Buke human-machine interface - anti-interference countermeasures of PLC controller and human-machine interface

1. Adopt a power supply with excellent performance to suppress the interference introduced by the power grid.

For the power supply supplied by the PLC controller, the non-power line power supply should be adopted, and the special line power supply should be used 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 added before the isolation transformer. For the power supply of transmitters and shared signal meters, a power distributor with small distributed capacitance and multiple isolation and shielding and leakage inductance technology should be selected. 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 not supply power to all kinds of peripheral sensors as much as possible, so as to reduce the interference of the PLC controller from the internal or power supply line short circuit fault of the peripheral sensor. 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 isolation from the power grid, which can improve the safety and reliability of power supply. For some important equipment, the AC power supply circuit can adopt a dual power supply system.

2. Correctly select the cable and implement the laying, and eliminate the space radiation interference of the programmable controller and the human-machine interface

Different types of signals are transmitted by different cables, using far away technology, signal cables are laid in layers according to the type of transmitted signals, and the same type of signal lines adopt twisted pair mode. It is strictly forbidden to use different wires of the same cable to transmit power supply and signal at the same time, avoid parallel laying of signal lines and power cables, and increase the angle between cables to reduce electromagnetic interference. In order to reduce the radiated electromagnetic interference of power cables, especially the feed cables of frequency conversion devices, and block the intrusion of interference from the interference path, shielded power cables should be used.

3. Anti-interference measures for the input and output channels of PLC controller

The filtering of the input module can reduce the differential-mode interference between the lines of the input signal. In order to reduce the common-mode interference between the input signal and the earth, the PLC controller should be well grounded. When there is an inductive load at the input, capacitors and resistors can be connected at both ends of the load for AC input signals, and continuous current diodes can be connected for DC input signals. 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 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 use of output point series intermediate relay or photoelectric coupling measures can prevent the output point of the PLC controller from being directly connected to the electrical control loop and completely isolated electrically.

4. Software measures for PLC controller anti-interference

Due to the complexity of electromagnetic interference, it is not enough to take hardware anti-interference measures, and the software anti-interference technology of PLC controller should be used to further improve the reliability of the system. Measures such as digital filtering, power frequency shaping sampling, and timing correction of reference point potential are adopted to effectively eliminate periodic interference and prevent potential drift. Information redundancy technology is used to design the corresponding software markers; Indirect redirects, software protection, etc. For example, the switching input signal is read multiple times by using the timer delay, and the result is consistent and then confirmed to be valid, which improves the reliability of the software.

5. 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 the occasional voltage impulse hazard 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 and low-level control device, and it should be directly grounded. In order to suppress the interference added to the power supply, input and output, the PLC controller should be connected with a special ground wire, and the grounding point should be separated from the grounding point 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 grounding point should be as close to the PLC controller as possible. The centrally arranged PLC controller is suitable for parallel one-point grounding mode, and the central grounding point of the cabinet of each device is led to the grounding electrode with a separate grounding wire. The PLC controller with scattered arrangement should be connected in series with a point grounding. 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 grounding point of the PLC controller must be more than 10m away from the grounding point of the strong electrical equipment. If an extension unit is to be used, its grounding point should be connected to the grounding point 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 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.

6. Equipment selection

When choosing equipment, we should first understand the anti-interference indicators given by domestic PLC manufacturers, such as common mode rejection ratio, differential mode rejection ratio, withstand voltage capacity, how much electric field strength and how high frequency of magnetic field strength environment are allowed to work, etc., and choose products with high anti-interference ability, such as the use of floating technology, programmable controller with good isolation performance, and human-machine interface HMI.

The anti-interference problem of programmable controllers and human-machine interfaces in field applications is complex and detailed. Anti-interference design is a very complex systematic engineering, involving specific input and output equipment and the specific environment of the industrial site, requiring us to comprehensively consider all aspects of factors, must be based on the actual situation of the site, from the reduction of interference sources, cut off the interference path and other aspects of comprehensive consideration, make full use of various anti-interference measures to carry out the design of programmable controller, 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.

Buke human-machine interface - anti-interference countermeasures of PLC controller and human-machine interface

Buke human-machine interface - anti-interference countermeasures of PLC controller and human-machine interface

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