The anti-interference problem in the field application of PLC and man-machine interface is complex and meticulous. Anti-interference design is a very complicated system engineering, involving specific input and output equipment and the specific environment of the industrial site. It requires us to comprehensively consider various factors, and must reduce the interference source and cut off according to the actual situation on site. Comprehensive considerations such as interference channels, and make full use of various anti-jamming measures to design programmable controllers and human-machine interfaces. In order to truly improve the anti-interference ability of the programmable controller and human-machine interface HMI field application, ensure the safe and stable operation of the system.

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

For the power supply of the PLC controller, it should be powered by non-power line, and directly from the main bus of the low-voltage power distribution room by dedicated line. The isolation transformer is selected, and the transformer capacity should be 1.2 to 1.5 times larger than the actual requirement. It is also possible to add a filter before the isolation transformer. For transmitters and common signal instrumentation, choose a distributor with small distributed capacitance, multiple isolation and shielding and leakage inductance technology.

The controller and I/O system are powered by their respective isolation transformers and are separated from the main circuit power supply. The 24V DC power supply of the PLC controller should not supply power to various peripheral sensors as much as possible to reduce the interference of the internal controller or the short-circuit of the power supply line to the PLC controller. In addition, in order to ensure that the grid feed is not interrupted, the online uninterruptible power supply (UPS) can be used for power supply. The UPS has the functions of overvoltage, undervoltage protection, software monitoring, and isolation from the grid, which can improve the safety and reliability of the power supply. For some important equipment, the AC power supply circuit can use a dual power supply system.

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

Different types of signals are transmitted by different cables, and the signal cables are layered according to the type of transmission signals. The same type of signal lines are twisted. It is strictly forbidden to use the different wires of the same cable to transmit power and signals at the same time, to avoid the signal wires and power cables being laid close to each other, and to increase the angle between the cables to reduce electromagnetic interference. In order to reduce the radiated electromagnetic interference of the power cable, especially the feeder cable of the inverter, and to block the intrusion of interference from the interference path, a shielded power cable is used.

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

Filtering of the input module can reduce differential mode interference between the lines of the input signal. In order to reduce the 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, for the AC input signal, the capacitor and the resistor can be connected at both ends of the load. For the DC input signal, the freewheeling diode can be connected in parallel. In order to suppress parasitic capacitance between input signal lines, and induced electromotive force generated by parasitic capacitance or coupling between other lines, an RC surge absorber can be used.

The output is an AC inductive load, and the RC surge absorber can be connected in parallel across the load; if it is a DC load, the freewheeling diode can be connected in parallel, as close as possible to the load. For the case of a digital output, a surge absorber or a thyristor output module can be used. In addition, the use of output point series intermediate relay or photoelectric coupling measures can prevent the PLC controller output point from directly connecting to the electrical control loop, and is completely electrically isolated.

4, PLC controller anti-interference software measures

Due to the complexity of electromagnetic interference, it is not enough to take hardware anti-interference measures. It is necessary to use the software anti-jamming technology of PLC controller to further improve the reliability of the system. The use of digital filtering and power frequency shaping sampling, timing correction reference point potential and other measures to effectively eliminate periodic interference and prevent potential drift. Use information redundancy technology to design the corresponding software flag bits; use indirect jumps, set software protection, etc. For example, for the digital input signal, the timer delay is used to read in multiple times, and the result is consistent and 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 reliable operation of the PLC controller, which can avoid accidental voltage shock hazards and can also suppress interference. A perfect grounding system is one of the important measures for the PLC controller to resist electromagnetic interference.

The PLC controller is a high-speed low-level control device and should be grounded directly. In order to suppress the interference applied to the power supply, input and output, the PLC controller should be connected to a dedicated 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 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 as possible to the PLC controller.

The centrally arranged PLC controller is suitable for parallel grounding, and the center grounding point of each cabinet is led to the grounding pole by a separate grounding wire. The distributed PLC controller should be connected in series with one point. The grounding resistance of the grounding pole is less than 2Ω, and the grounding pole is preferably 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 expansion 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 layers should be connected to each other. Select a proper grounding point for single point grounding to avoid multi-point grounding.

6, equipment selection.

When selecting equipment, we must first understand the anti-interference indicators given by domestic PLC manufacturers, such as common mode rejection ratio, differential mode suppression ratio, withstand voltage capability, and allow for working in large electric field strength and high frequency magnetic field strength environment. It is necessary to select products with high anti-interference ability, such as floating technology, programmable controller with good isolation performance, and HMI.