In recent years, with the development of society, plc in industrial production has been widely used, but its maintenance and repair methods and techniques, many engineers are not law, this article describes in detail the experience of PLC use process and skills for reference.
1, PLC input and output
A small PLC to control a flexible system, you can see is the upper and lower row of staggered input and output relay terminals, the corresponding indicator and PLC number, like a few feet of the integrated circuit The Any one if you do not look at the schematic to repair the faulty device, will be helpless, find the speed of the fault will be particularly slow. In view of this situation, we draw a form according to the electrical schematic diagram, affixed to the console or control cabinet equipment, marked with each PLC input and output terminal number corresponding to the electrical symbols, the Chinese name, that is similar to the integrated circuit Foot function description. With this input and output form, for understanding the operation process or familiar with the equipment ladder diagram of the electrician can start maintenance. But for those who are not familiar with the operation process, do not look at the ladder diagram of the electrician, you need to draw a form: PLC input and output logic function table. This table actually describes the logical correspondence between the input loop (trigger element, associated element) and the output loop (actuator) during most operations. Practice has proved that if you can skillfully use the input and output correspondence table and input and output logic function table, maintenance of electrical failure, without drawings, but also easy.
2, input circuit maintenance
To determine the quality of a push button, limit, line input circuit can be in the case of PLC power (preferably in the non-operating state, to prevent equipment malfunction), press the button (or other input contacts), this When the corresponding PLC input terminal and the common terminal is shorted, the corresponding button corresponding to the PLC input light, indicating that the button and the line is normal. The lamp does not light, it may be bad, bad line or broken. If further judgment, if the button is good, then use a multimeter of a table pen, one end of the PLC input side of the common, the other end of the corresponding PLC input point (the above operation to be careful, do not touch 22 0V or 110V input terminal). At this point the indicator light, indicating that the line is faulty. The indicator does not light, indicating that the PLC input point has been damaged (this situation is rare, the general strong intrusion caused by).
3, the output loop maintenance
For the PLC output point (here only talk about the relay output type), if the action object corresponding to the indicator light does not shine, in determining the PL C in the running state, then this action object PLC input and output logic function is not satisfied, that is to say Input circuit failure, as mentioned earlier, check the input circuit. If the corresponding indicator light, but the corresponding actuators such as solenoid valve, contactor does not move, first check the solenoid valve control power and the fuse, the easiest way to use the pen to the corresponding PLC output point of the common terminal The The pen does not light, may correspond to the fuse fuse and other power failure. Electric pen light, indicating that the power is good, the corresponding solenoid valve, contactor, line failure. Excluding the solenoid valve, contactors, lines and other failures, is still not normal, the use of a table with a test pen, one by one corresponding to the output of the common terminal, the other end of the corresponding PLC output point, then the solenoid valve is still not action, Description The output line is faulty. If the solenoid valve moves, then the problem is at the PLC output point. As the pen sometimes false, can be used to analyze another method, with a multimeter voltage range PLC output point and the common voltage, the voltage is zero or near zero, indicating that the PLC output point is normal, the fault point in the periphery. If the voltage is high, indicating that the contact contact resistance is too large, has been damaged. In addition, when the indicator light does not shine, but the corresponding solenoid valve, contactor and other actions, which may be the output point due to overload or short circuit burned. Then the output of the external wiring should be removed, and then use the multimeter resistance to the output point of the output point and the common resistance, if the resistance is small, indicating that the contact has been bad, if the resistance is infinite, indicating that the contact is good , Should be the corresponding output indicator is bad.
4, the program logic inference
Now the industry often use a wide range of PLC, for low-end PLC, the ladder instructions similar, for high-end machines, such as S7-300, many programs are compiled with language tables. Practical ladder diagram must have a Chinese symbol annotation, otherwise it is difficult to read, if the ladder before the equipment can understand the process or operation process, it seems easier. If the electrical fault analysis, the general application of anti-check method or anti-push method, that is, according to the input and output corresponding table, from the point of failure to find the corresponding PL C output relay, began to check the logic to meet their actions. Experience shows that found a problem, the basic failure can be ruled out, because the equipment at the same time two or more than two points of failure is not much.

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