To improve the anti-interference ability of the PLC system, when carrying out the anti-interference planning of the detailed engineering, we can select products with high anti-interference ability, and adopt methods such as pressing the source of interference, blocking or attenuating the propagation path of electromagnetic interference, and using software means to improve the anti-interference ability of equipment and systems.
1. Choose a power supply with excellent function and control the disturbance introduced by the power grid
Regarding the power supply of the PLC controller, the non-power line power supply should be selected, and the special line power supply should be selected directly from the main bus of the low-voltage distribution room. The isolation transformer is selected, and the transformer capacity should be about 1.2-1.5 times larger than the actual need, and the filter can also be used before the isolation transformer.
For the power supply of transmitters and shared signal surfaces, it is necessary to select a power distributor with small distribution capacitance, repeated isolation, shielding and leakage skills. The controller and I/O system are powered by their own isolated transformers and separated from the main circuit power. The 24V DC power supply of the PLC controller should not supply power to all kinds of peripheral sensors as much as possible, so as to reduce the disturbance of the PLC controller caused by the short circuit fault inside the peripheral sensor or the power supply line. In addition, in order to ensure that the power grid feed is not interrupted, the on-line uninterruptible power supply (UPS) can be selected for power supply, which has the functions of over-voltage and under-voltage maintenance, software monitoring, and isolation from the power grid, which can improve the safety and reliability of power supply. For some important equipment, the communication power supply circuit can choose a dual-channel power supply system.
2. Correctly select and lay cables to eliminate the spatial radiation disturbance of programmable controllers and human-machine interfaces
Different types of signals are transmitted by different cables, and the signal cables are laid in layers according to the transmission signal varieties, and the same type of signal lines are selected by twisted pair method. It is strictly forbidden to use different wires of the same cable to transmit power supply and signal at the same time, so as to prevent the signal line from being laid in close parallel to the power cable, and increase the angle between the cables to reduce electromagnetic disturbance. In order to reduce the radiation electromagnetic disturbance of power cables, especially the feed cables of frequency conversion equipment, and isolate the intrusion of disturbance from the disturbance path, shielded power cables should be selected.
3. Anti-disturbance method of input and output channels of PLC manipulator
The filtering of the input module reduces the differential mode disturbance between the lines of the input signal. In order to reduce the common-mode disturbance between the input signal and the earth, the PLC operator should be well grounded. When there is an inductive load at the input terminal, the capacitor and resistor can be connected simultaneously at both ends of the load for the communication input signal, and the continuous current diode can be connected for the DC input signal. RC surge absorbers are used to compress the parasitic capacitance between the input signal lines, the parasitic capacitance between other lines, or the induced electromotive force generated by coupling.
The output is a communication inductive load, and RC surge absorbers can be connected in parallel at both ends of the load; In the case of DC loads, freewheeling diodes can be connected in parallel, also as close to the load as possible. For switching outputs, surge absorbers or thyristor output modules can be used. In addition, the output point is connected to the central relay or photoelectric coupling method in series, which can prevent the output point of the PLC controller from being directly connected to the electrical control loop and completely isolated electrically.
4. Software method for PLC manipulator to resist disturbance
Due to the complexity of electromagnetic interference, it is not enough to adopt the hardware anti-interference method, and it is necessary to use the software anti-interference technology of the PLC controller to cooperate with it to further improve the reliability of the system. The methods of digital filtering, power frequency shaping sampling, and punctual calibration of reference point potential can effectively eliminate periodic disturbance and prevent potential drift. Select information redundancy skills and plan corresponding software markers; Choose direct jumping, set up software maintenance, etc. For example, for the switching input signal, the method of timer delay is used to read it repeatedly, and the results are consistent and then confirmed to be useful, which improves the reliability of the software.
5. Correctly select the docking address and improve the grounding system
Good grounding is an important condition to ensure the reliable operation of the PLC manipulator, which can prevent the occasional voltage impulse hazard, and can also press and disturb. The perfect grounding system is one of the important ways for PLC manipulators to resist electromagnetic disturbance.
The PLC manipulator is a high-speed and low-level control equipment, and the direct grounding method should be selected. In order to press the disturbance added to the power supply and the input and output terminals, a special ground wire should be connected to the PLC controller, and the connection address should be separated from the connection address of the power equipment. If this requirement is not met, it must also be grounded in common with other equipment, and it is forbidden to be grounded in series with other equipment. The connection address should be as close as possible to the PLC manipulator. The centrally arranged PLC manipulator is suitable for parallel one-point grounding method, and the central address of each equipment is led to the grounding electrode with a separate grounding wire. For the PLC manipulator that is loosely arranged, the series point grounding method should be selected. The grounding resistance of the grounding electrode is less than 2Ω, and the grounding electrode is best buried 10-15m away from the building, and the PLC controller grounding address must be more than 10m away from the strong current equipment grounding address. If an extension unit is to be used, its address should be the same as that of the base unit.
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 manipulator side. When there is a joint in the center of the signal line, the shielding layer should be firmly connected and insulated, and the shielding layer should be well connected with each other. Select the appropriate grounding for single-point grounding, and prevent multi-point grounding.
6. Correctly select the docking address and improve the grounding system
When selecting equipment, it is first necessary to understand the anti-disturbance objectives given by domestic PLC manufacturers, such as common mode press ratio, differential mode press ratio, withstand voltage ability, how much electric field strength and how high frequency of magnetic field strength environment are allowed to operate, etc., to select products with higher anti-disturbance ability, such as the use of floating skills, programmable manipulators with good isolation functions, and human-machine interface HMI.
The anti-disturbance problem of programmable manipulators and human-machine interfaces in field applications is complex and detailed. Anti-disturbance planning is a very complex system engineering, involving detailed input and output equipment and the detailed environment of the industrial site, requiring us to comprehensively consider all aspects of the elements, must be based on the actual situation of the site, from the reduction of disturbance sources, interception of disturbance paths, etc., to carry out comprehensive consideration, make full use of various anti-disturbance methods to carry out programmable manipulator, man-machine interface planning. The ability to truly improve the anti-disturbance ability of programmable controllers and HMI in field applications ensures the safe and stable operation of the system.
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