PROC: PLC as flow totalizer

[The AList]

------------ Forwarded Message ------------
From: Roy Matson
To: AUTOMATION at CONTROL.COM
Subject: Re: PROC: PLC as flow totalizer

All these fancy timing methods are OK, but don't forget the accuracy of your flowmeter is probably +/- 0.5% at best. So what's the point in using super accurate timers?

Before real time clocks we used timers & counters with a few seconds correction at midnight with quite accurate results.

Roy


On July 1, 2008, Keith Hogan wrote:
> You will need to sample your flow rate at an accurate, constant rate (dT).  Depending on the accuracy required, you should also consider using something like trapezoidal rule numerical integration to get more accurate results:
Total(n) = Total(n-1) + (Flow(n) + Flow(n-1))/(2*dT)
> 
> Finally, you need to consider round-off errors as your total value becomes large in relation to the small increment in flow you add every calculation period.  Using double-precision floating point for the total calculation can help.
> 
> Rockwell Automation's ControlLogix and CompactLogix controllers have a built-in Totalizer instruction made to handle this exact application and which uses the above techniques to improve accuracy. <

( Complete thread: http://www.control.com/thread/1026246378 )