In modern industrial production equipment, there are a large number of digital and analog control devices, such as motor start and stop, solenoid valve opening and closing, product counting, temperature, pressure and flow setting and control, etc. For these automatic control problems in industrial sites, programmable logic controllers ( PLCs ) have become one of the most effective tools for solving them.
The following points should be considered when designing a PLC control system.
1. Selection of Programmable Logic Controllers (PLCs) and Programmers
There are many PLC products on the market. In addition to domestic brands, foreign brands include: OMRON, MITSUBISHI, FUJI, IDEC, HITACHI, and Panasonic from Japan; Siemens from Germany; and LG from South Korea. How should one choose a PLC product?
1) System
First, determine whether the system should be controlled by a single PLC or by forming a network of PLCs. Then, calculate the number of input and output (I/O) points. When purchasing a PLC, reserve a 10% margin based on the actual number of points required.
2) Determine the load type
The choice between relay output, transistor output, and thyristor output depends on factors such as whether the load connected to the PLC output is DC or AC, whether it's high-current or low-current, and the frequency of the PLC output's operation. Selecting the appropriate output method for different loads is crucial for the stable operation of the system.
3) Execution speed
Storage capacity and instruction execution speed are important indicators for PLC selection. Generally, the larger the storage capacity and the faster the speed, the higher the price of the PLC. Although the products of foreign manufacturers are largely the same, there are still some differences.
4) Selection of "COM" point
Different PLC products have different numbers of "COM" points. Some have 8 output points per "COM" point, some have 4 output points, and some have 1 or 2 output points. When there are many types of loads and the current is large, products with 1-2 output points per "COM" point are used. When there are few types of loads but a large number of loads, products with 4-8 output points per "COM" point are used.
5) Select appropriate PLC products
Because different manufacturers develop different software, system compatibility is also a key consideration when purchasing. Currently, no fully compatible products have been found, and PLC products should be selected appropriately based on the system requirements.
6) Purchasing a Programmer
PLC programming can be done in three ways:
1) Using a standard handheld programmer
It can only be programmed using statements from the manufacturer's specified statement list. The standard method is easy to debug on-site and has a small size and low cost, but it is inefficient and has limited compatibility with different machine types. It is more suitable for systems with small capacity and low usage.
2) Programming with a graphical programmer
This method uses graphical programming, which is convenient and intuitive. Electrical workers can usually master it in a short time, but the programmer is expensive.
3) Programming using IBM and its compatible personal computer + PLC software packages.
This method is the most efficient and the most commonly used, but most software packages are expensive.
4) Try to choose products from large companies.
Because their products are of guaranteed quality, have good technical support, and generally offer good after-sales service, it is beneficial for future product expansion and software and hardware upgrades.
2. Design of input and output circuits
1) Power supply circuit
PLCs are typically powered by AC85-240V (some are DC24V), which provides a wide range of power supply options. However, to mitigate interference, power purification components (such as power filters and 1:1 isolation transformers) should be installed.
2) Use of DC24V power supply on PLC
Most PLC products from various companies have a DC 24V power supply, but the capacity of this power supply is small, ranging from tens to hundreds of milliamps. When using it to drive a load, attention should be paid to the capacity, and short-circuit protection measures should be taken (because 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 DC24V power supply capacity on the PLC is insufficient, a DC24V power supply must be provided from an external source. However, the "one" terminal of this external power supply should not be connected to the "one" and "COM" terminals of the PLC's DC24V power supply, otherwise it will affect the operation of the PLC.
4) Input sensitivity
Each manufacturer specifies the input voltage and current for PLCs. When the input current of an input element exceeds the PLC's maximum input current or leakage current exists, malfunctions will occur, reducing sensitivity. Therefore, low-current inputs should be used, leakage current protection measures should be taken, and PLCs with leakage-type inputs should be selected.
5) Handling inductive loads
When an inductive load is connected to the input or output terminals, an impulse suppressor or diode should be connected in parallel across the load terminals, with the cathode of the diode connected to the voltage terminal side.
6) External interlocking and grounding
When using a PLC to control the forward and reverse rotation of a motor , an external interlocking circuit should be set up to prevent accidents and mechanical damage caused by abnormal PC operation.
Grounding: The GR terminal is an earth grounding terminal. Use a third grounding method (grounding resistance 100Ω or less) with a dedicated grounding wire (cross-sectional area of 2mm2 or more) to prevent induced current.
LG is the neutral terminal of the noise filter. If malfunction occurs due to excessive noise, or to prevent electric shock, LG and GR should be shorted, using a third grounding method. The length of the grounding wire should preferably be within 20m.
Sharing a grounding wire with other equipment or connecting it to the metal structure of a building can have the opposite effect and cause adverse effects.
7) PLC external drive circuit
If the PLC output cannot directly drive the load, an external drive circuit must be used. Solid-state relays or thyristor circuits can be used for driving, and protection circuits and surge absorption circuits should be used at the same time.
In addition, there are certain requirements for the input and output wiring of PLCs. Please refer to the instruction manuals of each company.
3. Selection of expansion modules
For small systems, such as those with 80 or fewer nodes, expansion is generally not necessary; however, larger systems require expansion. Different companies' products have limitations on the total number of system nodes and the number of expansion modules. When expansion still cannot meet the needs, a network structure can be used.
Additionally, some manufacturers' products do not support expansion modules for certain instructions; therefore, this should be considered when programming the software. When using simulation modules such as temperature, each manufacturer also has its own guidelines; please refer to the relevant technical manuals.
4. PLC Network Design
Designing a network using a PLC is significantly more complex than designing a standalone PLC. First, you should choose a PLC model you are familiar with, gaining a thorough understanding of its basic and functional instructions. The execution speed of the instructions and the storage capacity of the user program should also be carefully considered. Otherwise, the system may not be able to meet real-time requirements, leading to system crashes. Furthermore, the communication interface, communication protocol, and data transmission speed must also be taken into account.
Finally, you should seek network design and software support, as well as detailed technical information, from the PLC manufacturer. As for how many workstations to choose, it depends on the size of the system.
5. Software development
Before programming the software, you should first familiarize yourself with the manual of the selected PLC product. If you are using a graphical programmer or software package, you can program directly. If you are using a handheld programmer, you should first draw the ladder diagram and then program. This can reduce errors and speed up the process. After programming, run the machine without load. After all actions are normal, you can then debug it on the equipment.
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