Nowadays, there are many kinds of PLC used in industry. For the low-end PLC, the ladder diagram instruction is similar. For the middle and high-end machine, many programs are compiled in language. Useful ladder diagram must be annotated with Chinese symbols, otherwise it is very difficult to read. If you can understand the equipment technology or operation process before reading ladder diagram, it seems relatively simple. If we analyze the electrical faults, we usually use the reverse search method or the reverse deduction method, that is, according to the corresponding table of input and output, we can find the corresponding PLC output relay from the fault point, and start the reverse search to satisfy the logical relationship of its action. Experience shows that if one question is found, the problem can be eliminated. Since there are two or more problems with the equipment at the same time, there are not many.
5. PLC fault identification
Generally speaking, PLC is an extremely reliable equipment with low failure rate, but it can also be damaged due to external factors.
(1) A proximity switch with 220 V working power supply shares a 4-core cable with the two lead wires of its input PLC signal contact and the 220 V power line of the proximity switch. The proximity switch is damaged at one time. When the electrician replaced it, he mistakenly adjusted the zero line of the power supply and the common line of the input PLC, resulting in burning out three PLC input points during power transmission.
(2) The zero line row of the primary system power transformer was stopped due to corrosion, resulting in the plc220v power supply rising to 380V, which burned the power module at the bottom of the PLC, and the 380 / 220V isolation control transformer was added during the rectification.
(3) PLC output common terminal label 1L, 2L, etc., operation computer for acl1n indicates, + 24 V power supply for L + m indicates simple mistake for beginners or inexperienced. If l + m is taken as the 220 V power supply terminal by mistake, the plc24 V power supply will be burnt out in the moment of power transmission.
(4) According to the author's long experience, the probability of PLC, CPU and other hardware damage or software operation error is almost zero. If the PLC input point is not caused by the invasion of strong current, it will not be damaged. If the normally open point of PLC output relay is not the external load short circuit or unreasonable design, the load current exceeds the extra range, and the contact life is also very long. Therefore, we find the electrical fault point, the key point should be placed on the PLC peripheral electrical components, do not always doubt the PLC hardware or program, which is very important for the rapid repair of defective equipment, rapid recovery of production, so the author talked about the PLC control circuit electrical fault maintenance, the key point is not in the PLC itself, but the PLC control circuit in the peripheral electrical components.