When carrying out anti-disturbance planning for specific projects, it is necessary to select products with high anti-disturbance ability, and adopt methods such as pressing the disturbance source, cutting off or attenuating the propagation path of electromagnetic disturbance, and using software means to improve the anti-disturbance ability of equipment and systems.
1. Choose a power supply with excellent function to 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 barrier 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 participated in before the barrier transformer. For the power supply of transmitters and shared signal meters, it is necessary to select a power distributor with small distributed capacitance, repeated blocking and shielding and leakage inductance skills. The controller and I/O system are powered by their own barrier transformers and are separate 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 problem of the peripheral sensor or the power supply line. In addition, in order to ensure that the power grid is not interrupted, the online uninterruptible power supply (UPS) can be used for power supply, which has the functions of over-voltage and under-voltage protection, software monitoring, and blocking 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 man-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 avoid the signal line and the power cable being laid in close parallel, 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 block 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 controller
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 turbulence between the input signal and the earth, the PLC controller should be well grounded. When there is a rational load at the input, the capacitor and resistor can be connected 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 communicative rational 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 in series with the central relay or photoelectric coupling method, which can avoid the direct connection of the output point of the PLC controller to the electrical control loop, which is completely blocked electrically.
4. Software method for PLC controller to resist disturbance
Because of the complexity of electromagnetic interference, it is not possible to only adopt hardware anti-interference methods, and it is necessary to use the software anti-interference skills of PLC controllers to cooperate with them to further improve the reliability of the system. Digital filtering, power frequency shaping sampling, punctual correction of reference point potential, etc., can effectively eliminate periodic disturbance and avoid potential drift. Select information redundancy skills and plan corresponding software markers; Choose indirect redirects, set up software protection, etc. For example, for the switching input signal, the method of using the timer delay method is repeatedly read, and the results are unanimously recognized as useful, which improves the reliability of the software.
5. Correctly select the docking address and improve the grounding system
Excellent grounding is an important condition to ensure the reliable operation of the PLC controller, which can avoid accidental voltage impulse damage, and can also be disturbed. The perfect grounding system is one of the important methods for PLC controller to resist electromagnetic disturbance.
The PLC controller 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 is also necessary to achieve public grounding with other equipment, and it is forbidden to connect with other equipment in series. The connection address should be as close to the PLC controller as possible. The PLC controller that is assembled and placed is suitable for parallel connection of a point grounding method, and the center connection address of each equipment is led to the grounding electrode with a separate grounding wire. For the PLC controller that is dispersed, the series point grounding method should be selected. The grounding resistance of the grounding electrode is less than 2Ω, and the grounding electrode is buried 10-15m away from the building, and the PLC controller must be more than 10m away from the strong current equipment address. If an extension unit is to be used, its address should be connected to the address of the root 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 controller 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 point for single-point grounding, and avoid multi-point grounding.
6. Equipment selection
When selecting equipment, it is necessary to understand the anti-disturbance targets given by domestic PLC manufacturers, such as common mode ratio, differential mode ratio, withstand voltage ability, how much electric field strength and how high frequency of magnetic field strength are allowed to operate in the environment, etc., and select products with high anti-disturbance ability, such as the selection of floating skills, programmable controllers with good blocking functions, and human-machine interface HMI.
The anti-disturbance problem of programmable controllers and human-machine interfaces in field applications is complex and detailed. Anti-disturbance planning is a very complex system engineering, involving specific input and output equipment and the specific environment of the industrial site, requiring us to comprehensively consider all aspects of factors, it is necessary to consider comprehensively according to the actual situation of the site, from the reduction of disturbance sources, cut off the disturbance path, etc., and make full use of various anti-disturbance methods to carry out the planning of programmable controller and human-machine interface. In order to truly improve the anti-interference ability of programmable controllers and HMI field applications, and ensure the safe and stable operation of the system.
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