As a new generation of industrial controllers, PLC is widely used in electric power, machinery, textile, electronics, transportation, petrochemical and other industries due to its versatility, practicability, complete hardware, and easy programming. In the control system. PLC is specially designed for industrial control. It adopts multi-level anti-interference measures in the design and manufacturing process, so that the system can work with strong electric equipment in harsh industrial environment, and the stability and reliability of operation are very high. The average time between failures of the PLC machine can reach hundreds of thousands of hours.
With the development of related technologies, the functions of PLCs are becoming stronger and stronger, and the use is more and more convenient. However, the high reliability of the whole machine is only the premise to ensure the reliable operation of the system. In the process of designing and installing the PLC system, corresponding measures must be taken to ensure the reliable operation of the system. If the working environment of the PLC is too harsh, such as excessive temperature, excessive humidity, excessive vibration and shock, and serious electromagnetic interference or improper installation, it will directly affect the normal, safe and reliable operation of the PLC. If the anti-interference measures of the peripheral circuits are improper, the reliability of the entire control system is greatly reduced. Therefore, the system design should be fully considered, the appropriate configuration on the hardware, and the corresponding software to achieve system failure prevention. The reliability of the PLC control system directly affects the safe production and economic operation of the enterprise. The anti-interference ability of the system is the key to the reliable operation of the whole system. Therefore, it is necessary to analyze and study the reliability and anti-interference technology in PLC applications. To improve the reliability of the PLC control system, it is necessary to take measures on the hardware and design corresponding protection programs on the software.
1. Interference sources in the PLC control system
The interference sources of PLC systems are classified into internal interference sources and external interference sources according to their sources, and generally include the following aspects.
(1) Interference from inside the electrical control cabinet equipment
1 The interference from the PLC system is mainly generated by the electromagnetic radiation between the internal components of the PLC system and the circuit. For example, the mutual radiation of the logic circuit and its influence on the analog circuit, the digital ground, the analog ground and the systematic improper handling and mutual influence, And the mismatch between components and so on. This is the content of the PLC manufacturer's electromagnetic compatibility design inside the system, which cannot be changed as a user.
2 Electrically controlled cabinets use devices such as high-power inverters and AC contactors that are prone to interference. Such disturbances include oscillations or waveform distortion caused by improper selection of circuit parameters and operating points, rapidly rising pulse sources, and impedance mismatch during signal transmission, physical noise of the device (such as component thermal noise, contact thermoelectric potential, etc.) .
3 Internal signals are crosstalked due to unreasonable component layout. Parasitic oscillations caused by capacitive components present in the line and interference due to irrational circuit design and electrical design of the system.
(2) Interference from the outside of the electrical control cabinet
1 interference from the power supply. Since the normal power supply of the PLC system is powered by the power grid, there are many situations in which the PLC control system fails due to the interference introduced by the power supply, such as the influence of high-voltage circuit breakers, isolation switches, large-capacity transformers, etc., large-scale power equipment start-stop and AC-DC transmission. Harmonics caused by the device, short-circuit faults of various electrical equipment (motors, air switches, etc.), electric welders and power systems are transmitted to the primary side of the power supply through the transmission line. PLC power supplies usually use isolated power supplies, but the isolation process is not ideal due to factors such as manufacturing processes. Absolute isolation is not possible due to the presence of distributed capacitance.
2 interference introduced from the signal line. In addition to transmitting various types of effective information, various types of signal lines connected to the PLC control system are also subject to interference from space electromagnetic radiation, that is, external inductive interference on the signal line. This type of interference signal can cause the PLC's I/O signal to work abnormally.
3 Interference from the grounding system. Electrostatic coupling or electromagnetic coupling invaded by the ground can cause interference to the system. In the PLC control system, due to the uneven distribution of potential at each grounding point, there is a ground potential difference between different grounding points, causing ground loop current to form common mode noise, which affects the normal operation of the system. In addition, the shielding layer, the grounding wire and the earth may constitute a closed loop. Under the action of the changing magnetic field, an induced current may appear in the shielding layer, and the signal loop is interfered by the coupling between the shielding layer and the core wire. If the system ground and other grounding treatments are confusing, the generated ground circulation may generate unequal potential distribution on the ground line, affecting the normal operation of the logic circuit and the analog circuit in the PLC. Proper grounding can both suppress the effects of electromagnetic interference and suppress interference from the device. The wrong grounding will not only introduce interference signals, but the grounding wire itself will also radiate noise to the antenna, which will interfere with the normal operation of the PLC control system.
4 Buttons, relays, etc., arcs generated between contacts, spark discharge caused by static electricity, external high-frequency heaters, high-frequency quenching equipment, and disturbances caused by messy radio waves.
(3) Other interference
1 Overvoltage and overcurrent caused by lightning strikes.
2 Change in contact resistance caused by temperature change.
3 mechanical vibration.
2. Interference pathway
The main ways in which PLC control systems are subject to interference are power lines, input/output lines, and space propagation. After the power supply is disturbed, the power quality of the PLC control system deteriorates, which may cause PLC control failure. Input/output control disturbances occur after the input/output lines are disturbed. Airborne interference mainly causes the PLC's CPU to malfunction due to electromagnetic induction and electrostatic induction.
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