PLC add function

I replaced a flowmeter and now the flow from the new flowmeter shows up at the correct address but it doesnt add to 14 other flows as it should. I know the signal is coming into the plc it just doesnt add to the other flows as it should and used to.

OK, it's been a long day. I'm a little slow. What's the question here?

Keith

I make it standard design philosophy to never show two flow meter readings from different meters, or a totalized reading plus a single common flow meter reading, in a way that operators can see them both at the same time. The totals never never never agree exactly, it usually doesn't affect control precision one little bit. I get really tired of explaining tolerances, the loss of world perfection since Eden, and the ability of our superb control system to deal with life's little imperfections.

The system has been operating for six years, we monitor the 14 flows individually and the total input and output of the system, After replacing the flowmeter, the total input is exactly 20 gpm low which is the flowrate thru the replaced meter.

there are 14 add functions in the ladder to total the flow, it appears that the one for the new flowmeter is not adding for some reason. the address for the input from the flow meter shows the correct flow but the address for the total flow is off by exactly 20 gpm

Your problem would appear to fall into the "I'd have to see it to believe it" category.

It would probably help if you posted details like the make and model of both the PLC and the flow sensor. You should also post your logic. Are you sure that the only thing different between now and the last time it worked properly is the sensor? No changes to the PLC program such as 'work-arounds' to compensate for the old sensor having failed while you were waiting for the new sensor to arrive?

I cant change the program because i replaced the hard drive in the computer and the rslinx wont work, it is AB PLC 5 and SLC 500 that is monitored by Intouch. It seems to me that it may be that the ladder rung with the add function isnt "true" but i dont know how to check that

I get really tired of explaining tolerances, the loss of world perfection since Eden

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Tom where were you on the "how can I measure parts per hour" thread!
Dittos, mega style.

Can you say, "I don't know how to calibrate this device?"

dnelling please register and join the forum, there is allot of great people here but sometimes we get tired of hearing of the same problems. Most of the so called new problems have already been answered. Check the archives, (do a search on analog scaling). register and welcome to the only place where your gonna make a difference to some body, some day.

perhaps a bitless
but I'm a Bitmore

ps. sorry for the edit I only had one L in your name
oh and I forgot one thing, is it fixed now? I mean your Hard drive RSLinx other problem? Or what?

It seems to me that it may be that the ladder rung with the add function isnt "true"

Click to expand...

You're probably right.

but i dont know how to check that

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You really need to get in touch with somebody who does. If there is nobody at your facility who can get your programming software connected to the PLC, then you need to call in some outside assistance.

A Fable

I agree with Steve on this on.

My guess is that the story goes like this:

Once upon a time, there were 15 good little flowmeters at the plant, and they all were added up together.

Then one day, one of the little flowmeters got sick. The big bad plant engineer said, "If we keep adding the number in the address of the sick one to the rest of the flowmeters, we are going to get the wrong answer".

So he removed that address from the summation. And he didn't tell anyone (I said he was bad).

Then along came the heroic maintenance tech. He nurses the flowmeter back to health (probably by ashcanning it and replacing it with a new one).

And - hooray - the reading is back to what is should be. But what's this? It's not playing with other little flowmeters! Oh, can someone please help me?

And the folks at PLCs.net heard his pleas, but were puzzled, since we couldn't see what he sees, and we don't know what he knows.


So, dnelling, how does this story end?

All I did was replace a flowmeter. There has been no change to the plc program. It is not possible to change it at this time because the rslinx doesn't work. I know I can get it working if I have to but it doesn't seem that I should need it because everything was fine before I replaced the flowmeter. I just want to reboot or restart the PLC and see if that makes it add the flow from the replaced flowmeter to the total flow. I didn't turn off the system when I replaced the flowmeter so I think it might need to be rebooted. Does this make any sense?

Go ahead and try anything that you think you can do without hurting anybody.

If I were in your shoes, I would want to understand what's happening, so I'd get RSLinx working and connect up to the PLC and take a look at the raw data from the analog input as well as the ladder logic.

How do you know that you're getting data from the flow transmitter into the PLC if you can't monitor the analog input channel with your programming software?

I'll assume (gosh I hate that word) that your flowmeter uses a 4-20mA signal to tell the PLC how much flow is being measured ...

have you checked (with a meter in series) to make sure that the 4-20mA signal is ok? ... and that it has been hooked up to the PLC with the right polarity? ...

basic idea: IF you took out one flowmeter and replaced it with an identical one ... and IF the new one is properly calibrated ... and IF the signal wires are correctly attached ... then the new flowmeter should "add" to the PLC's flow value without any trouble ... specifically, you should NOT have to "reboot" the PLC ...

I use Intouch to monitor the system. I can see the flow rates thru Intouch and I have a print out of the ladder diagram so i know the addresses of the data in the plc. If I turn the key on the plc to program and back to run will it reset?

so you're saying that InTouch DOES see the flowrate correctly? ... if that is true, and we assume (gosh I hate that word) that InTouch gets all of its input information from the PLC, then the PLC must see the flowrate correctly also ... so that means that the actual input signal (4-20mA ???) is really ok ...

then it looks like you're going to have to get into the PLC and see why the "add" function isn't working ...

I sincerely doubt that cycling the keyswitch is going to help ... but like Steve says, if you're SURE that it won't hurt anything, go ahead and give it a try ...

care to tell us what PLC addresses all of your flowmeters use? ... here's just a wild guess: maybe InTouch is looking at the wrong flowmeter ... maybe it's ALWAYS been looking at the wrong flowmeter and nobody noticed until you changed the hardware over ... like I said, it's just a guess ... but things like that happen sometimes when you have 14 or 15 inputs and they all have similar addresses ...

once again, it wouldn't be a bad idea to double-check the actual 4-20mA signal with a meter ... that would just be a logical thing to do ...