There is a problem with PLC, how to quickly find the cause?

PLC is a small industrial computer. If there is a problem with it, you must first rule out whether it is a problem with the PLC itself or a 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 light will turn green. , if this kind of problem occurs on the body, the probability of successful repair is not high.

 

Some PLC maintain data through the battery inside. When the battery voltage is lower than a certain threshold, a battery alarm light will light up. At this time, the battery needs to be replaced, and it needs to be replaced with power. If the battery is completely out of power, or the battery needs to be replaced When operating without power, RAM data will often be lost. At this time, programs and data need to be refreshed. Therefore, during daily maintenance of PLC, you must have the habit of backing up programs and data. Otherwise, they will be lost at critical times, and you will have to reprogram and debug. .
Power failure will also account for a certain proportion of body failures. PLC input is generally 220 AC, and some have 24V or 12V input, but there are chips inside that require 5VDC or 3.3VDC, so there is a switching power supply step-down circuit, such Due to the large current and high temperature of the circuit, it is easy to cause problems in some harsh high temperature or dusty environments. If the PLC indicator light does not light up, it is usually because the switching power supply is broken. If the switching power supply is broken, it is not particularly difficult for people with some level of electronic maintenance skills to repair it. For example, the probability of the control chip 384X or the switch tube being broken is relatively high. Generally, it can be replaced.

 

If there is a red light flashing or the ERR light is on, it is often caused by a broken motherboard or a lost program. You can try re-instilling the program. If it doesn't work, there is not much repair value. At most, reset the watchdog IC of the chip. Replace the circuit or replace the crystal oscillator of the chip. If the problem still cannot be solved, it is recommended to give up. This thing is too highly integrated and is very difficult to repair. Moreover, the price of the motherboard is not particularly expensive and does not have much value.

 

A broken output point is also a relatively common PLC body failure, especially for PLCs with relay output types. In some situations with frequent switching actions, the relay will easily hang up. The service life of the relay is about 100,000 times, and it is often used for 3- After 5 years, it may be broken. This kind of problem is also easy to find. After all, the output indicator light is on, but if you use a multimeter to measure that there is no contact closure or disconnection, you can find the problem. Take it apart and find some similar small domestic relays to replace. , generally not a big problem. If the transistor is broken, it is usually because the input is connected to 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, you can analyze the input and output logic based on the circuit diagram and analyze it from a macro perspective and the operation of the equipment. You can also shield some input and output interlock points to test separately to find the problem. It is even simpler if there is a PLC program, because the PLC has an online diagnosis function, which can be executed separately to see if the status of the corresponding output point is normal where it is executed, and the fault point can be found.

 

The most difficult thing is some system problems, especially problems with circuits that some people have repaired. One year, there was a problem with Chimelong's big pendulum. Their equipment department had PLC programs, as well as programs for the industrial computer. The external circuit drawings were also complete, and they could diagnose various states online, but the big pendulum ran to a certain Based on the logic, it automatically tripped, and all power was cut off without any alarm or prompt. They struggled for a long time and couldn't find the problem. They also asked many people to look at it, but they couldn't find the fault point. Later, I went to help them check it out. There seemed to be nothing special in the control room, because others had checked everything that needed to be checked. I personally climbed to a height of more than 10 meters, and there was a small on-site controller with some relays inside. , because I repeatedly looked at their drawings and on-site work logic, and judged that the problem should be inside this box. I asked them to turn it on and operate it several times, and each time it reached a relay position and tripped the main circuit breaker, but this relay was just an ordinary DC relay.

 

I tried to remove the relay coil wire and let them operate it again. As a result, the equipment ran for a certain period of time and stopped again, but did not trip. I judged that there was something wrong with the relay. I removed it and checked carefully and found that it was the relay's continuation. The direction of the current diode is reversed, so that the relay coil is in a charged state, which is equivalent to a 12-volt DC power supply. It is directly added to the freewheeling diode to form a forward conduction. The 12-volt power supply is short-circuited, so the power supply trips and causes the system to All tripped, just turn the diode upside down. It was probably caused by someone connecting the wrong wires when checking the replacement site.

 

Therefore, this type of PLC peripheral failure does not have any alarm prompts. Everything seems normal and nothing is broken, but the system is not running normally. This kind of failure is the most difficult to eliminate because there are many people checking and After repairs, if you just look at the drawings and procedures, you will not be able to find the problem. You must go to the site to pay attention to the details.