Basic Knowledge of PLC

Shahzad Masood


Programmable Logic Controller (PLC) is a new type of general-purpose industrial automatic control device that integrates computer technology, microelectronics technology, automatic control technology, digital technology, and communication network technology based on traditional sequential controllers. It is an important pillar of modern industrial control.

Applications of PLC:

PLC applications can generally be divided into the following five categories:

1. Sequential Control:

   This is the most extensive field of PLC applications and the most suitable for PLC use. It replaces traditional relay sequential control. PLC is used in single machine control, multi-level group control, and production automatic line control.

2. Motion Control:

   PLC manufacturers currently provide single-axis or multi-axis position control modules for stepper motors or servo motors. In most cases, the PLC sends data describing the target position to the control module, which outputs to move one or several axes to the target position. Each axis moves with appropriate speed and acceleration to ensure smooth motion.

3. Process Control:

   PLC can control a large number of physical parameters, such as temperature, pressure, flow, liquid level, and speed.

4. Data Processing:

   In mechanical processing, PLC is used as the main control and management system in CNC systems to complete a large amount of data processing work.

5. Communication Network:

   PLC communication includes communication between the host and remote I/O, communication between multiple PLCs, and communication between PLC and other intelligent control devices (such as computers, inverters, CNC devices, etc.). PLC, together with other intelligent control devices, can form a distributed control system with centralized management and decentralized control.

Characteristics of PLC:

Programmability: PLC can perform various logical operations, timing control, and data processing functions through programming. Users can define PLC behavior by writing programs, making modifications, and adjustments as needed to meet different control requirements.

Reliability: PLC has high reliability and stability. They undergo rigorous testing and quality control and use reliable hardware designs and protective measures to handle harsh working environments and electrical interference.

Flexibility: PLC can be configured and expanded flexibly according to needs. Users can select appropriate input/output modules, communication modules, and function modules according to specific application requirements to meet different control needs.

Real-time Performance: PLC can process input signals in real time and respond quickly to output signals based on preset logic and programs, achieving real-time control and monitoring of the system.

Ease of Maintenance: PLC has good maintainability. They usually have self-diagnostic functions that can detect and report faults and provide corresponding troubleshooting and maintenance guidance. Moreover, PLC programming and parameter setting are relatively simple, making maintenance and modifications easier.

Scalability: PLC systems can be expanded and upgraded as needed. Users can add new modules or functions to meet system expansion requirements without large-scale hardware replacement.

Ease of Integration: PLC can be integrated with other devices and systems, such as sensors, actuators, and human-machine interfaces. Through appropriate communication interfaces and protocols, PLC can exchange data and communicate with upper-level computers, SCADA systems, and other control devices.

Overall, PLC has features such as programmability, reliability, flexibility, real-time performance, ease of maintenance, scalability, and ease of integration, making it widely used in the field of automation control.

Classification of PLC:

PLC is a product of the needs of modern production, and its classification must meet the requirements of modern production. Generally, PLC can be classified from three perspectives: control scale, performance, and structural characteristics.

Classification by Control Scale:

1. Small PLC:

   Usually, the I/O points are less than 256, single CPU, 8-bit or 16-bit processor, with a user program memory capacity of less than 4KB, mainly for discrete control.

2. Medium PLC:

   Generally, the I/O points are between 256 and 2048, dual or multi-CPU, with a user program memory capacity of 2-8KB or more. It has both discrete and analog control functions and stronger digital computation capabilities.

3. Large PLC:

   Usually, the I/O points are more than 2048, dual or multi-CPU, 16-bit or 32-bit processor, with a user program memory capacity of 8-16KB or more.

Classification by Control Performance:

1. Low-end PLC:

   This type of PLC has basic control functions and general computing power, slower working speed, can handle fewer input and output modules, and has fewer types of I/O modules.

2. Mid-range PLC:

   This type of PLC has strong control functions and computing power. It can perform not only general logical operations but also more complex trigonometric, exponential, and PID calculations. It works faster and can handle more input and output modules, with a wider variety of I/O modules.

3. High-end PLC:

   This type of PLC has powerful control functions and computing power. It can perform logical operations, trigonometric, exponential, PID, and complex matrix calculations. It works very fast and can handle a large number of input and output modules with comprehensive I/O module types.

Classification by Structure:

PLC can be divided into integral type and modular type based on structure.

1. Integral Type:

   This is the most common and basic type of PLC. It consists of a central processor module, input module, output module, and communication module. The central processor executes programs, performs logical operations and control, the input module receives external signal input, the output module controls external device operations, and the communication module exchanges data with other devices.

2. Modular Type:

   The modular PLC structure divides the PLC system’s components into several modules by function, such as CPU module, input module, output module, power supply module, etc. These modules can be inserted into a frame or baseboard to form a complete control system.

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