What are the common faults of PLC?

After the PLC power is turned on, the POWER indicator is on, indicating that the power is normal. If the POWER indicator is off, it indicates that the power supply is partially faulty. You should check whether the power cable is disconnected, and whether the terminal is in poor contact.
2. When the system is in the running or monitoring state, the PUN light on the host does not light, which means that the basic unit is faulty.
3. When an input contact is turned on, the corresponding LED does not light, indicating that the input module is faulty. When the output LED is on and the corresponding output contact is not activated, the output block is faulty.
4. When the BATTERY light is on, it indicates that the lithium battery voltage is not enough, and the battery should be replaced in time.
5. When the ERROR light flashes, there may be a degree of syntax error, PLC internal fault, external interference, insufficient lithium battery voltage, and so on.
6. When the ERROR light is on, the PLC has malfunctioned. The watchdog timer returns the CPU to normal operation. This condition may be caused by interference or internal failure.
PLC is composed of semiconductor devices. The aging phenomenon after long-term use is unavoidable. Therefore, PLC should be checked and maintained regularly. Maintenance time is usually 1-2 times a year. If working in harsh environments, the frequency of inspection and maintenance should be increased according to the actual situation. In the ordinary times, dry rags can be used to dust the surface of the PLC and between the conductors. Keep the work environment clean and hygienic. The main items of overhaul are the following:
1. Overhaul power supply. It is possible to detect whether the voltage variation range is within the allowable ±10% at the power supply terminal.
2. Work environment. Focus on checking whether the temperature, humidity, vibration, dust, and interference meet the standard working environment.
3. Input, output power supply. The voltage change range can be measured at the corresponding terminals to meet the specifications.
4. Check the installation status. Check whether the cables connected between each module and the module and the cables between the modules are loose and whether the screws of the external accessories are loose. Whether the first year of aging and so on.
5. Check whether the post-service battery voltage meets the standards, whether the metal parts are rusted or not.
In the course of inspection and maintenance, if any conditions are found that do not meet the requirements, they should be adjusted, corrected and recorded for future reference.
The common failures of PLC can be roughly divided into several categories. According to the classification, it can help you analyze the location of the fault and the cause:
1. External device failure. Such failures come from external devices such as various sensors, switches, actuators, and loads. Failure of this part of the equipment will directly affect the control function of the system. This type of failure is generally caused by the quality and life of the device itself.
2 system failure. This is a global failure that affects the operation of the system. System failure can be divided into fixed failure I accidental failure. If a fault occurs, it can be restarted to return the system to normal, and it is the same as the computer system. It can be considered as an accidental failure. Conversely, if the system cannot be properly re-started without restarting and the hardware or software needs to be replaced, it can be regarded as a fixed failure. This kind of failure is usually caused by the system design being incorrect or the system running for a long time.
3. Hardware failure. This kind of failure mainly refers to the failure of the modules in the system, such as CPU, memory, power supply, I/O modules (especially I/O modules). This kind of failure is generally better than
Obviously, and the influence is also accompaniment. They are mainly caused by improper use or long use time and aging of components in the module.
4. Software failure. This type of failure is caused by errors in the software itself. Mainly due to poor software design. If it is not fulfilled, it will be triggered once the condition is satisfied. In practical engineering applications, due to the complexity of software work and the large workload, software errors are almost impossible to avoid, which raises the issue of software reliability.
If a PLC control system fails during operation, as long as the CPU of the CPU can run, it can use its own self-diagnosis function to determine the fault. PLC self-diagnosis has many fault diagnosis and adjustment functions. However, due to different PLC manufacturers and models, the self-diagnosis function is not very consistent. What specific self-diagnosis function of a certain type of PLC product is in the end, we need to check the corresponding manual, a PLC system, can provide the following aspects Self-diagnosis function:
1. Use the system status word and control to determine the fault. We can usually use programming software to read the contents of the system's status word or control word directly on the screen. According to the status word, it can be seen whether the hardware and software arrangements are reasonable. Is the I/O address, expansion rack, etc. in use or has failed. It is also possible to analyze an address number based on the readout. Rack, slot, or even an I/O point has failed. With the increase of CPU performance, the more status words the PLC can provide, the stronger the self-diagnosis capability.
2. Use the PLC's interrupt or stack to determine the fault. Interrupts and stacks are specific data storage areas in PLC operation. They are formed automatically by system self-diagnosis software and can be displayed. We can also judge the fault by adjusting the content.
3. Use the PLC programmer to diagnose the fault. The PLC programmer provides some of the adjustment and diagnostic functions and has a memory comparison. System parameter modification, program comparison, program verification, operation status testing. Input status testing and display, output status force and anti-true features. Using these functions, we can find errors and malfunctions during program tuning or trial run.