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Old July 26th, 2018, 10:22 PM   #1
IdealDan
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Strange plc problem

Hi Friends,
I have a strange situation at hand. I wrote a Logic for Tank Over fill (5 Tanks). When Level switch senses level, it triggers Alarm.
Same code duplicated for the 5 Tanks but I noticed that when PLC is Restarted, a False Alarm Triggers for Tank 5.
I have checked the code and I/O Assignments, everything is tight. replaced the Level switch on that Tank 5, checked wiring, used another input point on PLC (MELSEC iQ-FX5U)
Yet no joy.

Has any one here met this type of issue? or could it be PLC Battery as there is no battery in it and I could not see one locally.

I'll appreciate your input.
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Old July 26th, 2018, 10:32 PM   #2
GaryS
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It would help you could post your code for the alarm.
One thing I have leaned id to put a 5 to 10 sec delay in the alarms this helps prevent false alarms.
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Old July 27th, 2018, 05:21 AM   #3
taward31
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Could be a dodgey sensor or transistor input circuit,

Maybe rewire that sensor to another input position. See if the problem follows or remains.

Also good practice to have a PLC Power up timer. Stop all logic from working until X amount of time after power up.

And also a debounce timer on the alarm as GaryS Suggested.
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Old July 27th, 2018, 07:38 AM   #4
AustralIan
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Troubleshooting is like invading a small to medium sized country - divide and conquer.

Write out everything involved in the water level high alarm, and pinpoint where it is.

Water -> level sensor -> cable - junction box - cable - terminals - plc card - backplane - adapter - Comms cable - Comms card - CPU - code - Comms card - cable - HMI - HMI program

There are also power supplies to various things.
Put a latch in the plc. Does it trigger? If so ignore the Comms to the HMI.
Disconnect the wire from the plc card and reboot. Does it trigger? If not, ignore your code and look at the electrics.
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Old July 27th, 2018, 09:23 AM   #5
James Mcquade
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I always put a power on and stable logic into the program.
when the system power up, have a 1 second timer to ensure power is ok before continuing to the remainder of the program.
I have had to do this with several sensors.
24 seconds with Cognex camera interface board.

james
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Old July 27th, 2018, 01:01 PM   #6
IdealDan
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Honestly, I have done almost all of this things without result,
When I added a code Reset everything for the first 5secs of powering PLC, it resolves it but that same Tank 5 will no longer work

Could lack of battery cause this?
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Old July 27th, 2018, 01:09 PM   #7
James Mcquade
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I just thought of this.

I copied programs from one system to the other (identical systems on the same machine) and had the same issue.

turns out I forgot to confirm the I/o addresses and failed to change an address.
also, I forgot to change a bit address from station 4 and left it on station 5.

that gave me fits for a good while. You might want to check that.

james
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Old July 27th, 2018, 01:11 PM   #8
damica1
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What PLC are you using?
__________________
David M. Camp

Marshall Electric / https://mei-tech.com/monitor-and-control

Please Download from our download section "Monitor and Control" .pdf.
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Old July 27th, 2018, 01:18 PM   #9
IdealDan
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Quote:
Originally Posted by damica1 View Post
What PLC are you using?
MELSEC iQ-F.
FX5U
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Old July 28th, 2018, 11:10 AM   #10
Ronnie Sullivan
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It's always a programming error.
You know it so well you don't see that M434 should be M443 etc.
Post the code.
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Old July 28th, 2018, 03:27 PM   #11
damica1
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Quote:
Originally Posted by IdealDan View Post
I have a strange situation at hand. I wrote a Logic for Tank Over fill (5 Tanks). When Level switch senses level, it triggers Alarm.
Same code duplicated for the 5 Tanks but I noticed that when PLC is Restarted, a False Alarm Triggers for Tank 5.
I don't work with the brand of Plc that you are using, but I think the logic would almost be the same over all plc's.

I always used a simple timer for all my alarms, this would be for debounce.
Example: the alarm condition has to be in the "Alarm Condition" at lease for a predetermined amount of time before action is taken.

2nd. If I am using any "latches" I always reset these latches on a power cycle because if the alarm condition is still present the latch will set again.

I agree with Ronnie Sullivan "Lets see the code" if this is a copy & paste type of duplication then one of the variables did not get updated.
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Marshall Electric / https://mei-tech.com/monitor-and-control

Please Download from our download section "Monitor and Control" .pdf.
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