In the PLC control system, the power supply occupies a very important position. The power grid interference series-connected PLC control system is mainly coupled by the power supply of PLC system (such as CPU power supply, I/O power supply, etc.). ), transmitter power supply and instrument power supply directly connected with PLC system. At present, for the power supply of PLC system, the power supply with good isolation performance is generally used, but the transmitter power supply and the instrument power supply directly connected with PLC system have not been paid enough attention. Although some isolation measures have been taken, they are generally not enough. The main reason is that the isolation transformer used has large distribution parameters and poor interference suppression ability, and the * * * mode interference and differential mode interference are connected in series through power coupling. Therefore, for the power supply of transmitter and signal instrument, the distributor with small distributed capacitance and large suppression band (such as multiple isolation shielding and leakage inductance technology) should be selected to reduce the interference to PLC system.
In addition, this location ensures that the power grid feed point will not be interrupted, and online uninterruptible power supply (UPS) can be used for power supply, which improves the safety and reliability of power supply. Moreover, UPS has strong interference isolation performance and is an ideal power supply for PLC control system.
2, cable selection ao set
In order to reduce the electromagnetic interference radiated by power cables, especially the feed cables of frequency conversion devices. In a project, the author used copper tape armored shielded power cable, thus reducing the electromagnetic interference in power line production. After the project was put into production, satisfactory results were achieved.
Different types of signals are transmitted by different cables, and signal cables should be installed in different layers according to the types of transmitted signals. It is forbidden to use different wires of the same cable to transmit power and signals at the same time, so as to avoid parallel installation of signal wires and power cables and reduce electromagnetic interference.
3. Hardware filtering and software anti-interference measures
Before the signal is connected to the computer, connect a capacitor in parallel between the signal line and the ground to reduce the * * * mode interference; Adding a filter between the two poles of the signal can reduce the differential mode interference.
Due to the complexity of electromagnetic interference, it is impossible to eliminate the influence of interference. Therefore, in the software design and configuration of PLC control system, anti-interference treatment should be carried out in the software to further improve the reliability of the system. Some common measures: digital filtering and power frequency shaping sampling can effectively eliminate periodic interference; Regularly correcting the reference point potential and adopting dynamic zero point can effectively prevent potential drift; Using information redundancy technology, the corresponding software flag bits are designed; Using indirect jump and setting software traps to improve the reliability of software structure.
4. Choose the right grounding point and improve the grounding system.
Grounding usually has two purposes, one is for safety, and the other is to suppress interference. Perfect grounding system is one of the important measures to prevent electromagnetic interference of PLC control system.
There are three grounding modes in the system: floating grounding, direct grounding and capacitive grounding. For PLC control system, it is a high-speed and low-level control equipment, which should be directly grounded. Due to the influence of distributed capacitance of signal cables and filtering of input devices, the signal exchange frequency between devices is generally lower than 1MHz, so the grounding wire of PLC control system adopts one-point grounding and one-point grounding in series. Centralized PLC system is suitable for one-point grounding in parallel, and the central grounding point of each equipment cabinet is led to the grounding electrode with a separate grounding wire. If the equipment spacing is large, one point in series should be used for grounding. Connect the central grounding point of each equipment cabinet with a large section copper bus (or insulated cable), and then connect the grounding bus directly to the grounding electrode. The grounding wire shall be made of copper wire with a cross section greater than 22mm2, and the bus bar with a cross section greater than 60mm2 shall be used. The grounding resistance of grounding electrode is less than 2Ω, and the grounding electrode should be buried at a distance of 10 ~ 15m from the building, and the grounding point of PLC system must be more than 10m from the grounding point of high-voltage 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 connector 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 multiple measuring point signals is connected with the main shielded cable of multi-core twisted pair, the shielding layers should be connected and insulated. Select the appropriate grounding point for single point contact.