Key Points in Designing PLC Control Systems: A Must-Read!

[Professional Accumulation] A Comprehensive Guide to O-Ring Design, It Might Be Useful for You…

[Made in China] A Bulletproof Plate That Completely Impresses Foreigners with “Made in China”

[Professional Accumulation] 33 Classic Mold Dynamic Diagrams, Easy to Understand!

[Broad Knowledge] A Collection of Three Automation Videos, Machines Have Fully Replaced Manual Labor

In modern industrial production equipment, there are numerous digital and analog control devices, such as motor start/stop, electromagnetic valve opening/closing, product counting, and setting and controlling temperature, pressure, and flow. For these automatic control issues in industrial sites, Programmable Logic Controllers (PLCs) have become one of the most effective tools for solutions.

Key Points to Consider When Designing a PLC Control System:

1. Selection of Programmable Logic Controller (PLC) and Programmer

Currently, there are many PLC products on the market. In addition to domestic brands, foreign brands include: Japan’s OMRON, MITSUBISHI, FUJI, IDEC, HITACHI, Panasonic; Germany’s Siemens; and Korea’s LG. How to select PLC products?

1. System

First, determine whether the system will use a standalone PLC or a network of PLCs, and calculate the number of input and output (I/O) points. When purchasing a PLC, reserve an additional 10% on top of the actual required points.

2. Determine Load Type

Based on whether the load connected to the PLC output is DC or AC, high current or low current, and the frequency of the PLC output point’s action, determine whether to use relay output, transistor output, or thyristor output. Choosing the appropriate output method for different loads is crucial for the stable operation of the system.

3. Execution Speed

The storage capacity and instruction execution speed are important indicators for selecting a PLC. Generally, the larger the storage capacity and the faster the speed, the higher the price of the PLC. Although products from foreign manufacturers are generally similar, there are still some differences.

4. Selection of “COM” Points

Different PLC products have varying numbers of “COM” points. Some have one “COM” point with 8 output points, some with 4 output points, and others with 1 or 2 output points. When the types of loads are many and the current is large, use a product with one “COM” point and 1-2 output points. When the types of loads are few but the quantity is large, use a product with one “COM” point and 4-8 output points.

5. Reasonable Selection of PLC Products

Due to the different development software from various manufacturers, system compatibility is also a key point during selection. Currently, no fully compatible products have been found, so PLC products should be selected reasonably based on the system.

6. Selection of Programmer

PLC programming can be done in three ways:

1 is using a general handheld programmer, which can only program using the statements specified in the manufacturer’s statement table. This method is easy for on-site debugging and is compact and low-cost, but it is less efficient and suitable for fewer types of machines, making it more appropriate for small systems with low usage.

2 is graphical programming, which uses a graphical method for programming, making it convenient and intuitive. Generally, electrical personnel can quickly adapt to it, but the programmer is relatively expensive.

3 is programming using IBM and compatible personal computer PLC software packages, which is the most efficient method and the most commonly used, but most software packages are expensive.

7. Prefer Products from Large Companies

Because their product quality is guaranteed, and they provide good technical support, generally with better after-sales service, which is beneficial for future product expansion and software/hardware upgrades.

2. Design of Input and Output Circuits

1. Power Supply Circuit

PLC power supply is generally AC85-240V (there are also DC24V), with a wide range of power supply adaptability. However, to resist interference, power purification components (such as power filters, 1:1 isolation transformers, etc.) should be installed.

2. Use of DC24V Power Supply on PLC

Most PLC products from various companies have a DC24V power supply, but the capacity is small, ranging from tens to hundreds of milliamps. When using it to drive loads, attention should be paid to the capacity, and short-circuit protection measures should be taken (as overload or short-circuit of this power supply will affect the operation of the PLC).

3. External DC24V Power Supply

If the input circuit has proximity switches, photoelectric switches, etc., powered by DC24V, and the capacity of the DC24V power supply on the PLC is insufficient, an external DC24V power supply should be provided; however, one terminal of this power supply should not be connected to the “COM” terminal of the PLC’s DC24V power supply, otherwise it will affect the operation of the PLC.

4. Input Sensitivity

Each manufacturer has specifications for the input voltage and current of the PLC. If the input current of the input element exceeds the maximum input current of the PLC or there is leakage current, it will cause false actions and reduce sensitivity. Therefore, weak current inputs should be used, and protective measures against leakage current should be taken, and PLCs with leakage-type inputs should be selected.

5. Handling Inductive Loads

When connecting inductive loads at the input and output ends, a surge suppressor or diode should be connected in parallel across the load, with the cathode of the diode connected to the positive voltage side.

6. External Interlock and Grounding

When using PLC to control the forward and reverse rotation of motors and other actions, to avoid abnormal actions of the PLC causing accidents and mechanical damage, an external interlock circuit should be formed.

Grounding: The GR terminal is the ground terminal. Use a dedicated grounding wire (with a cross-sectional area of more than 2mm2) and adopt the third grounding method (grounding resistance below 100Ω).

LG is the neutral terminal of the noise filter. If false actions occur due to high noise, or to prevent electric shock, short-circuit LG to GR, using the third grounding method. The length of the grounding wire should preferably be within 20m.

Using the grounding wire in conjunction with other devices or connecting it to the metal structure of the building can have adverse effects and be negatively impacted.

7. External Drive Circuit for PLC

If the PLC output cannot directly drive the load, an external drive circuit must be used, which can use solid-state relays or thyristor circuits for driving, and protective circuits and surge absorption circuits should also be used.

Additionally, the wiring of the PLC’s input and output also has certain requirements; please refer to the user manual of each company.

3. Selection of Expansion Modules

For small systems, such as those with fewer than 80 points, expansion is generally not required; when the system is larger, expansion is necessary. Different manufacturers have limitations on the total number of points and the number of expansion modules for the system. If expansion still cannot meet the needs, a network structure can be adopted.

Additionally, some manufacturers’ products do not support certain instructions for expansion modules, so care should be taken during software programming. When using temperature or other analog modules, each manufacturer has specific regulations; please refer to the relevant technical manuals.

4. PLC Network Design

When designing a network using PLCs, the difficulty is much greater than that of standalone PLC control. First, select a model you are familiar with, and have a deep understanding of its basic and functional instructions, as well as the execution speed of instructions and user program storage capacity. Otherwise, it may not meet real-time requirements, leading to system crashes. Additionally, consider communication interfaces, communication protocols, and data transmission speeds.

Finally, seek network design and software support and detailed technical information from the PLC manufacturer. The number of layers of workstations should be determined based on the size of the system.

5. Software Development

Before developing software, first familiarize yourself with the user manual of the selected PLC product. Once familiar, proceed with programming. If using graphical programming or software package programming, you can program directly. If using a handheld programmer, first draw the ladder diagram, then program, which can reduce errors and speed up the process. After completing the programming, first run it empty, and once all actions are normal, then debug on the equipment.

Editor-in-ChiefFriend(WeChat: 1624753095) to apply for group entry!

Disclaimer: This article is a network reprint, and the copyright belongs to the original author. If there are any copyright issues, please contact for deletion! The content of the article only represents the author’s personal views, and reprinting does not imply that this platform agrees with or holds the same views.

FollowMechanical Engineers to gain more mechanical professional knowledge