PLC 5 Temp Rate Change Alarm

[NucorElec]

We are using PLC 5 and currently have "Temp Rate Change Alarm" that is basically doing a compare every 8 seconds and alarming on any change of over ten degrees. We have recently been getting some nuisance alarms due to noise or interference causing temporary spikes. I am trying to think through ways of writing the code so that the rate of change is either averaged over that scan time or at least that the alarm threshold has to be met (or sampled) twice over an 8 sec period before giving the alarm. Help me out gurus! Thanks!

 

[RonJohn]

Welcome to the forum! I only have a brief moment, so let me suggest you search this site for terms like "moving average", "analog filter", etc. This same dilemma has been presented and resolved on numerous occasions here.

 

[NucorElec]

Thanks!

 

[Ron Beaufort]

if your existing method of determining a "rate-of-change" problem is working OK – and depending on how quickly the nuisance trippings are occurring (and "clearing") – then you might try this "debounce" idea:

leave your existing rate-of-change alarm "as is" – but instead of directly making use of your existing alarm condition – program that condition as the input to a TON timer ... then if the condition stays ON for too long (determined by the Preset of the timer) then use the timer's Done bit to signal (by latching or by "sealing-in") the actual alarm ...

basic idea:

if the "bad condition" only stays on for a brief period, then the "bad condition" doesn't generate an alarm ... instead you only declare an actual "alarm" if the "bad condition" stays "bad" for a certain continuous amount of time ...

the following link might be helpful ...

http://www.plctalk.net/qanda/showthread.php?p=538924&postcount=5

 

[Mickey]

We have recently been getting some nuisance alarms due to noise or interference causing temporary spikes.

Why not try to find out what is causing the above.

 

[NucorElec]

Thanks Ron B that's what I was looking for.

Mickey we will be finding and correcting the issue but we can't get on the furnace roof while arching/making steel so I was just looking for a temporary fix until our next maintenance down day. Put a Band-Aid on it! Ha