PLC is a small industrial computer. If there is a problem with it, first of all, it is necessary to eliminate whether it is the PLC itself or the peripheral problem. If there is a problem with the PLC itself, the err light will often light up, or the red light will flash. Normally, the run operation green light is on. If there is a problem with the PLC itself, the probability of successful repair is not high.
Some PLCs keep data through the battery inside. When the battery voltage is lower than a certain threshold value, the battery alarm light will be on. At this time, the battery needs to be replaced and it needs to be charged. If the battery is completely dead or there is no charged operation when the battery is replaced, the RAM data will often be lost. At this time, the program and data need to be refreshed. Therefore, We should have the habit of program and data backup, otherwise we will not have it at the critical time, and only have to reprogram and debug.
Power failure also accounts for a certain proportion of the body failure. The PLC input is usually 220 AC, and some things are input by 24 V or 12 v. however, there is a chip inside, which requires 5VDC or 3.3vdc. Therefore, there is a switching power supply step-down circuit. This circuit is prone to problems in some harsh high temperature or dust situations because of large current and high temperature. If the PLC indicator lights are not on, the switching power supply is generally broken. The switching power supply is broken. It is not particularly difficult for people with some level of electronic maintenance. For example, the control chip 384x or the switch tube has a high probability of being broken. It is generally good to replace it.
If the red light flashes suddenly or the err light is on, it is often caused by the main board is broken or the program is lost. You can try to re instill the program. If not, there is not much maintenance value. The most you can do is to replace the watchdog IC and reset circuit of the chip, or replace the crystal oscillator of the chip. If you still can't solve the problem, you are advised to give up. The integration of this device is too high and it is very difficult to maintain, And the price of the motherboard is not particularly expensive, not much value.
The output point is broken, which is also a common PLC body fault. In particular, the PLC with relay output type, it is easy to hang up the relay on the occasion of frequent switching action. The service life of the relay is about 100000 times, and it may be broken after 3-5 years. This problem is also easy to find. After all, the output indicator light is on, but the problem can be found when no contact is closed or disconnected by measuring with a multimeter, Find some similar small relays made in China to replace them. Generally, there is no problem. If the transistor is broken, it is common that the input is connected to a high voltage, and the input and output may be damaged. It is often necessary to replace the optocoupler and some amplifying transistor circuits.
If it is a peripheral problem, the logic of input and output can be analyzed according to the circuit diagram, from the macro perspective and the operation of the equipment, or some input and output interlocking points can be shielded to separate test and find problems. If there is a PLC program, it is simpler. Because PLC has online diagnosis function, it can be executed separately. If you see where the output point is normal, you can find the fault point.
The most difficult problem is still some system problems, especially the circuit problems repaired by some people. One year, there was a problem with the large pendulum of Chimelong. Their equipment department had the PLC program and the upper industrial control computer program. The external circuit drawings were complete, and various states could be diagnosed online. However, when the large pendulum moved to a certain logic point, it automatically tripped, and all power was cut off without any alarm and prompt. They tossed about for a long time and couldn't find the problem. They also found many people to look at it, but they couldn't find the fault point. Later, I went to help them look at it. It seemed that there was nothing special in the control room, because other people had checked it. I climbed to a height of more than 10 meters. There was a small controller on the site, and there were some relays inside. Because I read their drawings and the working logic on the site repeatedly, I judged that the problem should be in the box. I asked them to start the machine for several times, and each time they reached a relay position, they tripped the main switch. However, this relay is an ordinary DC relay.
I tried to remove the coil wire of the relay and let them operate it again. As a result, the equipment stopped after running for a certain time, but it did not trip. I judged that there was a problem with the relay. After removing it, I found that the direction of the free wheeling diode of the relay was reversed. In this way, the coil of the relay was charged, which was equivalent to 12 V DC power supply, It is directly added to the freewheeling diode to form forward conduction. The 12V power supply is short circuited, so the power supply trips and causes the system to trip. It is good to invert the diode. It is estimated that it was caused by the wrong connection when someone inspected and replaced the site.
Therefore, there is no alarm prompt for this type of PLC peripheral fault. Everything seems normal and nothing is broken. However, the system is not operating normally. This fault is the most difficult to eliminate because many people have inspected and repaired it. If you only look at the drawings and procedures, you cannot find the problem. You must go to the site to grasp the details.
