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 the setting and control of 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 resolution. So, what should we pay attention to when designing PLC programs? Let’s analyze it briefly.
Selection of Programmable Logic Controllers (PLC) and Programmers
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 should we select PLC products?
·System
First, determine whether the system will use a standalone PLC for control or form a network of PLCs. This will help calculate the number of input and output (I/O) points, and when purchasing a PLC, it is advisable to reserve an additional 10% on top of the actual required points.
· 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.
· Execution Speed
The storage capacity and execution speed of instructions 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.
· Selection of “COM” Points
Different PLC products have varying numbers of “COM” points; some have one “COM” point with 8 output points, while others have 4, 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.
· 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.
· Selection of Programmers
PLC programming can be done in three ways:
First, 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, 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.
Second, graphical programming, which uses a graphical method for programming, is convenient and intuitive, and generally, electrical personnel can use it proficiently in a short time, but the programmer is relatively expensive.
Third, using an IBM-compatible personal computer with a PLC software package for programming, which is the most efficient and commonly used method, but most software packages are expensive.
· 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.
Input and Output Circuits
·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.
· Use of DC24V Power Supply on PLC
Most PLC products from various companies have a DC24V power supply, but this power supply has a small capacity, ranging from tens to hundreds of milliamps. When using it to drive loads, attention should be paid to its 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).
· External DC24V Power Supply
If the input circuit has proximity switches, photoelectric switches, etc., powered by DC24V, and the capacity of the PLC’s DC24V power supply is insufficient, an external DC24V power supply should be provided; however, one terminal of this power supply should not be connected to the “-” of the PLC’s DC24V power supply or the “COM” terminal, otherwise it will affect the operation of the PLC.
· Sensitivity of Inputs
Each manufacturer has specifications for the input voltage and current of the PLC. When the input current of the input element exceeds the maximum input current of the PLC or there is leakage current, false actions will occur, reducing 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.
· 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.
· External Interlock and Grounding
When using PLC to control the forward and reverse rotation of motors and other actions, to avoid accidents and mechanical damage caused by abnormal actions of the PLC, 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 with 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 lead to interference.
· External Drive Circuit for PLC
If the PLC output cannot directly drive the load, an external drive circuit must be used, which can be a solid-state relay or thyristor circuit, and protective and surge absorption circuits should also be employed.
Additionally, the wiring of the PLC’s inputs and outputs has certain requirements; please refer to the user manual of each company.
Selection of Expansion Modules
· For small systems, such as those with fewer than 80 points, expansion is generally not required; however, for larger systems, expansion is necessary. Different manufacturers have limitations on the total number of points and the number of expansion modules. 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.
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 the communication interface, communication protocol, data transmission speed, etc.
· 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.
Software Development
· Before programming, first familiarize yourself with the user manual of the selected PLC product. Once familiar, you can start programming. If using graphical programming or software package programming, you can program directly. If using a handheld programmer, first draw the ladder diagram and then program, which can reduce errors and speed up the process. After programming is completed, first run it empty, and once all actions are normal, debug it on the equipment.
