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PICList Thread
'Data Filtering'
2000\05\09@121322 by Mark Peterson

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I am working on an application based around a PIC16C715 that monitors the
density a gas in a closed vessel.  If there is a leak in the system, I need
to be able to predict a time-to-alarm value.  I have sensors and
conditioning circuitry that provide pressure and temperature analogs to the
PIC every five minutes.  From these two values, I calculate the density of
the gas.  I want to use the slope of roughly an hour's worth of density
values to calculate the time-to-alarm value.

The basic part of this scheme is fairly straight forward.  Where I am
having trouble is with filtering spurious data values.  In the environment
in which this monitor will be installed, there will definitely be
occassional spike values that if not filtered out, could yield a
time-to-alarm value that would cause undo action.  The filtering must be
done in the PIC code.

I have played all kinds of games with routines to pick out single oddball
values by comparing the relationship with points before and after each
value and throwing out values that are out of whack, by using linear
regression to smooth out an hour's worth of data, etc.  I dropped out of my
stats class in college and struggled through numerical methods.  Now I'm
hurting for it.  Can anybody point me to a source that could provide some
information and methods for data smoothing and filtering?

Thanks.

Mark P

2000\05\09@122608 by Andrew Kunz

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I think what you want is a median filter.  Take multiple samples, sort them,
then pick the one in the middle of the list.  Your spurious readings are
extrema, right?

Andy

2000\05\09@124441 by Alan B Pearce

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Go to this web site and download the original QST article for a GPS based
frequency standard (originally published in QST, July 1998). The software in the
PIC has filtering to stop glitches upsetting the VCO which is locked on to the
satellite. This sounds like the sort of filtering software you are after. You
may be able to get him to supply you with the source code, which you may well be
able to cut and paste into your application.

2000\05\09@130321 by Alan B Pearce

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This time I will try putting the web address in
http://www.rt66.com/~shera/

2000\05\10@020141 by Dr. Imre Bartfai

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Hi,

you,

On Tue, 9 May 2000, Mark Peterson wrote:

> I am working on an application based around a PIC16C715 that monitors the
> density a gas in a closed vessel.
[snip]
> I have played all kinds of games with routines to pick out single oddball
> values by comparing the relationship with points before and after each
> value and throwing out values that are out of whack, by using linear
> regression to smooth out an hour's worth of data, etc.  I dropped out of my
> stats class in college and struggled through numerical methods.  Now I'm
> hurting for it.

Could we learn what was the problem with trials described above?

Regards,
Imre

2000\05\10@042451 by Alan B Pearce

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I did not see my correction for the missing URL posted on the list, so will
repost it now. Sorry if this results in a repeat message in your mailbox.

Go to this web site and download the original QST article for a GPS based
frequency standard (originally published in QST, July 1998). The software in the
PIC has filtering to stop glitches upsetting the VCO which is locked on to the
satellite. This sounds like the sort of filtering software you are after. You
may be able to get him to supply you with the source code, which you may well be
able to cut and paste into your application.

This time I will try putting the web address in
http://www.rt66.com/~shera/

2000\05\14@165722 by Alan B Pearce

face picon face
I did not see my correction for the missing URL posted on the list, so will
repost it now. Sorry if this results in a repeat message in your mailbox.

Go to this web site and download the original QST article for a GPS based
frequency standard (originally published in QST, July 1998). The software in the
PIC has filtering to stop glitches upsetting the VCO which is locked on to the
satellite. This sounds like the sort of filtering software you are after. You
may be able to get him to supply you with the source code, which you may well be
able to cut and paste into your application.

This time I will try putting the web address in
http://www.rt66.com/~shera/

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