1. CPU Abnormality
When a CPU abnormality alarm occurs, check all devices connected to the internal bus of the CPU unit. The specific method is to replace the potentially faulty units one by one to identify the faulty unit and take appropriate action.
2. Memory Abnormality
When a memory abnormality alarm occurs, if it is a program memory issue, the fault will reoccur after reprogramming. This situation may be caused by noise interference leading to program changes; otherwise, the memory should be replaced.
3. Input/Output Unit Abnormality, Expansion Unit Abnormality
When such alarms occur, first check the insertion status of the input/output unit and expansion unit connectors, as well as the cable connection status. After determining which unit has failed, replace that unit.
4. Program Not Executing
Generally, checks can be performed according to the steps: Input — Program Execution — Output.
(1) Input checks can be done using input LED indicators or an input monitor made with a writer. When the input LED is off, it can be preliminarily determined that there is an external input system fault, and a multimeter should be used for further checks. If the output voltage is abnormal, it can be determined that there is an input unit fault. If the LED is on but the internal monitor shows no display, it can be considered a fault in the input unit, CPU unit, or expansion unit.
(2) Program execution checks can be done through the monitor on the writer. When the state of the ladder diagram contacts does not match the results, it indicates a program error (for example, double usage of internal relays) or a fault in the calculation part.
(3) Output checks can be recognized using output LED indicators. When the calculation result is correct but the output LED indicates otherwise, it can be considered a fault in the CPU unit or I/O interface unit. If the output LED is on but there is no output, it can be judged as a fault in the output unit or an external load system fault.
Additionally, due to different PLC models, the connection methods for I/O and LEDs vary (some are connected to the I/O unit interface, while others are connected to the I/O unit). Therefore, the fault range determined by the LED also differs.
5. Partial Program Not Executing
The inspection method is the same as the previous item.
However, if the input time for counters, step controllers, etc., is too short, a no-response fault may occur. In this case, the input time should be verified to ensure it is sufficiently large, checking can be done according to the relationship: input time < input unit’s maximum response time + calculation scan time multiplied by 2.
6. Short Power Outage, Program Content
May Also Disappear
(1) In this case, in addition to checking the battery, the following checks should also be performed.
(2) Check by repeatedly turning the PLC power on and off. To ensure the microprocessor starts correctly, the PLC has a reset point circuit and a program retention circuit when the power is disconnected. If these circuits fail, the program cannot be saved. Therefore, checking can be done by turning the power on and off.
(3) If battery abnormality alarms still occur after replacing the battery, it can be determined that it is caused by increased leakage current in the memory or external circuits.
(4) The power on and off is always synchronized with the machine system, so the noise influence generated by the machine system can be checked. This is because the power disconnection often occurs simultaneously with the machine system operation, and most of the time it is caused by strong noise from motors or windings.
7. PROM Cannot Operate
First, check whether the PROM is properly inserted, then determine if the chip needs to be replaced.
8. After Power Reset or Reconnection, Operation Stops
This fault can be considered caused by noise interference or poor internal contact in the PLC. The noise is generally caused by reduced capacitance of small capacitors on the circuit board or poor component performance. The contact issue can be checked by gently tapping the PLC body. Also, check the insertion status of cables and connectors.
9. Interference from Frequency Converter on PLC Analog Signal
In automation control systems, the use of frequency converters is becoming more widespread, and the issue of frequency converters interfering with PLC analog signals is also becoming prominent. Below is an example of frequency converter interference with PLC analog signals and a solution using signal isolation modules to overcome such interference.
Phenomenon Description: In Siemens PLC, the AO point emits a 4-20mA current control signal, outputting to a Siemens frequency converter, which cannot control the startup of the frequency converter.
Troubleshooting:
① Suspected analog output board issue, measured 4-20mA output signal with a multimeter, the signal is normal;
② Suspected that there is an issue with the control signal input terminal of the frequency converter, replaced it with another same model frequency converter, but the problem persists;
③ Used a handheld signal transmitter to generate a 4-20mA output signal, outputting a standard current signal to the frequency converter, which successfully started, thus ruling out faults in the analog output board and the frequency converter;
④ It is inferred that the interference signal from the frequency converter is transmitted to the analog channel;
⑤ To verify, a signal isolation module TA3012 was installed in the PLC analog 4-20mA output channel, connecting terminals 5 and 6 to the analog output module, and terminals 1 and 2 to the frequency converter, with terminals 3 and 4 connected to an external 24VDC power supply, and the frequency converter started normally;
⑥ Thus, it is concluded that the root cause of the problem is the interference from the frequency converter to the analog channel.
Many control engineers have encountered the issue of frequency converters interfering with PLC analog signals during system debugging, so I would like to share my debugging insights here.
In control systems where PLCs and frequency converters are used simultaneously, attention should be paid to the following points:
① The power supply for the PLC and the power supply for the power system (frequency converter power supply) should be separately configured, and the power supply for the PLC should use an isolation transformer; ② Power lines should be kept separate from signal lines, and signal lines should be shielded; ③ Regardless of whether it is analog signal input or output, signal isolation modules should be used for all analog channels; ④ Software filtering design should be implemented in the PLC program; ⑤ Signal ground and power ground should be designed separately; by ensuring these five points, the issue of frequency converter interference with PLC analog signals can be easily resolved.
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