Ladybug

Compact Logic 1769

6 posts in this topic

I have just started programming PLC's and the first thing I have came up against is a 4-20ma signal to a analog card for a Turbidity meter. The problem I am having is the reading is very erratic. Is there any way to get it from jumping around so much?

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Why did you post the question twice?

If the signal is erratic you may have a wiring issue. Grounding the shield on both ends can turn the shield into an antenna. Loose connections can cause noise.  The sensor could be screwed up. 

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It may be changing frequently because it is read frequently and the result is changing.  There may be a way to make the module average several values, which will smooth it out a bit.  Or you can write something like that in code.  I'm not completely familiar with AB command set or analog ocnfiguration.

In some of the analog modules I have used in the past from other vendors, you can set the module to internally average a number of samples to provide a smoother result.

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Can you provide the catalog number of your analog input module. Some have build-in filter options.

See text below for a filter, you can program your self.

 

Quote

FV=FV+C(NV-FV) Where: FV= Filtered value C= constant (range 0-.99) The smaller the number the more dampening. NV= New Value Filtered Value= Filtered Value+Constant*(New Value-Filtered Value) New Value = Unfiltered Value Constant= 0.00-.99 The smaller the number the more damping.

 

Filtering.txt

Edited by Mickey

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Admittedly turbidity is probably not a safety related IO point, however, rather than patch the problem with filters on the IO point, why not figure out why the reading is noisy? Turbidity shouldn't be a fast moving IO point. I'm more inclined to believe there is a noise issue.

Slowing down or filtering an IO point is not something we do regularly in the oil and gas industry. We like our IO points to arrive intact and unmolested. If they are noisy we fix the problem. But, most of our IO is subject to Process Safety Management regs.

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Thank you all for the comments. I will try them and see what happens. Will let you know what happens.

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