PLC using experience skills
PLC input and output
A small PLC flexible control of a complex system, you can see is the upper and lower rows of staggered input and output relay terminals, the corresponding indicator and PLC number, like a dozens of feet of integrated circuits . Anyone who does not look at the schematic to troubleshoot a failed device can do nothing about it, and finding a fault can be extremely slow. In view of this situation, we draw a table based on the electrical schematic diagram, affixed to the device console or control cabinet, indicating each PLC input and output terminal number corresponding to the electrical symbol, the Chinese name, that is, similar to the integrated circuit tubes Functional description of the foot. With this input and output table, the electricians who are familiar with the device's ladder can understand the operation or can start overhaul. But for those who are not familiar with the operation process, do not look at the ladder of the electrician, you need to draw a table: PLC input and output logic menu. This table shows the logical correspondence between the input circuit (trigger element, associated element) and output circuit (actuator) during most of the operation. Practice has proved that if you can proficient use of input and output mapping table and input and output logic menu, troubleshooting electrical problems, without drawings, but also easily.
Input circuit repair
Judge a certain button, limit, line input circuit is good or bad, in the case of PLC power (preferably in non-operating state, to prevent equipment malfunction), press the button (or other input contacts), which When the corresponding PLC input terminal and the public terminal is shorted, the corresponding PLC input indicator lights, indicating that the button and the line is normal. Light is not bright, may be bad button, bad line or broken line.
Output circuit overhaul
For the PLC output point (here only refers to the relay output type), if the corresponding object of the action object does not light, when it is determined that the PLC is running, it means that the PLC input / output logic function of this action object is not satisfied, that is, the input Circuit failure, as mentioned above, check the input circuit. If the corresponding indicator light, but the corresponding implementation of components such as solenoid valve, contactor does not move, first check the solenoid valve control power and fuses, the easiest way to use the amount of electricity to the PLC output point of the common terminal . Pen does not light, may correspond to the fuse blown power failure. Electric pen light, indicating that the power is good, the corresponding solenoid valve, contactor, line failure. Excluding solenoid valves, contacts, lines and other failures, is still not normal, the use of a multimeter pen, one after the corresponding output of the common terminal, the other end of the corresponding contact with the PLC output point, then the solenoid valve is still not action, The output line is faulty. If this time solenoid valve action, then the problem at the PLC output point. As the electric pen sometimes false, available another method of analysis, with a multimeter voltage file PLC output point and the common voltage, voltage is zero or close to zero, indicating that the PLC output point is normal, the point of failure in the periphery. If the voltage is high, indicating that the contact resistance is too large, has been damaged. In addition, when the indicator light is not on, but the corresponding solenoid valve, contactor and other actions, which may be the output point due to overload or short circuit burn. At this time should be the output of the external wiring removed, and then use the multimeter resistance to the amount of resistance output point and the public end of resistance, if the resistance is small, indicating that the contact has been bad, if the resistance of infinity, indicating that this contact is good , It should be the corresponding output indicator has been broken.
Program logic inferred
There are a wide variety of PLCs that are commonly used in industry. For low-end PLCs, ladder instructions are more or less identical. For mid- to high-end machines such as S7-300, many programs are written in a language table. Practical ladder diagram must have Chinese symbols, or reading is very difficult to see before the ladder if you know about the process equipment or operation process, it seems relatively easy. If the electrical fault analysis, the general application of anti-law or anti-push method, that is, according to the input and output table, find the corresponding output relay from the point of failure, began to reverse the logic to meet its action. Experience has shown that the detection of a problem, the basic failure can be ruled out, because equipment occurs at the same time two and more than two failure point is not much.
PLC own fault judgment
In general, PLCs are extremely reliable devices with low failure rates, but can also cause PLC damage due to external causes.
A working power supply 220V proximity switch, the input signal contacts PLC and two wires close to the switch 220V power line share a 4-core cable, once the proximity switch is damaged, the electrician to replace the wrong line of the zero line The public line with the input of PLC wrong, resulting in burned when the power 3 PLC input points.
A system power transformer zero line due to corrosion and interruption, leading to access PLC220V power rose to 380V, burn out the bottom of the PLC power module, after the rectification increased 380 / 220V isolation control transformer.
Siemens S7-200 PLC output common target 1L, 2L, etc., the working computer for the AC L1 N said +24 V power supply for the L + M said that for beginners or inexperienced easy to make mistakes. If the wrong L + M as 220V power supply terminal, power transmission is about to burn PLC24V power supply.
PLC, CPU and other hardware damage or software running error probability is almost zero, PLC input point is not caused by strong intrusion, almost no damage, PLC output relay normally open, if not the external load is short circuit or the design is not reasonable , The load current exceeds the rated range, the contact life is also very long. Therefore, we find the electrical fault point, the focus should be placed on the external electrical components of the PLC, do not always suspect PLC hardware or program problems, which quickly repair the faulty equipment, rapid recovery of production is very important, so PLC control loop Electrical troubleshooting, the focus is not on the PLC itself, but the PLC control circuit in the peripheral electrical components.
