With the development of society, PLC programmable controller has been widely used in industrial production, but its maintenance and overhaul methods and skills, many engineers are not lawful, the following is to introduce the experience and skills of PLC use process.
1. PLC input and output
A small PLC flexibly controls a complex system, and all you can see is the upper and lower rows of staggered input and output relay terminals, corresponding indicators and PLC numbers, like an integrated circuit with dozens of legs. Anyone who doesn't look at the schematic diagram to troubleshoot faulty equipment will be helpless and will be particularly slow to find faults. In view of this situation, we draw a table according to the electrical schematic diagram, paste it on the console or control cabinet of the equipment, and indicate the electrical symbol corresponding to the number of each PLC input and output terminal, the Chinese name, that is, the functional description of each pin similar to the integrated circuit.
With this I/O table, it is time for an electrician who understands the operation process or is familiar with the ladder diagram of the equipment. But for those electricians who are not familiar with the operation process and do not know how to read the ladder diagram, they need to draw another table: PLC input and output logic function table. The table actually illustrates the logical correspondence between the input loops (trigger elements, associated components) and output loops (actuators) during most operations. Practice has proved that if you are proficient in using the input-output correspondence table and the input-output logic function table, you can troubleshoot electrical faults without drawings.
2. Input circuit overhaul
To judge the quality of an input circuit such as a button, limit, line, etc., you can press the button (or other input contact) when the PLC is energized (preferably in a non-running state to prevent the equipment from malfunctioning), then the corresponding PLC input point terminal and the common end are short-circuited, and the PLC input indicator light corresponding to the button is on, indicating that the button and the line are normal. If the light is not on, it may be twisted, the line contact is poor, or the wire is broken.
If further judgment, if the button is good, then use a watch pen of the multimeter, one end is connected to the common end of the PLC input terminal, and the other end is to touch the corresponding PLC input point (be careful with the above operation, do not touch the 220V or 110V input terminal). At this time, the indicator light is on, indicating that there is a fault in the line. If the indicator light is not on, it means that the PLC input point has been damaged (this situation is rare, generally caused by strong current intrusion).
3. Overhaul of output circuit
For the PLC output point (here only the relay output type), if the indicator light corresponding to the action object is not lit, after determining that the PLC is in the running state, then it means that the PLC input and output logic function of the action object is not satisfied, that is to say, the input circuit is faulty, according to the previous mention, check the input circuit. If the corresponding indicator light is on, but the corresponding actuator such as solenoid valve and contactor does not operate, first check the solenoid valve control power supply and fuse, and the easiest way is to use a pen to measure the common terminal of the corresponding PLC output point.
If the pen does not light up, it may be a power failure such as a blown fuse. The electric pen is bright, indicating that the power supply is good, and the corresponding solenoid valve, contactor, and line are faulty. After eliminating the fault of solenoid valve, contactor, line, etc., it is still abnormal, use a multimeter pen, one end is connected to the corresponding output common terminal, and the other end is in contact with the corresponding PLC output point, at this time, the solenoid valve is still not moving, indicating that the output line is faulty.
If the solenoid valve is in action at this point, then the problem is at the PLC output point. Because the electric pen sometimes falsely alarms, another method can be used to analyze, use the voltage of the multimeter voltage level PLC output point and the common end, the voltage is zero or close to zero, indicating that the PLC output point is normal, and the fault point is in the periphery. If the voltage is high, the contact resistance of this contact is too large and has been damaged.
In addition, when the indicator light is not lit, but the corresponding solenoid valve, contactor, etc. operates, this output point may be burned due to overload or short circuit. At this time, the external wiring of this output point should be removed, and then the resistance of the output point and the common end should be measured with the resistance of the multimeter resistance file, if the resistance is small, it means that this contact has been broken, if the resistance is infinite, it means that this contact is good, and the corresponding output indicator has been broken.
4. Program logic inference
Nowadays, there are many kinds of PLCs often used in the industry, for low-end PLCs, ladder instructions are similar, for high-end machines, such as S7-300, many programs are programmed in language tables. The practical ladder diagram must have Chinese symbol annotations, otherwise it is difficult to read, if you can roughly understand the equipment process or operation process before looking at the ladder diagram, it seems easier.
If the electrical fault analysis is carried out, the reverse check method or the reverse deduction method is generally applied, that is, according to the input and output corresponding tables, the output relay of the corresponding PLC is found from the fault point, and the logical relationship that satisfies its action is started to be reversed. Experience shows that when a problem is found, the fault can basically be eliminated, because there are not many points of failure of the equipment with two or more failures at the same time.
5. PLC self-fault judgment
In general, PLCs are extremely reliable devices with a low failure rate, but they can also cause PLC damage due to external causes.
(1) a working power supply is the proximity switch of 220V, and two leads of its input PLC signal contact share a 4-core cable with the power line of 220V of proximity switch, and this proximity switch is damaged once, and when the electrician replaces, mistakenly adjust the neutral line of the power supply and the common line of the input PLC, and cause the 3-way PLC input point to be burned out when the power is transmitted.
(2) The neutral line row of the primary system power transformer was interrupted due to corrosion, resulting in the power supply of the access PLC220V rising to 380V, burning out the power module at the bottom of the PLC, and adding a 380/220V isolation control transformer during the rectification.
(3) Siemens S7-200 PLC output common end mark 1L, 2L, etc., the working computer is AC L1 N means, +24V power supply is L+M means that it is easy to make mistakes for beginners or inexperienced people. If L+M is mistaken for a 220V power terminal, the PLC24V power supply will be burned out at the moment of power transmission.
The probability of hardware damage such as PLC and CPU or software operation error is almost zero, and the PLC input point will hardly be damaged if it is not caused by strong current intrusion, and the normally open point of the PLC output relay is not a short circuit or unreasonable design of the peripheral load, the load current exceeds the rated range, and the life of the contact is also very long.
Therefore, we find the electrical fault point, the focus should be on the peripheral electrical components of the PLC, do not always suspect that there is a problem with the PLC hardware or program, which is very important for the rapid repair of faulty equipment and rapid recovery of production, so the electrical fault repair of the PLC control loop, the focus is not on the PLC itself, but on the peripheral electrical components in the PLC control loop.
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