Fault phenomenon: The day before yesterday, when operating the roller frame to rise and fall on the P01 control panel and touch screen, the DP bus momentarily disconnected, then automatically returned to normal.
Solution process:
1. On September 26th, the repairmen spent the whole morning trying to fix the machine but couldn't find the cause. My supervisor asked me to check it out. When I arrived, I asked about the fault and what they had been doing. They said that operating the S162 switch or touching the button on the touchscreen would disconnect the bus at station P01 12, and both the output and input signals would flicker. They replaced the output module and the DP connector, but the fault persisted.
2. Following their lead, I considered whether the problem might be caused by a faulty load on the output module. Upon checking, I found that all the load indicators were connected. I then asked them to test a module plug-in with only the power supply connected and all the indicator lights disconnected. The fault was the same.
3. I then wondered if there was a problem with the DP cable. I first clarified the DP cable routing and switched the DP ON and OFF positions. The bus routing is shown in the diagram:
I tested station 12 of P01 separately from the PLC, and the fault symptoms were the same.
4. On the second day, due to the rush to produce, production could continue even without opening the roller support. I asked if the problem only occurred when the south roller support was open, and they said they hadn't tried that. On the third day, I tested it and confirmed that only the south roller support had the problem; the north roller support was fine. Could it be a problem with the inverter? I checked the schematics and found that the LENZE inverter uses 24V power in two places: one for the brake and one for the inverter control board. Could this be causing the malfunction by pulling down the 24V power supply? I reconnected these two power supplies separately and tested, but the problem persisted. Then I found a relay station and tested the inverter via a separate bus, but the problem remained the same. There was no other way but to replace the inverter. I first backed up the LENZE inverter's parameters using a control panel.
In parameter management, I checked C20 and then pressed SHIFT+PRG to back it up. After replacing the inverter, I found C10 and pressed SHIFT+PRG to copy it to the new inverter. After powering on and testing, the fault was still the same. At this point, I was at my wit's end. I asked them to restore the inverter and then checked the diagnostic cache records. The bus interruptions were 14, 15, and 16. Why didn't the last one, number 12, interrupt?
5. We had to go back to DP analysis. We decided that the line from station 11 to the field runs through cables 14, 15, 16, and 12. I tried replacing cable 11 to 14 because it was the longest and it was indeed in the same cable trench as the power line from the frequency converter to the motor. The power line is a shielded cable, and if there's a problem with the communication cable, it could cause interference. After all, the equipment is 11 years old, so aging is normal. Based on the analysis, we decided to try replacing it, as shown in the diagram:
After replacing the line, the roller support on the south side was tested and found to be without any faults. The support could move up and down into place, and it worked normally after multiple tests. Haha, the ghost has finally been found.
6. The problem is: the communication cable is aging and the power cable is interfering with it, so the problem only occurs when the south bracket is opened, while the north bracket does not interfere with this line. This kind of problem is really troublesome!
7. On October 5th, the maintenance personnel replaced the DP cable when they had some free time. Looking at the picture, I wanted to cry and laugh at the same time; I was speechless. How could communication be good if the cable was wrapped in cloth?
It took me a day and a half to fix this problem, but the repairman probably only took 10 minutes. He was rushing through it and wasn't being meticulous. Because of the high turnover rate of our first-phase electrical repair team—three of them left—I don't know if it was the current repairman who did it. I doubt anyone would admit to it. However, I hope that everyone in this industry will take the shielding of DP communication lines seriously and do every task carefully. Don't use this kind of method to handle it.
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