Troubleshooting a Micrologix 1200

[mtn71]

Hello everyone. First post here; hope it's not a dumb one.

The plant where I worked shut down three years ago. I was out of work for two years and went back to school to get an electronics degree at a local community college. I learned some basics about PLCs there; enjoyed it and did very well. Now I'm working for a very small company and one of our systems is having issues. The PLC is an AB Micrologix 1200. Their is no documentation about the programming. We just bought an RSLogix Micro program and cable that I was told will work with it, though I haven't tried it yet.

There are two 20hp fan motors, primary and backup, and their corresponding dampers controlled by the PLC, as well as a couple pumps. There are several things that can make the fan trip over to the secondary side, and I have mapped out all the inputs and outputs to understand what happens.

It has worked fine until several days ago when there was a momentary power failure due to weather. Now it seems stuck on the secondary. When I have the fan selector set on Hand it will stay on fan 1, but both fan 1 and 2 dampers are open (not right- these should never be open at the same time). When I switch to Auto it instantly switches to fan 2. Fan dampers also stay open even when selector is set to Off (also not right- I have never seen it do this in the past).

Of course I don't expect anyone to understand the system without seeing it, but it sure seems like a PLC issue. I have listed and understood all the inputs and outputs (took me a while without documentation), and all inputs are normal. I have played with it quite a bit, watching input and output indicators as it switched, and can't see anything coming in telling it to misbehave.

I talked to the salesman that sold me the RSLogix program, and he said it sounded like a stuck bit (?) due to the power failure. We cycled the power a couple times with no change. I haven't had the chance to hook up the computer to it and see what the program is doing, waiting for the right window and this system can't be down very long. I'm also a little concerned that I might do something wrong and screw it up worse. Top grades in a couple classes don't translate into very much know-how, and I am vaguely aware of just how much I don't know.

Any tips or ideas would be greatly appreciated. I hope I'm not being presumptuous with such a long-winded question on my first post. Thank you for any and all responses.

 

[jstolaruk]

Welcome to the forum.

It sounds like you have electrical drawings or at least you know what each input & output's function is assigned to.

No contact with the original author of the program? Even a hardcopy printout would be helpful.

Did you say you have previous experience with RSLogix 500? If you don't, google for a free training or primer document that can help you get around the program.

Once connected to the plc with the software, I make a backup of what is in the PLC. I copy that backup to a working copy and annotate the inputs & outputs with the descriptions that were known from drawings (or other).

Check the actual input & output states online in the software against what is measured at their terminals to verify everything is as it should be.

Now the fun part starts because there will be internal bits & words that don't have descriptions so you'll have to see how the inputs & outputs interact with them and fill-in the description with what you think they are used for.

If you believe a output should be on, trace it down and look to see what enables or disables the output state in their output rung. Hopefully it will be easy. Otherwise get back with the forum to see if anyone here can give you some pointers.

Hope this helps get you started.

 

[Lancie1]

It has worked fine until several days ago when there was a momentary power failure due to weather. Now it seems stuck on the secondary. When I have the fan selector set on Hand it will stay on fan 1, but both fan 1 and 2 dampers are open (not right- these should never be open at the same time).

If it "worked fine until a momentary power failure", then the Micrologix PLC program is not at fault - it will work the same way every time, unless someone changes it. I would first check the fan damper limit switches (probably open and closed switches on each damper). A lightning surge could have damaged (burned) one of the switch contacts, or a gust of wind could have bent one of the damper operator arms.

Bottom line: (1)First check all physical inputs and outputs BEFORE you start thinking that something is wrong with the program. (2) Next, check the PLC operation to make sure it was not damaged by an electrical surge. (3) Then check to see if the last good hard copy of the program IS the one actually inside the PLC. PLC programs have a way of mysteriously getting swapped at plants where the personnel are just barely PLC-literate. (4) If the above items produce no results, only then take a look to see if there was some hidden program error that slipped by all the people who have gone before you!

I talked to the salesman that sold me the RSLogix program, and he said it sounded like a stuck bit (?) due to the power failure.

