I don't understand why you would want to reset the "I" value....
Maybe someone can clear this up for me...

It seems to me that when a PI loop is stable you will not have any
proportional contribution to the output, just an integral contribution to
maintain an output.
As your system gets close to setpoint, your proportional calculation works
it way
down to 'zero'.  As the loop settles down, using  a "P" only, you will have
an 'offset' from the
desired setpoint. This is because when SP=PV there is no proportional output.
 To reach setpoint you need to the "I" term.  This used
to be called "reset", and can be looked at like the old proportonal only
controlers, where the offset from setpoint was removed by the operator
'resetting' the setpoint  to get the Process Variable at the 'desired
setpoint'.

If you are worried about 'low freq' oscillations, and have to clear the "I"
term for some reason, then I don't think you have your loop tuned correctly.
If tuned correctly,  you would not normaly have any overshoot, or ringing
action in the loop..

Maybe I am missing the point.... if you are trying to make this system
look like the old cook oven controlers, so that you actualy do get a
constant swing of the PV around the setpoint, and don't really want
to end up with SP=PV... Then you would not want to use the "I" term
in your loop equations..  Otherwise  you would... Right????

Have a nice day...    AG


At 09:21 AM 5/18/99 -0500, you wrote:
>In my oven, when it is too hot I turn the output to the heaters off, and
>also set the I term to zero.  The controller can't GO any lower than off, so
>I term doesn't accomplish anything, and I don't want positive values of I
>stacking up and canceling the negative values of I which will be seen after
>the oven cools below the setpoint.  I think you need to dump the I term
>every once in a while.  When P=0 would work great if you have proportional
>control of your output.
>
>
>
>-----Original Message-----
>From: Tjaart van der Walt <tjaart@WASP.CO.ZA>
>To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
>Date: Monday, May 17, 1999 11:54 PM
>Subject: Re: PID algorithms in PIC C
>
>Good advice. I have noticed another thing that can make your
>system much more immune to the low-frequency I-oscillations :
>Reset the I-value to zero whenever P=0.
>
>