Curriculum Requirements:
3. Basic Components and Internal System Configuration of PLC
(1) Understand the definition, characteristics, and classification of PLC.
(2) Understand the basic components, working principles, and commonly used programming languages of PLC.
(3) Be able to wire the input and output of FX2N series PLC.
(4) Understand the internal register configuration of FXzN series PLC.
1. Understand the definition, characteristics, and classification of PLC
(1) Definition of PLC
PLC, which stands for Programmable Logic Controller in Chinese.
Definition: It is a digital operation electronic system designed specifically for industrial environment applications. It uses a programmable memory to store instructions for executing logical operations, sequential control, timing, counting, and arithmetic operations, and controls various types of mechanical equipment or production processes through digital or analog input/output.
We can think of PLC as an “industrial brain” or “intelligent controller that replaces traditional relay control cabinets.” PLC is also known as an industrial control machine, abbreviated as industrial control machine.
(2) Characteristics of PLC
The reason why PLC can become the pillar of modern industrial automation is mainly due to the following outstanding characteristics:1. High reliability and strong anti-interference ability *This is the core characteristic of PLC. It employs a series of hardware and software anti-interference technologies (such as shielding, isolation, filtering, self-diagnosis, etc.) to adapt to harsh industrial environments (such as dust, high temperature, humidity, electromagnetic interference), ensuring long-term stable operation without frequent failures.2. Simple programming and easy to masterThe most commonly used programming language for PLC is ladder diagram, which has graphical symbols and expressions very similar to traditional relay control circuit diagrams.3. Strong versatility and flexible applicationWhen the control object or production process changes, usually there is no need to change hard wiring; just modify the user program inside the PLC to achieve “soft wiring.” This greatly shortens the cycle for equipment modification and upgrades.4. Complete functions and strong expansion capabilityModern PLCs not only have basic logic control, timing, and counting functions, but also integrate various advanced functions such as data processing, analog processing, motion control, PID regulation, and network communication. By adding corresponding functional modules (such as analog input/output modules, communication modules), its functions can be easily expanded.5. Convenient installation and maintenance
PLC and its I/O modules have standard interfaces and installation methods, making wiring simple. When a system fails, the built-in diagnostic indicators and programming software monitoring functions of the PLC can quickly locate the fault point, greatly reducing maintenance time.
(3) Classification of PLC
1. Classification by I/O point count and functional capacity(1) Small PLC
I/O points < 256 points Compact structure (integrated), small size, low price, mainly for logic control. Used in single devices, simple production lines, such as elevator control, packaging machines, injection molding machines.(2) Medium PLC
256 points ≤ I/O points < 2048 points Modular structure, rich functions, combining logic control and complex analog control, network communication functions. Used in large equipment, complex process control, main control of production lines, such as central air conditioning, metallurgy production lines.(3) Large PLC I/O points ≥ 2048 points Modular, high performance, with powerful computing, control, and network management capabilities, often using multi-CPU structures. Used in large-scale process control, distributed control systems (DCS), factory automation networks, such as automotive manufacturing assembly lines, large water treatment plants.2. Classification by structural form(1) Integrated (unit type)
CPU, power supply, I/O interfaces, etc., are all integrated within one shell, compact structure, small size, low price. Limited expansion capability, usually increasing I/O points by connecting expansion units.(2) Modular (combinational)
Parts such as CPU, power supply, and I/O are made into independent modules, assembled together through a backplane (or rail) and interface connectors. Flexible configuration, easy expansion, and during maintenance, faulty modules can be directly replaced. This structure is commonly used in medium and large PLCs.3. Other classification methods:By brand: such as Siemens (German), Mitsubishi/Omron (Japanese), Rockwell (American), Delta (Taiwan, China), etc.
