I'm writing something purely theoretical lately because I've been thinking about how to explain PLC to someone with absolutely no prior knowledge. In other words, how do you get a complete novice interested in PLCs and then gradually guide them through the learning process? I've thought about it a lot, even digging out my old textbooks from school and trying to recall how I got started. Several days have passed with no clear direction, so I put together this piece first, but it still seems too theoretical.
Main components of a PLC
1. CPU module
The CPU module mainly consists of a microprocessor (chip) and memory, much like a home computer (PC), which acts as the brain and heart of a human. It aggregates, analyzes, and processes information collected from other modules, ultimately outputting the results to the system.
The main function of a CPU is to execute user programs, which programmers implement by writing user programs.
The memory is used to store programs and data, and it is similar to the RAM in a home computer PC.
CPU module is also called CPU for short.
2. I/O module
Input/output modules, or I/O modules for short, are like the eyes, ears, nose, hands, and feet of a person; they are the bridge connecting external information with the brain (CPU).
Input: input
Output:
The input module is used to acquire various signals from field devices, such as switching signals from buttons, selector switches, limit switches, proximity switches, and pressure relays. In addition to receiving these switching signals, it also receives analog signals from potentiometers, pressure gauges, ammeters, and voltmeters. These signals are characterized by continuous change and require continuous acquisition.
Therefore, there are two types of input modules:
Digital Input Module (DI)
Analog Input Module (AI)
There are also two types of corresponding output modules:
Digital Output Module (DO)
Analog Output Module (AO)
Digital output modules are mainly used to control devices such as relays, contactors, solenoid valves, and indicator lights, which only have two states: on and off.
Analog output modules are used to control devices that require continuous adjustment, such as regulating valves, frequency converters, and speed controllers.
Definitions:
Digital signals: These are signals with only two states, on and off, and are composed of 0s and 1s in digital devices. These signals are discontinuous, having only two opposite states.
Analog signals: continuous signals such as voltage and current, whose amplitude changes continuously over time.
Why do we need an I/O module?
The CPU typically operates at 5V, while the circuit voltages of PLC peripherals are generally higher. The control cabinet primarily handles two control voltages: DC 24V and AC 220V. Introducing such high voltages would inevitably damage the CPU. Therefore, optocouplers, photothyristors, and miniature relays are used in the I/O modules to isolate the voltage before it is input to the PLC, providing protection.
Therefore, the main functions of the I/O module are: signal transmission, level conversion, and isolation.
In small PLCs, the CPU module and I/O module are usually integrated together. When the integrated input/output is insufficient, dedicated I/O modules can be added. Medium and large PLCs mostly have a separate CPU module, with appropriate I/O modules configured according to the actual number of input points.
3. Programmer
The primary function of a programmer is to generate user programs and to edit, inspect, modify, and monitor them. Programmers were mainly used in the early days before PCs.
Currently, PLC programming, debugging, and monitoring are mainly done using programming software directly installed on a computer. Siemens S7-200 programming software is STEP7Micro/WIN. Ladder diagrams and statement lists can be written on the computer, and switching between different programming languages is possible.
Programming software can be used to compile programs and download them to the PLC, or user programs in the PLC can be uploaded to a computer.
4. Special function modules
Special function modules, as the name suggests, are dedicated modules used in special occasions, such as weighing modules, communication modules, and high-speed counting modules.
5. Power Module
PLCs use AC220V or DC24V power supplies. The main function of a switching power supply is to provide different levels of DC voltage to each module. Generally, small PLCs do not have dedicated power supply modules; only medium and large PLC systems require dedicated power supply modules.
