How to average in a SLC

In this application I need to monitor duct pressure with an analog input, and control an air make up damper with an analog output to maintain a set-point. My problem is the input could fluxuate greatly. Is there any way to average this value?

Welcome to the forum.

See the sample program right here: http://www.plctalk.net/qanda/showpost.php?p=260919&postcount=5

That program was written for a MicroLogix that supports 32 bit long integers. If this is for a SLC then I caution you to watch out for integer overflow if using N addresses instead of F addresses.

Maybe a filter would work.

FV=FV+C(NV-FV)
Where:
FV= Filtered value
C= constant (range 0-1) The smaller the number the more dampening.
NV= New Value

Click to expand...

I recommend trending it both filtered and unfiltered, and you might find averaging after a filter works best for such a sensitive reading as duct pressure which is libel to carry harmonics from the fans which are pulling/pushing against it...

Also, investigate filtering options in certain SLC input cards.

But when trending the signal in RSLogix, and comparing it to the results of the filtered signal, you can easier judge what you want your SLC control algorithm to "see" and select/adjust filter, average, time weighted average or some other combinations until it looks right.

Give us part numbers and details if you can for you input cards, SLC CPU, Software you're using ... as much as you can share will get great results here.

Paul

Thanks for the information everybody, this was very helpful. I appreciate the quick responses, the PLC community is a good one.

I had a problem with conveyor motors using a Micrologix 1100 that I averaged out with a sequencer and was wondering if your FIFO running average would be a better way to go.

Hmmm. I guess I don't have RSLogix on this PC yet. But for sure if Alaric linked you up with an averaging algorithm, it will work as advertised and not cause a problem. I can't imagine using a sequencer for this so I will have to peak at this code tomorrow if I get my panel building further along.