Scaling Anlog Input Channel in a Periodic Task -L5K

shiftedbits

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May 2007
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- Controllogix L61 PLC
- 1756- IF8 Analog Input Card set at RP1 of 75ms

I need to Totalize flow from a Mass flowmeter. Analog 4-20ma output from the flowmeter is the only available option as Pulse input from this flowmeter would require a High speed counter to be added. Latter is not an option due to rack space. Mass Flow Totalizing is done anywhere from 1 min to 5 minutes for a given totalizing cycle. Flowrate max is around 2k Grams/min. Accuracy being the most important, and this is the most critical of all the rest of the logic in the PLC, I have placed the totalizing routine inside a 100msec periodic task. I am also scaling the analog input for this channel in this same periodic task before the totalizing rung.

My question is, is scaling the Analog input inside a "Periodic Task" bad practice?

What about using a 25msec periodic task? Overkill?

Analog Card's minimum RPI is 18ms so I assume setting the Periodic task below this value is not going to buy anything in terms of Accuracy. Am I correct in this assumption?

Thanks-
 
Scale the input to engineering units on the analog input module, this releases controller resources. And anyway, if you did scale in the controller, and not in the periodic task, you will be using "stale" data. Do it on the I/O module.

The IF8 "Comm Format" also allows a "High-Speed" mode, which can RPI at 6mS.

Also check the input module RTS (Real-Time Sampling) rate as well, there's no point in a fast RPI if the analog module is converting every 100mS.

There is a good reference document that gives great detail on RTS/RPI settings - trying to find link to it.....
 
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Thanks Daba for the help and suggestions.

Minimum I can go down to on this card is 90ms for RTS.


Quick question regarding scaling. If I scale it in the module to EU vs Scaling in the controller to EU would I loose resolution?

Thanks
 
Thanks Daba for the help and suggestions.

Minimum I can go down to on this card is 90ms for RTS.


Quick question regarding scaling. If I scale it in the module to EU vs Scaling in the controller to EU would I loose resolution?

Thanks

No - the module does the same scaling calculation you could do in the controller...
 
If you have the function block language extension for RSLogix5000 then use the TOT instruction (Totalizer). It has built in timing and scaling features should you need them in addition to the in module scaling.
 
I am currently commissioning similar totalizator via 4-20ma inputs as well and am having excellent results. The periodic task must be highest priority to prevent any interruptions or increased processing time due to other higher priority tasks interrupting it. My setup is a control logix with flow meters wired to 1794-ie8 flex io cards. I have to scale this in the logic, and I do that just prior to the totalizator logic. I am within .2-.5 tenths of a LB on the local totalizer display on the flow meter. Extremely consistent. You may want to filter out the any low low flow input so the totalizer isn't adding tiny amounts of flow that in reality don't exist, might want to see if the meter has a min flow cutoff so you can ensure similar filtering in your logic.

I will add that my dosing ranges from 100lbs to 4000lbs with varying raw materials with different viscosities and my totalizator is spot on across all materials.
 
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