Introduction to PLC programming
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The programmable logic controller (PLC) is a digital electronic system designed specifically for industrial environments. It uses a programmable memory to store and execute instructions such as logic operations, sequence control, timing, counting, and arithmetic functions. This allows the PLC to manage and control various types of machinery or production processes through both digital and analog input/output signals. The design of PLCs and their associated peripherals ensures ease of integration into industrial control systems and flexibility in expanding their functionality.
2. Classification of PLC
PLCs come in a wide range of models, each with different specifications and performance levels. They are typically categorized based on their structural form, functional capabilities, and the number of I/O points they support.
2.1. Structural Classification
Based on structure, PLCs can be divided into two main types: integral (monolithic) and modular.
(1) Integral PLC
An integral PLC integrates the power supply, CPU, and I/O interface within a single chassis. This compact design makes it cost-effective and ideal for small-scale applications. It usually consists of a base unit (or host) and expansion units that add more I/O points. The base unit includes the CPU, I/O interface, and communication ports, while the expansion unit only provides additional I/O and power. These units are typically connected via a flat cable. Integral PLCs may also include special function modules like analog or position control units to enhance their capabilities.
(2) Modular PLC
Modular PLCs divide the system into separate modules such as CPU, I/O, power supply, and specialized function modules. These modules are mounted on a rack or frame, allowing flexible configuration and easy expansion. This design is common in large and medium-sized PLCs, offering greater scalability and ease of maintenance.
Some PLCs combine features of both monolithic and modular designs, creating a "stacked" configuration. In this case, the CPU, power supply, and I/O are individual modules that can be stacked, providing both flexibility and compactness.
2.2. Functional Classification
PLCs are also classified by their functional capabilities into low-end, mid-range, and high-end models.
(1) Low-End PLC
Low-end PLCs offer basic functions like logic operations, timing, counting, and simple data handling. They are suitable for small-scale control tasks, such as single-machine or sequence control.
(2) Mid-Range PLC
Mid-range PLCs have enhanced capabilities, including advanced analog I/O, data transfer, and remote I/O. They can handle more complex control tasks, such as PID control and subroutine execution.
(3) High-End PLC
High-end PLCs provide powerful features such as matrix operations, advanced math functions, and robust communication capabilities. They are used in large-scale process control and distributed network 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:
(1) Small PLC
Small PLCs have fewer than 256 I/O points and use 8-bit or 16-bit processors. They are often used for simple control applications.
(2) Medium PLC
Medium PLCs have between 256 and 2048 I/O points and feature dual CPUs for more complex tasks.
(3) Large PLC
Large PLCs support over 2048 I/O points and use multiple CPUs with 16-bit or 32-bit processors. They are used in extensive industrial systems.
PLC products vary by region, with distinct styles from American, European, and Japanese manufacturers. While American and European PLCs focus on larger systems, Japanese PLCs are known for their affordability and simplicity, particularly in small-scale applications.
Second, PLC's Functions and Application Areas
PLCs combine the advantages of relay-based control systems with the flexibility of computers. This makes them highly reliable, versatile, and widely used in industries such as metallurgy, energy, chemicals, and transportation.
1. PLC Function
As an industrial control device, PLCs integrate computer technology, automation, and communication. They are known for their reliability, compact size, and ease of use. Their key functions include:
(1) Switching Logic Control
PLCs replace traditional relays in logic and sequence control, making them ideal for automation and assembly lines.
(2) Analog Control
With A/D and D/A modules, PLCs can process analog signals like temperature and pressure, enabling precise control in industrial processes.
(3) Process Control
Modern PLCs include PID modules for closed-loop control, ensuring stable operation in complex systems.
(4) Timing and Counting
PLCs have built-in timers and counters, allowing users to set and adjust time and count values easily.
(5) Sequence Control
Using step instructions or shift registers, PLCs can manage sequential operations in manufacturing.
(6) Data Processing
PLCs can perform arithmetic operations, data sorting, and table lookups, making them useful for data-driven control systems.
(7) Communication and Networking
Most PLCs support communication protocols like RS-232 and RS-485, enabling networked control and data exchange.
2. Application Field of PLC
PLCs are used across many industries, including steel, chemical, automotive, and food processing. Common applications include:
(1) Logic Control
Replacing relay circuits for sequence and single-device control.
(2) Analog Control
Controlling continuous variables like temperature and pressure.
(3) Motion Control
Driving motors and positioning systems in machines and robots.
(4) Process Control
Maintaining stable conditions in industrial processes using PID algorithms.
(5) Data Processing
Analyzing and managing data in large control systems.
(6) Communication
Connecting PLCs to other devices and forming automated networks.
Third, Basic Structure and Working Principle of PLC
As an industrial control computer, a PLC has a similar structure to a regular computer but is optimized for industrial environments.
1. PLC Hardware Components
The hardware of a PLC includes a CPU, memory, I/O modules, programmer, power supply, and peripheral interfaces. In integral PLCs, all components are housed in a single unit, while modular PLCs allow for flexible configuration through separate modules.
The CPU acts as the central processor, executing program instructions and coordinating system operations. Memory stores both system and user programs, with different types like ROM, RAM, and EPROM. Input/output modules connect external devices, translating signals between the PLC and field equipment.
Programmers are essential for writing and debugging user programs, while power supplies convert AC to DC voltages required by internal circuits.
Peripheral interfaces enable communication with other devices, forming a networked control system.
2. PLC Software Composition
PLC software consists of system programs and user programs. System programs are preloaded by the manufacturer, while user programs are written by the operator to meet specific control needs.
3. Basic Working Principle of PLC
PLCs operate in three main stages: input sampling, user program execution, and output refresh. During input sampling, the PLC reads all inputs and stores them in memory. Then, it executes the user program in a scan cycle, updating the output status based on the results. Finally, the outputs are refreshed to reflect the current state of the system.
This scanning method introduces some delay, which depends on the program length and complexity. However, it ensures consistent and reliable operation in industrial environments.
Overall, PLCs play a vital role in modern automation, offering flexibility, reliability, and efficiency in controlling industrial processes.