PLC is a small industrial computer. If there is a problem with the PLC, first of all, it should be ruled out whether it is the PLC itself or the peripheral equipment. If there is a problem with the PLC itself, the err light will often light up or the red light will flash. Under normal conditions, the green light for operation is on. If the PLC itself has problems, the probability of successful repair is not high.

Some plc stores data through internal battery. When the battery voltage is lower than a certain threshold, the battery warning light will be on. At this time, the battery needs to be replaced and charged. If the battery is completely dead or there is no charging operation when replacing the battery, RAM data will often be lost. At this time, you need to refresh the program and data. Therefore, we should form the habit of program and data backup, otherwise we will have to reprogram and debug at the critical moment.

Power failure also accounts for a certain proportion of the body failure. The input of PLC is usually 220 AC, and the input of some things is 24V or 12vl, but there is a chip inside, which requires 5VDC or 3.3vdc. Therefore, there is a switching power supply step-down circuit. The circuit is prone to problems under some harsh high temperature or dust conditions due to high current and high temperature. If the PLC indicator light is not on, the switch power supply is generally broken. The switch power supply is damaged. This is not particularly difficult for people with a certain level of electronic maintenance. For example, the control chip 384x or switch tube has a high probability of damage. Generally speaking, just replace it

If the red light suddenly blinks or the err light is on, the main board is usually broken or the program is lost. You can try to inculcate the program again. If not, there is not much maintenance value. The most you can do is replace the watchdog IC and reset circuit of the chip, or replace the crystal oscillator of the chip. If you still can’t solve this problem, I suggest you give up. The integration of this device is too high, and it is very difficult to maintain. Moreover, the price of the motherboard is not particularly expensive, and there is little value.

The output point is broken, which is also a common PLC body fault. In particular, the PLC with relay output type is easy to hang the relay in the case of frequent switch action. The service life of the relay is about 100000 times, and it may be damaged after 3-5 years of use. This problem is also easy to find. After all, the output indicator light is on, but when measured with a multimeter, the problem can be found when no contact is closed or opened. Find some similar small relays made in China to replace them. Generally speaking, there is no problem. If the transistor is broken, usually the input terminal is connected with high voltage, and the input terminal and output terminal may be damaged. It is often necessary to replace the optical coupler and some amplifying transistor circuits.

If it is a peripheral problem, the logic of input and output can be analyzed from the macro perspective and operation according to the circuit diagram, or some input and output interlocking points can be shielded, tested separately and found problems. If there is a PLC program, it is much simpler. Because PLC has online diagnosis function, it can be executed separately. If you see that 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 Chimelong clock. The equipment department has PLC program and upper industrial control computer program. External circuit drawings are complete, and various states can be diagnosed online. However, when the pendulum moves to a certain logical point, the pendulum automatically trips and cuts off all power supplies without any alarm and prompt. They struggled for a long time, but still didn’t find the problem. They also found many people to check, but they could not find the fault point. Later, I went to help them. There doesn’t seem to be anything special in the control room because others have checked it. I climbed to a height of more than ten meters. There is a small controller on the site, which contains some relays. Because I have repeatedly read their drawings and the working logic of the site, I judged that the problem should be in the box. I asked them to start the machine several times. Every time they reached a relay position, they would touch the main switch. However, this relay is an ordinary DC relay.

I tried to remove the coil of the relay and let them operate it again. As a result, the device stopped after running for a period of time, but it did not do so. The coil of the relay was charged, equivalent to 12v DC power supply, which was directly applied to the freely rotating diode to form a positive conduction. The 12V power supply is short-circuited, and the power supply trips, causing the system to trip. It is good to reverse the diode. It is estimated that the connection error was caused when someone inspected the replacement site. Travel. I judged that there was something wrong with the relay race. After removal, I found that the free rotation diode of the relay is in the opposite direction.
Therefore, there is no alarm prompt for this type of PLC peripheral fault. Everything looks normal, nothing bad. The system is running abnormally. This fault is the most difficult to eliminate because many people have checked and repaired it. If you only look at the drawings and procedures, you can’t find the problem. You must go to the scene to master the details.