The programmable controller, also known as a PLC (Programmable Logic Controller), is a digital electronic system specifically designed for use in industrial environments. It utilizes a programmable memory to store and execute instructions such as logic operations, sequence control, timing, counting, and arithmetic calculations. These controllers are capable of managing various types of machinery or production processes by receiving input signals through digital or analog I/O interfaces and generating output signals accordingly. The design of PLCs emphasizes ease of integration into existing industrial control systems and the ability to expand functionality as needed.
An image illustrating an introduction to PLC programming is shown below:
2. Classification of PLC
PLCs come in a wide variety of models, each with different specifications and performance levels. They are typically categorized based on their structure, functionality, and the number of I/O points they support.
2.1. Structural Classification
Depending on their physical configuration, PLCs can be classified into two main types: integrated and modular.
(1) Integrated PLC
An integrated PLC houses all essential components—such as the power supply, CPU, and I/O interface—within a single chassis. This compact design offers cost-effectiveness and is commonly used in small-scale applications. The basic unit contains the CPU, I/O interface, and expansion port, while the expansion unit provides additional I/O points without a CPU. These units are usually connected via a flat cable. Integrated PLCs can also include specialized modules like analog or position control units to enhance their capabilities.
An image showing an integrated PLC is provided below:
(2) Modular PLC
Modular PLCs consist of separate, interchangeable modules such as CPU, I/O, power, and functional units. These modules are mounted on a frame or backplane, allowing flexible configuration and scalability. This type is ideal for larger systems where expansion and maintenance are key considerations. Large and medium-sized PLCs often adopt this modular approach.
An image depicting a modular PLC is displayed below:
Some PLCs combine features of both integrated and modular designs, forming what is known as a stacked PLC. These systems feature independent modules that can be stacked and connected via cables, offering both flexibility and compactness.
2.2. Functional Classification
PLCs can also be classified according to their functional capabilities, typically divided into low-end, mid-range, and high-end categories.
(1) Low-End PLC
Low-end PLCs provide basic functions such as logic operations, timing, counting, and data transfer. They may include limited analog I/O and simple communication features, making them suitable for small-scale applications like single-machine control or basic sequence control.
(2) Mid-Range PLC
Mid-range PLCs offer more advanced features, including strong analog I/O, data communication, remote I/O, and subroutine execution. Some models support PID control and other complex functions, making them ideal for more sophisticated industrial applications.
(3) High-End PLC
High-end PLCs incorporate advanced mathematical operations, matrix processing, and specialized functions. They are equipped with robust communication capabilities, enabling them to manage large-scale process control or form distributed control systems for factory automation.
2.3. I/O Point Classification
PLCs can also be categorized based on the number of I/O points they support, typically falling into three groups: small, medium, and large.
(1) Small PLC
Small PLCs have fewer than 256 I/O points and are typically equipped with an 8-bit or 16-bit processor. They are used for simpler control tasks and are common in small-scale applications.
(2) Medium PLC
Medium PLCs support between 256 and 2048 I/O points and often include dual CPUs. They are used in more complex systems where higher performance is required.
(3) Large PLC
Large PLCs have over 2048 I/O points and are equipped with multiple CPUs and 16-bit or 32-bit processors. They are used in large-scale industrial systems requiring extensive control and data processing.
In the global market, PLC products are generally divided into three regional categories: American, European, and Japanese. While American and European PLCs are known for their advanced technology and large-scale applications, Japanese PLCs focus more on small and mid-sized systems.
Common examples of PLCs are illustrated in the following images:
Second, PLC Functions and Application Areas
Designed with the advantages of relay contactor control and computer flexibility, PLCs have become a cornerstone of modern industrial automation. Their reliability, versatility, and ease of use make them widely adopted in fields such as metallurgy, energy, chemicals, transportation, and power.
1. PLC Function
PLCs are general-purpose industrial control devices developed using computer, automation, and communication technologies. They offer high reliability, compact size, strong functionality, and easy program design, making them one of the three pillars of modern industrial control alongside robots and CAD/CAM systems. Their functions can be broadly categorized as follows:
(1) Switching Logic Control
PLCs excel at performing logic operations, replacing traditional relay circuits in applications such as single-device control, multi-machine group control, and automated assembly lines.
(2) Analog Control
With A/D and D/A conversion modules, PLCs can process analog signals from temperature, pressure, and flow sensors, enabling precise control in continuous processes.
(3) Process Control
Modern PLCs often include PID control modules for closed-loop regulation, ensuring stable operation in industries like chemical and thermal processing.
(4) Timing and Counting Control
PLCs offer numerous timers and counters, allowing users to set time and count values flexibly for various control applications.
(5) Sequence Control
Using step instructions or shift registers, PLCs can implement sequential control in manufacturing and automation systems.
(6) Data Processing
Advanced PLCs can perform arithmetic operations, data sorting, and table lookups, enabling complex data analysis and reporting.
(7) Communication and Networking
Most PLCs support communication protocols such as RS-232 or RS-485, allowing them to network with other devices and exchange programs and data.
2. Application Fields of PLC
PLCs are widely used in various industries, including steel, oil, chemical, power, construction materials, machinery, automotive, textile, transportation, environmental protection, and entertainment. Their applications can be roughly divided into the following categories:
(1) Switching Logic Control
This is the most fundamental application of PLCs, replacing relay circuits in single-machine or automated line control.
(2) Analog Control
PLCs can process continuous analog signals like temperature and pressure, enabling precise control in industrial processes.
(3) Motion Control
PLCs are used to control motion in machines, tools, robots, and elevators, often through dedicated motion control modules.
(4) Process Control
PLCs are used in closed-loop control systems for temperature, pressure, and flow, ensuring consistent production quality.
(5) Data Processing
Modern PLCs can handle complex data operations, making them ideal for large-scale control systems such as flexible manufacturing and process control.
(6) Communication and Networking
PLC communication allows for seamless integration with other smart devices, supporting real-time data exchange and monitoring in industrial networks.
Third, Basic Structure and Working Principle of PLC
As an industrial control computer, the PLC shares similarities with standard computers but is optimized for industrial environments. Its hardware includes components such as the CPU, memory, I/O modules, programmer, power supply, and peripheral interfaces.
1. PLC Hardware Components
The basic block diagram of a PLC shows its core components: CPU, memory, I/O units, peripheral interfaces, communication interfaces, and power supply. In integrated PLCs, these components are housed in a single enclosure, while modular PLCs allow for flexible configuration using separate modules connected via a rack.
The CPU serves as the control center, executing instructions and coordinating system operations. Memory stores system programs and user data, while I/O modules interface with field devices. Programmers are used to write, debug, and monitor programs, and the power supply converts external power into the internal voltages required by the PLC.
2. PLC Software Composition
PLC software consists of system programs and user programs. System programs, developed by manufacturers, handle diagnostics, input processing, and communication, while user programs are created by end-users to meet specific control requirements.
3. Basic Working Principle of PLC
The PLC operates in a scanning mode, consisting of three stages: input sampling, user program execution, and output refresh. During input sampling, the PLC reads all input states and stores them in the I/O image area. The user program is then executed, and the results are used to update the output status during the output refresh stage.
Input/output hysteresis occurs due to the scanning method, which introduces a slight delay in response time. However, this delay is typically negligible in most industrial applications.
Overall, PLCs are essential in modern automation, offering reliable, flexible, and efficient control solutions across a wide range of industries.
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