"Stuck" bit, that is funny - a salesman's answer to all the wiring failures. I would stay away from the advice of salesman. If they knew much about electrical troubleshooting, they would be doing something more than selling....

 

[MarkNightingale]

I agree with what Lancie1 has said, a program does not suddenly change due to a lightening strike.

There well may be a bit stuck on, but there is something that is making that bit stick on, and without seeing it my best guess would be an input is on that shouldn't be on, and input isn't on that should be on, or an output is on that shouldn't be.

If you can go online with the PLC and you have an I/O list, even without a commented program you have a chance of finding the fault.

Mark

 

[jrwb4gbm]

I agree with Lancie1 about the dampers. You should check to see if the output indicator is on for both dampers. Also, don't just check the indicator light for the output, check for power going to the damper also. It is possible that an output welded a contact if it is a relay putput module, or a triac shorted if it is a SSR type output. That would mimic the salesman's "Stuck Bit" condition. Not really a "Stuck Bit" but a failed output.

 

[mtn71]

I wondered if it could be a damaged output relay or something. The indicator light for the secondary damper never turns off unless the power is off. I have a partial electrical drawing that has helped me understand the inputs and outputs, but no documentation at all for the program- hard copy, soft copy or anything. I even called the equipment manufacturer and the company that installed it.

I have worked with RSLogix 500, but only at school and that was a year ago. I found some info online so between that and my memory from school I think I can figure it out. I'm looking forward to using it and learning more because I would like to go further into controls as a career field.

The interruption wasn't a lightning strike, as far as I know. I wasn't there when it happened, but the guys said the power just flickered off for less than a minute then came back on. All the other equipment recovered except the primary fan. The dampers don't have limit switches, they have spring operated openers that are just controlled on or off by PLC outputs- no feedback for the PLC to know if they are actually open or not, and they are physically open when the indicators say they are. In other words, damper 1 indicator is on and damper 1 open when fan 1 is running in Hand, but damper 2 indicator is also on at the same time when it is not supposed to be (and damper 2 physically open). Both damper output indicators are on when the fan switch is Off (and dampers open), which is way wrong.

The funny thing is that I did troubleshooting on this system months ago and it was then that I figured out all the inputs and outputs and how it all works. It turned out to be an external problem at that time, but I kept all my notes and documentation, including notes about the states of all inputs and outputs when it was running normally on either fan. What confuses me is that all the inputs indicators are normal, every single one as they were when it was operating fine. The only difference is the damper outputs coming on when they're not supposed to, and something is telling it to switch to fan two when I put it in auto.

I suppose it's not likely an output relay problem if both of them will come on when they are not supposed to? I'm really curious to see the program; hope to get my window to do that on Monday. Thank you all for your suggestions, and your patience with my inexperience.

 

[shawn_75]

How long was the power out?

I've seen programs that written in such a way that the first pass bit is used to clear thing out and reset the program back to a default state. If the power was not out long enough to cause the 1200 to do a first pass scan, perhaps it thinks things are running that actually are not running and preventing what the OP would consider normal operation. This is all just guessing and without a copy of the program i can't be sure. Reading that the dampers do not have real world indication feedback to the PLC leads me to believe that the program tracks damper position by memory and it is very likely that the programmer used the first pass bit to reset the program.

If your system can take it, shut it down, change the PLC from run to program and back then start it all back up.

Shawn

 

[jrwb4gbm]

One more wild guess. If your PLC has an EEPROM, perhaps the plc loaded an old program from the EEPROM when the power failed.
By the way, since you are somewhat unfamiliar with the particular PLC, here is a link to some manuals.
http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1762-um001_-en-p.pdf
http://literature.rockwellautomation.com/idc/groups/literature/documents/in/1762-in006_-en-p.pdf
http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1762-rm001_-en-p.pdf
http://literature.rockwellautomation.com/idc/groups/public/documents/webassets/browse_category.hcst

The 4th link takes you to the Literature Library. About halfway down the right side of the page, Click on Programmable Controllers, then, Click on Micrologix 1200 system.

 

[mtn71]

I'll have to ask the guys Monday exactly how long the power was out. They indicated it was just a short time.

