Limit Switch Errors. | PLCS.net - Interactive Q & A - PLC Talk

CallumMacEwen said:

If you are looking for more details, kindly visit What Triggers a Limit Switch?.

...Does anyone have any other theories in what it could be?

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The electrical and software aspect of these machines are only part of the puzzle here. It's still quite possible that something is at fault with these limit switches themselves or even the program for these machines has a glitch. But I would always recommend people to step back from a process and consider the installation as a whole. Then develop likely theories and prioritise them. Then start checking from the top of the list down.

You should also consider other parts of the process which may be common to all machines. Hydraulics have been mentioned. Pneumatics may also be a possible cause here. If these were pneumatic cylinders extending and retracting and a common air supply pressure for the Plant was fluctuating or dropping intermittently then a PLC watchdog timer may possibly be timing out as the limit switches are slower to switch at certain times. This could explain why it happens across many of the 12 machines. I've seen that many times with air pressure faults. It could be an intermittent compressor fault, etc. If applicable, do the machines have a low pressure alarm?

If there are watchdog timers in a program then I often place logic to trap the longest duration the signals took to change beyond the watchdog time so as to see what is happening in the real world. The trapped durations are not reset with the watchdog timer and the longest duration will always be there to view if there are issues with that particular alarm condition.

I don't know enough about the installation to get into more specifics than that but my best guess from experience is that this will most likely be a single fault common to all the machines and not a different individual fault per machine.

Regards,
George

The electrical and software aspect of these machines are only part of the puzzle here. It's still quite possible that something is at fault with these limit switches themselves or even the program for these machines has a glitch. But I would always recommend people to step back from a process and consider the installation as a whole. Then develop likely theories and prioritise them. Then start checking from the top of the list down.You should also consider other parts of the process which may be common to all machines. Hydraulics have been mentioned. Pneumatics may also be a possible cause here. If these were pneumatic cylinders extending and retracting and a common air supply pressure for the Plant was fluctuating or dropping intermittently then a PLC watchdog timer may possibly be timing out as the limit switches are slower to switch at certain times. This could explain why it happens across many of the 12 machines. I've seen that many times with air pressure faults. It could be an intermittent compressor fault, etc. If applicable, do the machines have a low pressure alarm?If there are watchdog timers in a program then I often place logic to trap the longest duration the signals took to change beyond the watchdog time so as to see what is happening in the real world. The trapped durations are not reset with the watchdog timer and the longest duration will always be there to view if there are issues with that particular alarm condition.I don't know enough about the installation to get into more specifics than that but my best guess from experience is that this will most likely be a single fault common to all the machines and not a different individual fault per machine.Regards,George

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