>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 <spam_OUTtjaartTakeThisOuTWASP.CO.ZA>
>To: .....PICLISTKILLspam@spam@MITVMA.MIT.EDU <PICLISTKILLspamMITVMA.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.
>
>

>
> 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????
>

> > an 'offset' from the
>
> May be this question/idea is related to the behave of the two different
> PID
> algorithms - the positional PID and velocity PID.
>
> If I've understood this correctely the difference is like below:
>
> Positional PID:         Y =      P + I + D
> Velocity   PID:         Y =  Y + P + I + D + M
>
> The velocity  PID has the possibillity to be controlled mannualy ( M )
> while
> setting the control loop up.
>
> Also the reset of I in the velocity algorithem makes sense when you have
> reached
> the setpoint. Otherwise your loop will oscillate around the setpoint.
>
> If possible I would appreciate some comments confirming or correcting my
> statement's above.
>
> Kind regards,
>
>         Stefan Sczekalla-Waldschmidt
>         sswspam_OUToikossw.de
>

>Hi,
>
>Arnie Grubbs wrote:
>>
>> I don't understand why you would want to reset the "I" value....
>> Maybe someone can clear this up for me...
>>
>May be this question/idea is related to the behave of the two different
>PID
>algorithms - the positional PID and velocity PID.
>
>If I've understood this correctely the difference is like below:
>
>Positional PID:         Y =      P + I + D
>Velocity   PID:         Y =  Y + P + I + D + M
>
>The velocity  PID has the possibillity to be controlled mannualy ( M )
>while
>setting the control loop up.
>
>Also the reset of I in the velocity algorithem makes sense when you have
>reached
>the setpoint. Otherwise your loop will oscillate around the setpoint.
>
>If possible I would appreciate some comments confirming or correcting my
>statement's above.
>
>Kind regards,
>
>        Stefan Sczekalla-Waldschmidt
>        @spam@sswKILLspamoikossw.de