PLC corrupt?

[jcraft]

We have a small Mitsubishi plc that controls a shrinkwrap machine. Called to service it yesterday and found a bad 24vdc power supply in it that was basically cycling on and off. It would come on for about 1 second then immediately shutdown, therefore it was powering the plc up and down constantly. It did it probably 50 times while I was there and who knows how many times prior to that. We changed the supply and everything powered up and the plc is in run mode. However,parts of the machine are working others are not. Some I/O is turning on and off. At this point we don't have any schematics for the machine nor do we have the original plc program or the software to link up. Anyways, I am assuming that continuous power up and down of the plc could likely cause something to become corrupt within the program as i don't think the plc even reached its run state before losing power again. Let me know your thoughts on that. Thanks

 

[geniusintraining]

.... Some I/O is turning on and off. ...

If you replace the power supply... it still may not of been that, the new power supply may just be good enough to overcome a large load on it.

If it were I... start unplugging the loads on the new power supply and see if you have one that is dragging your voltage down, what is your voltage on your system now?

It may be just a short in the field

Thats were I would start.

 

[Paul T]

Um. Generally, the PLC would fail it's own self test if the logic didn't load correctly. The PLC is running from the 24VDC supply too?

If IO is turning "on and off," I like git's idea. Check the loads, especially if you've recently added any. Unplugging loads may not let you troubleshoot it though. If it's a foldover supply, then you won't see a problem until there are too many loads - it'll be fine when you measure it with the machine off. Got caught by that years ago...

Anyway, look for a short that's dropping the PS out again; look for a bad sensor that may be halting the machine cycle, or aborting it, or causing it to get lost; similarly, are the outputs really actuating - SV's moving, stuff like that. Sounds more like you have something bad on the machine that's killing the cycle, more than a corrupted PLC. Does anything happen normally?

The other thing to try is just forcing all the outputs... with a piece of wire... and when stuff is in position, looking to see that the corresponding inputs light up on the PLC cards. Lots of fun with no schematics... I hope it's not a big machine!

 

[Stationmaster]

We have a small Mitsubishi plc that controls a shrinkwrap machine. Called to service it yesterday and found a bad 24vdc power supply in it that was basically cycling on and off. It would come on for about 1 second then immediately shutdown, therefore it was powering the plc up and down constantly. It did it probably 50 times while I was there and who knows how many times prior to that. We changed the supply and everything powered up and the plc is in run mode. However,parts of the machine are working others are not. Some I/O is turning on and off. At this point we don't have any schematics for the machine nor do we have the original plc program or the software to link up. Anyways, I am assuming that continuous power up and down of the plc could likely cause something to become corrupt within the program as i don't think the plc even reached its run state before losing power again. Let me know your thoughts on that. Thanks

Of the couple of hundred times I've been asked to work on equipment where the owner or his electrician was sure that the PLC program was the problem, I think maybe TWICE it turned out to ACTUALLY be the case.

While the cycling power you described would qualify as something that COULD cause a program to be corrupt, try not to fall into the trap that "it's probably the PLC". It's a common plight, mostly because the PLC is inaccessible and therefore mysterious. SO "since we don't see anything ELSE, the problem must be in the PLC".

Do a systematic check of all the inputs first. If you don't have a print, pull the input leads back to the switch, or sensor. Trick the sensor or switch on and off and make sure the input indicator changes state appropriately.

The same can be done with the outputs, though I'm not going to recommend it because I don't know your skill level and it can be dangerous, both to personnel AND the machine.

Document as you go and you've BASICALLY sketched a print. You'll need that ANYway if someone is going to try to write a new program if it is determined that yours is really corrupt.

Stationmaster

 

[Longhorn]

I would be inclined to revert the system back to its original state by putting the old power supply back on and then follow what geniusintraining is suggesting, as i have seen this too many times in the past where some hardware is changed and the fault changes somewhat.

 

[Den Mark]

try to isolate outputs first. maybe there is a device connected to the output pulling the voltage down.

 

[PAINT]

We have a small Mitsubishi plc that controls a shrinkwrap machine. Called to service it yesterday and found a bad 24vdc power supply in it that was basically cycling on and off. It would come on for about 1 second then immediately shutdown, therefore it was powering the plc up and down constantly. It did it probably 50 times while I was there and who knows how many times prior to that. We changed the supply and everything powered up and the plc is in run mode. However,parts of the machine are working others are not. Some I/O is turning on and off. At this point we don't have any schematics for the machine nor do we have the original plc program or the software to link up. Anyways, I am assuming that continuous power up and down of the plc could likely cause something to become corrupt within the program as i don't think the plc even reached its run state before losing power again. Let me know your thoughts on that. Thanks

In my opinion. You shoud removing plc hardware and check electronic circuits whether tight or not tight.And solder if loose. Good luck!!!