In the factory, the main function of PLC is to receive the field signal. After centralized processing, the field equipment is controlled according to the operation logic. The greatest advantage of PLC is to use 24V to control 380V. Therefore, most of the signals accepted by PLC are weak electrical signals, including digital and analog signals.
The factory site environment is generally very complex, there are weak electricity, strong electricity, inverter, servo drive, in short, the electromagnetic environment is very complex, the purity of the grid is very low. This makes the anti-jamming capability of PLC greatly challenged. But PLC itself is also an electronic equipment, although it has the ability to deal with interference, but only rely on the anti-interference of PLC itself is not enough, we need to use some external measures to improve the anti-interference ability of PLC, making the control more perfect.
One: the use of PLC anti-interference capability - filtering, what is the filtering of PLC?
Filter time can be selected according to the site conditions.
PLC filter can deal with the interference signal is too fast change frequency signal, some PLC itself has this function, only need to change from the settings, this function is easy to understand, that is, PLC will filter out the small signal less than the set time, such as I 0.0 filter time set to 12 ms, then less than this function. Time signal, PLC will not accept it, that is, invalid, there are some advanced PLC filter control algorithm, but the general function is similar.
According to this principle, this function has a big drawback, that is, there is a delay, that is, poor timeliness.
For less important signals, a little delay is irrelevant, but for positioning sensors, encoders or weighing sensors that require precise weighing is a fatal flaw, so to deal with these situations can not use PLC filtering, can only rely on other measures.
Second, the most widely used, the lowest cost and the most effective measures to eliminate interference, we should all know that is shielding. How many kinds of shielding are there? How should I answer it?
There are mainly two kinds of shielding used in the field. The first kind of shielding is to use metal grooves or other similar sealed metal boxes to separate and wrap the strong and weak electric cables, so as to reduce the strong electromagnetic radiation and noise interference from the source.
The second kind is the shield of the cable itself. The shielded cable is mainly used for the connection between the instrument or sensor of precision measurement and PLC input. It can eliminate the static electricity signal and export the useless electromagnetic signal from the shielding layer to ensure the accuracy of measurement. This is the last layer of protection of the external cable layer, so it is very important. We generally say that the shielding of cables requires a technical term "single ended grounding". How do I pick up the single end?
The general factory has PLC dedicated ground wire, which is only used for weak current system. Single-end grounding, we usually connect the shielding layer to the shielding ground wire in the electrical cabinet, the scene that we just suspended. What happens if the field is not suspended or grounded? That's the end of the scene because of electromagnetic interference, there is likely to be a potential difference relative to the PLC ground wire, which sometimes makes the signal interference more serious. So for the PLC control system, we will shield layer single-ended grounding, and connected to the ground used in PLC, resolutely eliminate double-ended grounding.
Ground wire in electrical cabinet
Three: electrical isolation. Electrical isolation is also divided into two types, one is power isolation, the other is signal isolation. If it is suspected that the purity of the power supply itself is not high (for example, close to the inverter and servo control power), then we can use an isolation transformer to make the power supply more pure. Signal isolation is the use of signal isolator transmitter or instrument signals, modulation and transformation through electronic devices, and isolation of the conversion to the desired signal. This is also a very effective way to eliminate interference.
Fourth: The above is a common factory interference elimination measures, but in addition to this we have some small techniques can also be used to eliminate interference;
1 Cables intertwined, such as what we call shielded twisted pair, as well as the network we see are connected in this way, which can maximize the elimination of the signal cable itself magnetic field, for weak signal transmission has a great advantage.
2. The signal cable is wound on the side of the magnetic ring in the form of a transformer coil by using a magnetic ring. This method can also effectively reduce the interference and convert the interference magnetic field into heat energy.
Communication isolation module and other protective devices, and some PLC 232,485 communication is also interfered with, then we use communication isolation module, which is similar to the signal isolator, using electronic devices to convert the signal. For the complex factory status quo, we must reasonably analyze the possible interference reasons, find out the interference sources, and give a reasonable solution。