In order to improve the anti-interference ability of the PLC system, we should start from the design. In the anti-interference design of specific projects, products with strong anti-interference ability can be selected to improve the anti-interference ability of equipment and systems by suppressing interference sources, cutting off or attenuating the transmission path of electromagnetic interference, and using software means. Here are some considerations when designing:
1. Adopt a power supply with excellent performance to suppress the interference introduced by the power grid
The power supply of the PLC controller is powered by a non-power line, and is directly supplied by the main bus of the low-voltage distribution room. The isolation transformer should be selected, and the transformer capacity should be 1.2-1.5 times larger than the actual need, and the filter can be added before the isolation transformer.
The power supply of transmitters and common signal meters should be a power distributor with small distributed capacitance, multiple isolation, shielding and leakage inductance technology. The controller and I/O system are powered by their respective isolation transformers and are separate from the main circuit power. The 24V DC power supply of the PLC controller should try not to supply power to various peripheral sensors, so as to reduce the interference of the internal short circuit fault of the peripheral sensor or power line to the PLC controller. In addition, in order to ensure that the power grid is not interrupted, the online uninterruptible power supply can be used for power supply, and the UPS has the functions of over-voltage, under-voltage protection, software monitoring, and isolation from the power grid, which can improve the safety and reliability of power supply. For some important equipment, the AC power circuit can adopt a dual power supply system.
2. Correctly select the grounding point and improve the grounding system
Good grounding is an important condition to ensure the reliable operation of the PLC controller, which can avoid accidental voltage impulse damage and suppress interference. The perfect grounding system is one of the important measures for PLC controller to resist electromagnetic interference.
The PLC controller is a high-speed low-level control device that should be directly grounded. In order to suppress interference with the power supply, input and output terminals, a special grounding wire should be connected to the PLC controller, and the grounding point should be separated from the grounding point of the power supply equipment. If this requirement cannot be met, it must also be grounded together with other equipment, and it is forbidden to connect it in series with other equipment. The grounding point should be as close as possible to the PLC controller, and the centrally arranged PLC controller is suitable for parallel connection to the grounding mode, and the central grounding point of the cabinet of each device is led to the grounding electrode with a separate grounding wire.
When the signal source is grounded, the shield should be grounded on the signal side; When the signal source is not grounded, it should be grounded on the PLC controller side. When there is a joint in the middle of the signal line, the shielding layer should be firmly connected and insulated, and the shielding layer should be connected to each other. Choose the appropriate grounding for single-point grounding, and avoid multi-point grounding.
3. Reasonable selection of products and equipment
When choosing equipment, we should first understand the anti-interference indicators given by domestic PLC manufacturers, such as common mode rejection ratio, differential mode suppression ratio, withstand voltage capacity, allowable electric field strength and high-frequency magnetic field strength environment, and products with strong anti-interference ability should be selected.
4. Anti-interference measures for the input and output channels of PLC controllers
The filtering of the input module can reduce the differential mode interference between the lines of the input signal. In order to reduce common-mode interference between the input signal and the ground, the PLC controller should be well grounded. When there is an inductive load at the input, the AC input signal can be connected in parallel with capacitors and resistors at both ends of the load, and the DC input signal can be connected with a current diode in parallel. RC surge absorbers are used to suppress the induced electromotive force generated by parasitic capacitance between input signal lines, parasitic capacitance with other lines, or coupling.
The output is an AC inductive load, and the RC surge absorber can be connected in parallel at both ends of the load. For switching outputs, surge absorbers or thyristor output modules can be used. In addition, the output point of the PLC controller can be connected directly to the electrical control circuit by means of series intermediate relays or photoelectric coupling.
5. Software measures for PLC controller anti-interference
Due to the complexity of electromagnetic interference, hardware anti-interference measures are not sufficient. The software anti-interference technology of PLC controller is adopted to further improve the reliability of the system. The use of digital filtering, power frequency shaping sampling and reference point potential timing correction effectively eliminates periodic interference and prevents potential drift. Using the information redundancy technology, the corresponding software marker position was designed. Indirect jumps and software protection are employed.
The anti-interference problem in the field application of programmable controllers and human-machine interfaces is a very complex system engineering, involving specific input and output equipment and specific environments of industrial sites. It requires us to take into account a combination of factors. According to the actual situation of the site, it is necessary to comprehensively consider the aspects of reducing the interference source and cutting off the interference channel, and make full use of various anti-interference measures to design the programmable controller and man-machine interface. In order to truly improve the anti-interference ability of programmable controllers and HMIs in field applications, and ensure the safe and stable operation of the system.
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