With the expansion of automation production scale of modern enterprises, PLC is more and more widely used in industrial automation control. PLC is specially designed for industrial control. Generally, it can be used directly in industrial environment without any special measures. But in the PLC control system, if the environment is too harsh or improper installation and use, it will reduce the reliability of the system. Among the common faults that cause PLC, they are mainly divided into two categories: functional faults and hardware faults, of which the hardware part faults account for more than 80%.
1. Failure of peripheral circuit components
Such failures often occur in the PLC after a certain period of time. If there is a component damage failure in the PLC control loop, the PLC control system will automatically stop working immediately. The input circuit is the port that the PLC accepts input signals such as switching and analog signals. The quality of its components, the wiring method and whether it is reliable are also important factors that affect the reliability of the control system. For the switching output, the PLC output has three forms: relay output, thyristor output, and transistor output. The specific choice of output should be determined according to the load requirements. Improper selection will cause the system reliability to be seriously reduced and cause the system to fail. normal work. In addition, the load capacity of the PLC's output terminals is limited. If the specified maximum limit is exceeded, an external relay or contactor must be connected. To work properly. The quality of external relays, contactors, solenoid valves and other actuators is an important factor affecting system reliability. Common faults include short-circuiting of the coil and mechanical faults that cause the contacts to be immobile or have poor contact.
2. Poor contact of terminal wiring
This type of fault occurs with the increase in the frequency of equipment actions after the PLC works for a certain period of time. Due to wiring defects in the control cabinet or increased vibration during use and mechanical life, the terminals or component terminals are prone to loosening and cause poor contact. The troubleshooting method for this type of fault is to use a multimeter, and use the control system schematic diagram or PLC logic ladder diagram for fault diagnosis and maintenance. For the connection of some more important peripheral terminal blocks, in order to ensure reliable connection, the method of welding cold-pressed pieces or cold-pressed pins is generally used.
3. The PLC has a malfunction that is not working properly
3.1 Power and ground protection
The anti-interference ability of PLC itself is generally very strong. Usually the power supply of the PLC and the power equipment of the system are separately wired to interfere with the power line, and generally have a strong enough suppression ability. However, the power supply interference is particularly serious. An isolation transformer with a shielding layer can be added to reduce the interference between the equipment and the ground and improve the reliability of the system. If a system contains an expansion unit, its power supply must share the same switch control as the basic unit, which means that they must be powered on and off at the same time. In order to suppress the interference added to the power supply, input end, and output end, the dedicated ground wire should be connected to the PLC. The ground wire diameter should be thick enough, the ground resistance should be less than 4Q, the ground point should be as close as possible to the PLC, and the ground point should be separated from other equipment . For the strong electrical equipment in the power supply system, the metal components such as the shell, cabinet, frame, machine base and operating handle must be protected and grounded. The PLC internal circuit includes CPU, memory and other interfaces connected to digital ground. The external circuit includes A / D, D / A, etc. to connect analog ground and connect the PLC backplane to the central ground point in a star connection with thick and short copper wire to prevent indolent noise interference . When the PLC is not grounded, the PLC mounting bracket should be capacitively grounded to suppress electromagnetic interference.
3.2 Wiring arrangement
(1) Layout of wiring in electrical cabinet. Only shielded analog input signal wires can be installed in the same wire slot as digital signal wires. DC voltage digital signal wires and analog signal wires cannot be in the same wire slot as AC voltage wires. Only the shielded 220V power cable can be installed in the same cable slot as the signal cable. The shield of the cable plug of the electrical cabinet must be reliably grounded.
(2) Arrangement of external wiring of electrical cabinet. The digital signal lines and analog signal lines of DC and AC voltages must use separate cables and shielded cables. The signal cable can be installed together with the power cable in a wire slot. To improve the noise resistance, it is recommended to ensure that the interval is more than 250px.
3.3 Shielding
The shielding of the PLC housing should generally be guaranteed to float with the electrical cabinet. Install an equivalent shielding plate (generally galvanized plate) on the bottom plate of the PLC shell. Protectively use copper wires to maintain a little connection with the bottom plate. Its cross-sectional area should be no less than 10 mm to form an equivalent shielding body, effectively eliminating Interference from external electromagnetic fields. The shielded bus for analog signals can be insulated and connect the central point to the reference potential or ground (GND). The two ends of the cable of the digital signal line can be grounded to ensure that high-frequency interference is better eliminated.
3.4 Anti-noise measures
4. The PLC periodically crashes
The characteristic of the PLC periodic crash is that the PLC crashes or the program is chaotic every time it runs for a certain time, or a different interruption fault display appears, and everything is normal after restart. According to practical experience, the most common cause of this phenomenon is due to the long-term accumulation of dust in the PLC body. Therefore, the slot interface of the PLC rack should be purged regularly. When blowing, you can use compressed air or a soft brush to blow away the dust on the control board and each slot, and then wipe the slot and the control board plug with 95% alcohol. After cleaning, carefully set 3.3 PLC program without reason. PLC program loss is usually caused by several reasons such as poor grounding, wiring errors, operation errors and interference: (1) The PLC host and module must have a good grounding. (2) The phase and neutral wires of the power cord of the host must be wired correctly. (3) Prepare the package in advance for backup. (4) When using the handheld programmer to find the fault, the lock switch should be placed in the vertical position. Pull out to protect the memory. (5) The PLC program is lost due to interference. For the treatment method, please refer to the treatment of failure caused by PLC interference.
When the PLC malfunctions, as long as it is judged according to the general fault law, the fault should be eliminated accurately and quickly. Normal operation can be resumed after power on.
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