Find PLC fault method – Mitsubishi PLC

PLC is an industrial small computer, it has a problem, the first thing to clean up is the PLC ontology problem is still a peripheral problem, if there is a problem with the PLC ontology, often the ERR light is on, maybe the red light is flashing, the normal situation is generally RUN operation green light on, if the ontology produces this kind of problem, the probability of successful correction is not high.

Some PLC insists on data through the battery inside, when the battery voltage is lower than a certain threshold, there will be a battery alarm light on, at this time the battery needs to be replaced, and the demand is charged to replace, if the battery is completely out of power, perhaps the replacement of the battery is not live, often the RAM data is lost, and the need to rewrite the program and data from scratch, so the PLC usually maintains the habit, there must be a habit of program and data backup, otherwise there is no time to the critical moment, Only programming and debugging from scratch is done.

Find PLC fault method – Mitsubishi PLC

Power supply defects will also account for a certain proportion of ontology faults, PLC input is generally 220 AC, there are some things 24 volts or 12 volt input, but there is a chip inside, the need for 5VDC or 3.3VDC, so there is a switching power supply step-down circuit, this circuit due to the large current, high temperature, in some harsh high temperature or dust occasions simple problems, if the PLC indicator light is not on, it is generally the switching power supply is broken. The switching power supply is broken, for people who generally have a little electronic repair level, it is not particularly difficult to repair, for example, the control chip 384X may have a high probability of breaking the switch tube, and it is generally good to replace it.

If there is a red light flashing or the ERR light is on, it is often the motherboard is broken, or the program is lost, you can try to pour the program from the beginning, if not, there is not much repair value, at most replace the watchdog IC and reset circuit of the chip, or replace the crystal oscillator of the chip, if it is still unable to solve the problem, it is also advocated to give up, this thing is too integrated, it is very difficult to repair, and the price of the motherboard is not particularly expensive, there is not much value.

The output output point is bad, it is also a more common PLC body problem, especially the PLC of the relay output type, in some occasions of frequent switching action, it will simply hang up the relay, the service life of the relay is about 100,000 times, often used for 3-5 years, it may be broken, this problem is also simple to find, after all, look at the output indicator light, but with a multimeter to measure that there is no contact closure or disconnection to find the problem, disassemble, find some similar small relays in China to replace, generally not a big problem. If the transistor is broken, it is common for the input to be connected to the high voltage, and the input and output may be damaged, and the optocoupler and some amplified transistor circuits often need to be replaced.

If it is a peripheral problem, you can analyze the input and output logic according to the circuit diagram, analyze it from the microscopic level and the operation status of the equipment, and you can also shield some input and output interlocking points to separate the test to find the problem. If there is a PLC program is simpler, because the PLC has an online diagnosis function, it can be executed separately, and see if the corresponding output point is normal in the place where it is executed, and the fault point can be found.

The most difficult thing is still some system problems, especially the circuits that some people have repaired. One year, Chimelong's big pendulum had a problem, their equipment department had a PLC program, there was also a program of the industrial computer on the top, and the circuit drawings on the outside were also complete, and various conditions could be diagnosed online, but the big pendulum ran to a certain logical point, it took the initiative to trip, and all the power was cut off, without any alarm and prompt. They tossed for a long time and couldn't find a problem, and they also looked for many people to see, but they couldn't find anything wrong. Later, I went to help them see, it seems that there is nothing special in the control room, because the others who should check have checked, I personally climbed to a height of more than 10 meters, there is a small controller on the spot, there are some relays in it, because I repeated their drawings and the operation logic of the site, the problem should be in this box. I asked them to turn on the machine several times, and each time they ran to the direction of a relay and then jumped the main switch, but this relay was a general DC relay. I tried to remove the relay coil wire, let them operate again, the result of the equipment running to a certain time, and stopped, but did not trip, I judged that there was a problem with this relay, removed it and looked carefully, and found that the direction of the relay freewheeling diode was reversed, so that the coil of the relay was electrified, which was equivalent to a 12-volt DC power supply, directly added to the freewheeling diode to form a positive guide, and the 12-volt power supply was short-circuited, so the power supply tripped and caused the system to trip. It would be nice to turn this diode upside down, and estimate that it was caused by the wrong tap when someone looked at the replacement site. Therefore, this type of PLC peripheral fault, there is no alarm prompt, everything looks normal, and nothing is broken, but the system operation is not normal, this problem is the most difficult to sweep away, because there are many people to view and repair, if you just look at the drawings and procedures, it is impossible to find the problem, you must go down to the scene to grasp the details.

Find PLC fault method – Mitsubishi PLC

Sections viewed

Contact

working hours