Deal with PLC work environment interference measures

This paper discuss the three kinds of PLC controller anti-jamming technology. 2 anti-jamming analysis of the technical countermeasures to prevent interference, can use the hardware and software anti-interference measures, among them, the hardware anti-jamming is one of the most basic and most important anti-jamming measures, from the two aspects of the resistance and the general to suppress and eliminate the interference sources, to cut off the interference on the system of the coupling channels, reduce the sensitivity of the system of the interference signal. 2. 1 the interference of power supply system is introduced into the grid disturbance, the frequency fluctuations, will directly affect the stability and reliability of PLC controller system. How to suppress the interference of power supply system is to improve the main steps of PLC anti-jamming performance. ( 1) Equipped with filtering, isolation, shielding, a switching power supply system. Set the filter is used to suppress interference signals from the power cord conduction into the system, using isolation transformer, must pay attention to: shielding layer should be good grounding; Secondary cable to use double winding ( Reduce the interference between the wire) , isolation transformer primary winding and secondary winding should be shielded layer respectively, junior shield connect ac power grid of the zero line; Shield of secondary and primary shielding layer between the dc side. In order to suppress the large capacity power grid equipment start-stop ( Such as send water pumps, etc. ) Cause power grid voltage fluctuations, keep the power supply voltage regulator, can use the first regulated power supply. ( 2) Separation of power supply system of PLC controller and I/O system shall be provided by the respective isolation transformer power supply, and is separated from the main power supply, so when the input power supply power output, will not affect the controller of power supply. As shown in figure 1. Figure 1 separate power supply system in figure 2. 2 interference repress grounding system introduced PLC system is divided into logic circuit grounding and power circuit, with a total of floating, floating and casing and circuit and so on three ways. Generally USES the controller and other equipment grounding method is best, when grounding note: grounding line as far as possible, generally greater than 2 was grounding line; Ground should be close to the controller, the distance between the ground and controller is not more than 50 m; Earth wire should be as far as possible to avoid high voltage circuit and main circuit of the wire, can not avoid, should be vertical intersection, should try to shorten the length of the parallel to walk. Practice proved that the earth is often important means to suppress noise and interference, good grounding method can suppress internal noise coupled to a great extent, the invasion of the external disturbance and improve the anti-interference ability of the system. 2. 3 the introduction of input and output circuit interference suppression in order to realize the input and output circuit is completely isolated, in recent years is widely applied in the control system of photoelectric coupling, has become one of the most effective measures to prevent interference. Photoelectric coupler has the following characteristics: first, because it is sealed in a tube and shell inside, not influenced by the disturbance of external light; Second, since by optical transmission signal, cut off the circuit ground connection between various components; Third, light-emitting diodes (leds) dynamic resistance is very small, and the interference sources of resistance is very big, usually capable of transmitting to the interference of photoelectric coupler input and output signal becomes very small; Fourth, photoelectric coupler compared to the magnification of the transmission ratio and the transistor, generally small, far less so sensitive to disturbance signal transistor, and photoelectric coupler leds light. Only when the current through a certain Therefore, even in the case of interference voltage amplitude is higher, because there is no enough energy, still cannot make light-emitting diodes (leds, which can effectively suppress interference signals. As a result of the photoelectric coupler linear area normally only in a certain range, therefore, should guarantee by the change of the signal range is always in the linear zone. In order to guarantee the linear coupling, both must strictly selected photoelectric coupler, and the corresponding nonlinear calibration measures, otherwise will produce a large error. ( 1) Photoelectric coupling input circuit is shown in figure 2. The figure 2 ( a) , figure 2 ( b) Use more, the high electricity at ordinary times into form, low level input into the form. Figure 2 ( c) For differential connection, it has two constraints, has obvious superiority to prevent disturbance, serious environment is suitable for the external interference, when the external device current is larger, the transmission distance of up to 100 ~ 200 m, figure 2 ( d) Considering the COMS circuit output drive current is small, can't directly drive the light emitting diode, so added level as power amplifier transistor, light-emitting diodes (leds) need to be aware of is the picture and the photosensitive triode should respectively by two power supply, resistance depending on the selected voltage levels. Figure 2 photoelectric coupling input circuit ( 2) Photoelectric coupling output circuit is shown in figure 3. In order to get and the input signals in phase (figure 3) can be used to a) Form. If you want output and input reverse phase, you can connect the figure 3 ( b) Form. When driven by output circuit element is large, can be added to meet level 1 transistor as the driving power amplifier, the connection as shown in figure 3 ( c) Shown below. Sometimes in order to get a better output waveform, the output signal by schmidt circuit plastic. Figure 3 photoelectric coupling output circuit are two processing method of the switch input and output signals, and to the analog input and output signals, in order to eliminate the influence of industrial field transient interference on it, in addition to add A/D, D/A conversion circuit and photoelectric coupling, can according to need to take software digital filter technology, such as median method, first order recursive digital filtering method is presented. 3 conclusion anti-interference design of the PLC control system is a complicated system engineering, involving specific input and output devices and industrial field environment, in the design of anti-jamming system requirements to comprehensively consider all aspects of the factors.
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