PLC hardware problems

Can someone briefly describe hardware problems that occur in a PLC that an in plant PLC person would repair? Our plant electrician occasionally writes up a job ticket and all he says is repaired PLC. Job security I guess. I would imagine there are fuses and maybe plug in circuit boards that can be replaced. Thx.

gord1 said:

Can someone briefly describe hardware problems that occur in a PLC that an in plant PLC person would repair? Our plant electrician occasionally writes up a job ticket and all he says is repaired PLC. Job security I guess. I would imagine there are fuses and maybe plug in circuit boards that can be replaced. Thx.

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If you don't know how can you question him?

Modules can fail.
Processor can be poorly programmed & fault.
I've had a ML1000 put negative accumulated values in the timer file, with nothing else writing to .acc
Stuff does happen, and if you don't even know what could, why do you question?

"If you don't know how can you question him?" I do know the guy's personality. I'm trying to upgrade my maintenance abilities since our plant is closing. I took a PLC course at the local college which was of little value and it was years ago. I've been going through the tutorial on programming but there are hardware issues also. Just trying to learn.

A plant which wishes to minimize future problems will note for each instance:

The symptom - what was going wrong in general. Usually this is noted by an operator or other floor personnel. It is best to encourage the floor personnel to be as specific as possible. Not just "it doesn't work" but "it gets to this step and stops"

The specific cause - what the technician found to be specifically the fault. This should be backed up with the method of location of the problem (hooked up a PC - observed location X, used a voltmeter, measured voltage X - etc)

The correction - what was done to set everything right. - Reset counter X and added rung to automatically reset. Replaced solenoid X which was shorted and also replaced output module X which was damaged by the short.

This type of information would be useful in a historical context, not only on this piece of equipment but on other similar types or on other equipment with the same type PLC, output module, solenoid etc. You could see - "We've lost a solenoid type X on 3 different machine. We may need to check them all."

This type of record keeping may have to be implemented from the top down. You may be able to ask the PLC technician if he has any type record - even informal - that could help you to learn.

My old boss always wanted to know what id done to repair a machine and next time it broke down he assumed it was one of the previous faults and kept saying have you checked this and that so i just refused to tell him what id done or told him we had problems with the foofoo valve. he used to refer to these constant problems with the foofoo valves at production meetings it goes to show in the wrong hands a little knowledege is a dangerous thing.

In my opinion i was paid to repair things and if i could do this why should i teach somebody with no electrical experiance how to tinker with things they neednt touch

davefinic said:

My old boss always wanted to know what id done to repair a machine and next time it broke down he assumed it was one of the previous faults and kept saying have you checked this and that so i just refused to tell him what id done or told him we had problems with the foofoo valve. he used to refer to these constant problems with the foofoo valves at production meetings it goes to show in the wrong hands a little knowledege is a dangerous thing.

In my opinion i was paid to repair things and if i could do this why should i teach somebody with no electrical experiance how to tinker with things they neednt touch

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I agree that this is the main reason he doesn't share the knowledge. We just like to harass him some as is common in factorys. I've been through three years electronics school which I did pretty good at but there was no PLC training. I'm just trying to expand.

I agree that this is the main reason he doesn't share the knowledge.

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Knowledge not shared is knowledge lost.

Over many years and in many different plants, I have observed the efforts of electricians, instrument mechanics, and other crafts to protect their turf. The net result: a big fat zero. Many plants that had a strong union are now closed, the jobs moved to places where commmunications are required between all parties, but at a much lower wage rate.

I am working on the design for a new Honda non-union plant here in the US, but I know that many GM UAW plants are closing or cutting back. This is the new reality.

i have no problem in sharing knowledge at the factory i worked as a maintenance electrician i trained 2 apprentices and would help the other electricans and since working for myself i have trained 3 apprentices and i have been happy to help anybdy on this forum with any problems, my concern was to give information to people who would use it in the wrong way as my boss used to, we were working on interlinked hydraulic presses with decoilers and auto feeds so just because it stopped for one reason one day he would automatically tell me to check that part if it stopped the next day. this is not a skill but a waste of time as i do not check componenets unless i suspect there is a fault.

Any idiot can go to a machine and replace plug in sensors willy nilly, it does not mean that the machine will work and is a waste of time and parts, and when it still doesnt work you get called to the machine and they say theve do nothing to it you have to find two faults, the original one and the one they made like fitting an NPN sensor instead of a PNP sensor because they all looked the same.

I sometimes think that the best troubleshooting technicians are the ones with the shortest memories.
I have run into guys that can remember every single fault on a piece of equipment in the last 20 years and systematically try all the previous fixes without actually logically looking at the problem at hand.
These guys can fix better than half of the problems they get but always dump the 'never seen that one before' jobs on me. I may have worked on the machine dozens of times but because of my memory impairment every time I open it up again it's like it's the first time (only slight exageration). I consult the manual, talk to operators, run the machine through it's paces before jumping to any conclusions. Just had to add my piece because I abhor memory experts. Probably just jealous.

Brian.

All of our machines log the last 500 errors with time/date stamp. Talk about an invaluable tool for troubleshooting.

Back to the original question "Can someone briefly describe hardware problems that occur in a PLC that an in plant PLC person would repair?"
Speaking only of hardware (not the PLC program) you've got input cards, output cards, communications cards, fuses (depending on PLC) that, CPUs, backplanes (or racks), and power supplies; all of which can fail for any number of reasons. You also may find an input or output card with only one bad input or output point. If a replacement card is not available then moving the wires to an unused input or output and updating the software for this change is also pretty common.

It could be as simple as -

Repaired PLC = Turned off power to PLC, turned power back on to PLC

or

Repaired PLC = Placed PLC in Program mode, placed PLC in Run mode.

Tark, you are correct, but if you're power cycling or run mode cycling isn't that really a software issue?

Could be "I was trouble shooting, and blew a fuse, so I'll blame it on the PLC, and nobody else will know the difference."

Most common troubles have already been mentioned. Fuses, Cards, Power Supplies, on rare occaisions a memory dump caused by a combination of a bad battery, and power loss.

I had one electrician tell me, "I swear, I only hit one button."

Most often, will be non PLC hardware issues. Sensors, valves, relays, limit switches and so on... These items also get blamed on the PLC. Not to mention My friend the forklift driver. PLC's are an easy target in a lot of mills where managers don't understand what a control system really is.

marksji said:

Tark, you are correct, but if you're power cycling or run mode cycling isn't that really a software issue?

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Not necessarily. Brown outs and power surges can cause a PLC to lock-up.

I was just eluding to one reason why someone might write “Repaired PLC” on a work order. Regardless the reason, one does need better detain as to what was done to repair the problem. If in fact someone is cycling the PLC to “repair” it, one would need to know that in order to determine the root cause of the problem.

Tark said:

Not necessarily. Brown outs and power surges can cause a PLC to lock-up.

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OK, I'll buy that; though I would hope that the PLC is protected from surges and that its firmware recovers from brown outs better than that even though I know that's a pipe dream.