Plc designers must know the five insider - Solutions - Huaqiang Electronic Network

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First, the selection 1. Select the model familiar with the programming software often receive some letters, especially some users who use the programmable controller (plc) for the first time, which type of PLC is better? The selection is a Old problem. Because there are many manufacturers of PLCs, sometimes they feel that they have no way to solve the problem. In general, you know which company's PLC is much more familiar with which type of PLC, especially if it is familiar with its instructions and programming software. Because the products of various companies are similar in terms of reliability and performance indicators. If you have used a certain type of PLC on your equipment (or product) or imported equipment, if you want to use PLC to develop new products, if you meet the process conditions, it is recommended to use the PLC you have used. Ok, in this way, the benefits of resource sharing can be achieved. 2. Choose the model of the joint venture factory to choose imported PLC or domestic PLC. Some PLC production plants in China, especially some joint-venture PLC production plants, the performance of PLC is the same as that of imported PLC, and the domestic PLC manufacturer has after-sales service. Spare parts are easy to solve.
The price of domestic PLC is also about 1/3 cheaper than imported PLC. Of course, imported PLCs, especially PLCs produced by some well-known international companies, especially large or very large PLCs, are the preferred objects in major projects. 3. There is another important issue in selecting a PLC with the same performance. The performance is equivalent. If there are only a dozen or so switching input and output engineering projects, the PLC model with analog output input is selected, which is overkill. At this time, as long as the PLC with the same performance is selected, the price can be greatly reduced. 4. Choosing a new model Due to the rapid replacement of PLC products, it is necessary to select the corresponding new model. I once encountered a PLC user. On a device imported in the 1980s, I used a F1-20MR PLC to break. The factory tried to buy a PLC of the same model instead. To. In fact, it is completely replaceable with a FXON-20MR PLC. Of course, other models can also be used instead. Second, installation and wiring 1. Installation location PLC can not be installed in the same switch cabinet with high-voltage electrical appliances. Inductive components such as relays and contactors that are mounted in the same switchgear as the PLC should be connected in parallel with the RC arc-extinguishing circuit. The PLC should be kept away from strong interference sources such as high-power thyristor devices, high-frequency welders and large power equipment. 2. Connection line Usually, the switch signal provided by external electrical components such as buttons, limit switches and proximity switches does not have strict requirements on the cable, so a general cable can be selected. If the signal transmission is far away, shielded cable can be used; shielded cable is also used for analog signal and high-speed signal line. 3. The wiring signal line and power line should be routed separately. The power cables should be routed separately; different types of wires should be installed in different pipe slots, and the signal wires should be installed in the dedicated cable ducts, as close as possible to the ground wire or grounded metal conductor. When the signal line length exceeds 300m, the intermediate relay transfer signal or the remote I/O module using the PLC should be used. The power cable and I/O cable should also be routed separately and kept at a certain distance. To route in the same conduit, use a shielded cable.
Different lines should be used for the line for the AC circuit and the line for the DC circuit. 4. Signal transmission Generally, when the analog input and output signals are far away from the PLC, the current transmission mode of 4-20 mA or O-10 mA should be used instead of the voltage transmission mode. The shielded wire that transmits the analog signal shall be grounded at one end. In order to bleed high frequency interference, the shielding layer of the digital signal line should be connected in parallel with the potential equalization line, and the two ends of the shielding layer should be grounded. Third, the power supply guarantee PLC reliable work is one of the goals pursued by the project. When designing the system, measures to improve reliability should be taken to eliminate the influence of interference and ensure the normal operation of the system. Power supply interference is mainly generated by impedance coupling of the power supply line. Power is one of the main ways to interfere with entering the PLC. If conditions permit, a separate power supply circuit can be used for the PC to avoid interference from other equipment to the PLC. In the case of strong interference or high reliability requirements, shielded isolation transformer and low-pass filter can be added to the AC power input end of the PLC. The isolation transformer suppresses external disturbances from the power line, and the low-pass filter absorbs most of the "glitch" interference in the power supply. The power part, control part, PLC and I/O power supply should be separately wired, as shown in the attached figure. The isolating transformer is connected to the PLC and I/O power supply by twisted pair. The power line of the system should have sufficient cross section to reduce the line pressure drop. Fourth, a large number of external sensors have shown that the failure rate of the PLC control system is caused by an external sensor failure. In particular, the failure rate of some mechanical type of travel switches and limit switches is often much higher than the failure rate of the PLC itself, so corresponding measures should be taken when designing the PLC control system. If the high-reliability proximity switch is used instead of the mechanical travel limit switch, the high reliability of the PLC control system can be guaranteed. V. User program protection and backup debugging Good PLC user program is often placed in the RAM for backup power supply. In general, this method is insurance, but in the environment of strong interference, BAM User programs may also be overwritten or flushed. At present, the common practice is to use the PLC ROM writer to write the user program to the EPROM for long-term storage. When the system is running, the EP ROM should be write-protected to prevent it from being overwritten.