Help with Filtering

Tim Ganz · Feb 1, 2011

I have a question on filtering. I am working with a center winder that is ran by a compact logix brick processor and the program is filtering a laser sensr like this

Expression Local:3:I.Ch0Data/40+(Diameter_Input_Filtered*39/40

Destination Diameter_Input_Filtered

Can someone help me understand how this works? I am really struggling and math is not my strongest subject.

rdrast · Feb 1, 2011

Write it out and look. You basically have:

Output = Input/40 + (Output * 39/40)

That is just a slightly strange form of an averager, without having to devote a bunch of memory locations.
It integrates every scan, but might never actually converge.

Just being lazy and dropping that into Excel, even after 250 scans with a raw input of 20, it only hits 19.9661.

Tim Ganz · Feb 1, 2011

Is there a better way to average it in compact logix? It still is not filtered enough using what is currently there.

Peter Nachtwey · Feb 1, 2011

There is nothing strange about the filter. It is a low pass IIR fillter.
Y

=A*Y(n-1)+B*X

Where A=0..1 and B=1-A
If you want smooth the data even more then make A bigger. Right now A is 39/40. Try 0.99 and B=0.01
There is a lot information about IIR low pass filters on the web. One can calculate A and B in terms of a response time constant or cut off frequency.

Mickey · Feb 1, 2011

Here's another .

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

Tim Ganz · Feb 1, 2011

So i would do

Output = Input/0.01 + (Output * 0.99/0.01)

Is that correct?

Mickey · Feb 1, 2011

Tim Ganz said:
So i would do

Output = Input/0.01 + (Output * 0.99/0.01)

Is that correct?

No, that would give you a very large number. Run the math to see for
yourself.

jjh1982 · Feb 1, 2011

Tim Ganz said:
I have a question on filtering. I am working with a center winder that is ran by a compact logix brick processor and the program is filtering a laser sensr like this

Expression Local:3:I.Ch0Data/40+(Diameter_Input_Filtered*39/40

Destination Diameter_Input_Filtered

Can someone help me understand how this works? I am really struggling and math is not my strongest subject.

What type of laser sensor? Is it a "noisy" signal, or a "noisy" medium which the laser is sensing? There are some great laser sensors out there which I have personally used in high noise environments(material being sensed and high freq.) for dynamic height control. They filter the "environment's noise" for you!

creativepaper · Feb 1, 2011

I've attached a spreadsheet which graphically shows these 2 filters that they are mentioning.
I put in process change and noise amplitude.
I set the sample as twice the time so a small step will be seen for unfiltered.

Playing with the constants on the filters always helped me.

Tim Ganz · Feb 1, 2011

Thank you all for trying but it looks like i am in over my head with this one. This is some of my first plc work and i thought i would try to fix the problem but it will have to wait till the plc guru is back from vacation.

bernie_carlton · Feb 1, 2011

In your expression provided:

Local:3:I.Ch0Data/40+(Diameter_Input_Filtered*39/40

To make MORE filtering (and slower response) then Increase 39 and 40 by 1 at a time. The number at the '39' position will always be 1 less than the number at the '40' positions.

So for twice the filtering you could change it to:

Local:3:I.Ch0Data/80+(Diameter_Input_Filtered*79/80

And, as was being pointed out, more filtering will decrease the effect of noise and very short terms stuff but will make it slower to react to longer term changes.

Help with Filtering

Tim Ganz

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rdrast

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Tim Ganz

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Peter Nachtwey

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Mickey

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