Anti-interference performance measures of Mitsubishi PLC system

1. Choose a power supply with excellent performance and press the disturbance introduced by the power grid

In the PLC control system, the power supply occupies a very important position. The power grid disturbance series into the PLC control system is mainly entered through the coupling of the power supply of the PLC system (such as CPU power supply, I/O power supply, etc.), the power supply of the transmitter and the external power supply with direct electrical connection with the PLC system. Now, about the power supply of PLC system, the power supply with good barrier performance is generally selected, and the power supply of the power supply of the transmitter and the power supply of the appearance of the PLC system with direct electrical connection have not been satisfied. Therefore, for the power supply of transmitters and shared signal surfaces, it is necessary to select a power distributor with small dispersion capacitance and large pressing belt (such as the selection of repeated blocking, shielding and leakage inductance skills) to reduce the disturbance of the PLC system.

In addition, the bit ensures that the grid feed point is not interrupted, and the on-line uninterruptible power supply (UPS) can be used to provide power safety and reliability. In addition, UPS also has strong disturbance and blocking performance, which is an aspirational power supply for PLC control system.

Anti-interference performance measures of Mitsubishi PLC system

Anti-interference performance measures of Mitsubishi PLC system

2. Ao set for cable selection

In order to reduce the radiation electromagnetic disturbance of power cables, especially the feed cables of inverter equipment. In a certain project, the author selected copper tape armored shielded power cable, and then reduced the electromagnetic disturbance of power line production, and the project achieved satisfactory results after it was put into operation.

Different types of signals are transmitted by different cables, and the signal cable should be layered according to the type of transmission signal, and it is strictly forbidden to use different wires of the same cable to transmit power supply and signals at the same time, so as to prevent the signal line from being close to the power cable parallel to the cable, so as to reduce electromagnetic disturbance.

3. Hardware filtering involves software anti-as-case methods

Because of the complexity of electromagnetic disturbance, it is impossible to fundamentally eliminate the impact of electromagnetic disturbance, so when the software design and configuration of the PLC control system is designed, anti-disturbance treatment should also be carried out in the software to further improve the reliability of the system. Some commonly used methods: digital filtering and power frequency shaping sampling, which can effectively eliminate periodic disturbances; Punctual correction of the reference point potential, and the selection of dynamic zero point, can effectively prevent potential drift; Select information redundancy skills and design corresponding software markers; Choose direct jumps, set software traps, etc. to improve the reliability of the software structure.

Before the signal is connected to the accounting machine, the capacitor is connected between the signal line and the ground to reduce common mode disturbance; Installing a filter between the two poles of the signal can reduce differential mode turbulence.

Analog signals with lower signal-to-noise ratio. It often fluctuates greatly due to instantaneous disturbance in the field, and large errors will occur if only instantaneous sampling plants are used for control and accounting, so digital filtering methods can be selected.

The on-site analog signal is converted into a discrete digital signal after A/D conversion, and the resulting data is then stored in the PLC memory in a time series. Then use the digital filter program to process it, filter out the noise part to obtain a simple signal, and the input signal can be replaced by the average value of m sampling values at that time, but the well is not the general per sample. Finding the average value once, but each sampling is added with the nearest m-l historical sampling value, this method has a fast response speed, has a good real-time, the input signal is processed and then displayed with a dry signal or adjusted by the loop, which effectively presses the noise disturbance.

Due to the harsh environment of dry industry, there are many disturbance signals, and the transmission distance of I/O signals is long, which often makes the transmitted signals incorrect. In order to improve the reliability of the system operation, so that the PLC can find the fault in time in the case of signal error, and can eliminate the impact of the fault and continue to operate, the software fault tolerance skill can be selected in the programming.

4. Correctly select the docking address and improve the grounding system

There are generally two purposes of grounding, one is for safety, and the other is for pressing and disturbing. The perfect grounding system is one of the important methods for the PLC control system to resist electromagnetic disturbance.

There are three methods of system grounding: floating grounding method, direct grounding method and capacitor grounding. For the PLC control system, it is a low-speed and low-level control equipment, and the direct grounding method should be selected. Because of the influence of signal cable dispersion capacitance and input device filtering, the signal exchange frequency between devices is generally lower than 1MHz, so the grounding wire of PLC control system selects one-point grounding and series one-point grounding methods. The centrally placed PLC system is suitable for parallel connection of a one-point grounding method, and the central grounding address of each equipment is led to the grounding electrode with a unique grounding wire. If the distance between the equipment is large, the series grounding method should be used. Use a large-section copper bus (or insulated cable) to connect the center connection address of the cabinet of each equipment, and then connect the grounding bus directly to the grounding electrode. The grounding wire is a copper wire with a cross-section greater than 22 mm2, and the copper bar with a cross-section of more than 60 mm2 is used for the total bus. The grounding resistance of the grounding electrode is less than 2Ω, and the grounding is best buried at a distance of 10 - 15m away from the building (or not more than 50m between the grounding and the controller), and the connection address of the PLC system must be more than 10m away from the connection address of the strong current equipment.

When the signal source is grounded, the shield should be grounded on the signal side; When it is not grounded, it should be grounded on the PLC side; When there is a joint in the middle of the signal line, the shielding layer should be firmly connected and insulated, and multi-point grounding must be prevented; When the shielded twisted pair of signals of multiple measurement points is connected with the multi-core twisted pair main screen cable, each shielding layer should be connected to each other and insulated. Pick the appropriate grounding point at a single point of contact.

Anti-interference performance measures of Mitsubishi PLC system

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