Ghost Short Circuit

I have a working system that is experiencing some very intermittant unrepeatable errors. (See seperate thread for misbehaving axis...)

I have a Logix 5555 processor in a 13 slot chassis, with sercos card (1756M08SE), ethernet card (1756-ENBT/A), 6 input Cards (1756-IB32/B), and 3 output cards (1756-OB32), and one blank. Program runs fine, communications are fine. The customer has reported (I haven't seen it) that occasionally one of the input cards detects a short cicuit and so faults out. They go online, clear the fault, restart the machine and work away for days/weeks until it happens again. Always an input card. The problem moves cards, so don't think it is faulty card. Seems to be no correlation to program, start/stop procedure, or any other conceiveable physical occurance. There is a slight possibility of electrical noise in the machine, which we are working to solve now. Could this cause enough current to produce something the cards would see as a short?

Does anybody have any further suggestions?

Thank you,
JEFF

Any pneumatic cylinders on the system with reed switches fitted?

Had a similar problem and found the reed switch cable chafing eventually exposing conductors and shorting out intermittently.

Jeff,

Because it always happens on a input card could be significant. Noise could be the problem. What is the exact fault indication? Does the PLC code say "short circuit on Input Card, Slot x" or something similar? Or is it more general and just shows some type of fault? In other words, how does the customer know that it is a "short circuit" and NOT some other type of fault?

silva.foxx, there are reed Switches all over the machine, though very few are moving, probably not this problem since it happens on various cards, and would be highly unlikely to happen multiple times over a relatively new machine at the same time, but will keep it in mind.

Lancie1, can't tell you exact wording, since it never happens when I am there, and can only rely on 2nd hand info. But the report is that it is definitely a "short on card X" sort of problem, coming from somebody with decent programming experience.

Wanderer,

You have identified part of the problem (second hand information). Talk to the local guy and ask him to make an exact recording of the fault indication on the next occurrance. If he has a camera, ask him to make a picture, or least write down exactly the series of events and what the fault indicator says, letter for letter. Misunderstatnding the nature of a fault can lead to some wild-goose chases.

We actually video-taped a ghost machine so we could play back the operation right before the ghost failure.

On another ghost, we found that the inductive kick on deactivating dc servos caused all sorts of untimely situations.

How about this one



A servo system would position the female knives on a shaft for a slitter rewinder. All of a sudden the mechanism grabbing the knives would grab the knives to early but only sometimes ( I am using a prox to sense the knives). Also the some of the knives would be out of position but only sometimes. The customer is not local. I had the maintenance checking encoder wiring and a bunch of other things from servo tuning to what ever I could think over the phone. Anyway one of the maintenance guys saw a knife move when the knife gabber passed over a knife when it was not engaged. The female knives came back from the sharpener magnetized. This threw my magnetic prox off and the metal grabber would move the knives without touching depending on how much the knife was magnetized. The knives were demagnetized and all is fine



I just wanted to share. The system went from working fine to trash all of sudden but only sometimes. Ya gotta love it.



[font=&quot]You can find the ghosts, you have to be lucky or have maintenance guy with a sharp eye!!!!

GO JETS
[/font]

As paulcs said, "inductive kicks". I used an RC circuit in p//el with AC heater contactor coils to prevent input card errors in same cabinet.

Does the IB32 have short circuit detection? I thought there was a different module with diagnostics.

Anyhow: I think that everyone has given you good advice. I thought there might be something else for you to consider. Do you have pushbuttons on the inputs? If so, have you wired them as described in the manual for the module? If you're using short circuit or open circuit detection, you need to take a careful look at your circuit. Wiring things the "standard" way might not work with a fault-detecting module.

AK