One more question before I start on it Monday (please don't laugh at me); the PLCs we had were I worked before, and the ones at school, had a switch on front for Run, Remote, and Program. This Micrologix 1200 doesn't have that at all, that I can see. Am I supposed to just plug in the cable, establish connection, and switch it in the program?

 

[jstolaruk]

One more question before I start on it Monday (please don't laugh at me); the PLCs we had were I worked before, and the ones at school, had a switch on front for Run, Remote, and Program. This Micrologix 1200 doesn't have that at all, that I can see. Am I supposed to just plug in the cable, establish connection, and switch it in the program?

correct

 

[mtn71]

Wow, that was quick. Thank you!

I will post more when I get a look at the program hopefully Monday. Thank you all for the great info and ideas. This is a great site!

 

[mtn71]

I finally got a chance to try it today, but must be doing something wrong. I connected the cable, autoconfigured and started the driver in RSLinx, but was unable to upload the existing program to my PC. I hit upload but the message was that no matching file could be found. Do I need to be online first? It gave me the same message when I tried to go online.


I admit to being both green and rusty; most of what little I know I learned in school, and that was over a year ago. I must be missing a step or something. I'm a little embarrassed to ask, but any help would be appreciated.

 

[OkiePC]

I hit upload but the message was that no matching file could be found. Do I need to be online first?

That just means that you don't have an offline file in the RSLogix search path which matches the online program.

You have two choices:
1) (preferred) Browse to a file with comments and symbols with the closest match to the running program.

2) Create a new file. This method will still give you all the logic and data, but none of the internal addresses or I/O will be commented.

 

[mtn71]

Thank you. I was hoping it was something easy. I did see the "new file" button. I'll try that tomorrow. Choice #1 is not an option here. There are no existing files at all other than what is currently in the PLC. I'm probably being overly careful, as I'm learning here and don't want to do anything that could mess anything up, either PLC or program.

 

[Lancie1]

I have a partial electrical drawing that has helped me understand the inputs and outputs, but no documentation at all for the program- hard copy, soft copy or anything.

Choice #1 is not an option here. There are no existing files at all other than what is currently in the PLC.

I think you will eventually be able to upload a copy of the program. However you are in for some pain while trying to recreate the symbols and comments that go along with the program.

Hold on to that partial electrical drawing. It is better than nothing at all, and may allow you to put in address descriptions for each Input and Output. Once the Inputs and Outputs are identified, a bunch of the other MicroLogix PLC addresses will also become obvious (to an experienced electrical/programmer guy). Maybe if you post a copy of the drawing and the program, some of us here can help you figure out how it works, and what symbols, descrptions, and comments should be added to your new copy of the ladder program.

The dampers don't have limit switches, they have spring operated openers that are just controlled on or off by PLC outputs- no feedback for the PLC to know if they are actually open or not, and they are physically open when the indicators say they are.

Mtn,
Are you sure that there is NO feedback to the PLC at all? What about motor starter auxiliary contact feedback inputs? Surely you do have a motor starter or contactor that the PLC is energizing to run the fan? A good deisigner would have at least use an auxiliary contact from the motor starter to tell the PLC "yes, this motor should be running" (in other words, use that contact as a seal-in to keep the starter on after a start signal). If one of the starter auxiliary contacts stuck on (welded shut), that could create the condition that you described. It would tell the PLC that the motor is on when really it is off, and also would cause other screwy situations in the program. If this is the case, then the program can probably be improved somewhat to help prevent this particular situation from happening again.

A lightning surge can hit the power line a mile away, run down the line, jump the transformer, and weld some contacts together in your plant, or do other damage. It happens all the time, and a lot of times there was no lightning nearby.

I suppose it's not likely an output relay problem if both of them will come on when they are not supposed to?

NO. It is very likely, if both fan outputs come on when they are not supposed to, that you could have one or more malfunctioning "stuck on" or welded-shut MicroLogix 1200 outputs. Do you know what kind of outputs your MicroLogix has? It can be relays, or AC transistors (triacs), or DC transistors. If you get the model number of the PLC, then we can look up the type and find out more about it.