PLC functional fault and its treatment

First, the PLC is caused by interference

PLC interference will affect the system signal, resulting in reduced system control accuracy, PLC internal data loss, machine malfunction, serious personal and equipment accidents may occur. It is necessary to take corresponding technical measures to enhance the anti-interference ability of the PLC system. Interference has internal and external interference. External interference in the field environment is random, independent of the system structure, and can only limit the source of interference for specific situations. Internal interference is related to the system structure. By carefully designing the system wiring or system filtering, the interference can be suppressed to the utmost. Anti-interference technical measures at the PLC production site are usually considered from the aspects of grounding protection, grounding, shielding and anti-noise. For the strong electric equipment of the power supply system, the metal components such as the outer casing, the cabinet, the cabinet frame, the base and the joystick must be protected from the ground; the internal circuit of the PLC includes the CPU, the memory and other interfaces connected to the digital ground, and the external circuit includes A/ D, D/A, etc. are connected to the analog ground, and the PLC floor is connected to the central grounding point with a short copper wire to prevent noise interference. When the PLC is not grounded, the mounting bracket of the PLC should be grounded to suppress electromagnetic interference. In the PLC system, the wires mainly include a PLC and a load power line, a digital signal line of an AC voltage, a DC digital signal line, and an analog signal line. According to the function of the wiring, the anti-interference measures are as follows:

(1) Wiring arrangement in the electrical cabinet: Only the shielded analog input signal line can be installed in the same cable trough as the digital signal line; the DC voltage line and the analog signal line cannot be installed in the same cable trough with the AC voltage line; The 220V power cord can be installed in the same cable slot as the signal cable; the inlet and outlet shields of the electrical cabinet must be grounded.

(2) Wiring arrangement outside the electrical cabinet: digital signal lines for DC and AC voltages and analog signal lines (for shielded cables) must be used with separate cables; signal cable and power cable are installed in the same cable tray. However, in order to improve the anti-noise performance, it is recommended to be spaced about 10cm.

(3) Shielding the shield of the PLC casing. Generally, the casing and the electrical cabinet are floated. An equal-position shielding plate is installed on the floor of the PLC casing. The protective grounding maintains a certain connection with the floor. When the copper wire is used, its cross-sectional area Not less than 10mm2 to form an equipotential shield, effectively eliminating electromagnetic interference. Cable shielding, generally for the simulator signal line carrying small signal (mV or μV), the end of the cable shielding body in the electrical cabinet should be connected to the shielding mother, and the shielding of the digital signal line should not exceed the shielding mother; The shield bus can be insulated. Connect the center point to the reference potential or ground; grounding the digital signal line at both ends eliminates high frequency interference.

(4) Anti-noise measures: metal shielding is applied to the part of the strong magnetic field (such as transformer), and fluorescent lighting is not used in the electric control cabinet. In addition, the PLC system control power supply should also adopt corresponding anti-interference measures. Because the power supply of the PLC control system is generally 220V mains, the transient process of the power grid is often happening. The inductive load with large power supply fluctuation or the switching of the thyristor device can easily cause voltage gap or glitch. If the power supply is directly through the PLC and I/O templates will cause adverse consequences. The PLC system power supply anti-interference method includes: using an isolation transformer; using a low-pass filter; applying a spectrum equalization method. The isolation transformer is commonly used, because the PLC, I / O module power supply is commonly used DC24V, need to be stepped down by the isolation transformer, and then rectified by the rectifier bridge.

Second, PLC periodically crashes

The characteristic of PLC periodic crash is that the PLC crashes for a certain time, the program is confusing, different fault displays appear, and everything is normal after restarting. On-site practice believes that long-term accumulation of dust is the most common cause of PLC periodic crashes, and the PLC rack slot interface should be cleaned regularly. When cleaning, you can use the compressed air or "skin tiger" to blow off the dust on the control panel, then use 95% alcohol to clean the slot and control panel plug. After cleaning, carefully assemble and restore the boot to normal operation.

Third, the PLC program is lost

The loss of the PLC program is usually caused by poor grounding or incorrect wiring, incorrect operation and interference.

(1) The PLC host and module must have good grounding. Usually, the main unit casing and the switch cabinet housing are connected to the ground. When there is poor grounding, multi-strand copper wire should be considered. Grounding directly from the main ground terminal ensures good grounding. . In addition, the I/O module 24V DC power supply negative terminal should be well grounded.

(2) The phase connection of the host power supply terminal must be connected correctly, otherwise the host will not start, and often the error or program loss occurs.