block Diagram. Additionally, the Siemens S7-1200 series PLC has a wide range of communication options, including Ethernet and USB, making it easy to integrate into modern automation systems. These features make the Siemens S7-1200 series PLC a popular choice for engineers and technicians who are new to PLC programming.

PLCs are widely used in various industrial applications, including:

1. Automated manufacturing processes, such as assembly lines, packaging systems, and material handling systems.

2. Control of production machines and equipment, such as lathes, mills, and presses.

3. Control of conveyor systems and material handling systems.

4. Building automation, such as lighting control, HVAC control, and security systems.

5. Process control, such as water treatment, chemical processing, and oil and gas production.

6. Automated test equipment, such as for medical devices, electronic components, and automotive parts.

7. Robotics control, such as for pick-and-place robots, welding robots, and material handling robots.

8. Agricultural automation, such as for grain storage, silo management, and irrigation systems.

These are just a few examples of the many industries and applications that use PLCs to improve efficiency, accuracy, and reliability in automated processes.

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Updated 30.01.23