What is PLC

What is PLC?

PLC (Programmable Logic Controller) is a small industrial computer specifically designed to control machines or production lines, such as controlling the on and off of elevators or the rhythm of factory assembly lines.

Analogical understanding
Imagine you are the owner of a factory and need to direct the workers to operate the machines.
The PLC is the "super supervisor" you hire. It can stay awake for 24 hours and precisely control all the equipment according to the rules you write.

Ii. Composition of PLC (Hardware Section)
Iii. 1. CPU (Brain)
Responsible for thinking: reading input signals (such as sensors), running the programs you write, and then directing output devices (such as motors).
Just like your brain, it processes information and makes decisions.

2. Input Module (Sensory)
Connect buttons, sensors, switches and other devices to inform the PLC of what is happening outside.
For example: The temperature sensor detects overheating → The input module informs the CPU.

3. Output Module (Hands and Feet)
Connect devices such as motors, bulbs, and valves, and execute CPU commands.
For example: The CPU determines overheating → The output module starts the cooling fan.

4. Power Module (Heart)
Supply power to the entire PLC to ensure its stable operation.

5. Programming Interface (Remote Control)
Connect here with a computer and write the control program to the PLC.

Iii. How does a PLC work? (Three-step cycle)
Step 1: Check the input
Check the status of all input devices (such as whether the buttons are pressed and whether the sensors are triggered).

2. Step 2: Run the program
Based on the logic you wrote (such as "If the button is pressed, the motor will be turned on"), the CPU quickly calculates what to do.

3. Step 3: Modify the output
Based on the calculation results, control the output devices (such as starting the motor and closing the valve).

The PLC repeats these three steps hundreds of times per second, so its response is extremely fast!

Iv. PLC Programming (Software Section)
1. Common programming language: Ladder Diagram
Program like drawing a circuit diagram, using symbols to represent switches, coils, timers, etc.
- Example:

[Switch]-- [Timer]-- (Motor)
Meaning: When the switch is pressed, the timer starts to count down and the motor starts when the time is up.


2. Core logic: Condition → Action
All programs are "If the XX condition is met, the YY action will be executed."
For example: "If the temperature exceeds 100℃ → the alarm light is on + the heater is turned off."

V. What can PLC Do?
Factory automation: Controlling robotic arms, conveyor belts, and packaging machines.
Building control: Manage elevators, air conditioning and lighting systems.
Infrastructure: Traffic light control, sewage treatment.
Home scenarios: Controlling lights and curtains in smart homes (small PLC).

Vi. Why Use PLC? Not using an ordinary computer?
1. Strong anti-interference: The PLC can operate stably in factory environments with noise, vibration and high temperatures.
2. High real-time performance: The response speed of PLC is at the millisecond level, which is beyond the reach of ordinary computers.
3. Simple and reliable: The program is fixed and is less likely to freeze or get infected.


Vii. Learning Suggestions (Three-Step Approach)
The first step: Learn basic logic
Understand the logic of "and, or, and not", and master the basic symbols of ladder diagrams.

2. Step Two: Use simulation software
Download free PLC simulation software (such as LogixPro, CODESYS) to simulate programming.

3. Step 3: Purchase the kit for practical use
Get started with PLC learning kits (such as Mitsubishi FX series, Siemens S7-1200), connect sensors and motors for practical operation.

PLC is the "logic robot" in industrial scenarios. You write the rules, and it is responsible for strictly implementing them.