2. Understand the basic components, working principles, and commonly used programming languages of PLC.
(1) Basic components of PLC
1. Central Processing Unit (CPU) Function: The “brain” of the PLC. Responsible for executing user programs, processing data, performing logical and arithmetic operations, and coordinating the entire system’s operation.2. Memory Function: Used to store programs and data. Types: (1) System memory: Stores the factory-set system management program of the PLC, which cannot be modified by the user. (2) User memory: User program memory: Stores the control programs written by the user (such as ladder diagrams). Mitsubishi PLCs use RAM (with battery backup) or flash memory (EEPROM) for storage. User data memory: Stores intermediate states during program execution, current values of counters/timers, and other data.3. Input/Output Unit (I/O Unit) Function: The bridge connecting the PLC to external devices (buttons, sensors, contactors, etc.). (1) Input Unit (X) Receives switch signals (such as buttons, limit switches) or analog signals (such as temperature, pressure sensors) from field devices. It converts external signals into standard level signals recognizable by the CPU and provides optical isolation to resist interference. Mitsubishi example: `X0`, `X1`, `X2`… typically correspond to a physical wiring terminal. (2) Output Unit (Y): Converts the results processed by the CPU into signals that can drive field actuators (such as contactor coils, indicator lights, solenoids). Mitsubishi example: `Y0`, `Y1`, `Y2`… common output types include relay output (strong driving capacity), transistor output (high frequency), and thyristor output.4. Power Supply Function: Converts external AC power (such as 220V AC) into the DC low voltage required by the PLC (such as 5V DC, 24V DC), powering the CPU, memory, and I/O units. Mitsubishi feature: The FX series integrated machine comes with a power supply module.5. Programmer Function: A device used to write, input, debug, monitor, and modify user programs. Modern form: Usually a computer with Mitsubishi GX Works2 or GX Works3 programming software installed, connected to the PLC via a programming cable (such as USB-SC09).
(2) Working principle of PLC
1. Input Sampling Stage The PLC reads the ON/OFF status of all input terminals (X) in a scanning manner sequentially and stores these statuses in the “input image register.”
During this stage, regardless of how external input signals change, the content of the input image register remains unchanged until the next scanning cycle’s input sampling stage. This ensures data consistency during the program processing of the current cycle.2. User Program Execution StageThe CPU of the PLC starts from the first instruction of the user program, sequentially interprets and executes each instruction, and finally writes the execution results into the “output image register.”
During program execution, the input states on which the calculations are based come from the input image register, rather than directly reading the real-time changing input terminals. The intermediate results of the calculations are also not directly driving the outputs but are temporarily stored in the output image register.3. Output Refresh Stage After all user instructions have been executed, the CPU transfers the states in the output image register to the output latches at once, driving the corresponding output terminals (Y) to control external loads.Output signals are also refreshed collectively, rather than changing an output immediately after executing an output instruction.
Scanning Cycle: The time required to complete the above three stages. It is a very important concept that determines the response speed of the PLC.
(3) Commonly used programming languages for PLC
The programming language standard for PLC is IEC 61131-3. In Mitsubishi PLCs, the most commonly used are the following two:
1. Ladder Diagram
It follows the form of relay control circuits, is intuitive and easy to understand, making it very suitable for learners with electrical backgrounds. It consists of contacts, coils, and function blocks.
(1) Normally open contact: `-| |-` represents input conditions, such as buttons.
(2) Normally closed contact: `-|/|-` represents non-conditions, such as stop buttons.
(3) Coil: `-( )-` represents output results, such as contactors, indicator lights.
Example:
—-|X0|—-|X1|——-(Y0)—- |
| |
|–|Y0|–|
Explanation: When `X0` (start button) is pressed, `Y0` (motor) is powered and self-locks through its normally open contact. When `X1` (stop button, normally closed) is pressed, the circuit breaks, and `Y0` loses power.
2. Instruction List
Features: A mnemonic language similar to assembly language, consisting of a series of instructions. The program is compact and executes quickly, but is not very intuitive.
Mitsubishi example (to achieve the same start-hold-stop circuit):
LD X0 // Load the status of X0
OR Y0 // OR the status of Y0 (self-lock)
ANI X1 // AND NOT the status of X1 (stop)
OUT Y0 // Output to Y0
3. Sequential Function Chart: Suitable for sequential control processes, describing the flow of the program.
4. Function Block Diagram: Similar to digital circuits, using function blocks to represent functions.
5. Structured Text: A high-level text language similar to Pascal or C, suitable for complex algorithms.