Re: [foxboro] 12 hour ave

I think there is some confusion about the "rolling average".  When I hear
"rolling average", I think of the sum of values for the last 12 hours
divided by the number of samples.  This is not the same as a first order lag
calculation (BTW, if you want a first order lag, you can use the CALC block
steps here or just use a LAG block)

For a simple example, consider sampling once an hour, so that the number of
samples for any given time is 12.  In Ray's lag CALC block, M02 is 12.
Assume the measurement has been zero for at least 12 hours, so that RI01,
RO01, M01 are all zero.  The measurement is now 12 for the hourly sample.
Therefore the average would be ((11 * 0) + 12)/12 or 1.  In Ray's lag
calculation, RI01 = 12, M01 = 1 = RO01 with M02 being 12.  So far, they are
the same.  The measurement now stays at 12 for an additional hour, so the
next sample measurement is also 12.  The 12 hour rolling average is now ((10
* 0) + (2 * 12))/12 or 2.  The lag calculation yields RI01 = 12, M01 = 1.92,
or about a 4% error from the actual rolling average.  Note that this problem
would become more pronounced if we didn't have a step function in
measurement, but allowed it to vary more randomly, or if we increase the
sample rate from 1/hour to 1/second or 3600/hr.

Note that the LAG block is similar a weighted average, with more recent
values given more weight based on the time constant.

Hope this helps,

Jim Mowrey   

-----Original Message-----
From: foxboro-bounce@xxxxxxxxxxxxx [mailto:foxboro-bounce@xxxxxxxxxxxxx] On
Behalf Of Ray J Chouinard
Sent: Friday, February 27, 2004 1:13 PM
To: foxboro@xxxxxxxxxxxxx
Subject: Re: [foxboro] 12 hour ave

Take the 1 sec analog measurement into a calc block with a period time of 
1 sec. through RI01.
and send the 12 hr rolling average out RO01.
Step 1   IN RI01
Step 2   SUB M01
Step 3   DIV M02
Step 4   ADD M01
Step 5   STM M01
Step 6   OUT RO01
Step 7   END

Where M02 is a constant equal to (1time constant in seconds) / block 
period in seconds.
8hrs = approx 1 time constant = 28800 seconds
M02 = 28800 / 1 = 28800

M01 is the storage for the running average (first order lag filter).

Ray Chouinard
Process Control Eng
MeadWestvaco
Escanaba, MI







"Brian Long" <blong@xxxxxxx>
Sent by: foxboro-bounce@xxxxxxxxxxxxx
02/25/2004 08:08 PM
Please respond to foxboro

 
        To:     <foxboro@xxxxxxxxxxxxx>
        cc: 
        Subject:        Re: [foxboro] 12 hour ave


I need a rolling 12 hour average that the operators can watch, otherwise 
I'd
use a reduction group.  Thanks for the help.

-----Original Message-----
From: foxboro-bounce@xxxxxxxxxxxxx
[mailto:foxboro-bounce@xxxxxxxxxxxxx]On Behalf Of Schouten, Frits JF
Sent: Wednesday, February 25, 2004 6:24 PM
To: 'foxboro@xxxxxxxxxxxxx'
Subject: Re: [foxboro] 12 hour ave


Do you want a rolling average or an average per shift or what?
Reduction groups can help you a fair bit.
Filtering and cascading blocks can do it too.
Perhaps use cascading DTIME blocks.
Utilise the bucket array to first average 10 seconds, then feed the 
average
into the next DTIME block and average 10 of 10, and then 10 of 100 and 
then
10 of 1000 and then......
12 hour seconds is 43200 samples. Fair bit to go.

Cheers,
Frits.


-----Original Message-----
From: Brian Long [mailto:blong@xxxxxxx]
Sent: Thursday, 26 February 2004 12:54
To: Foxboro
Subject: [foxboro] 12 hour ave


I need to take an analog input running at 1 sec and average it over 12
hours.  I need to display the value so I can't use the historian 
functions.
Anybody got a simple solution?
Thanks,
Brian Long
E&I Superintendent
Green Bay Packaging
Arkansas Kraft Division




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