1. The choice of cable is set to reduce the radiated electromagnetic interference of the power cable, especially the inverter cable. In a project, the author used a copper tape armored shielded power cable to reduce the electromagnetic interference generated by the power line. The project achieved satisfactory results after it was put into production. Shenzhen Ruidian Technology CO., Ltd , https://www.wisonen.com
Different types of signals are transmitted by different cables. The signal cables should be layered according to the type of transmission signals. It is strictly forbidden to transmit power and signals simultaneously with different wires of the same cable to avoid the parallel connection between the signal wires and the power cables to reduce electromagnetic interference.
2. The power supply with excellent performance is used to suppress the interference introduced by the power grid. In the plc control system, the power supply occupies a very important position. The grid interference into the PLC control system is mainly coupled through the PLC system's power supply (such as CPU power, I / O power, etc.), transmitter power supply and instrument power supply with direct electrical connection with the PLC system. Now, for the power supply of the PLC system, the power supply with better isolation performance is generally used, and the power supply for the instrument with direct electrical connection between the power supply of the transmitter and the PLC system has not received enough attention, although it has taken certain The isolation measures, but generally not enough, mainly use the isolation transformer with large distribution parameters, poor interference suppression capability, and common mode interference and differential mode interference through power supply coupling. Therefore, for the power supply of the transmitter and the common signal meter, the distributor with small distributed capacitance and large suppression band (such as multiple isolation and shielding and leakage inductance technology) should be selected to reduce the interference of the PLC system.
In addition, the position ensures that the grid feed point is not interrupted, and the online uninterruptible power supply (UPS) can be used to improve the safety and reliability of the power supply. And UPS also has strong interference isolation performance, is an ideal power supply for PLC control system.
3. The correct choice of grounding point and the grounding of the grounding system are usually two, one for safety and the other for suppression of interference. The perfect grounding system is one of the important measures for the anti-electromagnetic interference of the PLC control system.
The system grounding methods are: floating mode, direct grounding mode and capacitor grounding. For the PLC control system, it is a high-speed low-level control device, and should be directly grounded. Due to the influence of signal cable distribution capacitance and input device filtering, the signal exchange frequency between devices is generally lower than 1MHz. Therefore, the PLC control system ground wire adopts one-point grounding and one-point grounding in series. The centrally arranged PLC system is suitable for parallel grounding, and the grounding point of the cabinet center of each device is led to the grounding pole by a separate grounding wire. If the device spacing is large, a one-point grounding method in series should be used. Use a large cross-section copper busbar (or insulated cable) to connect the center ground of the cabinet to each unit, and then connect the grounding strap directly to the grounding pole. The grounding wire is made of copper wire with a section larger than 22 mm2, and the total busbar is a copper row with a section larger than 60 mm2. The grounding resistance of the grounding pole is less than 2Ω, and the grounding pole is preferably buried 10 to 15m away from the building (or no more than 50m from the controller), and the grounding point of the PLC system must be more than 10m away from the grounding point of the strong electrical equipment.
When the signal source is grounded, the shielding layer 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 avoided; When the shielded twisted pair of the point signal is connected to the multi-core twisted pair main screen cable, the shield layers should be connected to each other and insulated. Select the appropriate grounding point for the single point contact.
4, hardware filtering and software anti-measures due to the complexity of electromagnetic interference, it is impossible to eliminate the interference impact, so in the software design and configuration of the PLC control system, it should also be anti-interference processing in software. Further improve the reliability of the system. Commonly used measures: digital filtering and power frequency shaping sampling, can effectively eliminate periodic interference; timing correction reference point potential, and use dynamic zero point, can effectively prevent potential drift; use information redundancy technology, design corresponding software flag; Use indirect jumps, set software traps, etc. to improve software structure reliability.
Before the signal is connected to the computer, a capacitor is connected between the signal line and the ground to reduce common mode interference; adding a filter between the two poles of the signal can reduce differential mode interference.
For analog signals with low signal-to-noise ratio, they often cause large fluctuations due to instantaneous interference at the scene. If only the instantaneous sampling plant is used for control calculation, a large error will be generated. For this reason, a digital filtering method can be used.
The live analog signal is converted into a discrete digital signal by A/D conversion, and then the formed data is stored in the PLC memory in time series. Then use the digital filtering program to process it, filter out the noise part to obtain a simple signal, and replace the current value with the average value of the m samples of the input signal, but the well is not the usual per sample. The average value is obtained once, but each time it is sampled, it is added to the nearest ml history sample value. This method has a fast response speed and good real-time performance. The input signal is processed and used for dry signal display or loop adjustment. Ground noise is suppressed.
Due to the harsh industrial environment, there are many interference signals, and the I/O signal transmission distance is long, which often causes the transmitted signal to be incorrect. In order to improve the reliability of the system operation, the PLC can find errors in time under the condition of signal error, and can continue to work by eliminating the influence of errors. Software fault tolerance technology can be used in programming.
November 30, 2022