ROC Alarm

New member here... I've got an issue & I could use your help.

I need help configuring a Rate-of-Change alarm in ControlLogix. The bin has a capacity of around 5,000 pounds with a weigh feeder discharging around 14 lbs/min out the bottom. This feedrate is variable depending on the overall system feedrate. When the bin weight falls to around 2000 pounds, it adds around 3000 pounds of mix from the premix system. The refill phase usually lasts around 3 to 4 minutes depending on the mix.

My ROC alarm needs to be configured to monitor the refill phase. That's easy enough but my issue is how to allow for the loss in weight of 14 lbs/min occurring during the refill phase. I only have the bin weight & the weigh feeder feedrate to work with. The premix system isn't on load cells.

Thanks in advance.

14lb/min or 14lb/sec ?
14lb/min would not seem significant enough to worry about during a 3 - 4 min discharge.

Sounds like you are trying to get your ROC alarm set too fine.

You stated that the refill usually takes about 3-4 minutes to dump 3000 lbs into the bin.

So if you assume that the refill rate is (worst-case) 750 lbs/min, subtract the 14 lbs/min being fed out of the bin, with everything working as it should your ROC of the refill will be 736 lbs/min.

But you also said "...depending on the mix...", so it would be wise not to set your ROC alarm anywhere near that figure, as the refill rate may be fluctuating wildly "..depending on the mix."

I would probably allow 5 minutes for the refill, and set the ROC alarm at 600, and see how it goes from there. You can always move it closer later.

You also said this is a ControlLogix system... are you aware of the ROC alarm built into the analog input module? You don't need to "roll your own".

daba said:

Sounds like you are trying to get your ROC alarm set too fine.

You stated that the refill usually takes about 3-4 minutes to dump 3000 lbs into the bin.

So if you assume that the refill rate is (worst-case) 750 lbs/min, subtract the 14 lbs/min being fed out of the bin, with everything working as it should your ROC of the refill will be 736 lbs/min.

But you also said "...depending on the mix...", so it would be wise not to set your ROC alarm anywhere near that figure, as the refill rate may be fluctuating wildly "..depending on the mix."

I would probably allow 5 minutes for the refill, and set the ROC alarm at 600, and see how it goes from there. You can always move it closer later.

You also said this is a ControlLogix system... are you aware of the ROC alarm built into the analog input module? You don't need to "roll your own".

Click to expand...

I see. It makes it much clearer to consider both in terms of lbs/min. I'll look at my ROC alarm template Monday & see if I can make this work. By the way, I am familiar with the built-in ROC alarm instruction but I've never used it. I've always been a "find existing code that works, copy it & load in my tags" kinda' guy. But if I could find some good, proven examples of how to use it I'd be more than willing to try.

I know the feed rate seems like a small amount in terms of the premix coming in, but the feed in/feed out ratio can tighten up quite a bit depending on the system feed rate & the density of the premix coming into the bin. The bin also has a "load out" feature to reduce the refill parameters as the product run nears its end. I need the alarm to work reliably no matter how close the feed in/feed out ratio becomes.